Characterization of synthetic polymers by MALDI-MS

Prog. Polym. Sci. 31 (2006) 277–357 www.elsevier.com/locate/ppolysci

Characterization of synthetic polymers by MALDI-MS Giorgio Montaudo a,*, Filippo Samperi b, Maurizio S. Montaudo b b

a Chemistry Department, University of Catania, Viale A. Doria 6, 95125 Catania, Italy Institute of Chemistry and Technology of Polymers, CNR, Viale A. Doria 6, 95125 Catania, Italy

Received 23 June 2005; received in revised form 30 September 2005; accepted 20 December 2005

Abstract In recent years, matrix assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectroscopy has become a routine analytical tool for the structural analysis of polymers, complementing NMR and other traditional techniques, a noteworthy change with respect to the past, when mass spectrometry (MS) was seldom used. In this review, we discuss salient aspects of MALDI. First, we devote a section to fundamentals and practice in MALDI of polymers (such as the laser, ion source, ion optics, reflectron, detector, ionization efficiency) as well as to some basic concepts of sample preparation (such as the MALDI matrix and cationization agents). Then, we focus on measurable quantities of polymers: average molar masses, the chemical formula and the structure of the monomer (actually of the repeat unit), the masses of the chain end groups, etc. In-depth coverage is given of coupling MALDI with liquid chromatography (LC), since often LC offers valuable help in exploring macromolecules. The final section is devoted to recent applications, with a detailed discussion of MALDI of addition polymers, condensation polymers, polymers with heteroatoms in the chain, copolymers and partially degraded polymers. q 2006 Elsevier Ltd. All rights reserved. Keywords: MALDI; Polymers; Copolymers; Polymer degradation; LC-MALDI

Contents 1. 2.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fundamentals and practice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Ion extraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. Resolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5. MALDI matrices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6. Developments in sample preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.7. Sample preparation for low molar mass compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.8. Doping agents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.9. Ionization efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.10. Measurement of molar mass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.11. Coupling MALDI with devices that separate macromolecules by size . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.12. Coupling MALDI with devices that separate macromolecules by functionality or by composition . . . . . .

* Corresponding author. Tel.: C39 95339926. E-mail address: [email protected] (G. Montaudo).

0079-6700/$ - see front matter q 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.progpolymsci.2005.12.001

278 279 279 279 280 280 282 283 286 287 287 290 294 298

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G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357

2.13. Structure determination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 300 2.14. End group determination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303 2.15. Tandem mass spectrometry for structure determination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 304 2.16. Copolymer characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 308 2.17. Bivariate distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 309 3. Recent applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 309 3.1. Polystyrene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 309 3.2. Polymethylmethacrylates and acrylic polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 311 3.3. Other polymers with an all-carbon main chain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315 3.4. Polymers with heteroatoms in the main chain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315 3.5. Polysiloxanes, poly(silsesquioxane)s and polysilanes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315 3.6. Polyethers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315 3.7. Polyesters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 316 3.8. Polycarbonates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 317 3.9. Polyamides and polyimides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 317 3.10. Polymers with phenyl and other cycles in the main chain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319 3.11. Copolymer studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 320 3.12. Polymer degradation studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 327 Appendix A. Size exclusion chromatography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 341 Appendix B. Copolymer composition from MS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 342 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 344

1. Introduction Polymers display a variety of structures, including linear, cyclic, and branched chains, copolymers with various architectures, dendrimers, and star polymers with different number of arms. The structural characterization of a polymer sample usually involves: evaluation of the average molar mass (MM) and of the molar mass distribution (MMD); determination of the repeat units structure; copolymers sequence analysis; end group identification; detection and identification of impurities and additives. Modern mass spectrometry (MS) offers the opportunity to explore the finest structural details in polymers [1–10]. Matrix assisted laser desorption/ionization time-offlight (MALDI-TOF) has dramatically increased the mass range of MS; it provides mass-resolved spectra up to 50–70 kDa and above, allowing the detection of quite large molecules (106 Da), even in complex mixtures, at the femtomole level [3]. Peaks in the spectra originate from ions of intact polymer chains, and, therefore, allow structural identification of single oligomers. The last few years have witnessed outstanding progress in the application of MALDI to open problems concerning the characterization of polymers. Initially, MALDI-TOF instruments had poor spectral resolution (M/DM about 500): i.e. mass-resolved spectra usually did not go beyond 10,000 Da. This caused structural identification problems, even in the lowest mass range. Therefore, MALDI-TOF spectral

data on polymers that appeared in earlier papers (up to about 1998) may need updating. MS yields information on the masses of individual oligomers, a remarkable difference with respect to NMR, which is an averaging method. Therefore, besides providing unequivocal information on the chemical structure of polymeric materials, MALDI allows the identification of chain end groups, including species present in minor amounts in a polymer sample. End group identification is so crucial in polymer analysis that its importance cannot be overemphasized; in fact, it has been one of the most popular applications of MALDI to polymers. The determination of the end group structure of intact polymer samples often has interesting side effects, namely, the identification of procedures used in the synthesis of research and industrial polymers, and the capture of information on the structure of capping agents and additives. Applications of the MALDI technique to the characterization of synthetic polymers have been summarized [3,4]. However, this field has recently experienced considerable progress, and it is our purpose to illustrate advances in the fundamental and practical aspects of the MALDI-MS technique related to polymer analysis. Among other topics, our attention is focused on recent advances in: MALDI sample preparation, achieving high mass resolution, detailed structure identification in polymers and copolymers, accuracy of molar mass determination, functional and end group identification, copolymer sequence analysis, in situ


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detection of photo and thermal oxidation products of polymers, monitoring polymerization reactions, and coupling MALDI with liquid chromatography. We discuss MALDI literature concerning selected classes of polymers, and our bibliography includes about 400 references selected from papers that appeared in 2000– 2005. MALDI applications are classified on the basis of the polymer backbone: polymers of styrene and its derivatives [11–63], acrylic and methacrylic polymers [64–102], other all-carbon polymers [103–127], polymers with heteroatoms in the chain: namely polysiloxanes, poly(silsesquioxane)s and polysilanes [128–148], polyethers [149–205], polyesters [206–250], polycarbonates [251–261] polyamides and polyimides [262–283], polymers with an aromatic ring in the backbone [284–311], and copolymers [312–365]. Studies of partially degraded polymers are discussed separately [366–393].

potential, ranging from 15 up to 35 kV. Homemade instruments may use lower voltages, but they require careful tuning. For instance, the first MALDI instrument (used by Hillenkamp and coworkers in Mu¨nster for their pioneering experiments [1]) had only 3 kV. Depending on their mass-to-charge ratio m/z, the ions have different velocities when they leave the acceleration zone and enter a field-free flight tube (drift-tube) 1 or 2 m long. After a time-of-flight of the order of 100 ms, the ions impact onto an ion detector, often formed by two microchannel plates connected in series. The detector produces a signal (proportional to the number of ions arriving at the detector), which is processed by an ADC converter (using a clock with a time base of 2 ns or better). The ADC is connected with a computer, in which the resulting MALDI spectrum can be stored and processed (e.g. for smoothing, etc.).

2. Fundamentals and practice

The detector amplifies the signal by a factor of about 107; and therefore, it is a very-high-gain amplifier. This gain cannot be achieved without the presence of a ‘pool’ of secondary electrons and, when the amplifier’s task is too demanding, the ‘pool’ may become empty. This annoying effect is called detector saturation. The presence of low molar mass compounds in the polymeric sample can cause detector saturation and, in turn, the amplification of the signals due to low and high molar mass ions becomes uneven, the latter being much reduced. This effect is particularly evident in samples difficult to desorb at moderate laser power. The user has various possibilities in order to avoid (or at least to reduce) the negative consequences of detector saturation [4,5]. First, MALDI mass spectrometers are equipped with an electrostatic device, the deflector, which acts as a cut-off, since it pulses away low molar mass compounds and does not allow them to reach the detector. The efficacy of this device is high and thus the use of a deflector is very popular. However, it has a drawback: namely, the on–off switching needs a small (but not infinitesimal) time, and thus, the trajectories of ions possessing masses close to the cut-off are not straight and can appear in the spectrum as artifacts. The second possibility is to leave the detector off initially and to turn it on when high-mass ions start hitting the detector. In this way, the pool of secondary electrons is fully available for high-mass ions. However, the two procedures described above ‘neglect’ low molar mass compounds, assuming that they are unimportant, and this assumption is not necessarily true. As an

2.1. Ion extraction MALDI makes use of short pulses of laser light to induce the formation of intact gaseous ions. Analyte molecules are not directly exposed to laser light, but are homogeneously embedded in a large excess of ‘matrix’, which consists of small organic molecules. The matrix molecules strongly absorb the laser light to allow for very efficient energy transfer to the analyte (in our case, the polymer). The high energy density obtained in the solid or liquid matrices (even at moderate laser irradiance) induces instantaneous vaporization of a microvolume (called a ‘plume’), and a mixture of ionized matrix and analyte molecules is released into the vacuum of the ion-source. The laser pulse must not be too long, otherwise analyte molecules do not all desorb at the same time. On the other hand, there is no advantage in using ultrashort pulses (fractions of picoseconds) and there are many disadvantages (for instance, a laser which generates ultrashort pulses is expensive and bulky). The nitrogen laser, operating at a wavelength of 337 nm, has a very compact design, it is pulsed and its shots last about 3 ns, which is perfect for the scope of MALDI. In commonly used instruments (those equipped with a time-of-flight tube), the laser pulse is followed by a time delay, lasting 300–800 ns, before the application of an extraction voltage, which brings the ions out of the ion source. After this time delay, the packet of ions generated in the process is accelerated by an electric

2.2. Detector

280


alternative, one can replace microchannel plates with another type of detector (for instance a hybrid detector, in which the signal coming from the first microchannel plate is processed by another detector, which is also a high dynamic-range amplifier). In this way, low molar mass compounds are not neglected. 2.3. Calibration The MALDI-TOF mass spectrum is then obtained by recording the detector signal as a function of time. According to Eq. (1), the square of the flight time is proportional to the m/z ratio m=z Z 2Vt2 LK2

(1)

where m is the mass of the ion, z is the number of charges, V is the accelerating voltage, t is the ion flight time, L is the length of the flight tube. In principle, since V and L are known, the m/z ratio can be calculated solely from Eq. (1). In practice, exact values for the mass scale are obtained using another (empirical) formula m=z Z at2 C b

containing the known ions is recorded at the beginning of the MALDI session to obtain a and b. Thereafter, other spectra are recorded and it is assumed that a and b do not change. This procedure is called ‘external calibration’, since the two peaks do not belong to the spectrum under analysis. Another procedure, called ‘internal calibration’, for time-to mass conversion yields more accurate m/z values since the two peaks used to determine the constants a and b are internal (i.e. they belong to the spectrum under study) [161]. However, internal calibration is not used frequently, owing to the fact that the calibrant must be selected carefully to avoid loss of spectral quality.

(2)

because of uncertainty in the determination of flight time due to a short delay in ion formation after the laser pulse. Hence, the true starting time of the ion is not identical with the time of the laser pulse (which provides the starting signal for the measurement of flight time). The constants a and b in Eq. (2) are estimated from the measured flight times of two ions with known masses. Usually, a preliminary calibration spectrum

2.4. Resolution Most MALDI-TOF instruments possess a device to enhance the resolution, a reflectron, which consists of a series of electrodes placed at the end of the flight tube. The reflectron must be used in conjunction with an additional detector (usually called the reflectron detector) placed at the opposite side with respect to the other (ordinary) detector. Fig. 1 shows the scheme of a reflectron MALDI-TOF mass spectrometer [7]. When the electrodes are turned off, the MALDI spectrum is recorded in the linear mode, whereas, if they are turned on, the spectrum is recorded in the reflectron (or reflected) mode. The reflectron causes a decrease in sensitivity, and therefore, it cannot be used with polymers which show a recalcitrance to desorb, such as dimethylsilaferrocenophane [81], polymers of high mass [4] (above 40 kDa), polymers terminated by

Fig. 1. Diagram of the TOF/TOF mass spectrometer showing the location of the collision chamber, the mass selection point for the first mass analyzer, and the second mass analyzer with a curved field reflectron. Reproduced from Ref. [7] with permission of the American Chemical Society.


bulky end groups, such as terpyridine [182,214,234] and polymers of, dibutyl-substituted thiophenes [287]. Commercial MALDI-TOF instruments became available soon after the first MALDI experiments. Some of them were equipped with a reflectron; but despite this, they had poor spectral resolution (M/DM about 500). This means that mass-resolved spectra usually did not go beyond 10,000 Da; and this caused structural identification problems, since the resolution in the 1000–2000 Da range was uncertain. However, the discovery that a time-delay produces MALDI spectra with better resolution came later in 1995; and it is now possible to build MALDI mass spectrometers with higher resolution by adding a fast high-voltage electronic switch that allows for a time delay between the laser shot and the extraction. This discovery was patented under the name ‘delayed extraction’ (DE) but it is also referred to as ‘time-lag extraction’ or ‘pulsed extraction’. A series of papers dealing with DE-MALDI spectra appeared soon after this discovery. In one of them, the authors recorded DE-MALDI spectra of bradikynin, cytocrome C, apomyoglobin and other peptides [394]. Another paper presented the DEMALDI spectrum of a mixture of two oligonucleotides [395], which were quite similar (the masses were 9471.2 and 9486.2 Da, respectively, and thus the mass difference was only 15 Da). Researchers in the field of polymers were immediately aware that a mass spectrometer with enhanced resolution could be particularly useful [396–399]. MALDI instruments built before the availability of ‘delayed extraction’ must be considered obsolete, and spectral data that appeared in earlier papers (up to about 1997–1998) may need updating. Obtaining good resolution with complex samples is important. Cai et al. [344] synthesized a Y-shaped copolymer via Michael addition of 2 equiv. of 2-hydroxyethyl acrylate to a commercial monoaminecapped poly(alkylene oxide), Jeffamine XTJ-507, followed by esterification using excess 2-bromoisobutyryl bromide. Fig. 2 reports the MALDI spectra of the initial Jeffamine sample (Fig. 2a) and of the Y-shaped copolymer (Fig. 2b), along with its structure. The latter spectrum suffers from poor resolution: peaks do not ‘pop out’, the valley between peaks is shallow and there is substantial overlap among neighboring peaks. On the other hand, it is quite apparent that the first spectrum is perfectly resolved: the valley between peaks is very deep (it touches the baseline) and neighboring peaks do not overlap. This is a rare case in which the problem of poor resolution can be circumvented. By comparing the spectra; it can be seen that the mass of the second

281

Fig. 2. MALDI spectra of a Jeffamine sample (a) and of a Y-shaped polymer derived from Jeffamine (b). The figure also reports the structure of the Y-shaped polymer. Reproduced from Ref. [344] with permission of the American Chemical Society.

sample is about 500 Da higher than that of the initial Jeffamine sample; and this is in line with the proposed structure. However, in general, spectra with poor resolution must be discarded. As an alternative to TOF analyzers, MALDI instruments can be equipped with Fourier transform ion cyclotron resonance (FT-ICR) analyzers. They are limited in molar mass, and give poor results when used to analyze polymers with masses of 20 kDa or above. However, for molar masses in the range 1000–8000 Da FT-ICR analyzers possess a distinct advantage, namely

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the resolution is astounding (one part in 30,000 and above). There are cases in which such resolution is needed. Mize et al. [213] recorded the MALDI spectra of two homopolyesters and two copolyesters, using a MALDI instrument equipped with FT-ICR. In one of the samples, two components, denoted E1n and E2n were present, E1n having a mass only 2 Da higher than E2n. Clearly, the isotopic patterns of the two were partially superposed. For example, E2n(0), the E2n chains with no 13C carbons, were (almost) isobaric with E1n(2), the E1n chains with two 13C carbons. As a result of the superposition, one of the two components (the second one) was hidden, and its presence was difficult to detect. However, E2n(0) and E1n(2) do not have exactly the same mass and the FT-ICR MALDI instrument was able to spot the very small difference in mass (only about 10 ppm at mass 1800) and two distinct MS peaks appeared in the spectrum, revealing the presence of the second species. In the case of a copolymer with ethylene and CO units [342], many oligomers are also almost isobaric, since the two repeat units are almost isobaric at about 28 Da), the difference being 36 mDa. In the mass region 639–640 Da, five ions are expected at masses 639.28, 639.32, 639.36, 639.39, 639.42 Da, respectively. Fig. 3 shows the expansion of the FT-ICR MALDI spectrum of the copolymer in this region and identifies the peaks. The resolution of the peak at 639.355 Da can be estimated directly from Fig. 3. In fact, the full width at

half-maximum is about 10 mDa, and thus the resolution is about 1 part in 64,000. Thanks to the high resolving power of FT-ICR, the five ions are fully resolved. Baker et al. [334] prepared a copolymer with units of glycidyl methacrylate (GMA) and butyl methacrylate (BMA) by using a free-radical initiator Vazo-52 and a cobalt chain-transfer agent Co(dimethylglyoximeBF2)2. Two types of copolymer chains are expected:

A huge number of almost isobaric structures are possible due to the fact that GMA and BMA have the same nominal mass but slightly different exact masses, with BMA 0.036 Da greater than GMA. When the authors used a TOF instrument to analyze the copolymer, the MALDI spectrum was characterized by insufficient resolution. However, there are reasons to believe that an FTICR instrument would have better chances to detect the cited isobar structures as separate peaks [365]. GMA– BMA copolymers obtained using the Vazo initiator were successfully characterized by electrospray ionization (ESI) [365]. 2.5. MALDI matrices

Fig. 3. Expanded view of the MALDI-FT spectrum of an E–CO copolymer showing resolutions of isobaric 19-mer oligomers. Peak shoulders arise from 109Ag isotopes of oligomers of the same monomer number with an additional unsaturation site. Reproduced from Ref. [342] with permission of the American Chemical Society.

In MALDI-MS analysis, a dilute solution of the analyte polymer is mixed with a more concentrated matrix solution. The number of molecules nmol formed in the desorption/ionization process decreases very fast as the laser irradiance is turned down (often nmol falls as the irradiance to the eighth power). However, it is well known that small values of nmol (e.g. 100 or 1000) are never found. There exists a threshold irradiance, peculiar to each matrix, below which ionization is not observed. Above this level, the ion production increases nonlinearly. The choice of a matrix tailored for a particular kind of polymer sample is crucial for successful characterization of the sample and is usually done in two stages. In the first step, only the backbone structure is considered and this implies that chain end groups and the average molar mass (which can be high and low) are unimportant. The MALDI user searches through the literature and retrieves a set of three or four candidate matrices, which are optimal for that kind of backbone structure. In order to speed up the process, one can use tabulations of MALDI matrices and sample


COOH

COOH OH

N

283

OH O

OH

N OH

HO HABA

DHB

Dithranol

COOH

H C

C

COOH

H

CN

C

COOH

C H

N OH

all-trans-retinoic acid

IAA

α -CHCA Fig. 4. The structure of some common MALDI matrices.

preparation recipes, such as the document which appears at the NIST website [16] or the listing produced by Nielen [8]. Then, the user records the MALDI spectra, using all the candidate matrices, and identifies the highest-quality spectrum to select the best matrix. Notably, for polar polymers the ‘optimal matrix’ is actually a set of two to four matrices, as stated early by Danis and Karr [241]. Fig. 4 gives the structures of some common matrices. 3-amino4-hydroxybenzoic acid and POPOP need high laser power, since they have a high threshold. a-cyanocinnamic acid is often used for fragmentation experiments [174,176,299], because it yields ions with a (slightly) higher internal energy than the others. Some MALDI matrices, such as all-trans retinoic acid, are particularly sensitive to impurities whereas for other matrices (like HABA and Dithranol) the loss of efficiency is small, and hence, the latter matrices are preferred when purification is a problem. Retinoic acid works with polystyrene but it must be doped, preferably with Ag salts. 5-clorosalicilic acid gives good MALDI spectra of nonpolar polymers, whereas nor-harmane [400] and trihydroxyacetophenone are general-purpose matrices. It sometimes happens that common MALDI recipes fail. The search is still performed by trial-and-error, since the exact role of the matrix is still not fully understood. Nevertheless, the search follows some broad guidelines, as discussed in the following. Three key functions of the matrix have been suggested, i.e. incorporation of the analyte into matrix crystals, a collective absorption and ablation event, and an active role of the matrix in ionization [401,402]. Until recently it was generally agreed that incorporation of individual analyte molecules into the crystalline host matrix is an

important prerequisite for a successful MALDI analysis. Nowadays, this incorporation is no more seen as mandatory, and some researchers prepare samples without such intimate contact between analyte and matrix. Generally, an ideal matrix should have the following properties: high electronic absorption at the employed laser wavelength, good vacuum stability, low vapor pressure, good solubility in solvents that also dissolve the analyte, and good miscibility with the analyte in the solid state. Recently, Hoteling et al. [42] suggested a method for finding the optimal matrix. The polymer and the matrix are injected in a reverse-phase HPLC equipment. The best MALDI spectra are obtained when matrix and polymer have retention times that closely match. As an example of polymer for which published MALDI recipes (and matrices) are not effective, Ameduri et al. [115] cited poly(vinylidene fluoride) (PVDF) and claimed that when sample preparation involving conventional matrices is used to record its MALDI spectrum (and, in general, the spectra of polymers with a high content of fluorine), they fail. They proposed a new sample preparation based on three new matrices: 7,7,8,8-tetracyanoquinodimethane, pentafluorobenzoic acid and 2,3,4,5,6-pentafluorocinnamic acid (PFCA). Fig. 5 shows the MALDI spectrum of poly(vinylidene fluoride) using the PFCA matrix. The spectral quality is excellent and the spacing between MS peaks is 64 Da, the expected value. 2.6. Developments in sample preparation In most cases, the pristine ‘dried droplet’ method [1–4], is utilized for sample preparation. Solutions of matrix, analyte and salts (cationizing agents) are mixed

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Fig. 5. MALDI-TOF mass spectrum of a poly(vinylidene fluoride). Reproduced from Ref. [115] with permission of the American Chemical Society.

and the mixture is spotted onto the MALDI target. Using the dried droplet method, one can prepare about 100 targets per hour. Meier et al. [46] showed that MALDI can be used to perform the ‘screening’ of polymers obtained using combinatorial chemistry (COCHE). This MALDICOCHE combination allows finding the optimal reaction parameters for a given monomer such as the optimal temperature, the optimal solvent, the optimal initiator, the optimal concentration of the reactant species, etc. However, the dried droplet method is too slow for this purpose. Meier et al. used, instead, a new automated (robotic) MALDI sample spotting technique that allows full integration of MALDI sample preparation. MALDI-COCHE is so demanding that other authors [56] decided to apply ink-jet technology to the automated preparation of MALDI target plates. They employed a multiple layering approach where the matrix, cation and analyte were deposited as separated layers and they noted that the spectral quality was good; mass-resolved peaks were observed up to 3500 Da in the analysis of PEG and PMMA samples. Under the dried droplet conditions, crystallization is relatively slow, thereby increasing the risk of segregation phenomena of analyte, matrix or cationization salt. If segregation occurs, significant variations of peaks, peak intensity, resolution and mass accuracy are observed by focusing the laser on different regions of the same spot. Optimum results are obtained when the polymer and the matrix are soluble in the same solvent. The dried droplet method cannot be used for analysis of the polymer samples that are insoluble or poorly soluble in organic solvents. Therefore, considerable efforts have been devoted to the development of new

sample preparation methods. The ‘solvent-free’ method consists in immersing the polymer sample in liquid nitrogen, followed by addition of powdered matrix. The resulting mixture is finely ground in a rotating-ball mill. The ‘solvent-free’ methodology has been applied to polymers such as polyetherimide [268,303,309], aromatic polyamides [270], poly(9,9-diphenylfluorene) [300], polycyclic aromatic hydrocarbons (PHAs) [304], etc. [298]. Contrary to the dried-droplet, where the solvent evaporation allows for very strong adhesion to the sample holder, in this case, the matrix/analyte powder must be carefully fixed on the MALDI sample holder [11]. MALDI optimization is slightly simpler, due to the absence of the solvent. However, MALDI spectra are still sensitive to the matrix, to mixing ratios of matrix/polymer/cationizing agent, and to the sample preparation procedure. The accuracy, sensitivity and resolution of the MALDI spectra obtained using solvent-free sample preparation are very similar to those obtained with traditional solvent-based methodology [11]. In several cases, an improved signal-tonoise ratio was obtained and also interference from the matrix was less intense. The major advantage of the solvent-free sample preparations is in the characterization of insoluble polymers. Recently, Gies et al. obtained MALDI spectra of wholly aromatic, poorly soluble and insoluble polyamides, Nomex and Kevlar oligomers, by using wet grinding methods [270] where, the matrix/sample mixtures is initially processed in the usual way (i.e. with a rotating-ball mill), and then the solvent is added. The authors found that the spectrum quality improves when steps are taken to break the hydrogen bonds that


Fig. 6. MALDI-TOF mass spectrum of Nomex oligomers in dithranol with KTFA using the R–E–G method. Reproduced from Ref. [270] with permission of the American Chemical Society.

join polyamide chains. Fig. 6 gives the MALDI spectrum of poly(m-phenylene isophthalamide) (Nomex) obtained by the so-called ‘resolvated-evaporation-grinding’ method (REG), using Dithranol (see Fig. 4) as the matrix and potassium trifluoroacetate (KTFA) as the cationizing agent. The spectrum shows a series of peaks separated by 238 Da, which were identified as Nomex cyclic oligomers [270]. Recently, a new sample preparation method has been reported [168], which can be considered as a variant of the above, since it involves two steps. The first step is the spraying of the analyte/matrix mixture on a substrate, followed by freeze-drying; the second step is the positioning of the resulting powder on a second substrate (the target). This method will be discussed in the section that deals with molar masses. The ‘dried droplet’ method gives quite irregular surfaces. This is inconvenient, since one must move the target, searching for a spot that gives abundant MALDI ions. This search constitutes a formidable obstacle for applications that require unattended MALDI analysis of hundreds of samples, such as MALDI sequencing of proteins and nucleic acids. Electrospray sample deposition (ES) is a sample preparation method where matrix and analyte solutions are sprayed on the target surface under the influence of a high-voltage electric field [33,36]. ES is reported to yield much better shot-to-shot and spot-to-spot reproducibility than the dried droplet method. The improved results are ascribed to the small and evenly sized crystals thus formed, and the consequent improved homogeneity of the MALDI sample surface [33,36]. Moreover, repeating the ES, the user can deposit two or

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more layers of matrix/analyte mixture, with negligible mixing between layers. This represents a fascinating example of flexibility. Wetzel et al. [33] prepared a large number of targets by ES, changing the deposition voltage (DEVO) over a broad range, using three narrow MMD polymers: polystyrene (PS), poly(ethylene glycol) (PEG) and poly(propylene glycol) (PPG). Peaks due to ion fragments were present in the MALDI spectra of PEG and PPG for all the matrices used, whereas PS, which is more thermally stable, did not show ion fragmentation. Both PEG and PPG showed increased ion fragmentation with increasing DEVO. The authors also used the MALDI spectra to determine the average molar masses and noted that the MMD values for PEG and PPG tended to decrease, owing to fragmentation, with increasing DEVO. On the other hand, the MMD values of PS did not change over the whole range of DEVO, indicating absence of fragments [33]. Therefore, the thermal stability of the polymer and possible fragmentation should be considered when using the ES deposition method. Progress was also made in the analysis of polyethylene (PE), by a substrate-assisted laser desorption/ionization MS method, which uses cobalt, copper, nickel or iron metal powders as sample substrates, and silver nitrate as the cationizing reagent. Intact ions of PE chains up to 5000 Da were characterized [117]. Sometimes MALDI spectral quality is so bad that the polymer cannot be characterized. In these cases, one may be able to chemically modify the polymer and record the spectrum of the modified polymer. Finding a successful chemical modification can be an extremely time-consuming process in MALDI sample preparation. For instance, poly(trimellitic anhydride-co-4, 4 0 -methylenedianiline), PI-PAA, gave no MALDI signals with classical sample preparation [271]. Modification of the polymer by reacting it with N-methylethanolamine failed to achieve the intended purpose. The polymer reacted with N-methylethanolamine was further reacted with 2-fluoro-1-methylpiridinium p-toluensolfonate and eventually the MALDI spectrum could be recorded. From it, the authors determined the structure of the modified polymer and inferred properties of the pristine polymer. In particular, one of the two trimellitic anhydride rings (the imide ring) could be found in the open form (i.e. amic acid), and this implies that PI-PAA is a copolymer with approximately 50% amic acid. Saturated polyolefins such as PE and polypropylene (PP) were derivatized prior to the MALDI analysis to produce intact macromolecules by MALDI [118]. The authors reacted

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fractions and of unfractionated sample, the former are found to have better resolution because they are free of low molar mass impurities. Gallet et al. [373] recorded MALDI spectrum of a sample obtained by injecting a PEO–PPO copolymer in an SEC device and collecting the fractions. The fraction eluting at 29.7 ml showed mass-resolved peaks in the 2000–3000 Da region. On the other hand, the MALDI spectrum of the unfractionated PEO–PPO copolymer (in the same mass region) showed mass-unresolved peaks, indicating a dramatic worsening of the resolution which is impossible to estimate, but is certainly larger than a factor four.

Fig. 7. MALDI-TOF MS spectrum of the PE sample SRM 1482. Reproduced from Ref. [118] with permission of the American Chemical Society.

the terminal vinyl groups of narrowly dispersion PE with a large excess of bromine, then reacted the resulting chain-end brominated polymer with triphenylphosphine, and recorded the MALDI spectrum [118]. Fig. 7 gives the MALDI spectrum of the modified PE sample coded as SRM 1482 [118]. The spectrum shows well-resolved mass peaks centered around 9000 Da. Although the masses of the macromolecules are sensibly larger than those discussed above (obtained using cobalt, copper, nickel or iron metal powders), the average molar masses measured by MALDI were sensibly lower than those estimated by conventional osmometry. In another study, the olefin ends of a series of polyisobutylene samples were sulfonated before MALDI analysis, and the measured molar masses agreed with values obtained by laser light scattering and vapor pressure osmometry, except for one sample, characterized by a high molar mass and a large polydispersity index [112]. Copolymers ethylene and CO units possess too many isobaric structures for their characterization to be accomplished using MALDI instruments equipped with TOF analyzers. Cox et al. [342] proposed two solutions to overcome the problem, the first one is the use of an FT-ICR machine. As an alternative, the copolymer can be derivatized. When the copolymer was reacted with a multifold molar excess of sodium borohydride or lithium aluminum hydride, so that the CO units were reduced to HCOH units, the MALDI spectrum of the resulting copolymer did not show isobar peaks and was therefore, easy to interpret. Among sample preparations, the SEC-MALDI method deserves a mention. In comparing MALDI spectra of size exclusion chromatography (SEC)

2.7. Sample preparation for low molar mass compounds For low molar mass compounds, the usual sample preparations cannot be applied. Even if the analyte molecules display negligible fragmentation, the MALDI matrix breaks apart, producing a variety of matrix-related ions, and thus the low-mass region of the MALDI spectrum is literally stuffed with peaks due to the matrix and its fragments. As a consequence, low molar mass compounds (m/z!500 Da) cannot easily be analyzed by MALDI, since the peaks due to the analyte and to the matrix show virtually inextricable overlap. Considerable efforts have been made and several alternative approaches have been developed to overcome this problem. One solution is to deposit the polymer on the target, without adding the matrix. This works for alanine and some peptides, but often the molar extinction coefficient of the analyte is too small, and it is necessary to switch to an instrument with a high-irradiance laser. In order to overcome this obstacle, a modified MALDI technique has been proposed, called DIOS (desorption/ionization on silicon), where the analyte is deposited on porous silicon. The latter acts as a matrix (in the sense that it adsorbs UV light and it is able to promote analyte ionization) with the advantage that it does not produce peaks in the spectrum. DIOS has been successfully applied to record the spectra of PEG samples [180,183], the presence of spurious peaks being rather limited. Soltzberg and Patel [403] employed as a matrix poly(3-n-octylthiophene-2-5-diyl) which has some interesting advantages: namely, it is commercially available, light-adsorbing and electrically conductive. They demonstrated that it could be used to analyze by MALDI some aliphatic and aromatic molecules possessing a carboxylic acid group. In another study [404], the surfactant cetrimonium bromide was added to the a-cyano-4-hydroxycinnamic acid matrix for the


successful analysis of a variety of low-mass molecules by MALDI. Low-mass components of polyesters such as poly(neopentylglycol adipate) were determined by MALDI analysis using a 10,15,20-tetrakis(pentafluorophenyl)porphyrin (F20TPP) matrix, which does not give matrix-related ions below m/zZ822 [242]. 2.8. Doping agents The ionization of synthetic polymers often occurs by metal clustering (cationization) rather than protonation. Since many polymers have relatively high cation affinities, they do not necessarily require a high cation concentration and thus cations present as impurities in matrix, reagents, solvents, glassware, etc. may suffice. However, Bahr et al. [1] put forward the following objection: if one relies on ‘adventious’ cationization, the yield of cationized species may turn out to be low or, more simply, the cationization can prove to be less than homogeneous. In order to avoid such drawbacks, they added an alkaline salt solution (LiCl, NaCl, KCl) to the matrix–analyte mixture (for polystyrene analysis, silver trifluoroacetate was added instead). Their objection has a sound foundation, and thus the addition of dopants is now routine in MALDI. There are some rough agreed-upon rules for selection of the dopant most effective for a given class of polymers, but they are obviously empirical. As an example, Trimpin et al. suggested the use of sodium for poly(vinyl pyrrolidone) [109]. Fig. 8 shows MALDI spectra of poly(vinyl pyrrolidone) using different cationizing agents with Na, K, Li and Ag. It can be seen that sodium indeed

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gives a good S/N ratio, whereas the number of counts (the ion yield) with Ag is unsatisfactory [109]. 2.9. Ionization efficiency A polymer system comprises hundreds of different analytes, each present in a given abundance: i.e. the mixture contains chains with different lengths, but also with different end groups and different backbone structures. If the MALDI ionization efficiencies of all these analytes are the same, they will all produce ions in amounts proportional to the abundance of each analyte in the polymer sample. Actually, however, the ionization efficiency changes with the analyte. Ionization efficiency and cation attachment are clearly connected and some authors believe that a cation attaches itself preferentially to small macromolecules instead of large ones. The total number of cationization sites CTOT is the sum of the cationization sites (CCHA) along the chain plus the cationization sites CEND at the end groups. Cyclic macromolecules do not have end groups and, to a first approximation, CCHA increases as the length of the chain increases, since each repeat unit represents a cationization site [405]. However, if the cyclic polymer chain forms a random coil, cationization sites that lie inside the coil cannot be accessed, and the effective CCHA is less than the total value for all repeat units in accordance with experimental evidence. The case of linear macromolecules is much more complex since they possess end groups and the cation affinity for the latter may be much higher than for the backbone. Ionization efficiency differences among

Fig. 8. MALDI spectrum of poly(vinylpyrrolidone). Reproduced from Ref. [109] with permission from Elsevier.

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chains with different lengths are often called ‘mass discrimination’ [3]. These differences can be estimated by simply measuring the changes in ionization efficiency when a low molar mass and a high molar mass sample are analyzed simultaneously. Mass discrimination implies that the MALDI response is not linear with respect to molar mass. The simplest (and most used) method to study ionization efficiency is to create a polymer mixture of known composition and to measure the relative amounts of ions. In describing these experiments, we shall differentiate between polymer mixtures with the same backbone and mixtures with different backbones. The second kind of mixture is more demanding. A mixture of two polymers having different end groups but the same backbone can be produced by mixing chains of the type G1-AAAAAAA-G2 with chains of the type G3-AAAAAAA-G4, where G1, G2, G3, G4 are end groups and A is the repeat unit. By recording the MALDI spectrum of the mixture and measuring MALDI peak intensities, we can estimate apparent the relative abundance of the chains. We discuss some cases of this type. Cox et al. [34] considered a series of PS samples terminated with hydroxyl, hydrogen, tertiary amine, and quaternary amine groups. They observed that the ionization efficiency of PS oligomers was affected exclusively by the chemical groups at the PS chain-ends. The PS samples terminated with a quaternary amine exhibit 10-fold greater ionization efficiency than the other PS samples studied. The authors analyzed nine MALDI spectra of 1:1 blends of these end-functionalized PS samples, finding that the relative ionization efficiencies of the polymer components varies dramatically with the laser power, and that spectra recorded at the threshold of the laser power give the most accurate representation of the blend composition. However, at this usual instrumental condition, owing to the large difference in ionization efficiency between the quaternary amine terminated PS (PSQ) samples and that for all the other functionalities, accurate quantization of the other component in mixtures with the PSQ samples is difficult. They also measured the relative intensity ratio in the MALDI spectra of the different end terminated PS blends as a function of the different blend composition, and found a linear trend for each blend. On the base of these data, assuming that the ionization efficiency of the PSQ samples is 100% and comparing mixtures with a common component, the authors calculated the ionization efficiency relative to the PSQ of all PS polymers investigated. They found that the samples terminated with OH and H end groups have similar

ionization efficiencies, with the tertiary amine slightly higher, depending on the average molar masses. Chen et al. [149] considered a mixture of PEGs with the following structures:

The MALDI spectral quality was good, since the masses were very low. They plotted the actual composition versus the composition measured using MALDI peak intensities. The points lie on a straight line, with a slope of 45.58, which does not differ much from 458, which indicates identical ionization efficiencies. Okuno et al. [179] analyzed mixtures of linear and branched polypropyleneglycol (PPG). They noted that composition (weight percent of branched chains) measured by MALDI was slightly different from the actual composition. They noted that the difference depended on sample preparation and, in particular, on the concentration of the polymeric solution, and that it tended to fade away as more concentrated solutions were used. Despite the fact that there are some cases in which MALDI is only semi-quantitative, many studies have appeared which assume that the ionization efficiency in mixtures with the same backbone is uniform. Maziarz et al. [138] considered a mixture of a,u-bis(t-butylamine-fumaryl-oxy-butyl) poly(dimethyl siloxane) (BAF-PDMS) and another polysiloxane (IMPUPDMS). The structure of the latter is quite complex, with two PDMS chains connected by a bridge (for brevity, we indicate only its empirical formula: C40H70N2O10Si2 [C2H6OSi]n [C2H6OSi]x). The MALDI spectrum is a little crowded, since the mass of IMPU-PDMS is approximately 12 Da greater than the mass of the corresponding BAF-PDMS. Taking the ratios of peak intensities (and applying some corrections), the authors were able to estimate the abundance ratio, i.e. BAF/IMPUZ67/33. Campbell et al. [13] analyzed a polystyrene sample, a mixture of three different types of chains denoted TDB0, TDB1 TDB2. Here, TDB stands for terminal double bonds, and chains TDB2 are terminated at both ends with a styrene molecule (and thus two double bonds), whereas chains TDB0 are terminated by two hydrogens. Using MALDI peak intensities, the authors were able to estimate the ratio of abundances TDB0/TDB1/TDB2Z 5/90/5. This result yields information about the polymerization process since it clearly indicates that TDB0 and TDB2 chains are side-reaction products.


Libiszowski et al. [257] recorded the MALDI spectrum of PDX, a polymer obtained by cationic ring-opening of 1,4-dioxan2one, and noted the simultaneous presence of peaks due to cyclic and linear PDX chains. From the MALDI peak intensities and they constructed a bar graph of the abundance of cyclic chains ACIC versus that of linear chains, ALIN. Summing all the bars in the bar graph, they were able to estimate the relative (total) abundances, which gave ACIC/ ALINZ59/41. This implies that the reaction produced large amounts of cycles. Luftmann et al. [159] reacted a commercial PPO sample with p-nitrobenzoyl chloride and obtained a diester- and monoester-terminated PPO chains. They recorded the MALDI spectrum and, using peak intensities, found that the diester/monoester mole ratio was 875/125. It is interesting to note that the NMR spectrum was useless for this purpose. As the authors state, it yielded diester/monoesterZ100/0; but this failure is due to the circumstances that the monoester and diester signals tend to overlap and that the integration has an error margin of about 10%. Ring–chain equilibration reactions are characterized by a critical concentration B 0 , and when the concentration is above B 0 , they yield a mixture of cyclic and linear macromolecules. Ke´ki et al. [209] obtained poly(lactic acid) by ring–chain equilibration. They measured the MALDI spectral intensities due to cyclic and linear chains and assumed them to have the same ionization efficiency. From the ratio of intensities, they were able to estimate B 0 and found that it varied from 100 to 1000 l/mol, depending on the temperature. Recording the MALDI spectrum of a mixture of two polymers having different backbones, one finds that MALDI peak intensities reflect in a distorted manner the abundances of the chains and the composition of the blend. In some cases, the distortion is small and thus MALDI is semiquantitative. In the following, we discuss some examples. Scamporrino et al. [406] showed that two instrumental parameters could affect peak intensities, thus falsifying the composition of the blend. The two parameters are the delay time (DETIM) and the voltage of a wire electrode (VOWIEL) measured in percent of grid voltage (the wire electrode acts on ion trajectories, attracting the ions). The authors studied an equimolar mixture of PEG and PMMA, recorded the MALDI spectrum of the mixture and found, on changing DETIM and VOWIEL, that the apparent blend composition changed from 100/0 to 50/ 50 to 0/100.

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Alicata et al. [273,274] considered a blend of Nylon 6 (Ny6) and poly(butylene terephthalate) (PBT) oligomers and noticed that their ionization efficiencies varied greatly, depending on the end groups. Fig. 9a shows the MALDI spectrum of the Ny6/PBT mixture. It can be seen that the peaks due to the Nylon 6 polymers terminated with carboxyl groups (Ph–Ny6–COOH) predominate over those for the PBT sample terminated with hydroxyl end groups (HO–PBT–OH) although the polymers used have similar molar mass distributions and were present as an equimolar blend. Similar behavior was observed in the analysis of an equimolar mixture of Ny6 terminated with amino groups (Ny6–NH2) and PBT–OH. However, MALDI spectra of blends of Ny6–COOH and PBT polymers terminated with carboxyl groups (PBT–COOH) show peaks of comparable intensity due to both polymers, as can be observed in Fig. 9b. When an equimolar mixture of PBT–OH and PBT–COOH was analyzed, MALDI spectra showed intense peaks due to PBT–COOH oligomers at lower mass, whereas above 2000 Da peaks due to PBT–OH became more intense, as the intensities of peaks due to the two PBT polymers tended to equalize. Since the number of end groups in both polymers decreased at higher mass, this result indicates the preponderant effect of the end groups on the ionization efficiency of oligomers. Yan et al. [141] recorded the MALDI spectrum of a mixture of PMMA and PDMS and found that PMMA peaks were absent, even when the mole fraction of PMMA exceeded the mole fraction of PDMS by a factor of four. The authors proposed an explanation based on the fact that the PMMA had a high molar mass and high polydispersity index (MnZ33,000 and MwZ 100,000). However, a simple calculation shows that such a sample should be characterized by a very large amount of low-molar mass oligomers and thus strong PMMA MALDI peaks were to be expected. Possible explanations are that the ionization efficiencies of PMMA and PDMS are very different or that the instrumental parameters DETIM and VOWIEL had values that favor the PDMS ions and suppress the PMMA ions. Chapman et al. [210] reported the MALDI spectrum of an almost equimolar mixture of poly(butylene glutarate) (PBuGu) and poly(butylene adipate) (PBA). Peaks due to PBA dominate the spectrum, and this clearly indicates that PBA is preferentially ionized. Measuring peak intensities and summing them, we found PBuGu/PBAZ30/70, which indicates a difference in ionization efficiency of a

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Fig. 9. Enlarged regions of the MALDI-TOF mass spectra (a) of equimolar mixtures of Ny6–COOH (MvZ2500) and PBT–OH (MvZ2300) and (b) equimolar mixtures of Ny6–COOH (MvZ2500) and PBT–COOH (MvZ2000), with Mv the viscosity-average molecular weight. The labels NA, NB, THC and TCA indicate the cyclic Ny6 oligomers, the Ny6–COOH oligomers, the PBT–OH and the PBT–COOH oligomers, respectively. Reproduced from Ref. [273] with permission of the American Chemical Society.

factor 2–3. Murgasova et al. [41] considered mixtures of PS and poly(a-methylstyrene) (PAMS) in which the blend composition (i.e. weight fraction of PS) varied from 0.2 to 0.75. They recorded the MALDI spectra of the mixtures and found peaks due to both PS and PAMS. Since the structure of PS is quite similar to the structure of PAMS, they assumed that the ionization efficiencies for PS and PAMS are the same. From the ratio of MALDI intensities, they were able to estimate the blend composition correctly.

2.10. Measurement of molar mass Mass spectrometry can be used to determine the molar mass (MM) of each polymer chain species and the molar mass distribution (MMD) of a polymer sample by measuring the intensity of each mass spectral peak corresponding to a molar mass Mi. Mass spectrometers are equipped with a detector that gives the same response if an ion with mass 1 kDa or 100 Da (actually any mass) strikes it. The detector measures the number fraction and this implies that the intensity of the


ith peak is proportional to Ni, the number of chains with mass Mi. The number average and weight average molar masses defined by nX oK1 X X X Mn Z Mi Ni = Ni Z Wi = Mi Wi (3) and Mw Z

X

Mi2 Ni =

X

Mi Ni Z

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Mi Wi =

X

Wi

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are readily obtained. Wallace et al. [50] developed an operator-independent approach to mass spectral peak identification and integration, which is claimed to increase the accuracy of the summations and thus the accuracy of the Mn and Mw values obtained. The method is straightforward and the calculations can be performed independently of whether the spectrum is mass-resolved. The definitions given by Eqs. (3) and (4) are quite old, certainly older than MALDI, and they may be used with other types of mass spectrometers such as FAB (fast atom bombardment), PD (plasma desorption), or LD (laser desorption). For instance, in 1986, Brown et al. [407] recorded the LD spectrum of a polymer sample with masses well beyond 5 kDa and extracted the average molar mass values (Mn and Mw) embedded in the mass spectrum. In order to obtain the MMD of the polymer, mass spectral data must be processed using a suitable transformation algorithm, and the quantities Ni (i.e. the fraction of the chains with mass mi) are transformed into Wi (the weight of the chains with mass mi). Barre`re et al. [137] applied the algorithm to a PDMS sample. The open circles in Fig. 10 represent the SEC trace for the PDMS sample. The figure also shows a series of

Fig. 10. MM distribution for a PDMS sample. The needles represent the result when the MALDI spectrum is processed to yield the resulting weight fraction MMD. The SEC trace (open circles) is also reported. Reprinted from Ref. [137] with permission from Elsevier.

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‘needles’ that represent the result when the MALDI spectrum is processed to yield the resulting weight fraction MMD. There is good agreement with the SEC trace. The ionization process must be ‘soft’. If hard ionization occurs, chains are no longer intact (fragmentation occurs) and the measurement will be affected by a systematic error toward the bottom (i.e. underestimation of Mn and Mw). Since fragmentation is an annoying concern, some authors developed a protocol to avoid it. They noted that the extent of fragmentation decreases when the laser power is lowered and also when a large excess of matrix is used in sample preparation. Thus, the protocol consists in using low laser power (close to the threshold) and in using a matrix-toanalyte ratio of at least 10,000:1. In our opinion, few polymers undergo fragmentation; the overwhelming majority remain intact during desorption, with fragmentation close to zero. Fig. 11 shows the MALDI mass spectrum of a poly(butylene glutarate) (PBG) sample [211]. In the mass region between 1000 and 5000, there are no peaks. This implies that the ionization process is soft and that the bonds forming the PBG backbone are quite strong. The small number of polymers subject to fragmentation includes polyethylene [118] and some hyperbranched polymers [190,218]. Laine et al. [87] studied fragmentation in MALDI and carefully labeled the fragmentation peaks. They studied a variety of samples and deliberately switched to conditions different from those indicated by the above-cited protocol. They found that results are biased when one abandons the protocol. The method described above for extracting the average mass information (Mn and Mw) embedded in the mass spectrum of a polymer sample has been used extensively and a huge number of authors (we counted at least 100 reports) compared Mn and Mw values for the polymer sample with M n and M w obtained by traditional methods for MM (molar mass) determination (SEC, viscometry, light scattering, etc.). Considering narrow-MMD polymer (with Mw/Mn! 1.10–1.20), as those that can be obtained by anionic or cationic polymerization, most authors found the agreement within 10–15%, or even better, i.e. it can be considered excellent [3,4,396–399]. However, researchers at NIST (formerly the National Bureau of Standards) noted that authors knew in advance the molar mass averages of their samples and thus the results could be biased. For this reason, they sponsored an interlaboratory comparison among 23 laboratories, based on a polystyrene sample of which the participants did not know the average

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Fig. 11. MALDI-TOF mass spectrum of an almost monodisperse poly(butylene glutarate) sample, using IAA as the matrix. Reproduced from Ref. [211] with permission of the American Chemical Society.

mass. After receiving the results of the MALDI analysis, researchers at NIST made public that the sample was obtained by anionic polymerization and had a narrow MMD centered on 7 kDa. Fig. 12 reports one MALDI spectrum [16] of the sample. It can be seen that the strongest peaks are around 7 kDa and this implies that MALDI estimates are in accordance with estimates obtained by traditional methods for MM determination. On the other hand, when the MALDI spectrum is used to estimate Mn and Mw values for a broadMMD polymer, a discrepancy is always noticed: i.e. MALDI underestimates both Mn and Mw [408]. As already noted, this problem is usually called ‘mass discrimination’. Some authors [5] believe that its nature is ‘instrumental’ (i.e. connected with the ion-source design or with the detector as discussed at length above) whereas others [34,141,273,274] believe that its nature is ‘chemical’: the cation

attaches itself preferentially to small macromolecules instead of large macromolecules. Regardless of its causes, mass discrimination must be avoided or t somehow circumvented. Many authors have proposed remedies when using MALDI to estimate Mn and Mw of polymers. First, we consider methods that aim to obtain a size-independent ionization efficiency of sample molecules, and then we turn to methods, which assume that ionization efficiency changes as a function of the molar mass and try to correct for this effect. Some authors [47,65,168] claim that proper MALDI sample preparation minimizes mass discrimination. A simple recipe [47] consists in avoiding common matrices (such as HABA dithranol DHB) and switching to another matrix, 2-[(2E)-3-(4-tert-butylphenyl)-2methylprop-2-enylidene]malononitrile (TEBUMAL). The reason why this recipe works well is unclear, but probably it is connected with the fact that TEBUMAL

Fig. 12. MALDI-TOF mass spectrum of the PS 7000 sample used in the Interlaboratory Comparison, using retinoic acid as a matrix and AgTFA. Reproduced from Ref. [16] with permission of the American Chemical Society.


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Fig. 13. MALDI-TOF MS spectrum of PEO 100,000. Reproduced from Ref. [168] with permission from Wiley.

possesses a very low (laser) threshold. Unfortunately, TEBUMAL is not yet commercially available and it must be synthesized following the route developed by Ulmer et al. [409]. Recently, a new sample preparation method has been reported [168] involving flash spraying/freezing of the analyte/matrix mixture, followed by freezedrying. This protocol was used successfully to acquire correct molar mass distribution (MMD) estimates for polydisperse samples of poly(vinylpyrrolidone) (PVP), poly(ethylene oxide) (PEO), dextran, lichenan and nigeran. Fig. 13 displays the MALDI mass spectrum of PEO 100,000, obtained by the above freeze-drying preparation, showing a MMD from 10,000 to above 250,000 Da. The Mn and Mw values calculated from this spectrum are in good agreement with those obtained by conventional techniques [168]. Electrospray sample deposition is known to improve the homogeneity of the MALDI sample surface and also the signal strength, in comparison with the dried droplet method, potentially enabling the use of MALDI for some MMD measurements of polymers [33]. Some authors have tried to cope with the problem of mass discrimination in MALDI by performing an ‘off-line’ correction of the detector response, eliminating spurious components in the signal and generating a new spectral baseline from which the molar mass of the polymer can be calculated [410]. The method utilizes the MALDI spectrum in continuous extraction to get the full ion yield from the detector and the estimation of Mn and Mw using integral forms of Eqs. (3) and (4) as the asymptotic

limits of these parameters obtained on calculation with increasing upper mass integration limit Mup. This procedure has been applied with encouraging results to several widely polydispersed polymers, such as PDMS, PMMA and a bisphenol-A copolyether sample [410]. A further procedure [411] for the correction of decreasing detection response in MALDI-TOF spectra with increasing ion mass is based on the use of PMMA standards of known MM to calibrate the detector response. Mize et al. [213] tried to overcome mass discrimination in another way. Fig. 14a shows a MALDI-FT-ICR spectrum of a poly(hexanediol-altazelaic acid) (poly(HEX-AZ)) homopolymer. The peak intensities were used to compute MM averages MnZ1083, MwZ1210, Mw/MnZ1.12; but they display considerable differences from SEC data from the supplier (1040, 2590 and 2.5, respectively). Therefore, the mass spectrum of the whole sample appears to suffer from mass discrimination of high molar mass oligomers. The authors used SEC to prepare low polydispersity samples and, in order to cope with the problem of mass discrimination, they developed a reconstruction algorithm, thus generating a ‘projection’ mass spectrum. The intensity of each peak in the projection spectrum is the sum of 15–20 terms, each term being the product of intensity of the corresponding peak in a given fraction multiplied by the area under the SEC fraction. Fig. 14b shows the projection mass spectrum of the poly(HEX-AZ) polymer reconstructed from 15 SEC fractions. The improvement is remarkable, especially for masses above 4000 Da.

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Fig. 14. (a) Single-shot-mass spectrum of poly(HEX-AZ). (b) Mass spectrum reconstructed from the sum of 15 mass spectra, each representing ions from 1-min fractions of a 15-min capillary SEC separation. Insets show the presence of higher mass species than those detected in the unfractionated sample. Reproduced from Ref. [213] with permission from IM publications.

2.11. Coupling MALDI with devices that separate macromolecules by size Size exclusion chromatography is a very popular method for polymer characterization, however, it must be noted that SEC calibration is an error-prone task. Furthermore, it needs mass calibration by an absolute method of MM measurement. Appendix A deals in detail with SEC elution procedures and algorithms. One method for measuring the calibration constants consists in preparing a mixture of five or more polymer samples with the same repeat unit, each having a narrow MM distribution and known MM (so-called SEC primary standards). The mixture is injected in the SEC

apparatus and the resulting chromatogram is recorded. Measuring the elution volumes and plotting them against the logarithm of the mass, yields the parameters that identify the calibration line (see Appendix A). Actually, the slope of the calibration line is relatively insensitive to the type of polymer injected whereas the intercept changes from one polymer to another. Intercept values correspond to the molar mass, and these changes may be quite large. The reliability of SEC results strongly depends on the availability of a set of polymers of known MM and narrow MM distribution (primary standards) with the same structure as the polymer of interest. However, a set of such calibration standards is often unavailable for a specific


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polymer. As a rough approximation, some authors assume that the parameters, which appear in the SEC calibration equation, are independent of the polymer structure. Thus, a common procedure is to use a mixture of polystyrene primary standards to construct an SEC calibration line supposedly valid for any type of polymer. Unfortunately this assumption is rarely valid. For instance, when tetrahydrofuran (THF) is used as the solvent, polycarbonate gives an SEC mass value only about half the mass of polystyrene of the same mass [254]. To solve this difficulty, the SECMALDI method can be used. The polymer is injected in the SEC apparatus and fractions are collected. Then, selected fractions are analyzed by MALDI. SEC calibration constants (see Appendix A) are easily obtained by combining MALDI mass data with the elution volumes of the fractions. The major application of the SEC-MALDI method [3,4] has been to obtain accurate values Mn and Mw when the sample is a broadMMD polymer or copolymer. Several papers have recently reported applications of SEC-MALDI for the analysis of synthetic polymers [412– 416], and Murgasova and Hercules [413] reviewed the entire SECMALDI field. Different approaches have been proposed to couple ‘on-line’ MALDI to liquid separation methods, and it has been shown how SEC sample collection and subsequent preparation of samples for MALDI analysis can be automated [128–130,401]. Several interfacing methods have been proposed where the eluate stream is deposited on the MALDI target by a spray or drip process. The matrix is co-added to the eluate stream, or matrix-precoated targets are used [128,401]. However, some of these methods require an expensive robotic system with precise XYZ movements and some do not allow for varying the matrix/analyte ratio to optimize the MALDI spectral resolution of the higher molar mass fractions, which need a matrix/ analyte ratio very different from that of lower MM fractions. On the other hand, in ‘off-line’ SEC-MALDI the matrix/analyte ratio can be varied, and this represents a major advantage. A typical SEC-MALDI trace is illustrated in Fig. 15, which shows the SEC trace of a polydimethylsiloxane (PDMS) sample together with the MALDI mass spectra of four fractions obtained by SEC [131]. These data allowed calibration of the SEC curve against absolute molar masses and, thereafter, computation of the MM averages from the SEC curve according to the standard procedure adopted in SEC work [131]. MALDI spectra of high mass SEC fractions in the figure are not mass-resolved, whereas the MALDI spectrum of the last (low mass) SEC fraction is mass-resolved. Thus, the MALDI spectra of

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the SEC fractions containing the lowest molecular species allow the identification both of the polymer structure and the end groups. It is also possible to identify the presence of cyclic and open chain oligomers, a recurrent structural problem in polymer synthesis. Fig. 16 shows the mass spectra corresponding to SEC fractions of the DPMS polymer of very low mass reported in the previous figure. Peaks in Fig. 16a correspond only to linear oligomers, whereas in Fig. 16b, a distribution of peaks corresponding to cyclic oligomers can be detected besides that of linear chains [131]. If the SEC sample injected is a mixture of linear and cyclic macromolecules with the same backbone, it must be remembered that linear chains r and cycles have different hydrodynamic volumes (see Appendix A) since a linear chain occupies a larger volume than corresponding cycle and, therefore, linear chains and cycles are eluted at different times [417– 420]. A quantity of interest is the ratio DCLZMcyc/Mlin at a given elution volume, where Mcyc is the mass of cycles and Mlin that of linear chains. Theory [417] predicts that DCL is 1.25 for masses up to 100 kDa, then starts to increase towards higher values (1.30 and even higher). Wright and Beevers [417] report on early experiments in 1978 on the construction a preparative SEC device to determine SEC calibration curves for cyclic PDMS and linear PDMS. The speed of the analysis is very low (also due to the fact that it must be preceded by solvent extraction in water–methanol mixtures), and it is limited in mass since it cannot separate macromolecules above 70 kDa. In a single

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SEC-MALDI experiment, the calibration lines for linear and cyclic PDMS were determined up to 500 kDa, and DCL was estimated with a consistent increase of the useful range [3]. SEC-MALDI investigations have been used to explore the MALDI response to molecular association in poly(bisphenol A carbonate) (PC) [255]. Most PC samples are mixtures of PC chains with different chain ends (also cycles are present) but only PC chains terminated with one or two hydroxyl groups can undergo self-association by hydrogen bonding. In the presence of molecular association in a polymer sample in a particular solvent, the SEC method fails, and the usual Mn and Mw information cannot be obtained, since Mn and Mw values are affected by a systematic error. Chain self-association was observed when a 10 mg/ml PC solution was injected into SEC columns, using chloroform (CHC13) or tetrahydrofuran (THF) as the eluant. The presence of self-association in PC was revealed by the difficulty of obtaining SEC fractions with narrow molar mass distributions [255]. The MALDI spectra of the SEC fractions contained a number of species much higher than expected, covering huge mass ranges (10 kDa and even up to 20 kDa) clearly showing that narrow SEC fractions are not obtained and that the standard goal in SEC (i.e. to elute chains of different sizes at different volumes) is not achieved. Fig. 17a–c show [255] the MALDI spectra of

PC fractions collected at the same elution volume in three different SEC runs. As is shown in the spectra, a higher sample dilution, or the addition of a polar solvent, such as ethanol, to the CHCl 3 eluant, suppressed self-association in the PC samples. Chain association of PC produced molecular aggregates of relatively small molecules with high hydrodynamic volume, which were, therefore, eluted through SEC columns at the same elution volume as higher molar mass chains. However, the molecular aggregates were broken when the SEC fractions, containing a heterogeneous mixture of PC chains of different size, were diluted in the matrix used for the MALDI sample preparation. The most common MALDI matrices contain carboxylic acid units, which are able to break the hydrogen bonds responsible for the formation of the chain aggregates. The MALDI spectrum of one of these PC fractions (Fig. 17b) shows a bimodal distribution of peaks. The low-mass peaks are due to PC chains terminated with OH groups, whereas the ions at high mass correspond to PC chains capped at both ends (see oligomer structures depicted in the upper part of the figure). Chmelik et al. [139] were able to fractionate a poly(dimethylsiloxane) by liquid chromatography using supercritical CO2 as the eluent. Fig. 18 shows the chromatographic trace. The separation is apparent; peaks corresponding to chains of length 10, 20, 30, 40, 50 and 60 units are indicated. The authors also collected


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the fraction that eluted between 15 and 18 ml and labeled it Fraction F (see the lower part of the figure). The figure also shows the MALDI spectrum of fraction F. The spectrum is very clean, with well-resolved MS peaks in the mass region 1200–2200, thus confirming the power of the method. Kassalainen et al. [39] developed an alternative to SEC-MALDI; they used a thermal field-flow fractionation (TFFF) device to separate macromolecules of different length and equipped it with a detector to measure abundances. They studied two polystyrene samples whose TFFF traces turned out to have quite different shapes, as shown in the inset of Fig. 19. They collected TFFF fractions and analyzed them by MALDI. Fig. 19 also shows the MALDI spectra of five selected TFFF fractions. The molar masses of the fractions increase with elution time giving 8, 12,

20, 45, 60 kDa, respectively. In the MALDI spectra of the first and the second fraction, ions due to chains with different size are detected as separate peaks. At higher masses, some overlap shows up. This phenomenon is very common. One can introduce a useful quantity, REGmax, which is the experimental mass (or the mass region) in which the MALDI spectrum is still mass-resolved. For homopolymers, REGmax may reach values as high as 100,000, whereas for copolymers REGmax is five times smaller and rarely equals (or exceeds) 12,000. A possible explanation for these findings is low resolution. Indeed, the resolution degrades at high masses. However, the relation between the resolution associated with a peak and the molar mass in MALDI is quite involved, and the derivative K of the full width at half maximum (FWHM) of MALDI

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Fig. 18. Chromatographic trace of PDMS in supercritical CO2. The inset displays the MALDI spectrum of the indicated fraction F. Reproduced from Ref. [139] with permission from Wiley.

peaks with respect to mass, can take both positive and negative values. Nevertheless, K is always negative above 5 kDa. HPLC can be used to separate macromolecules by size, but above 10–50 kDa, chains with different sizes are coeluted. Thus, if high masses are absent from the sample, one can collect fractions and analyze them by MALDI. The HPLC calibration line is readily obtained, and the HPLC trace yields Mn and Mw values for broad-MMD polymers. For copolymers, great care must be used since HPLC ordinarily uses a UV detector, and its response seldom reflects accurately the abundance of copolymer species [421] (see Appendix B). MALDI can also be combined with HPLC for the purpose of purity control. 2.12. Coupling MALDI with devices that separate macromolecules by functionality or by composition It has been shown that some liquid chromatography devices are able to separate macromolecules by functionality; in particular, macromolecules having the same

backbone but different end groups may elute at different times. Under suitable conditions, HPLC separates macromolecules having different end groups. In an early study, Pasch and Rode [240] showed that it is possible to collect HPLC fractions of poly(decamethylene adipate) and to identify the oligomers present in the fractions by recording their MALDI spectra. The HPLCMALDI method is time-consuming and, therefore, its use is mostly limited to cases where the MALDI spectrum of the unfractionated sample is too complex to give a complete picture of the sample properties. Peetz et al. [285] prepared poly(2,5-diheptyloxy-1,4-divinyl-benzene) by acyclic diene metathesis. In order to isolate oligomers with different sizes (the trimer, the tetramer and the pentamer) they processed their sample by column chromatography, collected the fractions and then recorded their MALDI spectra. They noted that peaks due to impurities are absent from the spectra (each fraction displayed a single peak). Fig. 20 shows the MALDI spectrum of an HPLC fraction; it contains a single peak due to the pentamer. There is a good match


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Fig. 19. TFFF traces of two poly(styrene) samples fractionated by TFFF (right) along with MALDI spectra of five TFFF fractions (left). Reproduced from Ref. [39] with permission of the American Chemical Society.

between the calculated intensity of each isotopic and corresponding observed spectral peak. Liquid chromatography at the critical condition (LCCC) is performed at the elution–adsorption

transition. It can be used for a variety of separations; but here we focus exclusively on two types, namely. To separate macromolecules with different functionalities (mostly chain ends) and for block copolymers.

Fig. 20. MALDI-TOF mass spectrum of an isolated oligomer (the pentamer) of poly(1,4-diheptyloxy-2,5-divinyl-benzene). On the right, a comparison between calculated intensity of each isotopic and experimental peak. Reprinted from Ref. [285] with permission from Wiley.

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Keil et al. [170] showed that LCCC is able to separate macromolecules with different shapes from a sample in which three poly(propylene oxide) chains are bonded to a glycerol molecule. The MALDI spectrum of the unfractionated sample is very complex, whereas the MALDI spectra of LCCC fraction turn out to be simpler to interpret. LCCC is particularly well suited for AB copolymers with long AAAA. and BBBB. blocks, since these samples exhibit a most peculiar retention behavior. As a consequence of this behavior, the spectra of the fractions exhibit many fewer peaks and thus are much simpler than the spectra of the unfractionated sample. Park et al. [37] analyzed a styrene–isoprene copolymer. The polymer was injected in the LCCC apparatus and fractions were collected. Thereafter, selected fractions were analyzed by MALDI. Fig. 21 shows MALDI spectra of six fractions: denoted f1, f3, f5, f7, f9, f11. It can be seen that in the spectrum of fraction f1 all peaks are absent except those due to oligomers with three isoprene units, whereas fractions f3, f5, f7, f9 and f11 show exclusively MS peaks due to oligomers with 5, 7, 9, 11 and 13 isoprene units, respectively. These interesting MALDI results imply that the length of the isoprene block determines uniquely the retention behavior, which is completely

independent of the length of the styrene block. In other words, the authors succeeded in performing a perfect separation and MALDI was used to prove the absence of spurious copolymer chains in the fractions. This is a particularly happy case; usually some contamination occurs. For instance, MALDI analysis combined with LCCC separation has been used to characterize a poly( L -lactide)-block-poly(ethylene oxide)-blockpoly(L-lactide) (PLLA-b-PEO-b-PLLA) triblock copolymers [354]. In this case, the authors found that the molar mass and composition, and the distribution of two end blocks, affect chromatographic retention. In thin-layer chromatography (TLC), polymer chains terminated in different ways yields different spots. By scraping away the TLC spots and recording their MALDI spectra, one may be able to identify the terminal group. When a densitometer is available, one can estimate the fraction of macromolecules terminated with a certain end group. Ji et al. [111] analyzed two poly(butylene) samples by TLC; Fig. 22 gives a picture of the TLC plates and their MALDI spectra. One of the samples gives a single TLC spot, which suggests two hypotheses: that the sample is a complex mixture or that the TLC apparatus does not have sufficient resolving power to separate it. The second hypothesis is (more simply) that all chains are terminated in the same way. The MALDI spectrum of the TLC spot indicates clearly that the second hypothesis is correct. The other sample gives two TLC spots, which can be interpreted as evidence that the sample is a mixture of chains terminated in two different ways. The figure also shows the MALDI spectra of the two spots and, by inspection, it is seen that this interpretation is correct. 2.13. Structure determination

Fig. 21. LCCC fractionation of a styrene–isoprene block copolymer. LCCC trace (upper part) along with the MALDI spectra of LCCC fractions f1, f3, f5, f7, f9, f11. Reproduced from Ref. [37] with permission from Elsevier.

Polymers display a variety of structures, including linear and branched chains, copolymers with different sequences and star polymers with different numbers of arms. Because of the variety of possible structures, the process of analyzing a polymer has to answer several questions and logically proceeds by steps. The first step consists in the determination of the chemical structure of the backbone. The second step consists in finding out if the chains possess branching points and in the determination of the degree of branching. The third step consists in finding out what end groups lie at the chain ends, and therefore, also in detecting cyclic oligomers that may be present. The first step is quite simple. In fact, mass spectra of two polymers possessing different repeat units will produce widely different mass spectra, since the spacing between peaks is different: e.g. in


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Fig. 22. TLC of two poly(butadiene) samples. A photograph of the TLC plate (left) along with MALDI spectra (right) of spots (a)–(c). Reproduced from Ref. [111] with permission of the American Chemical Society.

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Fig. 23. MALDI spectra of complex mixture obtained when tetrakis(p-hydroxy-phenyl)porphyrin is allowed to react with chlorinated poly(ethylene glycol) methyl ether. The mixture contains a consistent fraction of star polymers with 2, 3, 4 arms. Reproduced from Ref. [184] with permission from Wiley.

poly(ethylene glycol) the spacing is 44.05 g/mol, in poly(lactic acid) t 72.1 g/mol, in poly(dimethyl siloxane) 74.1 g/mol, in poly(butylene terephthalate) 220.2 g/mol. This feature provides the polymer identification. In the case of branched polymers, quantities of interest are the number and the position of branching points. Their determination is sometimes impossible using MS, because branched and linear macromolecules often have the same mass (polyolefins are a typical instance). However, when they have end groups different from hydrogen atoms, and the spectra are mass-resolved, the masses of branched polymers and the corresponding linear ones differ. Trifunctional (or multifunctional) units may have different masses than the corresponding linear polymer (this circumstance often occurs in grafted copolymers). Sometimes one deals with a complex mixture of macromolecules with the same repeat unit but with different architectures. For instance, during the synthesis of star polymers with four branches, it may happen that stars with three or with two branches are also formed. It often happens that macromolecules belonging to such a complex mixture have different masses and, in these cases, MALDI can

discriminate between among them. In an interesting example [184], Fig. 23 gives the MALDI mass spectrum of the products obtained when tetrakis(phydroxyphenyl)porphyrin is allowed to react with chlorinated poly(ethylene glycol) methyl ether, providing an extremely complex mixture of macromolecules. Ions due to chains with one branch appear in the region 800–1200 Da, whereas ions due to chains with two, three and four branches appear at 900–1500, 1000– 2000 and 1500–2500 Da, respectively. Each type of architecture generates an envelope of peaks, and a line is drawn to indicate the envelope. The abundances of chains with one, two, three or four branches can be denoted as B1, B2, B3, B4. By measuring individual MALDI peak intensities, one can estimate the relative abundances and, in particular, the ratio B1/B2/B3/B4. Alternatively one can evaluate the ratio by measuring the area subtended by each envelope. Using MALDI, one can elucidate even more complex structures. Im et al. [32] recorded the MALDI spectrum of a complex mixture where chains with three, four and five branches, and other complex architectures, were present. The MALDI spectrum was


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Fig. 24. MALDI spectrum of a polyamide sample with a linear/cycle mole ratio close to 5/95. Reproduced from Ref. [267] with permission from Dekker.

very clean. Peak positions gave information on the structures present, whereas peak intensities gave a semiquantitative estimate of their abundances. In condensation polymers, one often deals with samples that are mixtures of linear and cyclic macromolecules. Fan et al. [267] synthesized an aromatic polyamide and found the sample to have a massive amount of cycles, the linear/cycle molar ratio being close to 5/95. The NMR spectrum was not very useful for quantitation; signals due to end groups were very weak whereas signals due to cycles were

superposed with signals due to backbone units. Fig. 24 shows the MALDI spectrum, from which the chemical structure of the cyclic macromolecules could be determined. The ions dominating the spectrum, indicated as C2, C3, C4, are due to the cyclic dimer, cyclic trimer, and cyclic tetramer, respectively. 2.14. End group determination The general structure of ions detected by MALDI is of the type

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G1–AAAAAAA–G2 /CC where G1 and G2 are end groups, CC is a proton or a cation and A is the repeat unit. End group determination by MS is done as follows. One considers the mass number of one of the MS peaks, subtracts the mass of C, and then repeatedly subtracts the mass of the repeat unit, until one obtains the sum of the masses of G1CG2. A linear best fit can also be used to find G1CG2. As a simple example, we recall the MALDI spectrum of the polystyrene sample with molar mass around 7 kDa used for the NIST-sponsored interlaboratory comparison [16]. The individual peaks are due to molecular ions (MC), and the repeat unit is C8H8, with a mass of 104.15 Da. If one subtracts an integral number of monomer units from the observed molecular ion, a ‘residual’ mass of 58 Da is obtained (e.g. 6307K60!104.15Z58). This represents the sum of the end group masses. A mass of 58 Da is consistent with a butyl group at one end of the chain and a hydrogen on the other: H–(St)n–C4H9. In this instance, the polymer was prepared anionically using tert-butyl lithium [16]. One should note that the lowest residual mass (in this case 58 Da) is not necessarily the correct sum of the end group masses. In principle, the sum of the end group masses might 162, 266, 370 Da, etc. (i.e. 104!nC58, with nZ1, 2,.). One may need additional information to elucidate the correct end groups: for example, other spectroscopic or chromatographic data and/or knowledge of the synthetic procedure that was used. 2.15. Tandem mass spectrometry for structure determination For structural studies, one may use a mass spectrometer equipped for tandem MS (see Fig. 1). Some ions, once generated, break apart quickly. This can occur by ‘in source fragmentation’ (INSF) inside the ion source or by ‘post-source decay’ (PSD) in the field-free region. In MALDI-TOF instruments equipped with a reflectron, utilization of PSD is appealing, since it can give information on the structure of ions. The method of analyzing PSD fragments bears distinct similarities to classical tandem MS (MS/MS) on double-sector instruments [422]. In the latter, one of the sectors is used to produce a narrow-mass spectral window (a beam of ions of approximately the same mass, ‘parent-ions’ or ‘precursor-ions’), whereas the other sector is used to separate and analyze the fragment ions, ‘daughter-ions’, produced in the collision chamber. The PSD method consists of three steps: (1) selecting a parent-ion by the ion-selector, (2) changing the voltages of the reflectron

electrodes, (3) recording the resulting spectrum, called the ‘PSD-MALDI spectrum of the parent-ion’ at the specified mass. The ion-selector (see Fig. 1) is an ultrafast electronic switch capable of pulsing away all the ions except those in a narrow temporal window (a couple of nanoseconds) corresponding to a mass window of 4 Da or less. Since MALDI is a soft-ionization method, the number of intact oligomer ions that break apart spontaneously is small, and it is necessary to increase it to improve the quality of the PSD-MALDI spectrum: i.e. increase the signal-to-noise ratio. For this purpose, MALDI-TOF instruments accommodate a collision chamber (see Fig. 1), in which the MALDI ions suffer hundreds of collisions with molecules of an inert collision gas (usually argon). The effect of the collision chamber is to increase the number of ions that break apart. In this way, one obtains collision-induceddissociation (CID), and the resulting spectrum is referred to as a CID-MALDI spectrum [423]. In recent years, there have appeared tandem time-of-flight (TOF/ TOF) instruments, which are especially important for the analysis of high mass, singly charged ions [422–424]. Hoteling and Owens [174], using PEG1000 as a model analyte, studied the effect of various instrumental parameters operating on the resolution and mass accuracy in MALDI-TOF PSD and CID analysis of polymers. In another study, Hoteling et al. [176] investigated the effect of the collision gas (He, Ar, air) and collision gas partial pressure, on the MALDITOF CID MS/MS spectra of PEG1000. Fournier et al. used PSD-MALDI to study fragmentation pathways of three nylon oligomers desorbed under MALDI conditions [272]. They found that the end groups and the length of the methylene units influenced the fragmentation of the different nylons and the relative abundance of the product ions. They observed competitive dehydration and deamination reactions, depending on the nature of the terminal groups and the repeat units. In all cases, the PSD spectra were very similar to the CID spectra recorded under low-energy conditions, indicating that the selected precursor ions had similar average internal energies. Fig. 25a shows the PSD spectrum of the parent ion with m/zZ599 corresponding to the protonated Ny6 tetramer dicarboxy terminated (Ny6– COOH), while Fig. 25b reports the MALDI-CID spectrum of the protonated Ny6–COOH trimer oligomer (m/zZ486). Peaks present in both spectra can be interpreted with the scheme in Fig. 25. The parent ion (at mass 599) eliminates a water molecule,


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Fig. 25. Fragmentation of NY6: effect of the collision cell. The upper spectrum is recorded on an instrument without a collision cell. Reprinted from Ref. [272] with permission from Wiley.

generating the ion at mass 581. The latter has a C-terminal (actually a COOH group) and an N-terminal, and therefore, there are two fragmentation pathways. The parent ion can suffer consecutive losses of a neutral lactam (with mass of 113 Da). MS peaks at masses 468, 355, 242 are due to fragment ions that retain the N-terminal. Alternatively, ions can retain the C-terminal and peaks at 471, 358, 245 are due to these fragment ions (the mass difference is always 113 Da). Ions giving peaks at 454, 341, 228 retain the C-terminal too, but they are generated by further scissions of the previous ions. In the scheme, the C-terminal and the N-terminal of the parent ion are drawn at the right and at the left, respectively. The ion

undergoes fragmentation, and thus tandem MS allows determination of the masses of the C-terminal and the N-terminal separately [272]. In another paper, Murgasova et al. [265] suggested from MALDI-CID spectra of hexamer, octamer and dodecamer linear Ny6 oligomers, that the fragmentation process includes cleavage of the end groups followed by dissociation of the m/zZ113 unit (the repeat unit). They also observed that the cleavage of the oligoamide chain occurs at the amide linkage, as well as at adjacent bonds. In the same work, they studied the effect of the matrix and cationization agent in MALDI-CID analysis of Ny6 and found that the DHB matrix and NaCl salt gave the best results.

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Arnould et al. [347] synthesized a branched polymer by reacting a mixture of two diacids, terephthalic acid (T) and adipic acid (A), with a mixture containing a large amount of a diol, neopentyl glycol (G), and a small amount of a triol, trimethylol propane (M). They recorded the MALDI spectrum of the polymer and they noted the presence of eight mass series. The first four mass series were assigned to linear chains with 0, 1, 2, 3 adipic acid residues along the chain. The absence of chains with more than three A units is consistent with the fact that adipic acid was by far less abundant than T in the feed. The other four mass series were assigned to branched chains with one or two branching points and different amounts of adipic acid. Clearly, the presence of branching points coincides with the presence of type M units. Although their structural characterization was complete, the authors also performed some fragmentation experiments. Fig. 26 shows the PSD-MALDI spectrum of the parent-ion at mass 1297, which corresponds to the sodiated pentamer H–(GT)5–G–OH. Peaks labeled L and C are due to linear and cyclic fragment ions, respectively; and the subscripts indicate the cation (H means protonated). It can be seen that linear chains (being flexible) bind NaC more strongly than cyclic chains and that cycles are more basic than linear fragments. The spectrum also contains several lowmass products that do not contain NaC (marked DH) and are due to consecutive dissociations of larger fragment ions. Thus, it is evident that structural information can be recovered from the PSD-MALDI spectrum.

Fig. 26. PSD-MALDI spectrum of the parent-ion at mass 1297, which corresponds to the pentamer H–(GT)5–G–OH/NaC. Reproduced from Ref. [347] with permission from Elsevier.

Laine et al. [87] observed systematic changes in fragmentation behavior of PMMA with increasing MM by PSD MALDI-TOF analysis of alkali–metal cationized PMMA 20, 60 and 100-mer. Using lithium, potassium and cesium salts, they observed that increasing MM of PMMA required increased cation size to optimize the intensity and the number of the fragments in the PSD spectrum. In fact, they obtained the best PSD spectra when PMMA 20-mer was cationized with lithium and 100-mer with cesium. In the last case, the best results were obtained from SEC fractionated PMMA sample. The authors postulated the advantages of various cations to be a consequence of the strength of interaction of the cations with isolated PMMA molecules and the PSD fragments [87]. An alkali cation effect has also been observed in the MALDI-TOF PSD analysis of ethoxylated polymers that are commonly used as surfactants [193]. Neubert et al. confirmed the dendrimer structures of a number of polypentylresorcinol samples by PSD MALDI-TOF MS analysis, using a-cyano-4-hydroxycinnamic acid (CHCA) matrix and alkali salts as the cationizing agent [299]. They observed heavy fragmentation using LiC and NaC cations, and on the basis of the corresponding PSD spectra proposed fragmentation pathways. The most abundant PSD fragments arose from loss of the terminal tetrahydropyranyl (THP) ether groups. In an interesting work, Gies and Nonidez [270] determined the lengths of blocks in poly(ethylene oxide)-b-poly(p-phenylene ethynylene) (PEO-b-PPE) by PSD MALDI-TOF. The fragment ion mass spectra revealed that the main fragmentation process involves cleavage of the ester function that links the two blocks. They also determined the composition of the PEO-bPPE copolymers from their PSD spectra. Recently, Rizzarelli et al. [362] showed by PSD MALDI-TOF analysis of poly(esteramide)s from dimethyl sebacate or sebacic acid and 1,2-ethanolamine or 1,4-buthanolamine, that the main cleavage of these polymers proceeded through a b-hydrogen transfer rearrangement. MALDI-TOF/TOF spectra acquired using Ar as a collision gas, showed new intense fragment ion peaks in the low mass range, mixed with the peaks present in the PSD-MALDI spectra of the same parent ion. These new fragment ions were diagnostic in establishing the random sequences of the ester and amide bonds in the copoly(esteramide)s [362]. Fig. 27 shows the MALDI-TOF/TOF-MS/MS spectrum of the sodiated diamino alcohol terminated oligomers at m/zZ1220 of the poly(esteramide) derived from sebacic acid and an excess of


307

722.42 747.46

949.55 974.59 707.80

725.46 764.49

700.0

819.57

992.72

861.57

779.6

859.2

938.8

1046.62

1088.73

1018.4

1098.0

495.29

520.32

498.30

477.29 480.33

592.37 536.37 523.35 541.38

502.30

472.0

514.4

634.42 648.45

606.40 563.36

556.8

599.2

662.48 676.47

641.6

684.0

268.15 271.18

293.18

365.21 309.19 407.24

266.19

275.17

264.16

279.19

260.0

297.19

336.23 322.17

301.2

347.22 363.22

342.4

379.21

375.23

452.25

435.29

449.31

421.28 393.25

383.6

424.8

466.0

210.15 62.05

228.16 55.03 65.99

69.05

54.0

253.18

84.05 88.06

225.11 98.06

95.08

94.8

112.07

180.07

125.04 138.05 152.07

135.6

167.05

176.4

194.08 208.11

223.15

250.14 229.15 237.11

217.2

258.0

m/z Fig. 27. Expansions of MALDI-TOF/TOF-MS/MS spectrum of sodiated diamino alcohol terminated oligomers at m/zZ1220 of a poly(ester amide) sample synthesized from sebacic acid and an excess of 1,2 ethanol amine. Reproduced from Ref. [362] with permission from Wiley.

308


O(CH2)2 NHCO(CH2)8CONH (CH2)2O

O

(CH2)2NHCO(CH2)8CONH(CH2)2 O

(a)

NH2CO(CH2)8CONH2

CH2 CHNHCO(CH2)8CONHCH CH2

M•Na+ 223.15

O

M•Na+ 275.17

(CH2)2NHCO(CH2)8COO

(CH2)2NH

(b)

CH2 CHNHCO(CH 2)8COOH + M•Na 250.14

NH(CH2)2 OCO(CH2)8COO (CH2)2NH

(c)

HOCO(CH2)8COOH M•Na+ 225.11

Scheme 1.

1,2-ethanolamine. As can be seen in Scheme 1, the fragment ions at m/zZ223 and 275, 250, and 225 are diagnostic of amide–amide, ester–amide, and ester– ester sequences, respectively. These data suggest that the ester and amide groups are distributed randomly in the copolymer chains [362]. Muscat et al. observed in-source fragmentation by MALDI-TOF analysis of hyperbranched polyesteramide, prepared from hexahydrophthalic anhydride and diisopropanolamine. The MALDI spectrum did not show signals due to the oligomers terminated with –OH groups, whereas peaks corresponding to protonated oligomers minus water [HPEAKH2OCH]C [425] did appear. These data provide evidence that the in-source decay of hydroxyl terminated HPEA chains causes end group loss. 2.16. Copolymer characterization Mass spectra of copolymers are significantly more complex than those of simple homopolymers, and thus, the task of peak assignment is more demanding. However, the procedure is the same: i.e. putting forward a reasonable hypothesis on the chemical structures that may be present in the sample, computing the masses of all the possible chains and checking if expected peaks are actually present in the spectrum. In AB copolymers, two repeat units with masses m1 and m2 alternate along the macromolecular backbone.

The case in which m1 is equal (or almost equal) to m2 is extremely rare, and thus, the identification of the copolymer backbone structure by MALDI is readily accomplished. For instance, MALDI spectra of styrene–methyl methacrylate copolymers display many peaks and the differences in mass between peaks are 108 and 100, which correspond exactly to masses m1 and m2. MS peak intensities can be used to determine copolymer composition [3,9,316], provided that the ionization method used to desorb and ionize the oligomers does not produce significant ion fragmentation. Appendix B describes in some detail how to use MS peak intensities to determine copolymer composition. The application of the MS method is based on the assumption that the intensities of peaks appearing in the mass spectrum of a copolymer are directly related to the relative abundance of oligomers present in the copolymer [3,9,315,316]. This condition is usually met, but there are a few exceptions. For instance, in the case of a random copolymer with units of isobutylene (IBU) and para-methylstyrene (pMST), the average molar composition determined from the MALDI spectrum of the copolymer was found to be skewed to higher methylstyrene content (36 mol%) as compared to that obtained by NMR (13%) [315]. The authors believe that the facile ionization of methylstyrene-rich oligomers caused the composition discrepancy between MALDI and NMR data.


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2.17. Bivariate distribution A bivariate distribution, as a function of mass and composition, is a feature peculiar to addition copolymers synthesized by high-conversion processes (as many industrial copolymers are) and it is frequently associated with a drift of the composition with growing length. The relevance of the bivariate distribution of chain length and composition has been discussed [3, 336,337]. The shape of the bivariate distribution has a sensible influence on copolymer properties, but its measurement has been somewhat laborious. In fact, the method traditionally used for measuring the bivariate distribution is ‘chromatographic cross fractionation’ (also referred to as ‘two-dimensional chromatography’ or ‘orthogonal chromatography’). Macromolecules of different compositions are separated in a silica column, and an SEC column is then used to elute chains of different sizes [418– 420]. As an alternative to conventional methods, MS can be used, and this has shown great advantage [3,9]. For low molar mass copolymers, MS alone is sufficient to monitor the compositional drift in AB copolymers, and the change in the mole fraction of the A units was followed in the range 2000–11,000 Da [326]. The method was used to estimate the weight fraction of copolymer chains, which possess a given composition and to draw the compositional distribution histogram for copolymers containing methylmethacrylate, butylacrylate, styrene or maleic anhydride units [326]. For copolymers of high molar mass some problems arise, owing to the fact that REGmax (see above) takes values of 12 kDa with a loss of spectral resolution above that value. Nevertheless, a variant of the SECMALDI method used to overcome this problem. The variant consists in fractionating the whole copolymer by SEC, collecting the fractions and recording both MALDI and NMR spectra [337]. In fact, because of the high sensitivity of the MS and NMR methods, the amount of sample in the narrow fraction provided by an analytical SEC device is sufficient to run both types of spectra. Fig. 28 shows the bivariate distribution of a commercial ST/MAH sample [325]. The sample was polymerized to high conversion, and it exhibits an asymmetric bivariate distribution, showing the composition drift expected for this type of polymer. 3. Recent applications In addition to the applications discussed above, MALDI-MS techniques have been used to analyze a variety of polymers; extensive listings of MALDI

Fig. 28. Bivariate distribution for ST-maleic anhydride sample. Reproduced from Ref. [325] with permission of the American Chemical Society.

studies on synthetic polymers have been published [3,4,8]. A survey of the most important MALDI literature on selected classes of polymers appearing in the last 5 years follows. 3.1. Polystyrene Narrow- and wide-dispersion polystyrenes (PS) synthesized by different methods and having different end groups have been extensively studied by MALDITOF [12–63]. Bartsch et al. [23] found that chlorine, amine and acrylate functionalized TEMPO-capped PS, and bisTEMPO-capped PS, could be analyzed by MALDITOF MS by utilizing gentle conditions of protonation with the DHB matrix. Campbell et al. [13] prepared PS by thermal polymerization of styrene between 260 and 340 8C, and characterized the product by 13C NMR, preparative SEC and MALDI-TOF MS. They measured the distribution of terminal unsaturation by the last technique. The MALDI data showed that the backbiting reaction, followed by a b-scission dominates the molar mass development by comparison with either termination or chain transfer processes. In other work, Schappacher and Deffieux [53] prepared macrocycles by direct coupling of a-acetal-u-bis(hydroxymethyl) heterodifunctional polystyrene at high dilution, and characterized the structure of the linear and cyclic PS chains by MALDI-TOF-MS [53]. Zettl et al. [29] obtained rhodamine-B-labeled polystyrene PS (PS-RhB) by reacting a large excess of the acid chloride of rhodamine-B with

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hydroxyl-terminated polystyrene (PS-OH). The MALDI spectrum of PS-RhB shows mass-resolved peaks in the 9000–13,000 Da, region, a good result, because rhodamine-B is a very complex, quite massive (479 Da) molecule and because the addition of a silver salt as a cationization agent to PS-RhB does not yield silver-cationized peaks. Francis et al. [27] prepared AA 0 2-type asymmetric stars and AB2-type miktoarm star polystyrenes using a precursor. The latter polymer consisted of a-bromopolystyrene chains (i.e. chains terminated on one side by a Y-shaped group with two Br atoms) obtained by atom transfer radical polymerization (ATRP) using ethyl 2-bromoisobutyrate as the initiator. Alberty et al. [25] prepared a series of polystyrene dianions and reacted them with dibromomethane (DBM). They obtained almost pure cycles (99%) and showed that the 1% impurity was due to linear chains containing a styrenic chain end. Cauvin et al. [18] studied cationic polymerization of p-methoxystyrene in a miniemulsion. They considered MALDI spectra of two polymer samples, PM2 and PM5, withdrawn after 2 days, and after 5 days, respectively. Sample PM2 is at almost 100 wt% conversion and its spectrum shows that all chains bear one methyl and one hydroxyl chain end provided by proton initiation and water termination, respectively. Side reactions do not occur. For instance, transfer to monomer would have generated ethylenic or indanyl terminations, but peaks due to both reactions are absent from the spectrum. Sample PM5, withdrawn 3 days after polymerization ceased, is partially degraded. The MALDI spectrum shows series of new peaks. From the position and the intensity of the new peaks, the authors were able to infer that chain-end dehydration, as well as chain scission, is responsible for the generation of short chains during the acidcatalyzed degradation reactions. The formation of disproportionation products was revealed during the synthesis of telechelic PS by atom transfer radical polymerization (ATRP) of styrene at 110 8C using various substituted 2-bromoisobutyrates as initiators and the homogeneous catalyst CuBr/1,1,4, 7,10-hexamethyltriethylenetetramine [30]. In another study, Deng and Chen used MALDI-TOF MS to confirm the branched structure of the core of a star polymer synthesized by ATRP of N-[2,(2-bromoisobutyryloxy)ethyl]maleimide and styrene [31]. Goldbach et al [19] obtained anthracene end-functionalized PS (PS-Ant) and anthracene end-functionalized PMMA (PS-Ant). Then, they synthesized a diblock

copolymer (PS-AntAnt-PMMA) via UV coupling of PSAnt with PMMA-Ant. When they reacted PS with 2(bromomethyl)anthracene, the MALDI spectrum revealed end-functionalized PS of the expected molecular weight as well as a considerable amount (O20%) of a product with exactly double the expected molecular weight. This impurity was assigned to ‘dimeric’ PS containing an anthracene middle group. The authors proposed a mechanism for the formation of this reaction byproduct by three steps: (a) SN2 substitution of bromine by polystyryllithium; (b) nucleophilic attack on the anthracene ring by a second PS chain; (c) rearomatization back to anthracene. Since the impurity level was unacceptably high in this preparation, the authors decided to react polystyrene with 1-phenyl-1-(2-anthryl)ethylene (PHANE). In this case, the MALDI spectrum also showed a peak due to the ‘dimeric’ impurity, but the peak was less than 5% of the total. In this way, MALDI aided the synthesis of PS-Ant, since the spectra indicated clearly that the synthetic route that uses PHANE is the better one. Tatro et al. [61] synthesized a series of polystyrene and PMMA samples with narrow MM distribution. They measured the viscosities and developed a method that uses MALDI to determine Mark–Houwink– Sakurada (MHS) parameters. The method requires samples with narrow MM distribution and may, therefore, be impractical for polycondensates. For this reason, Montaudo [62] proposed a modification based on the universal calibration concept and on the coupling of size exclusion chromatography with MALDI. The modified method was applied to two styrene–maleic anhydride copolymers and to a series of polymers and copolymers obtained by condensation. The MHS parameters were measured for all the samples and compared with values obtained by a method for predicting MHS parameters from first principles. Menoret et al. [20] synthesized polystyrene using a mixture of lithium hydride and triisobutylaluminum ([Al]/[Li]Z0.7). The MALDI spectrum shows a bimodal distribution, with the maxima centered at 2900 and at 6800 Da. Peaks in the low-mass region are due to ions of the type Bu–(St)n–H/AgC (BuZ CH3CH2CH2) whereas peaks in the high-mass region are due to H–(St)n–H/AgC ions. In another study [21], the mixture was slightly changed and triisobutylaluminum was replaced by a solution of n-butylmagnesium and n-octylmagnesium, the n-butyl/n-octyl ratio being about 75/25. Various [Mg]/[Li] ratios were used and the authors used MALDI to monitor the reaction products. The polystyrene obtained using a concentration ratio [Mg]/[Li]Z3, shows two series of peaks due to Bu–(St)n–H, AgC and to Oct–(St)n–H/AgC


(OctZoctyl). The latter are about three times more intense than the former (intensity ratio about 75/25), suggesting that the reactivities of n-butylmagnesium and n-octylmagnesium are comparable [21]. Park et al. characterized two highly branched PS samples by a combination of reverse-phase temperature-gradient interaction chromatography (RP-TGIC) and MALDI-TOF-MS. As expected the molar masses increase as integral multiples of the PS precursors [40]. They also used normal phase-TGIC-MALDI-TOF-MS methods to characterize air-terminated polystyryllithium [40]. Kassalainen and Williams [39] successfully combined thermal field-flow fractionation (ThFFF) with off-line MALDI-TOF MS analyze polydisperse PS and poly(2-vinylpyridine) homopolymers and their mixtures in the MM range from several kiloDaltons to several hundred kiloDaltons [39]. The data show that narrow polymer fractions can be obtained by ThFFF and, therefore, that combined ThFFF/MALDI-TOF MS technique may be a viable means for preparing standards from a widely polydisperse polymer sample. 3.2. Polymethylmethacrylates and acrylic polymers A host of papers on MALDI-TOF-MS of polymethylmethacrylates (PMMA) and acrylic polymers have appeared in the last 5 years [64–102]. For reasons of space, our comments will not be exhaustive. Norman et al. [71] synthesized a series of PMMA samples in which the chains were mainly terminated with an unsaturated end group (actually an MMA molecule). These polymers were produced by the addition of catalytic chain transfer agents at late stages of an atom-transfer polymerization. The percentage of unsaturated end groups, PEUN in one of the samples (denoted PMMA-A) was measured by 1H NMR, yielding PEUNZ0.76; and it was also calculated from the percent weight loss at 225–275 8C, by thermogravimetric analysis (TGA), yielding PEUNZ 0.78. The MALDI spectrum revealed only two types of ions, T1 and T2. T1 is due to unsaturated end groups, whereas T2 is due to ions with an end group (referred to as a lactonized end group), which contains a fivemembered heterocycle. T1 ions are three times more abundant than T2 ions, implying that PEUNZ0.75, in good agreement with TGA and NMR measurements. Favier et al. [76] studied the polymerization of an acylamide derivative, N-acryloylmorpholine, using azobis(isobutyronitrile) as the initiator and tert-butyl dithiobenzoate as a RAFT chain transfer agent. Fig. 29 shows MALDI mass spectra of the reaction products as

311

a function of conversion. The spacing between mass spectral peaks is quite large, due to the fact that the acylamide derivative is a massive group (it contains a phenyl ring). Theory predicts that the average molar mass increases linearly with the conversion, and that the proportionality constant is related in a simple manner to the monomer/initiator ([M]/[I]) ratio. In order to compare the theoretical prediction with experiment, the authors plotted the molar mass estimated using MALDI versus conversion. The agreement was good. The most intense peaks in the MALDI spectra are due to chains terminated by a dithiobenzoate group. The authors showed that, if suitably treated, they can further react to form longer chains. The matrix trans-2[3-(4-tert-butylphenyl)-2-methyl2-propenylidene)]malononitrile (DCTB) was used in experiments to determine the propagation rate coefficients Kp of PMMA prepared by pulsed-laser photopolymerization (PLP) [65]. The Kp determined by MALDI-TOF MS became constant after the first 100 propagation steps, whereas the values determined by SEC decreased with increasing chain length. Willemse et al. proposed that these differences are due to instrumental effects in SEC [65]. By increasing the laser intensity, using dithranol matrix and AgTFA as the cationizing agent, Nonaka et al. [66] demonstrated that partial dehalogenation occurs during the MALDI-TOF MS analysis of Clterminated PMMA, as well as for Cl-terminated poly(methyl acrylate) (Cl-PMA). Moreover, they observed that Cl-terminated PS is unstable under MALDI-TOF MS conditions. MALDI analysis of low MM PMMA polymers, obtained by anionic polymerization of methyl methacrylate (MMA) initiated by phenyllithium combined with MoCl5 or WCl6, has shown that initiation of MMA occurs by nucleophilic attack of HK on the monomer [67]. In addition, MALDI-TOF MS analysis indicates that iBu3Al controls the polymerization by improving the uniformity of the polymerization with respect to initiation and termination and by preventing a backbiting reaction [67]. MALDI-TOF MS analysis has permitted direct identification of Co–C bonds in polymethylacrylate (PMA) prepared by chain-transfer polymerization of methyl acrylate with a Co(II) complex [80]. MALDI analysis has also confirmed the total displacement of cobaloxime from PMA chains when the polymer is reacted with a-methylstyrene. SEC/MALDI-TOF MS has proved useful in elucidating the products, denoted as PBA-RAFT, from

312


Fig. 29. MALDI spectra of polymerization products of an acrylamide derivative at various conversions. Reproduced from Ref. [76] with permission of the American Chemical Society.

polymerization of butyl acrylate in the presence of the reversible addition-fragmentation chain-transfer (RAFT) agent, cumyl dithiobenzoate [75]. Fig. 30 shows the structures of the reaction products of PBARAFT with Br-terminated PBA initiated with Cu(I) and Cu(0). In the latter case, SEC/MALDI analysis of the polymeric material clearly revealed the formation of three- and four-arm stars. This constitutes the first example of the synthesis of a four-arm star through intermediate–intermediate radical polymerization. The authors postulated that this peculiar reaction pathway is

due to the much slower fragmentation rate in the BA system than in the styrene system [75] Jiang et al. fractionated PMMA obtained in RAFT polymerization by LCCC and then analyzed the fractions by off-line MALDI-TOF-MS and electrospray-ionization quadrupole-TOF-MS (ESI-QTOF-MS) [82]. Labile end groups of PMMA, such as the dithioester groups, were lost in MALDI-TOF experiments but were observed intact in the ESI-QTOF-MS spectra [82]. Schilli et al. [72] monitored by MALDI the polymerization of N-isopropyl-acrylamide (NIPAAm)


313

Fig. 30. Structure derived from MALDI-TOF-MS distributions for reaction of poly(butyl acrylate)–S–C–(Ph)aS with poly(butyl acrylate)–Br initiated with Cu(I) and Cu(0) Reprinted from Ref. [75] with permission of the American Chemical Society.

314


Fig. 31. Polymerization of acrylamide in the presence of chain transfer agents. Structure of six types of oligomers expected (upper part) together with MALDI spectra of the poly(acrylamide) sample (a) and an expansion (b) of the spectrum [1900–2020] with the theoretical isotope distribution. Reproduced from Ref. [72] with permission of the American Chemical Society.

carried out in the presence of a mixture of two dithiocarbamates, namely benzyl 1-pyrrolecarbodithioate and cumyl 1-pyrrolecarbodithioate, which act as chain-transfer agents. The polymerization is expected to give chains terminated in three different ways, namely: (1) with dithio (dit), (2) with H (H) (3)

with a double bond (doub). Furthermore, chains can occur with two starting groups, namely (1) with a cumyl group (cum) (2) with one-half azobis(isobutyronitrile) (in) and thus there are six possible combinations. Fig. 31 shows the MALDI spectrum of the NIPAAm polymer. There are six mass series, corresponding to


the six expected types of chains. The figure also shows expansion of the spectrum in the mass region 1900– 2020 Da, along with the simulated isotopic pattern. The experiment and simulation show some analogies but also some differences, probably due to the fact that the simulation was performed under the (incorrect) assumption that all chain species are equally abundant. 3.3. Other polymers with an all-carbon main chain MALDI spectra of poly(vinyl acetate) [103], poly(ethylene) [117–123], poly(butadiene) [125,126] and polyisoprene [116] have been recorded by various authors [103–127]. As a rule, MALDI analysis of polyethylene requires careful sample preparation, as discussed above; but low molar mass samples represent an exception [118–120]. Switek et al. [108] obtained polyisoprene using secbutyllithium or 1-tert-butyldimethylsiloxypropyllithium as the initiator. They reacted the resulting polymer with hexafluoropropylene oxide (HFPO) and obtained a three-arm star polymer, plus some sidereaction products. They proposed a mechanism, in which HFPO acts as a multifunctional coupling agent. The MALDI spectrum of the reaction products displays three mass series. The first series appears only at high mass, and peaks are due to the three-arm star polymer. The second series and the third are present exclusively in the middle range and in the low molar mass range, respectively. These peaks are due to two-chain ketone structures and to poly(isoprene) chains simply terminated by a proton. All MALDI peaks support the proposed coupling agent mechanism. 3.4. Polymers with heteroatoms in the main chain Polymers with heteroatoms in the chain have been analyzed by MALDI [128–283]. We shall consider polysiloxanes, poly(silsesquioxane)s and polysilanes [128–148], polyethers [149–205], polyesters, [206–250], polycarbonates, [251–261] polyamides and polyimides [262–283]. 3.5. Polysiloxanes, poly(silsesquioxane)s and polysilanes MALDI spectra of polysiloxanes are commented upon above in the discussion of: impurity detection, SEC-MALDI [128–131,138,140], using supercritical CO2 [139] and comparing MMDs obtained by SEC and by MALDI [130,131]. Poly(silsesquioxane)s are silicon-containing polymers with a peculiar stoichiometry;

315

their MALDI spectra are quite complex, but also very rich in structural information [143–148]. 3.6. Polyethers More than 50 reports on MALDI of polyethers have appeared in the period considered [149–205]. Gobom et al. [161] showed that it is possible to calibrate a MALDI-TOF spectrum with fantastic accuracy, namely to 10 ppm. In order to achieve their goal, they had to introduce an alternate function for time-to mass conversion and they had to select an analyte which produces many intense equally spaced peaks in the mass range 1000–7000 Da. Their choice was PPG or, more precisely, a mixture of four PPGs, each with a narrow distribution, with different masses that covered the entire mass range. Kricheldorf et al. [192] prepared poly(ether sulfone)s by polycondensation of silylated 4-tert-butylcatechol and 4,4-difluorodiphenylsulfone in N-methylpyrrolidone, varying the proportions of the reactants. The MALDI spectrum of the polymer prepared with nearly exact stoichiometry showed a single mass series, with peaks up to 19 kDa. Creaser et al. [175] analyzed PEG 1500 was by atmospheric pressure MALDI quadrupole ion-trap MS (AP-MALDI-QIT) and by classical vacuum MALDITOF MS. Contrary to the MALDI-TOF spectra that presented cationized oligomers, the AP-MALDI-QIT spectra showed dimetallated matrix–analyte adducts, in addition to the expected PEG-alkali ions, for all the matrix/alkali salts used. To study the interaction of PEG with ions likely to be found in the human body, Mwelase et al. [150] prepared complexes of bivalent ions of Mg, Ca, Cu, Zn and Pt with a PEG5000 at pH 7 and characterized them by MALDI-TOF MS and other techniques (UV, FT-IR, TG). From the MALDI spectra, they deduced that Cu(II), Zn(II) and Pt(II) are directly bound to PEG without water whereas Mg(II) and Ca(II) complexes hold four and six water molecules, respectively [150]. MALDI spectra of poly(3-ethyl-3-hydroxymethyloxetane) (poly-EOX) and of samples obtained by cationic polymerization of EOX in presence of an analogous polyether such poly(3,3-dimethyloxetane) (poly-DIOX) that does not contain OH groups, show that the intramolecular reactions (backbiting) observed during cationic polymerization are due to OH groups while the ether moiety does not participate in the backbiting processes [167]. Therefore, this reaction, limits the MM of poly-EOX allowing formation of

316


cycles. MALDI analysis of SEC fractions of a hyperbranched polyether, prepared by melt transetherification of 1-(2-hydroxyethoxy)-3,5-bis-(methoxymethyl)-2,4,6-trimethylbenzene with mesitol in presence of an acid catalyst, reveal two families of peaks due to simple branched structures and to macrocycles [203]. From the relative intensities of the two peaks, it is apparent that cyclization is favored at higher conversions in the melt transetherification process. Various end groups in oligo(isobutyl vinyl ether) (iBVE) have been characterized by MALDI-TOF MS [105,166], which reveals that oligo-iBVE containing aryl ketone ends are susceptible to elimination of the ultimate isobutoxy group in the presence of a strong Lewis acid, while the ketone groups are unaffected [105]. MALDI-TOF MS mass spectra of poly(propylene oxide), obtained by anionic polymerization of propylene oxide in the presence of alkali metal alkoxide initiators and trialkylaluminum, in addition to the intense peaks belonging to the expected oligomers terminated with alkoxide initiator and OH groups, showed weak peaks due to oligomers terminated with OH and unsaturated allyl groups, which were not detected by 1H NMR [162]. In the case of polyethers obtained by polymerization of isosorbide with 1,8-dibromo or dimesyl octane with phase transfer catalysts under microwave irradiation or by conventional heating, MALDI-TOF MS showed that the mechanism of chain termination is different in the two methods [163]. Polyethers prepared by conventional heating have short chains with hydroxylated ends, whereas under microwave irradiation the polymer chains are longer with ethylene end groups. MALDI-TOF MS has also been used to characterize ethylene oxide (EO) and propylene oxide (PO) copolymers [178,346]. Using homemade software, Terrier et al. [178] determined the copolymer composition of triblock copolyethers EO–PO–EO, PO–EO–PO, and a random EO/PO copolymer by MALDI-TOF. They observed that MALDI spectra of the triblock copolymers depend on experimental parameters, such as the number of laser shots relative to the polymer/salt ratio, and on the nature of the matrix. They detected the side-reaction products with unsaturated end groups by MALDI analysis of the crude copolymer and by MALDI analysis of SEC fractions. These results were confirmed by 1H NMR [178].

3.7. Polyesters There are a number of reports of characterization of polyesters by MALDI-MS [206–250]. Kricheldorf et al. [227] obtained an aliphatic polyester by condensation of isosorbide and suberoyl chloride. The MALDI spectrum showed three types of peaks (La, Lb, Lc) due to linear polyester chains terminated in three different ways plus peaks (C) due to cyclic polyester chains. The intensity ratios were C/La/ Lb/LcZ83/7/7/3. Since cyclodextrins and their partly methylated derivatives are commercially available and are known for their ability to thread a variety of linear polymers (depending on ring size and diameter of the polymer chains), the authors used this property to separate linear chains from their cyclic analogs. They added a concentrated solution of polyesters in dichloromethane dropwise to a refluxing concentrated solution of methylated b-cyclodextrin in methanol. Part of the dichloromethane evaporated, and the polyesters precipitated. The threading of the methylated cyclodextrin on the linear chains increased their solubility in methanol and accelerated their methanolytic degradation. The MALDI spectrum of the degradation product displayed the same peaks as the pristine polymer, but the intensity ratios changed to C/La/Lb/ LcZ96/1/3/0, implying that the content of linear chains had been significantly reduced. Pantiru et al. [208] recorded MALDI and NMR spectra of a poly(3-caprolactone) (PCL) sample terminated by ethylene glycol vinyl ether. The molar mass was low enough to allow for NMR estimation of the average molar mass (using end group signals); the result was MnZ1380. The MALDI spectrum indicated a high purity level (no terminal groups present apart from those from ethylene glycol vinyl ether) and the average molar mass computation yielded MnZ1400, in good accord with the preceding value. Ming et al. [221] recorded the MALDI spectrum of a poly(butylene adipate) (PBA) sample. Then, they reacted the PBA with a large excess of perfluorooctanoyl chloride and obtained a partially fluorinated polymer. The MALDI spectrum showed peaks due to unreacted PBA plus additional peaks, four or five times less intense, due to partially fluorinated chains. Assuming that the difference in cationization efficiency was not too large, this might indicate that modified chains are less abundant. Takashima et al. [217] recorded the MALDI spectrum of a poly(d-valerolactone) (PDVL) terminated with b-cyclodextrin (CD). The spectrum displays a series of peaks (separated by about 100 Da) due to


PDVL terminated with CD and also an intense peak at lower mass due to CD. Apparently, the polymer does not fragment, although the end group is bulky. This is further proof that, under appropriate MALDI conditions, the extent of fragmentation is low. Kricheldorf et al. [220] obtained polylactides using Bismuth(III) acetate as an initiator and 1,1,1-tri(hydroxy methyl)propane (THMP) as a co-initiator. The resulting macromolecules are three-arm stars having three CH–OH end groups. The MALDI spectrum confirmed the structure. In several studies, cyclic chains have been detected in hyperbranched polyesters (h-PEs) by MALDI-TOF MS analysis [218,219,228,231]; and in the case of aliphatic h-PEs synthesized by reaction of 2,2bis(hydroxymethyl)propionic acid with pentaerythrytol or trimethylolpropane [218], the MALDI spectra, besides the expected peaks, showed a series of small peaks corresponding to the formation of one cyclic branch per molecule by an intramolecular etherification side reaction and loss of water. The presence of etherified units (including the –CHaCH–O– moiety) was confirmed by 13C NMR [218]. To investigate mass discrimination effects in MALDI-TOF analysis of polydisperse polymers, Williams et al. optimized sample preparation and instrumental parameters, obtaining a uniform response for each component of an equimolar mixture of four poly(butylene glutarate) (PBG) oligomers [210,211]. They proposed this oligomer mixture as a mass calibration standard for MALDI analysis of polydisperse polymers in the mass range 780–6000 Da. MALDI-TOF analysis revealed that linear oligomers are formed during the melt polycondensation of D,Llactic acid at 100–120 8C, while both linear and cyclic products are formed at higher temperature (220 8C) [209]. MALDI-TOF analysis has been found useful to explain the mechanism of the reactions occurring during the synthesis of aliphatic polyesters from ethylene sulfite and succinic anhydride or its higher homologues in the presence of such catalysts as quinoline and a Lewis acid (BF3 or SnCl4). In fact, signals of sulfur-free polyesters were observed in the MALDI spectra of the polyesters synthesized [230]. Recently, low molar mass all-aromatic polyesters, derived from 6-hydroxy-2-naphthoic acid (HNA), 4-hydroxybenzoic acid (4-HBA) and 3-hydroxybenzoic acid (3-HBA), were characterized by MALDI-TOF MS [225]. The spectra of polymers from 3-HBA showed

317

solely one series of peaks, whereas spectra of polymers from 4-HBA showed two series of peaks, namely the expected main series and an ancillary series generated by depolymerization reactions via a quinomethide mechanism. 3.8. Polycarbonates Polycarbonates have been characterized by combining MALDI-TOF with chromatographic methods [254–256], and a few papers have reported on classical MALDI-TOF MS analyses [251–253,257–261]. Coulier et al. [256] used LCCC-MALDI to identify chemical changes due to hydrolytic degradation in a PC sample. The LCCC chromatogram in Fig. 32a of a PC sample aged for 12 weeks shows two very well resolved peaks. In Fig. 32b, the MALDI spectrum of the first peak presents two series of peaks corresponding to undegraded PC chains. The spectra of the second peak (Fig. 32c) eluted at about 6.50 min, whose intensity increases with the degradation time, present only a series of peaks due to degraded PC chains terminated with one cumyl group and one OH end group (species C). Scamporrino et al. synthesized poly(bisphenol-A carbonate) (PC) copolymers containing Cu-diimine(I) units with nonlinear optical (NLO) properties and characterized them by MALDI-TOF MS [258]. On the basis of the structure of the copolymer products identified—versus reaction time, the authors proposed the reaction mechanism depicted in Scheme 2. Fig. 33 shows the MALDI spectrum of the copolymer obtained by heating commercial PC with the Cu-diimine(I) complex (10 wt%) at 250 8C for 5 min; the corresponding assignments are reported recorded in Table 1. The same authors also prepared PC copolymers containing porphyrin or fullerene units and characterized them by MALDI-TOF MS [258]. 3.9. Polyamides and polyimides MALDI analysis has been applied to the characterization of a number of polyimides and polymides [262–283]. Gibson et al. [264] recorded the MALDI spectrum of an aramide formed by condensation of crown ethers and 4,4 0 -oxydianiline (ODA). The matrix/analyte mixture was doped with a silver salt, and thus silver-cationized ions were expected, but the results were slightly different, because of adventitious sodium and potassium cationization. Signals were detected up to m/zZ 9100 Da with a spacing of 700.27 Da, which

318


Fig. 32. Identification of peaks observed with LCCC using semi on-line coupling of MALDI-TOF-MS. Reprinted from Ref. [256] with permission from Elsevier.

corresponds to the repeat unit molecular weight (see Fig. 34). In the expanded spectrum in Fig. 34, only minor amounts of linear structures are detected, consisting of linear amino acids (AA) (m/zZ700nC 18), diacids (DAc) (m/zZ700nC536), and diamines (DAm) (m/zZ700nC200). The weak cluster at m/zZ 2858 Da corresponds to the K adduct of the tetrameric amino acid (AA4), (m/zZ700nC18C39). The cluster at m/zZ2866 Da corresponds to the silver adduct of the macrocyclic tetramer (M4) that has lost CO2, m/zZ 700nK44C106.95. The remaining signals for the predominant cyclic species are identified as (MC

H)C, (MCNa)C and (MCK)C adducts, as illustrated for the tetramers M4; the corresponding silver adducts (M4CAg)C can also be seen in Fig. 34. In another work, after optimization of condensation polymerization conditions, mainly cyclic polyamides were detected in the MALDI spectra of various samples (up to 10,000–13,000 Da) [262,263]. Only cyclic Ny6 oligomers were observed when 3-caprolactam and 3-aminocaproic acid were polycondensed to high conversion at 250 8C [263]. MALDI-TOF analysis of hyperbranched polyamide (h-PA) obtained from novel carboxyl and amine


CH3

[O

1)

C O

C

O

CH3

]n

+

N

319

N Cu

O

O

HO

N

OH

O CH3

[

H O

( ) CH3

2)

[O

C CH3

CH3 C O

C

O

CH3

]nOH

+

N

C

O C O

CH3

an d

N

(#) and

O OH

N

[O

CH3

CH 3 C O

C

O

CH 3

]nO

CH3

CH3 C O

C

O

CH3

+

N

( )

N

[O

an d

CH3

CH3 C O

C

O

CH3

]nO

O

C O O

(+)

Cu O

N Cu

O C

C

OH

( )

( )

CH3

O

]nO

Cu O HO

3)

N Cu

( )

O

HO

OH

( ) CH3 HO

[O

C CH3

CH3 C O

C

O

CH3

]nOH

(#) Scheme 2.

3.10. Polymers with phenyl and other cycles in the main chain

terminated caprolactams, showed that ring–chain equilibria lead to the formation of cyclic branches or end groups. The formation of anhydride, imides, amidines and secondary amine, due to a number of intramolecular or intermolecular side reactions, was also observed [218].

Polymers with phenyl and other cycles in the main chain are of great interest because they are often electrically conducting or possess light-emitting 2718

∗

100

2267

I% 75

+

2775 # ∗

100

#

#

2972

2702 +

I% ∗#

∗

∗

# + +

+

50

+

# ∗ 4045 # ∗ # #

∗

∗

#

∗ #

#

2700

+

# +

+

∗

#

# ∗ # +

3200

4600

2900

3000

m/z

∗

+ +

25

2800

5315

+ +

3045

3013

50

∗ #

+

3029 ∗ #

2956 +

2759

∗#

+

2791

+

6000

∗

#

∗ #

∗

#

∗

#

∗

#

7400

#

∗

m / z

Fig. 33. MALDI-TOF mass spectrum of complex mixture obtained by heating a commercial PC with a Cu–diimine complex at 250 8C for 5 min. Reproduced from Ref. [258] with permission from Wiley.

OH

320


Table 1 Structures and molecular ions of the species detected in the positive MALDI-TOF mass spectra of PC and heated mixtures of PC and copperdiimine complex I Structures

m/z Values CH3

O C O

(ο)

Na+

C

O

n

CH3

CH3

(∗)

CH3

C

O C O

CH3

C

CH3

C

O C O

O C O

n

CH3

CH3 HO

CH3

C

O

CH3

(#)

Na+

OH

n

CH3

O

CH3

(•)

O C O

C

O

CH3

C

O

CH3

C

1801, 2055, 2309, 2563, 2817, 3071, 3325, 3579, 3833, 4087, 4341, 4595, 4849, 5103, 5357, 5611, 5865, 6119, 6373, 6627, 6881, 7135, 7389, 7643 1759, 2013, 2267, 2521, 2775, 3029, 3283, 3537, 3791, 4045, 4299, 4553, 4807, 5061, 5315, 5569, 5823, 6077, 6331, 6585, 6839, 7099, 7347, 7601 1743, 1997, 2251, 2505, 2759, 3013, 3267, 3521, 3775, 4029, 4283, 4537, 4791, 5045, 5299, 5553, 5807, 6061, 6315, 6569, 6823, 7077, 7331, 7585 1775, 2029, 2283, 2537, 2791, 3045, 3299, 3553, 3807, 4061, 4315, 4569, 4823, 5077, 5331, 5585, 5839, 6093, 6347, 6601, 6855, 7109, 7363, 7617 1702, 1956, 2210, 2464, 2718, 2972, 3226, 3480, 3734, 3988, 4242, 4496, 4750, 5004, 5258, 5512, 5766, 6020, 6274, 6528, 6782, 7036, 7290, 7544

OH

Na+

CH3

Na+

n

CH3

Na+ N

N Cu

(♦)

O

O

CH3 H O

C

O C O

CH3

OH

O

n

Na+ N

(+)

N Cu

O CH3 C CH3

1940, 2194, 2448, 2702, 2956, 3210, 3464, 3718, 3972, 4226, 4480, 4734, 4988, 5242, 5496, 5750, 6004, 6258, 6512, 6766, 7020, 7274, 7528, 7782

O

CH3 O C O O

C CH3

O C O

n

OH

O

properties. MALDI of poly(thiophene) and its derivatives has been reported in many papers [284–311]. Chen et al. [305] obtained a series of conjugated polymers by Suzuki polycondensation and analyzed them by MALDI. The gap between the valence and conduction bands corresponds to blue light emission. One of the polymers, denoted P7, has a complex repeat unit with a mass of 745 Da and the empirical formula: C46H59N5O4. The MALDI spectrum shows only four peaks at 1491.0, 2236.4, 2981.8, and 3727.3, which are due to P7 chains (dimer, trimer, tetramer, and pentamer). Poly(1,3-cyclohexadiene) is an interesting polymer since it can be dehydrogenated to form poly(phenylene), but the control of MM is difficult. Quirk et al. [292] developed an innovative method to measure the amount of chain transfer to monomer, ACTM, during the alkyllithium-initiated polymerization of 1,3-cyclohexadiene. ACTM was evaluated by adding ethylene oxide

as a terminating agent and characterizing the resulting products. Two experimental procedures were investigated to detect chain transfer by ethylene oxide functionalization: (A) sec-BuLi in benzene at room temperature with 1,4-diazo-bicyclo[2.2.2]octane (DABCO), (B) n-BuLi/TMEDA in cyclohexane at 40 8C (TMEDAZN,N,N 0 N 0 -tetramethylethylenediamine). MALDI results showed that n-BuLi/TMEDA systems do not have a living character, since no reinitiation occurs. By contrastt, the sec-BuLi/DABCO systems possess a living character since reinitiation was observed. In this way, MALDI spectra were used to clarify some aspects of a controversial subject. 3.11. Copolymer studies Viala et al. [324] studied the radical emulsion copolymerization of methyl methacrylate and 1,1-diphenylethylene (DPE) in the presence of


321

Fig. 34. Polyamide containing crown-ether units, which can form rotaxanes. Structure (upper part) of the polyamide and expansion (lower part) in the mass region 2600–3000 of the MALDI spectrum of the polyamide. Reproduced from Ref. [264] with permission of the American Chemical Society.

ammonia, sodium dodecylsulfate and ammonium peroxodisulfate. DPE cannot polymerize by itself and this implies that DPE units will be found isolated along the chain. Depending on the reactants, various types of end groups are expected. Some examples follow. Termination by disproportionation can give –H terminal groups. Initiation with sulfate ion radicals can give –SO4 terminal groups. Initiation with hydroxyl radical or hydrolysis of a polymer chain ending in a sulfate end group can give –OH terminal groups. The same types of chain ends can also derive from initiation with a hydroxyl radical formed during peroxodisulfate decomposition. More complex end groups derive from termination by the disproportionation reaction of a polymer radical ending with DPE or by disproportionation of a polymer radical ending in MMA. The authors recorded the spectrum of the copolymer and found all the expected products. Impallomeni et al. [319] heated a blend of poly(4hydroxy butyrate) (PHB) and poly(3 caprolactone) (PCL) using p-toluenesulfonic acid (PTSA) as a catalyst. The ester–ester exchange produced a copolymer, and chains terminated with two different endgroups were expected, namely H/COOH and PTSA/

COOH. The authors used SEC, since chains terminated with PTSA are expected (due to their stiffness) to display different elution behavior than the other ones. Fig. 35 shows an expansion of the MALDI spectrum along with the m/z assignment of MS peaks. It shows two ion distributions: the first, centered at m/zZ3700, is due to sodiated ions of the co-oligomers terminated with tosyl-terminated and carboxyl groups (species 1 in Fig. 35); while the second, centered at m/z4Z4600, is due to the sodiated ions corresponding to the cooligomers terminated with OH and COOH groups (species 2 in Fig. 35). These data suggest that these oligomers have different elution behavior. Fig. 35 shows an expansion of the MALDI spectrum along with the m/z assignments of several of the MS peaks, along with the structures of the oligomeric ions giving rise to these peaks. The two types of chains are present and chains terminated with PTSA have distinctly higher mass, suggesting different elution behavior. Venkatesh et al. [343] investigated the copolymerization of methyl acrylate (MA) with 1-octene (OCT) using two different synthetic routes, namely atomtransfer-radical-polymerization (ATRP) and the usual free-radical (FR) reaction. The MALDI spectra of MAOCT copolymers obtained by ATRP and by FR showed

322


Fig. 35. MALDI-TOF mass spectrum of SEC fraction of P(HB-co-47% mol CL). The structures of the revealed copolymer chains are shown in the upper part. Reproduced from Ref. [319] with permission of the American Chemical Society.

a single mass series and five mass series, respectively. The fact that side-reaction products are more abundant in copolymers obtained by ATRP with respect to FR and this implies that ATPR is ‘cleaner’. During ATRP, most of the polymer chains are halide end-capped. However, during MALDI ionization, it is observed that a small percentage of the terminal Br undergo fragments. Milani et al. [341] used a palladium-complex catalytic system to obtain terpolymers with units of CO, styrene (ST) and 4-methyl-styrene (MST), a synthetic route that produces chains in which two consecutive identical units (CO–CO, ST–ST MST– MST) do not occur.

Fig. 36 shows MALDI spectra, in the region 750– 1200 Da, of three CO–ST–MST terpolymers synthesized using three different ST/MST mole ratios, namely 2/1, 5/1 and 10/1. The MALDI spectra consist of very well resolved clusters, each due to polymeric chains formed by the same number of repeat units, i.e. (mCn). Many authors agree on the fact that, for an AB copolymer, the most intense peak in the MALDI spectrum can be assigned to an ApBq oligomer and the A/B ratio in the oligomer reflects the A/B ratio in the copolymer and vice versa: if the copolymer is rich in B units, B-rich peaks will be intense. The application of this to the CO–ST–MST system is straightforward. Comparison of the three spectra evidences that, within each cluster, the relative abundance of the oligomer containing more styrene residues increases on increasing the initial amount of styrene in the feed. The authors found also that MST is more reactive than ST [341] The composition and microstructure of a low-MM ethylene/ carbon monoxide (E–CO) copolymer have been determined by MALDI-FT-ICR mass spectrometry [342]. A part of the mass spectrum is reported in the


323

Fig. 36. MALDI-TOF mass spectra of CO/styrene/4-Me-styrene terpolymers. Variations observed with styrene/4-Me-styrene ratios: (a) 2/1; (b) 5/1; (c) 10/1. An enlarged region. Reproduced from Ref. [341] with permission of the American Chemical Society.

section, which deals with resolution and MALDI-FTICR. The E/CO mole fraction computed from MS is 0.728, which compares well with the value 0.818 obtained using NMR Willemse et al. [313] monitored the synthesis of a block copolymer with styrene (ST) and isoprene (IPR) using MALDI. The synthetic procedure consisted in the sequential addition of the initiator, the ST monomer and then the IPR monomer. Fig. 37 gives the spectra of the reaction products before the addition of IPR (Fig. 37a), and those of the block copolymers after approximately 50% (Fig. 37b) and 100% (Fig. 37c) conversion of the isoprene monomer [313]. The three spectra have narrow MMDs (a typical feature of anionic synthesis) centered at 2200, 3000, 4000 Da, respectively. It is apparent that the MM of the copolymer grows as the conversion increases. Fig. 37d is an enlargement of Fig. 37c and shows that each peak has a width (strictly speaking FWHM) of about 0.4 Da, which implies (by definition) that the resolution is about one part in 10,000, which is satisfying at these masses. The peaks tend to form triangular clusters, the base of the triangle being about 30 Da. The major cause of these complicated patterns is the fact that, at these masses,

at least nine 13C isotopes are relevant for each structure (chains containing three 13C isotopes are the most abundant ones). There are other causes, namely that silver has two isotopes (nominal masses 107 and 109) and a difference of only 4 Da between 3 isoprene units and 2 styrene units. The MALDI spectral intensities were used to compute the ST/IPR mole ratio (i.e. the average copolymer composition), which agrees with that from 1H NMR. In the case of a random copolymer of isobutylene (IBU) and methylstyrene (pMST), Cox et al. [315] found that the average molar composition determined from the MALDI spectrum of the copolymer was skewed to higher methylstyrene content (36 mol%) as compared to that obtained from NMR (13%). They proposed that the facile ionization of methylstyrene-rich oligomers is the cause of the discrepancy. Melt mixing by heating together two homopolymers is a promising route for synthesis of block, segmented, or random copolymers [317]. The process proceeds through exchange reactions, such as ester–ester, ester– carbonate, ester–amide, amide–amide, siloxane–siloxane reactions, etc. This reactive polymer-blend technology currently encounters problems in control of

324


Fig. 37. MALDI-TOF-MS mass spectra (a–c) of the system polystyrene-block-polyisoprene after 0, 50 and 100% conversion of isoprene monomer and (d) an enlargement of (c) between 4000 and 4140 Da. Reproduced from Ref. [313] with permission of the American Chemical Society.


reaction parameters, due partially to lack of adequate monitoring methods and analytical protocols. However, another huge obstacle exists: the current practice is still that of mixing polymers without considering the chain ends, looking only at the repeat units of the two components. MALDI-MS analysis of the structure of copolymers obtained by reactive blending in the molten state of their corresponding homopolymers has been [318,321–323] reported recently. The experiments were mainly focused in three directions: (1) structure of the end groups of the reacting polymers; (2) copolymer yield as a function of melt mixing time; (3) composition and sequence of the copolymer formed. Fig. 38 shows the MALDI spectra of an equimolar mixture of carboxyl terminated Ny6,6 (Ny6,6–COOH) and high MM Ny6,10, both for the physical blend (Fig. 38a) and for melt-mixed blend held at 290 8C for 30 min (Fig. 38b). Ny6,6–COOH oligomers predominate in Fig. 38a, whereas there is a drastic change in the MALDI spectrum of the heated blend (Fig. 38b), hinting that the formation of Ny6,6/Ny6,10 copolymers by exchange reactions has occurred [321]. In fact, the most intense peaks are due to copolymer oligomers formed in the process of melt mixing. Theoretical matching of the experimental peak intensities (inset in Fig. 38b) was obtained for a Bernoullian distribution in the copolymer and a 50/50 mole ratio of the comonomers. As can be seen in the inset of Fig. 38b, the theoretical MALDI mass spectrum in the mass region 1050–1890 Da, generated for a random copolymer containing equimolar units of Ny6,6 (A) and Ny6,10 (B) units, matches well with the spectrum recorded for the Ny6,6–Ny6,10 melt-mixed for 30 min at 290 8C. This indicates the copolymer has a random sequence distribution, and equimolar composition of Ny6,6 and Ny6,10 units. Samperi et al. [318] applied chain statistics analysis to MALDI spectra of Ny6/Ny4,6 and Ny6/Ny6,10 random copolyamides synthesized by melt mixing of carboxyl terminated nylon-6 (Ny6–COOH) with high molar mass Ny4,6 or Ny6,10 at 290 8C for different times under N2 flow. The molar composition, sequence distributions, average sequence lengths and degree of randomness calculated by the chain statistics are in accord with calculations based on a model that uses the intensities of the carbonyl peaks in the 13C NMR spectra [318].In another study, MALDI analysis allowed structural identification of the copolyesteramide formed during melt mixing of Ny6/PET and Ny6/PBT blends [322,323]. This work revealed the essential role of carboxyl groups in the exchange

325

reaction, and allowed the formulation of a detailed mechanism for the reaction [322,323]. The composition, sequence distribution, degree of randomness and the yield of the copolyesteramide formed as a function of the melt mixing time were also calculated [322,323]. Tillier–Lefebre et al. [320] studied melt mixing of PET and 3-caprolactone. The MALDI spectrum of the resulting copolymer turned out to be very complex, and thus, the authors decided to gain better insight by fractionating the polymer using SEC and recording the MALDI spectra of the fractions. Fig. 39 shows the MALDI spectrum of SEC fractions 46, 48, 50. The fractions eluting first have higher masses, as expected. All three spectra exhibit two series of peaks. The intensity of the series corresponding to the highest masses increases with the increasing fraction number, i.e. with increasing elution volume. The peaks in the low-mass range are due to linear oligomers, whereas peaks in the highmass range are due to cyclic oligomers. Owing to their smaller hydrodynamic volumes the cyclic oligomers are eluted slightly later than their linear homologs, and in the MALDI spectra of the corresponding SEC fractions appear together with lower-mass linear oligomers the same elution volume [3]. Polce et al. synthesized a copolymer with units of phenylquinoxaline (PQ) and ethersulfone (ES) by combining selfpolymerizable quinoxaline monomers with a 1:1 molar mixture of 4,4 0 -dichlorodiphenyl sulfone and bisphenol-A. They noted differences in cationization efficiencies: oligomers containing at least one PPQ unit readily protonated in MALDI, whereas PES homopolymers required alkali metal ion addition to become detectable. The MALDI mass spectra of the polymers revealed that the major products up about 15,000 Da homopolymeric or copolymeric macrocycles. Linear byproducts are also observed, arising from nucleophilic ring opening of already-formed macrocycles. Montaudo et al. proposed a new model for the bivariate distribution of chain sizes and composition in copolymers [336]. They compared predictions of the model with MALDI data for a block copolymer of pivalolactone and 3-hydroxybutyrate, and with some published MALDI data on a block copolymer a-methyl styrene and methyl methacrylate. The new model considers a sum of two bivariate distributions; and it replaces an earlier model that deals only with a single distribution. The new model gives better results than the previous model because it fits better with the experimental compositional distribution histograms of the copolymer samplesKricheldorf et al. obtained a hyperbranched polymer by

326


Fig. 38. MALDI-TOF mass spectra of an equimolar mixture of Ny6,6–COOH (MvZ7200 Da) and high MM Ny6,10 (MvZ36,100): (a) physical blend; (b) melt mixed at 290 8C for 30 min. Part (B) shows an enlarged section of the calculated mass spectrum (above) and experimental mass spectrum (below) of the last sample. Reproduced from Ref. [321] with permission of the American Chemical Society.

condensing 4,4 0 -bis[p(acetoxy)phenyl] valeric acid (also called diphenolic acid, DPA) [325]. Careful assignment of the peaks in the MALDI spectrum revealed that the product was a copolymer of regular DPA units and modified DPA units possessing a phenol group. The MALDI analysis detected a drift in composition: the mole fraction of regular DPA units

changed from 0.80 to 0.95 in passing from low to high mass. Quirk et al. [12] studied the functionalization of PSLi with ethylene oxide. The reaction giving PSCH2CH2OH chains was rapid and quantitative. However, at long reaction times with 10 equiv. EO per mole of PSLi, oligomerization of the end group by further


327

Fig. 39. MALDI-TOF mass spectra of low molar mass fractions 46, 48 and 50 collected during SEC analysis of PET/CL (50/50 mol/mol) copolyester 2. The two distributions correspond to linear macromolecules (lower masses) and cyclic macromolecules (higher masses) eluted at the same elution volume. Reprinted from Ref. [320] with permission from Wiley.

reaction produce compounds PS(CH2CH2O)nH with nZ2 or 3 (denoted dimers or trimers, respectively), detected by MALDI mass spectra (see Fig. 40). After 12 h and after 4 weeks, respectively, the dimer amounted 4 and 34% of the functionalized chains; after 1 and 4 weeks the amounts of trimer were 1 and 4%, respectively. The MALDI results were in good agreement with 1H and 13C NMR analyses. Schmalz et al. [154] synthesized PS-b-PEO di-block copolymers by sequential anionic polymerization of styrene and EO in THF. Fig. 41 reports MALDI spectra of samples reacted at different times along with MALDI spectra of the homopolymer. It can be observed that, as reaction time increases, the spectra allow a number of copolymer compositions up to chains bearing seven EO units to be resolved while the initial EO end-capped chains almost disappear. The spectrum of the EO endcapped PS (PS-OH) also indicates that the end capping is quantitative since signals due to the PS precursors are absent [154]. Venkatesh et al. recorded the MALDI mass spectrum of a copolymer of methyl acrylate (MA) and allyl butyl ether (ABE) obtained by atom transfer radical polymerization (ATRP) using ethyl-2-bromoisobutyrate (EBr-ibu) as the ATRP initiator [332]. They found four mass series and labeled them E1–E4. The most intense peaks in the mass spectrum belong to the first series (E1) and are due to chains terminated with bromine. Kricheldorf et al. reacted dimercap-

toethane with phthaloyl chloride in the presence of pyridine and obtained a spirophthalide [248]. Then they used dibutyltin dimethoxide to obtain tin-containing PEGs and heated them with excess spirophthalide. The reaction products contain one, two, three, or more phthalate groups and thus are copolymers, since they also have variable amounts of ethylene oxide units. Fortunately, products with one phthalate group fall in a different mass range than products with two phthalate groups, making the MALDI spectrum easy to interpret. 3.12. Polymer degradation studies A number of studies have established MALDI as a unique technique for analyzing chemical modifications in the structure of synthetic polymers induced by degradation processes [366–393]. For example, the study of polymer degradation by MALDI involves the collection of MALDI spectra at different times and/or temperatures to detect structural changes induced by heat or light under an inert and/or oxidizing atmosphere. MALDI-MS has advantages because, unlike traditional mass spectral techniques such as GC-MS [422] and DPMS [422], it allows samples subjected to oxidation to be analyzed without further decomposition. When a polymer sample is partially degraded at a given temperature, in an inert atmosphere (e.g. nitrogen, argon) or in air, the MALDI spectrum will

328


Fig. 40. MALDI spectra of products obtained when poly(styryl lithium) is reacted with EO. Reprinted from Ref. [12] with permission of the American Chemical Society.

consist of a mixture of peaks from undegraded and degraded chains.Puglisi et al. [376] studied isothermal degradation of BPA-polycarbonate (PC) between 300 and 450 8C under a nitrogen stream. Their MALDI-

TOF spectra showed that a rearrangement of the carbonate group leads to the formation of several adjacent xanthone units in PC chains of sizeable molar mass [376]. Xanthones, found among pyrolysis


329

Fig. 41. MALDI-TOF mass spectra of samples taken during EO polymerization of S19EO38 (SZstyrene). Spectra were measured in reflectron mode using AgTFA as cationizing agent and dithranol as matrix. Reprinted from Ref. [154] with permission from Wiley.

D

D C

2793

2913

G 2809

G 3063 L 2955

B S 2821 E 2835

B

O N 2929 2875 P

V I 2885

E 3089

3033 T Q 2971

3019

2779

3047

2941

products, are believed to be precursors of graphite-like structures in the char residue that is produced at temperatures higher than 450 8C under an inert atmosphere. The structure of the species produced in the thermal oxidative degradation of PC has been also analyzed [377,378]. PC samples heated at 300 and 350 8C in air up to 180 min produced a THF insoluble gel at the longer heating times. The MALDI spectra of oxidized PC samples and of their SEC fractions, showed the presence of oligomers containing acetophenone (peak S, Fig. 42), phenyl substituted acetone (peaks E and I, Fig. 42), phenols (peaks L, O, P and Q, Fig. 42), benzyl-,alcohol (peak G, Fig. 42), and biphenyl terminal groups (peaks T and V, Fig. 42) [378]. The presence of biphenyl units among the thermal oxidation products confirms the occurrence of cross-linking processes, which are responsible for the formation of the insoluble gel fraction [377,378]. It has been proposed that the mechanisms accounting for the formations of thermal oxidation products of PC involve the simultaneous operation of several reactions: (i) hydrolysis of carbonate groups of PC to form free

S

I 3103

m/z 2800

2900

3000

3100

Fig. 42. Enlarged section of MALDI-TOF spectrum of SEC fraction, collected at 28 ml, of a PC sample oxidized at 300 8C. Reprinted from Ref. [378] with permission from Elsevier.

330


Bisphenol-A end groups (species C and D, Fig. 42); (ii) oxidation of the isopropenyl groups of PC; (iii) oxidative coupling of phenol end groups to form biphenyl groups [377,378]. Lattimer [370] carried out the thermal degradation of poly(acrylic acid) (PAA). Negative ion MALDI analysis of the pyrolysis residues provided direct evidence of dehydration and decarboxylation. Lattimer et al. [372,373] studied the pyrolysis of a segmented polyurethane consisting of 4,4 0 -methylenebis-(phenylisocyanate) (MDI), poly(butylene adipate) (PBA) and 1,4-butanediol (BDO) by MALDI-TOF [372]. Several pyrolysis products appeared in the range 800–10,000 Da. Dissociation of the urethane linkage to yield products with isocyanate and hydroxyl end groups occurred at the lowest temperatures (ca. 250 8C). Linear polyester oligomers with hydroxyl and/or vinyl end groups were detected, as were cyclic polyester oligomers. At higher temperatures (O300 8C) nitrogen-containing pyrolysis products were no longer present in the residue. Dehydration of the linear and cyclic polyester pyrolyzates occurs at these temperatures, producing olefinic end groups [372]. Star-shaped polymers with a fullerene core and six polystyrene arms [374] are thermally unstable, and decompose in toluene solution at around 100 8C. Since both C60 and PS are thermally stable, it is supposed that the thermal degradation reaction results from breaking of C–C bonds in the a and b positions to C60, and that the rate constant KD, is a combination of two contributions Ka, Kb, due to the indicated ruptures. The MALDI spectrum of (PS)6C60 shows two series of peaks with different intensities (the intensity ratio being 3:1) due to the ruptures in the a and b positions, respectively. The data confirm the proposed degradation route and suggest that Ka is threefold greater than Kb. Puglisi et al. subjected Ny66 to thermal degradation 290 and 315 8C in an inert atmosphere [366]. The formation of a gel fraction was observed after about 15 min of heating, and the addition of a condensing agent such as triphenyl phosphite (TPP) made the gelation complete in few minutes. The MALDI-TOF mass spectra of the soluble fraction showed that secondary amino groups and cyclopentanone chain ends were generated in the heating process, as was confirmed by MALDI analysis of heated Ny66 that had been terminated with specific amine and carboxyl end groups [366]. The gel fraction was partially hydrolyzed to destroy the network structure and the soluble material was analyzed by MALDI-TOF. The spectra of the hydrolyzed Ny66 gel revealed the presence of N,Nsubstituted amide (species M, N, P and Q, in Fig. 43,

B10

B11

D10

D11

C10

R9

G9

M8 S9

2300

N9

F10

E9

2400

C’10

P6 Q4

R10

C11

E10 M9

C’11

F11 P7

Q5 G10

S10 N10

2500

2600

m/z 2700

Fig. 43. Enlarged section of MALDI-TOF mass spectrum of hydrolyzed gel formed by heating Ny66 sample in the presence of TPP at 290 8C for 30 min. Reprinted from Ref. [366] with permission from Elsevier.

Table 2) as side chains generated by the condensation of carboxyl end groups with secondary amino groups and azomethyne structures (species R and S in Fig. 43, Table 2) originating from the reaction of cyclopentanone moieties with terminal amino groups. These structures were most likely responsible for the gel formation on heating Ny66. Comparison with similar experiments conducted on Ny6, showed that only secondary amino groups were formed in Ny6, leading to branched structures but not to crosslinking [366]. MALDI investigations by Samperi et al. [368,369] of PET and PBT samples isothermally degraded at processing temperatures in the broad range 270–370 and 270–350 8C, respectively, under nitrogen, showed that the butylene unit in PBT is apparently able to induce sensible differences in the isothermal degradation of PBT in comparison with PET [368,369]. On the basis of MALDI and NMR data, the authors proposed chemical degradation mechanisms (Schemes 3 and 4). Terephthalic anhydride-containing oligomers are clearly detected in the MALDI spectra of melt processed PET (Scheme 3), whereas unsaturated oligomers are absent [368]. The opposite is true in the case of PBT [369]. The b-CH hydrogen transfer reaction is very efficient in PBT (Scheme 4), and unsaturated oligomers are present in the MALDI spectra of the heated PBT samples. They appear to be the only decomposition products, whereas the formation of terephthalic anhydride-containing oligomers along the PBT chains, actually observed at 400 8C, does not occur at 270–350 8C [369]. The different thermal degradation behavior of PET and PBT was attributed to the different reactivities of the ethylene and butylene units, and to the greater chain

Table 2 Structural assignments of the peaks displayed in the MALDI-TOF mass spectra of the Ny66 samples, reported in figure 43 Species B

MCKC (n)

Structure

HO

Ny6,6

C

HO

Ny6,6 n

C0

CO(CH2)4COOH

O HO

Ny6,6

C n

O D

E F

Ny6,6

H2N(CH2)6NH

n

DA-CO-(CH2)4-CO

DA

2418 (10) 2644 (11) 2870 (12) 3096 (13) 2359 (9) 2584 (10) 2233 (9) 2459 (10) 2685 (11) 2911 (12) 2274 (8) 2500 (9) 2726 (10) 2952 (11) 2343 (8)a 2569 (9)a

H

Ny6,6

OH

Ny6,6 n

H n

G

DA

Ny6,6 n

M

DA

Ny6,6 x

CO(CH2)4CO-DA

CO(CH2)4CO-NH(CH2)6N

y

CO(CH2)4CO N

H2N(CH2)6NH

Ny6,6 x

CO(CH2)4CO-NH(CH2)6N

CO

Ny6,6

(CH2)6NH

Ny6,6

(CH2)6NH

O OH


2320 (10) 2546 (11) 2772 (12) 2998 (13) 2448 (10) 2674 (11) 2900 (12) 3126 (13) 2430 (10) 2656 (11) 2882 (12) 3108 (13)

H n

z

2517 (9)a 2743 (10)a

H

Ny6,6 y

H P

DA

Ny6,6 x

DA

CO(CH2)4CONH(CH2)6N (CH2)6NH

Ny6,6 t

CO(CH2)4CO

Ny6,6 y

CO(CH2)4CONH(CH2)6N

(CH2)6NH

H

Ny6,6 z

2473 (6)a 2699 (7)a

H

331

NH(CH2)6

z

NH(CH2)6 Ny6,6 u

Ny6,6

CO

H2N(CH2)6NH

x

Ny6,6 S

R

a

HO

The (n) values correspond to the sum of the repetitive units; DAZ–NH(CH2)9CH3.

Ny6,6

x

CO

N(CH2)6NH

Ny6,6

N(CH2)6NH

y

H

Ny6,6

y

H

DA CO(CH2)4CO CO(CH2)4CO t

Ny6,6 DA

y x

H

m

Ny6,6 CO(CH2)4CONH(CH2)6N (CH2)6NH Ny6,6 CO(CH2)4CONH(CH2)6N (CH2)6NH Ny6,6 DA Q

Structure Species

2401 (9)a 2627 (10)a

2302 (9)a 2528 (10)a


2485 (4)a 2711 (5)a

MCKC (n)

332

flexibility of the PBT chains. The unsaturated oligomers produced in the thermal degradation of PBT through the b-CH hydrogen transfer reaction can easily undergo another b-CH transfer process to yield butadiene. This process competes with the formation of terephthalic anhydride-containing PBT oligomers, and the latter reaction is suppressed in the temperature range explored here. In PET, the vinyl group formed in the primary degradation step is electronically conjugated to the adjacent ester group, and another b-CH hydrogen transfer reaction (to yield acetylene) is quite unlikely. Therefore, the vinyl group-ended oligomers may quickly react with carboxyl-ended oligomers, to yield terephthalic anhydride-containing oligomers. Alternatively, the Samperi et al. proposed an easy unimolecular extrusion of ethyleneoxide (acetaldehyde) from PET [368]. The formation of unsaturated oligomers in the thermal degradation of PBT has also been confirmed by direct bromination of heated PBT. In Fig. 44, MALDI spectra of unbrominated (Fig. 44a) and brominated (Fig. 44b) PBT are heated at 300 8C for 60 min. The spectrum of the latter sample (Fig. 44b) shows new peaks due to brominated species (peaks E4 Br 2, E 5Br2 , E400 Br2 , E500 Br2 ) [369]. In thermal degradation of PET carried out in the presence of p-toluene sulfonic acid (pTsOH) (0.5 wt%) at 270 and 285 8C, it was found that pTsOH induces a strong hydrolytic reaction with consequent increase of carboxyl-terminated polyester chains [368]. Weidner et al. [379,380] studied the oxidative and hydrolytic degradation in PET by MALDI-TOF and characterized the structures of oligomers formed during hydrolytic degradation. They found that an ester scission process generates acid-terminated oligomers H–[GT]m– OH and T–[GT]m–OH and ethylene glycol-terminated oligomers H–[GT]m–G, where G is an ethylene glycol unit and T is a terephthalic acid unit. The scission of ester bonds during the chemical treatment led to a marked decrease in the number of cyclic oligomers [GT]m. The presence of diacid-terminated species demonstrated a high degree of degradation [379,380]. MALDI analysis of a PEG sample (MMZ2000 Da), heated at low temperature (150–300 8C) in an inert atmosphere, showed that the initial pyrolysis products, obtained at 150 8C, have hydroxyl and ethyl–ether end groups formed via C–O homolytic cleavage followed by hydrogen abstraction [381]. At higher temperatures, the abundance of the ether end groups increases as more C–C cleavage occurs. Vinyl ether end groups increase at higher temperatures (250–300 8C), owing to dehydration of hydroxyl end groups [381]. The assignment


O

(CH2)2

O

O

O

C

C

333

-CH transfer n

1

O

O

O

C

C

O

CH2

CH2

O

O

O

O

O

C

C

-CH transfer

3

O

O

CH2

CH2

2

4

C O

O C

O

O

O

O

C

C

O

O

+

CH2

CH O C

C O

OH

+ CH3CHO

O O

C

5

O C

6 O

CH2

CH2

OH

O

O

C

C

+

CH3CHO

OH

Scheme 3.

of the end group structures was aided by tandem mass spectrometry (CI-MS/MS) and by deuteration of hydroxyl end groups in the pyrolyzate [381]. Lattimer characterized eleven series of oligomers in MALDI-MS analysis of poly(tetrahydrofuran) (PTHF) degraded at 175–350 8C in an inert atmosphere [371]. The pyrolysis products at about 175 8C, all have at least one hydroxyl end group, retained from the original low molar mass polymer, and the other end group is ethyl ether, propyl ether, butyl ether, or aldehyde. MALDI spectra of the pyrolysis products at higher temperatures (250–350 8C) show an increasing tendency to form products with a combination of alkyl ether and/or aldehyde end groups. The amount of pyrolysis products containing the hydroxyl end group diminishes at the higher temperatures, and butenyl ether end groups are observed to an appreciable extent. The latter functionality is apparently formed mainly via dehydration of oligomers terminated with OH groups. The author proposed a free radical mechanism to explain the main degradation products of PTHF [371]. Gallet et al. carried out the thermal oxidative degradation of a poly(ethylene oxide–propylene oxide–ethylene oxide) triblock copolymer (Poloxamer 407) at 80 8C in air for various times [373]. They found by combination of MALDI-TOF-MS for

the analysis of oligomers, solid-phase microextraction/gas chromatography-MS (SPME/GC-MS) for the analysis of low molecular weight compounds, and 1H NMR for chain-end determinations, that the thermal oxidation proceeds in three steps. After an induction period depending on the quantity of antioxidant present in the polymer (21 days for 100 ppm BHT), the degradation started through a six-ring intramolecular decomposition reaction of the PPO block of the copolymer. By SPME/GC-MS they found that the first volatile degradation product to appear was 1,2-propanediol-1-acetate-2-formate. This means that the secondary hydroperoxide formed on the PPO chain plays a major role in the thermoxidation of poloxamer materials. Finally, more chain scissions occurred both in the PPO and PEO blocks of the copolymer, leading to a dramatic decrease of the molecular weight and the appearance of formates, acetates, aldehydes and acids [373]. Although applications of MALDI to the study of polymer photo-oxidation processes are quite recent [382], results obtained for the systems so far investigated are highly informative as compared with previous studies based on such conventional techniques as UV and IR. Molecules formed in the photo-oxidation

334


O

(CH 2)4

O

O

O

C

C n

1

O

O

O

C

C

O

O

C

C

O

O

O

(CH 2)4 O

C

C

β -CH transfer

2

(CH 2)4

O

O

O

+

CH2

(CH2)2 O C

CH

C O

OH β-CH transfer

+

3

O

O

C

C OH

Scheme 4.

processes are often very reactive, do not accumulate, and are present only in minor amounts among the reaction products. Nevertheless, MALDI spectra yield precise information on the size, structure and end groups of molecules formed in the oxidation process, allowing discrimination among possible oxidation mechanisms. In a recent investigation, Ny6 films subjected to photo-ageing were analyzed by MALDI. The spectra show the presence of over 40 different types of oligomers, as compared to only three in a Ny6 blank sample (Fig. 45). Three photo-oxidation processes are occurring Ny6, as summarized in Scheme 5. The first process is a hydrogen abstraction from the methylene group adjacent to the amide NH, leading then to the formation of a hydroperoxide intermediate. The decomposition of this hydroperoxide by radical rearrangement reactions generates the final products of Ny6 photo-oxidation (Scheme 5a). Besides the hydrogen abstraction and subsequent hydroperoxide formation, which had been established in previous studies [382], two other major processes appear to be operating in Ny6, i.e. chain cleavage reactions of

Norrish types I and II (Scheme 5b and c). In Fig. 45, each peak carries a label. Letter A specifies any end group generated by the decomposition of the hydroperoxides (Scheme 5a); B specifies any end group generated by the Norrish type I chain cleavage (Scheme 5b); C specifies any end group generated by the Norrish type II chain cleavage (Scheme 5c); whereas E indicates just one of the end groups present in the original Ny6 sample. Since each oligomer has two ends, the notation B–A, for instance, means that a Norrish type I chain cleavage occurred at one end and that hydroperoxide decomposition occurred at the other end. There are five oligomers originating exclusively from Norrish I and four oligomers exclusively from Norrish II reactions. Furthermore, nine peaks are exclusively due to Ny6 oligomers originating only from hydroperoxide decomposition reactions [382]. In a similar study, the MALDI analysis of the Ny6,6 films photoxidized at 60 8C in air, yielded detailed information on the photodecomposition mechanism of Ny6,6 [383]. The results confirm previous insights about the hydrogen abstraction and subsequent


A=

O

O

C

C O (CH 2)4 O

B=

HO

O

O

O

O

C

C O (CH 2)4 O

C

C OH

n

E=

O

O

H 2C CH (CH 2)2O C

C

O

O

H 2C CH (CH 2)2O C

C

HO

O

O

O

O

C

C O (CH 2)4 O

C

C ONa

n

n

O (CH 2)4 O

O

O

C

C

OH

E'=

O

O

H 2C CH (CH 2)2O C

C

O (CH 2)4 O

O (CH 2)4 O

O

O

C

C

ONa

n

n

E''=

B'=

335

O

O

C

C

O(CH 2)2CH CH 2

n

Counts

40000

A6 + E5

A5 + E4

C5

C4 E’’4

A7 + E6

E’’5 C’4

20000

E’4

C6

E’5

C’5

C’6

C6

C5

Counts

E’’6 E’6

C4

1000 E 4Br 2 C’4

E 5Br 2 C’5

A6

C’6

E’’5Br 2

E’’4Br 2

1200

1400

1600

m/z Fig. 44. Enlarged sections of MALDI-TOF mass spectra of a PBT sample heated at 300 8C for 60 min: (a) unbrominated sample; (b) brominated sample. Reprinted from Ref. [369] with permission from Elsevier.

formation of a hydroperoxide intermediate and reveal, as well, that the Ny6 [382], Norrish I and Norrish II chain cleavage reactions play an important role in the photo-oxidation of Ny6,6. The MALDI spectra of Ny6,6 films photoxidized for short times show only oligomers produced by hydrogen peroxide decomposition, indicating that the Norrish I and Norrish II reactions occur at a later stage of irradiation [383]. An induction period before the occurrence of Norrish I and Norrish II reactions was also observed in the photoxidation of Ny6 and of aliphatic polyesters [382–384]. It is believed that the initial irradiation of these polymers triggers a-hydrogen abstraction, a lowenergy process, which induces polymer oxidation through the formation of hydroperoxides. The latter are thermally unstable at 60 8C and decompose,

forming oligomers with functional end groups that enhance the light absorption power of the oxidized polymer, thus allowing the Norrish reactions to take place [382–384]. MALDI spectra of poly(butylene succinate) (PBSu) photoxidized at 60 8C in air have revealed the formation of oxidized PBSu oligomers containing succinic acid, malonic acid, butyl ester, ethyl ester and butyl formate end groups, which have not been detected with other analytical tools. On the basis of the structure of photo-oxidized products observed, Carroccio et al. [384] proposed that process involves several reactions: (i) oxidation of hydroxyl end groups; (ii) a-H abstraction decomposition; (iii) Norrish I photocleavage. Carroccio et al. [385–387] have also investigated the photo and thermo-oxidation processes, respectively,

336


Fig. 45. Enlarged sections of MALDI spectra, obtained in reflectron mode, of 40 mm Ny6 film photo-oxidized for (a) 0 and (b) 289 h. In (c) is shown the deisotoping mass spectrum of sample (b). Reprinted from Ref. [382] with permission of the American Chemical Society.

occurring during exposure of the polyetherimide ULTEM in a QUV panel at 60 8C in air and during heating at 350 8C in air [385–387]. The photo-oxidative degradation produces a significant reduction of the molar mass of the ULTEM samples [385,386], whereas this is not observed in the thermal oxidation at 350 8C [387]. Most likely this difference is due to the abundant crosslinked insoluble residue formed during thermal oxidation, whereas no insoluble residue is formed in photo-oxidation process at 60 8C. The authors found that thermal oxidation produces charring only after only 15 h and that the formation of insoluble residue amounts to 50% after 180 h at 350 8C. Therefore, in the case of thermally oxidized ULTEM products, only the MALDI spectra of the fractions soluble in CHCL3 were recorded. MALDI spectra of both the photooxidized and thermo-oxidized samples, recorded using

HABA as a matrix, CHCl3 solvent and sodium trifluoroacetate salt as cationizing agent, showed the presence of polymer chains containing acetophenone, phenyl acetic acid, phenolic, benzoic acid and phthalic anhydride end groups [385–387]. Oligomers terminated with phthalic acid groups were observed in the spectra of photo-oxidized samples [385,386], whereas oligomers terminated with bisphenol A and phthalimide groups were observed in the spectra of thermo-oxidized samples [387]. According to the structure of the major oxidation products detected by MALDI, the authors postulated four photo-oxidation processes: (Scheme 6): (i) photooxidation of phthalimide units to phthalic anhydride end groups (P1 in Scheme 6); (ii) photo-cleavage of methyl groups of the N-methyl phthalimide terminal units (P2 in Scheme 6); (iii) oxidative degradation of the


337

CO (CH2)5 NH CO

NorrishI (B) CO (CH2)4 CH NH

H abstraction (A)

CO

CH3

CO

NH (CH2)2

CH CH2

CO

NH (CH2)3

CH3

C Norrish II (C) h CH2 CH2 CH2 CH2 CH2 NH CO

H

h

h

B

A

NH (CH2)4

CO CO (CH2)4 CH NH

CO

NH (CH2)4

O OH CO

CH3 COOH

NH (CH2)4

CHO

CO (CH2)5 NH2 CO (CH2)4

CO NH

CO (CH2)4 CO (CH2)4 NH (CH2)5

CO

CO (CH2)5 CO

COOH

NH

NH (CH2)3

CHO CH CH2

CHO CO NH2

Scheme 5.

isopropylidene bridge of BPA units (P3 in Scheme 6); (iiii) a photo-oxidation reaction introducing an oxygen atom in several isopropylidene bridges along the main chain (P4 in Scheme 6) [385,386]. In the case of thermal oxidation, they proposed a mechanism that involves three processes (Scheme 7): (1) thermal cleavage of diphenyl ether units (routes T1 and T2); (2) oxidative degradation of the isopropylidene bridge of BPA units (route T3); (3) thermal cleavage of phenyl-phthalaimide units (route T4). By comparison of the photo- and thermo-oxidation mechanisms (Scheme 6 and Scheme 7, respectively), it emerges that only process T3 corresponds to a pure thermo-oxidation reaction of ULTEM, whereas the degradation pathways T1, T2 and T4 (Scheme 7) are pure thermal scissions [387]. Both photo- and thermooxidative degradation of the isopropylidene bridge, are initiated by the extraction of a methyl hydrogen atom [386,387], yielding a methylene radical, which reacts with oxygen to form the hydroperoxide; and the decomposition of this group 350 8C leads

directly to the same oligomers listed in pathway P3 of Scheme 6. However, when the photo-oxidation is performed at 60 8C, the formation of further oxidation products such as B1–B3 can be detected (pathway P4, Scheme 6). Another apparent difference between the two processes is that the scission of the diphenyl ether units is not observed in photo-oxidation at 60 8C (Scheme 6), whereas the cleavage of the diphenyl ether units is detected in thermal oxidation at 350 8C [387]. The structures of the ULTEM thermal oxidation products were also confirmed by MS/MS analysis. Fig. 46 shows the CID MALDI-TOF/TOF spectrum of the parent ions at m/zZ1015.6 together with the structure of the fragment ions generated. These correspond to those expected for an oligomer having two hydroxyl groups attached to phthalimide units [387]. There is a large class of compounds, called generically ‘hindered amine light stabilizers’ (HALS), which are derivatives of 2,2,6,6-tetramethyl piperidine (TEMPIR). They do not absorb UV radiation, but act to

338

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357 O HN OH O

CH2

O

O

P4

CH3

P2 h ,O2

O

O CH3

N O

O

O CH3

P1 h ,O2 O O O

H2 O

N

O HO HO

O

Scheme 6.

Scheme 7.

CH3

O P3 O

CH2COCH3

O

COCH3

O

CH2OH

O O O

COOH OH CH2COOH


339

Fig. 46. CID MALDI-TOF mass spectrum of peaks at m/zZ1015.6 from ULTEM sample oxidized for 2 h at 350 8C. Reprinted from Ref. [387].

inhibit photo-degradation of polymers by slowing photochemically initiated degradation reactions. The advantage of using HALS is that no large thickness or concentration lower limit needs to be reached to guarantee good results. Significant levels of stabilization are achieved at very low concentration (70–100 ppm). HALS high efficiency and longevity are due to a cyclic process wherein the HALS are regenerated rather than consumed during the stabilization process. Ohtani et al. [390] studied Adekastab LA-68LD, a commercial oligomeric HALS in which the repeat unit has two TEMPIR moieties. They recorded the MALDI spectrum of the pure HALS and noted peaks up to 7 kDa. Inspection of the spectrum reveals three mass series, due to HALS chains terminated in three different ways, which the authors labelled an, bn, cn. After these preliminary measurements, they used MALDI to study the photostabilizing action of HALS in polypropylene (PP). They recorded the MALDI spectrum, shown at the bottom of Fig. 47, of a mixture of HALS and PP after UV irradiation for 700 h. Peaks due to PP are absent, probably because there is an enormous polarity difference between HALS and PP (actually, PP is virtually apolar).

The spectrum shows peaks (an, bn, cn) already present in the MALDI spectrum of pure HALS, along with additional peaks due to hydrolytic decomposition (dn), peaks due to oxidation ðan0 ;bn0 ;cn0 Þ and peaks labeled with a double prime (e.g. bn00 ) due to double oxidation. Fig. 47 shows the structures of d1 and b10 ions. Unfortunately, in the case of b10 , it is difficult to say which of the six TEMPIR moieties is oxidized. The presence of the dn compounds indicates that HALS is subject to hydrolysis induced by atmospheric moisture in the PP sample during irradiation. The figure also shows partial spectra in the mass range 1400– 1800 Da of the HALSCPP mixture before and after UV irradiation for 200 and 700 h, respectively. As expected, the intensities of the peaks due to decomposed and oxidized HALS chains increase as the irradiation time is increased. This confirms peak assignments and, at the same time, underlines the power of the MALDI method. Sato et al. report the application of MALDI for characterization of the products obtained by enzymatic degradation of polymer materials [388,389]. They treated PCL terminated by a-benzyloxy groups with cholesterol esterase at 37 8C in phosphate buffer at pH 7.0 for various times. Two additional mass series appear in the spectrum

340


Fig. 47. MALDI mass spectrum of HALS in UV-irradiated PP composites for 700 h obtained by the solid sampling method and partial spectra in n region observed for related PP composite samples: (a) before UV irradiation, (b) after UV irradiation for 200 h, and (c) after UV irradiation for 700 h. The figure displays structures of the original, oxidized and decomposed HALS sample. Reprinted from Ref. [391] with permission of the American Chemical Society.

of the PCL sample recovered after 36 h of enzymatic degradation (Fig. 48). These new series were assigned to ions without benzyloxy units (see Fig. 48 for assignment details), indicating that the enzymatic degradation of the PCL might proceed mainly in exo-cleavage mode from a-benzyloxy terminal groups [388]. In another example [389], the MALDI spectra of low molar mass octyl phenol polyethoxylate (OPEO) biodegraded by a pure culture of Pseudomonas under aerobic condition, showed the formation of OPEO oligomers with a carboxyl terminal of ethylene oxide (EO) chains with molar mass less than 600 Da [389]. From these data, the

biodegradation of OPEO would proceed by exo-scission of EO chain accompanied by oxidation of the hydroxyl end groups [389]. Random styrene-butadiene copolymers were distinguished from ABA block styrene-butadiene copolymers, by MALDI-TOF analysis of ozonolysis degradation products [391]. Several acrylonitrilebutadiene copolymers were also characterized using the same method [391]. The composition calculated from the oligomer distributions detected by MALDITOF, was close to the reported composition for these copolymers (typically within 5 wt%). The discrepancy


341

Fig. 48. Typical MALDI-MS spectra of (a) original PCL sample and (b) PCL sample recovered after enzymatic degradation for 36 h. Mass numbers in the expanded spectra indicate monoisotopic mass. Reprinted from Ref. [388] with permission from Elsevier.

in the values was explained, in part, by a compositional bias resulting from the ozonolysis process [391]. Appendix A. Size exclusion chromatography This appendix deals with SEC and SEC calibration. In SEC, it is generally observed that the molar mass M of macromolecules eluted at a given elution volume Ve decreases as Ve increases and the data usually conform to a relation of the type log M Z b0 Kb1 Ve

(A1)

where b0 and b1 are constants. The chromatogram of a standard polymer sample (say polystyrene) serves to calibrate the column for other samples of the same or similar polymer. It is found experimentally that b0 and b1 depend on the column and the solvent.

Another approach to SEC calibration is based on the hydrodynamic volume Rh of the polymer molecule, a quantity that is of particular relevance since it can be taken to determine Ve. Theory shows that Rh is proportional to the cube root of M[h] where [h] is the intrinsic viscosity (recommended IUPAC name: limiting viscosity number) of a polymer in solution. It turns out that Eq. (A1) can be recast in a form including the intrinsic viscosity of the sample log M Z Q0 C Q1 Ve Klogð½hÞ

(A2)

where Q0, Q1 are ‘universal’ constants. It is observed experimentally that Q0 and Q1 vary when the columnist changed, but remain unchanged from one polymer type to another. The intrinsic viscosity is related to M by the Mark–Houwink–Sakurada (MHS) equation ½h Z KM a

(A3)

342


A double logarithmic plot of [h] versus M is linear, and the MHS parameters K and a are obtained from the slope and intercept. The MHS parameters depend on the polymer, solvent, and temperature. With Eq. (A3) the universal calibration equation becomes log M Z Q0 C Q1 Ve Klog K Ka log M

(A4)

SEC devices are equipped with a refractive index detector, which gives a signal proportional to the weight of the chains eluted at a given volume, dW/ (dVe). This quantity must be converted into dW/(dM) using the chain-rule in differentiation to give dW=ðdVe Þ Z Cfac dW=ðdMÞ

(A5)

where the conversion factor, CfacZdM/dVe, is related to the calibration equation. The number fraction Ni of each chain species is Wi/Mi. The MMD is readily obtained using the SEC trace. The number- and weight-average molar masses defined above by Eqs. (3) and (4) in Section 2.10 may also be computed using the SEC trace, using the version in terms of Wi, which is proportional to the concentration measure in SEC; in some cases, the integral form of these expressions is applied, testing for an asymptotic limit to the parameters Mn and Mw as the upper bound on the integration is increased. The calibration of SEC traces of copolymers is a more complex problem, and requires additional effort than needed with homopolymers since copolymer chains having the same MM may differ in comonomer composition, and thus in overall chain dimensions. Consequently, isobaric molecules may have different hydrodynamic volumes and thus different elution volumes that depend on copolymer composition. This poses a serious problem for calibration of SEC traces. Runyon et al. proposed a method based on calibration lines obtained for homopolymer A and homopolymer B to compute Mn and Mw of an AB copolymer [355]. First, after constructing the calibration lines for the homopolymers, one records the SEC trace of the copolymer using an RI detector in series with a UV detector. Comparing the two detector responses, one obtains wA and wB, the weight fractions of A and B units at any point of the chromatogram. In the second step, the molar mass MC of the copolymer at any elution volume is assumed to obey the relation log MC Z wA log MA C wB log MB

(A6)

where MA and MB are the molar masses of the two homopolymers eluted at the same Ve. When the weight fractions of the two units in the copolymer are comparable (wAz0.5), the copolymer

line falls in the middle and one can draw it directly on the graph of the two-homopolymer lines. Unfortunately, the average molar mass averages of a copolymer sample obtained by the method of Runyon et al. are not reliable since the simple assumption of Eq. (A6) is not correct [426]. However, SEC-MALDI can be used to overcome this limitation [426]. Appendix B. Copolymer composition from MS This appendix describes use of MS peak intensities to determine copolymer composition. The method, based on chain statistics, has been widely applied to intensities derived from model sequence distributions. The relative abundance of all the oligomers of a defined chain length (dimers, trimers or higher oligomers) reflects the composition and monomer sequence in the copolymer [9]. Thus, the estimate of a sequence might be done restricting the analysis only to one group of oligomers; but of course, it is good practice to take the average of the single estimates (see below), and to keep in mind that higher oligomers are much more sensitive to subtle sequence differences [9]. When using a spectroscopic technique to obtain the copolymer sequence and composition, the essential step is to generate a theoretical spectrum, to be compared with the experimental one. The chain statistics approach allows discrimination among different sequence distribution models [9]. The process can be described as follows. For each copolymer composition, an arrangement of comonomer units along the chain is generated, according to a predefined model. Starting from any sequence, a theoretical spectrum can be generated, based on the assignment of each mass peak to a set of sequential arrangements of monomers. The quantity to be minimized is the agreement factor AF AF Z ðH1 =H2 Þ1=2 (B1) P 2 P 2 where H1 Z U1 and H2 Z U2 . The quantities U1 2 and U2 are defined as U1 Z ½IðAm Bn ÞKIexp 2 U2 Z Iexp , where Iexp and I(AmBn) are, respectively, the experimental and theoretical mole fractions of copolymer chains of type AmBn. The sums span all mass spectral peaks considered. The best-fit minimization procedure follows the scheme described (the parameters of the model are varied iteratively until convergence occurs), and yields the copolymer composition and sequence. Following this iteration, one records the spectrum, selects a model, compares the experimental and


theoretical intensities and performs a best-fit minimization [9], finds the minimum, and records the result. Then one selects a different model, and again finds a minimum. Finally, one selects the model that gives the best result. A problem frequently encountered in copolymer analysis is that the MS peaks can be assigned to two or more isobaric structures. In this case, the experimental peak intensity may come from several contributions. An automated procedure to find composition and sequence of a copolymer has been developed to cope with this problem of determining the sequence when a mass spectroscopic peak has a multiple structural assignment [9]. Bernoulli statistics predict [9] that the mole fraction, I(AmBn), of the oligomer AmBn is given by n IðAm Bn Þ Z fðm C ng!=½m!n!gcm A cB

(B2)

where cA and cB are the mole fractions of A and B units. The above equation is the well-known Newton formula; it predicts that the most abundant oligomer is that with x units of the type A and y units of the type B, i.e. the oligomer AxBy, where xZ(mCn)cA and yZ (mCn)cB. Copolymers with three and four components contain oligomers of the type AmBnCp, AmBnCpDq, respectively, where m, n, p, q are the numbers of the units of each kind in the molecule. The Bernoulli model [9] predicts that the mole fraction, I(AmBnCp), of oligomer AmBnCp is given by n p q IðAm Bn Cp Þ Z gABC cm A cB cC cD

(B3)

whereas the mole fraction, I(AmBnCpDq), of oligomer AmBnCpDq is given by n p q IðAm Bn Cp Dq Þ Z gABCD cm A cB cC cD

(B4)

where cA, cB, cC, cD, are the mole fractions of A, B, C, D units in the copolymers and gABC Z ðmC nC pÞ!=½m!n!p!; gABCD Z ðmC nC pC qÞ!=½m!n! p!q!. These are referred to as the Liebniz formulas. The above three equations allow one to generate theoretical mass spectra, with the remarkable result that the peak intensity patterns for any random copolymer of a given composition will be identical. Since each series of oligomers (dimers, trimers, etc.) allows an independent calculation of the copolymer composition and sequence distribution, the MS method provides an excellent way to evaluate the precision of these measurements [9]. The number-average length of sequences of like monomers hnAi is given by:

hnA i Z 1=ð1KcA Þ

343

(B5)

A random copolymer produced according to the Bernoulli model is compositionally homogeneous, i.e. the composition of the copolymer does not vary with the chain length. The sequence distribution followed by copolymers produced by conventional free radical processes at low conversion is the first-order Markoff distribution [9], which has an associated P-matrix. This model predicts [9] that the mole fraction of dimers is given by: IðA2 Þ Z cA PAA

(B6)

IðABÞ Z 2cA PAB

(B7)

IðB2 Þ Z cB PBB

(B8)

The corresponding equations for trimers and tetramers can be found elsewhere, along with the predicted for the number-average sequence lengths of like monomers [9]. The Markoff model also predicts that the resulting copolymer is compositionally homogeneous, i.e. that the composition of the copolymer does not vary with chain lengths. The number-average length of like monomers, hnAi, is given by: hnA i Z 1=ð1KPAA Þ

(B9)

The chain statistics method is of particular value when it is necessary to discriminate between a pure copolymer sample and a sample made from a physical mixture of two copolymers. This is a frequent case, since commercial copolymers are often obtained by mixing two copolymer batches. Let us consider a mixture of two random copolymers of the same chemical structure. The quantities of interest are dA and eA, the mole fractions of A units in the first and second copolymers, respectively, and X the mole fraction of the first copolymer in the mixture. The theory shows that the copolymer sequence distribution followed by mixtures of two copolymers is peculiar (sequences due to both components of the mixture are present) [9]. The overall composition of the mixed copolymer cA will be intermediate between the compositions of the two components cA Z XeA C ð1KXÞdA

(B10)

The model gives the mole fraction I(AmBn) of the oligomer AmBn as [9]

344

IðAm Bn Þ Z fðm C ng!=½m!n!gG3


(B11)

where G3 Z XðeA Þm ð1KeA Þn C ½1KXðdA Þm ð1KdA Þn . Explicit expressions for each oligomer may be derived from this compact formula [9]. The MMD of copolymers obtained by anionic synthesis is usually narrow, even narrower than the polymeric precursor produced before the addition of the second monomer. Their mass spectra show negligible mass discrimination. Thus, one can record the mass spectrum, derive Mn, Mw and cA (see formulas above) and then use the formulas that yield the numberaverage lengths of like monomers hnAi and hnBi: namely hnAiZcAMn/k1 and hnBiZcBMn/k1, where k1 is the mean mass of repeat units A and B.

[14]

[15]

[16]

[17]

[18]

References [1] Bahr U, Deppe A, Karas M, Hillenkamp F, Giessmann U. Mass-spectrometry of synthetic polymers by UV matrixassisted laser desorption ionization. Anal Chem 1992;64: 2866–9. [2] Schriemer DC, Li L. Detection of high molecular weight narrow polydisperse polymers up to 1.5 million daltons by MALDI mass spectrometry. Anal Chem 1996;68:2721–5. [3] Montaudo G, Montaudo MS, Samperi F. Mass spectrometry of polymers. In: Montaudo G, Lattimer RP, editors. Boca Raton: CRC Press; 2002 [chapters 2 and 10]. [4] Pasch H, Schrepp W. MALDI-TOF mass spectrometry of synthetic polymers, Springer: Berlin; 2003. p. 298. [5] McEwen CN. In: Ashroft AE, Brenton G, Monaghan JJ, editors. Recent developments in polymer characterization using mass spectrometry. In: Advances in mass spectrometry, London, vol. 16; 2004. [6] Peacock PM, McEwen CN. Mass spectrometry of synthetic polymers. Anal Chem 2004;76:3417–28. [7] Cotter RJ, Gardner BD, Litchenko S, English RD. Tandem time-of-flight mass spectrometry with a curved field reflectron. Anal Chem 2004;76:1976–81. [8] Nielen MWF. MALDI Time-of-flight mass spectrometry of synthetic polymers. Mass Spectrom Rev 1999;18:309–44. [9] Montaudo MS. Mass spectra of copolymers. Mass Spectrom Rev 2002;21:108–44. [10] Hanton SD. Mass spectrometry of polymers and polynmer surfaces. Chem Rev 2001;101:527–69. [11] Trimpin S, Rouhanipour A, Az R, Ra¨der J, Mu¨llen K. New aspects in matrix-assisted laser desorption/ionization time-offlight mass spectrometry: a universal solvent free sample preparation. Rapid Commun Mass Spectrom 2001;15:1364–73. [12] Quirk RP, Mathers RT, Wesdemiotis C, Arnould MA. Investigation of ethylene oxide oligomerization during functionalization of poly(styryl)lithium using MALDI-TOF MS and NMR. Macromolecules 2002;35:2912–8. [13] Campbell JD, Allaway JA, Teymour F, Morbidelli M. Hightemperature polymerization of styrene: mechanism determination with preparative gel permeation chromatography,

[19]

[20]

[21]

[22]

[23]

[24]

[25]

[26]

[27]

[28]

matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and 13C nuclear magnetic resonance. J Appl Polym Sci 2004;94:890–908. Lepoittevin B, He´mery P. Synthesis of multicyclic and grafted polystyrenes. J Polym Sci Polym Chem Ed 2001;39: 2723–30. David G, Boutevin B, Robin J-J, Loubat C, Zydowicz N. Synthesis of carboxy-terminated telechelic oligostyrenes by dead end polymerization. Polym Int 2002;51:800–7. Guttman CM, Wetzel S, Blair WR, Fanconi BM, Girard JE, Glodschmidt RJ, et al. NIST-Sponsored interlaboratory comparison of polystyrene molecular mass distribution obtained by matrix-assisted laser desorption/ionization timeof-flight mass spectrometry: statistical analysis. Anal Chem 2001;73:1252–62. Touchard V, Graillat C, Boisson C, D’Agosto F, Spitz R. Use of a Lewis acid surfactant combined catalyst in cationic polymerization in miniemulsion: apparent and hidden initiators. Macromolecules 2004;37:3136–42. Cauvin S, Sadoun A, Dos Santos R, Belleney J, Ganachaud F, Hemery P. Cationic polymerization of p-methoxystyrene in miniemulsion. Macromolecules 2002;35:7919–27. Goldbach JT, Russell TP, Penelle J. Synthesis and thin film characterization of poly(styrene-block-methyl methacrylate) containing an anthracene dimer photocleavable junction point. Macromolecules 2002;35:4271–6. Menoret S, Fontanille M, Deffieux A, Desbois P, Demeter J. Initiation of styrene retarded anionic polymerization using the combination of lithium alkoxides with organometallic compounds. Macromolecules 2002;35:4584–9. Menoret S, Deffieux A, Desbois P. Initiation of retarded styrene anionic polymerization using complexes of lithium hydride with organometallic compounds. Macromolecules 2003;36: 5988–94. Lin-Gibson S, Bencherif SA, Beers K, Byrd HCM. MALDITOF Mass spectral chracterization of covalently cationized polystyrene. Macromolecules 2003;36:4669–71. Bartsch A, Dempwolf W, Bothe M, Flakus S, SchmidtNaake G. Characterization of chlorine, amine- and acrylatefunctionalized TEMPO-capped and bis-TEMPO-capped polystyrene using MALDI-TOF MS. Macromol Rapid Commun 2003;24:614–9. Quirk RP, Hasegawa H, Gomochak DL, Wesdemiotis C, Wollyung K. Functionalization of poly(styryl)lithium with styrene oxide. Macromolecules 2004;37:7146–55. Alberty KA, Tillman E, Carlotti S, King K, Bradforth S, Hogen-Esch TE, et al. Characterization and fluorescence of macrocyclic polystyrene by anionic end to end coupling. Role of coupling reagents. Macromolecules 2002;35:3856–65. Vosloo JJ, De Wet-Roos D, Tonge MP, Sanderson RD. Controlled free radical polymerization in water-borne dispersion using reversible addition-fragmentation chain transfer. Macromolecules 2002;35:4894–902. Francis R, Lepoittevin B, Taton D, Gnanou Y. Toward an easy access to asymmetric stars and miktoarm by atom transfer radical polymerization. Macromolecules 2002;35: 9001–8. Kukula H, Schlaad H, Falkenhagen J, Kru¨ger R-P. Improved synthesis and characterization of u-primary amino-functional polystyrenes and polydienes. Macromolecules 2002;35: 7157–60.

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357 [29] Zettl H, Hafner W, Boker A, Schmalz H, Lanzendorfer M, Muller AHE, et al. Fluorescence correlation spectroscopy of single dye-labeled polymers in organic solvents. Macromolecules 2004;37:1917–20. [30] Otazaghine B, David G, Boutevin B, Robin JJ, Matyjaszewski K. Synthesis of telechelic oligomers via atom transfer radical polymerization, 1—Study of styrene. Macromol Chem Phys 2004;205:154–64. [31] Deng GH, Chen YM. A novel way to synthesize star polymers in one pot by ATRP of N-[2(2-bromoisobutyryloxy)ethyl] maleimide and styrene. Macromolecules 2004; 37:18–26. [32] Im K, Park S, Cho D, Ryu J, Kwon K, Lee W, et al. HPLC and MALDI-TOF MS analysis of highly branched polystyrene: resolution enhancement by branching. Anal Chem 2004;76: 2638–42. [33] Wetzel SJ, Guttman CM, Flynn KM. The influence of electrospray deposition in matrix-assisted laser desorption/ionization sample preparation for synthetic polymers. Rapid Commun Mass Spectrom 2004;18:1139–46. [34] Cox FJ, Johnston MV, Dasgupta A. Characterization and relative ionization efficiencies of end-functionalized polystyrenes by Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry. J Am Soc Mass Spectrom 2003;14: 648–57. [35] Kubo M, Hibino T, Tamura M, Uno T, Itoh T. Synthesis and copolymerization of cyclic macromonomer based on cyclic polystyrene: gel formation via chain threading. Macromolecules 2002;35:5816–20. [36] Hanton SD, Hyder IZ, Stets JR, Owen KG, Blair WR, Guttman CM, et al. Investigations of electrospray sample deposition for polymer MALDI mass spectrometry. J Am Soc Mass Spectrom 2004;15:168–79. [37] Park S, Cho D, Ryu J, Kwon K, Lee W, Chang T. Fractionation of block copolymers prepared by anionic polymerization into fractions exhibiting three different morphologies. Macromolecules 2002;35:5974–9. [38] Ji H, Sato N, Nonidez WK, Mays JW. Characterization of hydrxyl-end- capped polybutadiene and polystyrene produced by anionic polymerization technique via TLC/MALDI TOF mass spectometry. Polymer 2002;43:7119–23. [39] Kassalainen GE, Williams KR. Coupling thermal field-flow fractionation with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry for the analysis of synthetic polymers. Anal Chem 2003;75:1887–94. [40] Park S, Cho D, Ryu J, Kwon K, Chang T, Park J. Temperature gradient interaction chromatography and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry analysis of air terminated polystyryllithium. J Chromatogr A 2002; 958:183–9. [41] Murgasova R, Hercules DM. Quantitative characterization of a polystyrene/poly(a-methylstyrene) blend by MALDI mass spectrometry and size-exclusion chromatography. Anal Chem 2003;75:3744–50. [42] Hoteling AJ, Erb WJ, Tyson R, Owens KG. Exploring the importance of the relative solubility of matrix and analyte in MALDI sample preparation using HPLC. Anal Chem 2004;76: 5157–64. [43] De P, Faust R. Living carbocationic polymerization of p-methoxystyrene using p-methoxystyrene hydrochloride/

[44]

[45]

[46]

[47] [48]

[49]

[50]

[51]

[52]

[53]

[54]

[55]

[56]

[57]

[58]

345

SnBr4 initiating system: determination of the absolute rate constant of propagation for ion pairs. Macromolecules 2004; 37:7930–7. Kwak Y, Goto A, Komatsu K, Sugiura Y, Fukuda T. Characterization of low-mass model 3-arm stars produced in reversible addition–fragmentation chain transfer (RAFT) process. Macromolecules 2004;37:4434–40. Hanton SD, Parees DM. Extending the solvent-free MALDI sample preparation method. J Am Soc Mass Spectrom 2005; 16:90–3. Meier MAR, Hoogenboom R, Fijten MWM, Schneider M, Schubert US. Automated MALDI-TOF-MS sample preparation in combinatorial polymer research. J Comb Chem 2003;5:369–74. Staal B. PhD Thesis. Technical Univ Eindhoven 2005; the library http://w3.tue.nl/nl/diensten/bib. Byrd HCM, Bencherif SA, Bauer BJ, Beers KL, Brun Y, LinGibson S, et al. Examination of the covalent cationization method using narrow polydisperse polystyrene. Macromolecules 2005;38:1564–72. Basile F, Kassalainen GE, Williams KR. Interface for direct and continuous sample-matrix deposition onto a MALDI probe for polymer analysis by thermal field flow fractionation and off-line MALDI-MS. Anal Chem 2005;77:3008–12. Wallace WE, Kearsley AJ, Guttman CM. An operatorindependent approach o mass spectral peak identification and integration. Anal Chem 2004;76:2446–52. Cho D, Park S, Kwon K, Chang T, Roovers J. Structural characterization of ring polystyrene by liquid chromatography at the critical condition and MALDI-TOF mass spectrometry. Macromolecules 2001;34:7570–2. Sato H, Ichieda N, Tao H, Ohtani H. Data processing method for the determination of accurate molecular weight distribution of polymers by SEC/MALDI-MS. Anal Sci 2004;20:1289–94. Schappacher M, Deffieux A. a-Acetal-u-bis(hydroxymethyl) heterodifunctional polystyrene: synthesis, characterization, and investigation of intramolecular end-to-end ring closure. Macromolecules 2001;34:5827–32. Pascual S, Coutin B, Tardi M, Polton A, Vairon JP, Chiarelli R. Kinetic and mechanistic study of copper chloride-mediated atom transfer polymerization of styrene in the presence of N,Ndimethylformamide. Macromolecules 2001;34:5752–8. Arnould MA, Polce MJ, Quirk RP, Wesdemiotis C. Probing chain-end functionalization reactions in living anionic polymerization via matrix-assisted laser desorption ionization time-of-flight mass spectrometry. Int J Mass Spectrom 2004; 238:245–55. Meier MAR, de Gans B-J, van den Berg AMJ, Schubert US. Automated multiple-layer spotting for matrix-assisted laser desorption/ionization of synthetic polymers utilizing ink-jet printing technology. Rapid Commun Mass Spectrom 2003;17: 2349–53. Woldegiorgis A, Lo¨wenhielm P, Bjo¨rk A, Roeraade J. Matrixassisted and polymer-assisted laser desorption/ionization timeof-flight mass spectrometry analysis of low molecular weight polystyrenes and polyethylene glycols. Rapid Commun Mass Spectrom 2004;18:2904–12. Royo E, Brintzinger HH. Mass spectrometry of polystyrene and polypropene ruthenium complexes. A new tool for polymer characterization. J Organomet Chem 2002;663: 213–20.

346


[59] Shimada K, Lusenkova MA, Sato K, Saito T, Matsuyama S, Nakahara H, et al. Evaluation of mass discrimination effects in the quantitative analysis of polydisperse polymers by MALDI TOF mass spectrometry using uniform oligostyrenes. Rapid Commun Mass Spectrom 2001;15:277–82. [60] Gibson HW, Ge ZX, Huang FH, Jones JW, Lefebvre H, Vergne MJ, et al. Syntheses and model complexation studies of well-defined crown terminated polymers. Macromolecules 2005;38:2626–37. [61] Tatro SR, Baker GR, Fleming R, Harmon JP. Matrix-assisted laser desorption/ionisation (MALDI) mass spectrometry: determining Mark–Houwink–Sakurada parameters and analyzing the breadth of polymer molecular weight distributions. Polymer 2002;43:2329–35. [62] Montaudo MS. MALDI for the estimation of viscosity parameters. A modified method which applies also to polycondensates. Polymer 2004;45:6291–8. [63] Wetzel SJ, Guttman CM, Girard JE. The influence of matrix and laser energy on the molecular mass distribution of synthetic polymers obtained by MALDI-TOF-MS. Int J Mass Spectrom 2004;238:215–25. [64] Hirao A, Matsuo A. Synthesis of chain-end-functionalized poly(methyl methacrylate)s with a definite number of benzyl bromide moieties and their application to star-branched polymers. Macromolecules 2003;36:9742–51. [65] Willemse RXE, Staal BBP, van Herk AM, Pierik SCJ, Klumperman B. Application of matrix-assisted laser desorption ionization time-of-flight mass spectrometry in pulsed laser polymerization. chain-length-dependent propagation rate coefficients at high molecular weight: an artifact caused by band broadening in size exclusion chromatography? Macromolecules 2003;36:9797–803. [66] Nonaka H, Ouchi M, Kamigaito M, Sawamoto M. MALDITOF-MS analysis of ruthenium (II)-mediated living polymerisations of methyl methacrylate, methyl acrylate, and styrene. Macromolecules 2001;34:2083–8. [67] Ihara E, Tanaka S, Inoue K. Anionic polymerization of methyl methacrylate initiated with molybdenum and tungsten chloride/organolithium/triisobutylaluminum systems. Poly(3-ethyl3-hydroxymethyloxetane. J Polym Sci Part A Polym Chem 2002;40:4302–15. [68] Rodriguez-Delgado A, Chen EYX. Mechanistic studies of stereospecific polymerization of methacrylates using a cationic, chiral ansa-zirconocene ester enolate. Macromolecules 2005;38:2587–94. [69] Tatro SR, Baker GR, Bisht K, Harmon JP. A MALDI, TGA, TG/MS, and DEA study of he irradiation effects on PMMA. Polymer 2003;44:167–76. [70] Ihara E, Ikeda J, Itoh T, Inoue K. tBuOK/iBu(3)Al as new initiating system for controlled anionic polymerization of tertbutyl acrylate and methyl methacrylate. Macromolecules 2004; 37:4048–54. [71] Norman J, Moratti SC, Slark AT, Irvine DJ, Jackson AT. Synthesis of well-defined macromonomers by sequential ATRP-catalytic chain transfer and copolymerization with ehtyl acrylate. Macromolecules 2002;35:8954–61. [72] Schilli C, Lanzendo¨rfer MG, Mu¨ller AHE. Benzyl and cumyl dithiocarbamates as chain transfer agents in RAFT polymerization of N-isopropylacrylamide. In situ FT-NIR and MALDI-TOF MS investigation. Macromolecules 2002; 35:6819–27.

[73] Farcet C, Belleney J, Charleux B, Pirri R. Structural characterization of nitroxide-terminated poly(n-butyl acrylate) prepared in bulk and miniemulsion polymerizations. Macromolecules 2002;35:4912–28. [74] Otazaghine B, Boutevin B. Synthesis of Telechelic oligomers via atom transfer radical polymerization, 3a study of butyl a-fluoroacrylate. Macromol Chem Phys 2004;205:2002–11. [75] Venkatesh R, Staal BBP, Klumperman B, Monteiro MJ. Characterization of 3- and 4-arm stars from reactions of poly(butyl acrylate) RAFT and ATRP precursors. Macromolecules 2004;37:7906–17. [76] Favier A, Ladavie`re C, Charreyre T, Pichot C. MALDI-TOF MS Investigations of the RAFT polymerization of a watersoluble acylamide derivative. Macromolecules 2004;37: 2026–34. [77] Loiseau J, Doe¨rr N, Suau JM, Egraz JB, Llauro MF, Ladavie`re C, et al. Synthesis and characterization of poly(acrylic acid) produced by RAFT polymerization. Application as a very efficient dispersant of CaCO3, kaolin, and TiO2. Macromolecules 2003;36:3066–77. [78] Desponds A, Freitag R. Synthesis and characterization of photoresponsive N-isoprppylacrylamide cotelomers. Langmuir 2003;19:6261–70. [79] Ray B, Isobe Y, Matsumoto K, Habaue S, Okamoto Y, Kamigaito M, et al. RAFT polymerization of N-isopropylacrylamide in the absence and presence of Y(OTf)3: simultaneous control of molecular weight and tacticity. Macromolecules 2004;37:1702–10. [80] Roberts GE, Heuts JPA, Davis TP. Direct observation of cobalt–carbon bond formation in the catalytic chain transfer polymerization of methyl acrylate using matrix-assisted laser desorption ionization time-of-flight mass spectrometry. Macromolecules 2000;33:7765–8. [81] loninger C, Rehahn M1. 1,1-dimethylsilacyclobutanemediated living anionic block copolymerization of [1]dimethyl silaferrocenophane and methyl methacrylate. Macromolecules 2004;37:1720–7. [82] Jiang X, Schoenmakers PJ, van Dongen JLJ, Lou X, Lima V, Brokken-Zijp J. Mass spectrometric characterization of functional poly(methyl methacrylate) in combination with critical liquid chromatography. Anal Chem 2003;75:5517–24. [83] Sadhu VB, Pionteck J, Voigt D, Komber H, Fischer D, Voigt B. Atom-transfer polymerization: a strategy for the synthesis of halogen-free amino-functionalized poly(methyl methacrylate) in a one-pot reaction. Macromol Chem Phys 2004;205: 2356–65. [84] Singha NK, Rimmer S, Klumperman B. Mass spectrometry of poly(methyl methacrylate) (PMMA) prepared by atom transfer radical polymerization (ATRP). Eur Polym J 2004;40:159–63. [85] Cho D, Park S, Chang T, Ute K, Fukuda I, Kitayama T. Temperature gradient interaction chromatography and MALDI-TOF mass spectrometry analysis of stereoregular poly(ethyl methacrylate)s. Anal Chem 2002;74:1928–31. [86] Llauro MF, Loiseau J, Boisson F, Delolme F, Ladaviere C, Claverie J. Unexpected end-groups of poly(acrylic acid) prepared by RAFT polymerization. J Polym Sci Pol Chem 2004;42:5439–62. [87] Laine O, Trimpin S, Raäder HJ, Mu¨llen K. Changes in postsource decay fragmentation behavior of poly(methyl methacrylate) polymers with increasing molecular weight studies by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Eur J Mass Spectrom 2003;9:195–201.

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357 [88] M Kubo M, Nishigawa T, Uno T, Itoh T, Sato H. Cyclic polyelectrolyte: synthesis of cyclic poly(acrylic acid) and cyclic potassium polyacrylate. Macromolecules 2003;36: 9264–6. [89] Chernikova E, Morozov A, Leonova E, Garina E, Golubev V, Bui C, et al. Controlled free-radical polymerization of n-butyl acrylate by reversible addition–fragmentation chain transfer in the presence of tert-butyl dithiobenzoate. A kinetic study. Macromolecules 2004;37:6329–39. [90] D’Agosto F, Hughes R, Charreyre M-T, Pichot C, Gilbert RG. Molecular weight and functional end group control by raft polymerization of a bisubstituted acrylamide derivative. Macromolecules 2003;36:621–9. [91] Singha NK, de Ruiter B, Schubert US. Atom transfer radical polymerization of 3-ethyl-3-(acryloyloxy)methyloxetane. Macromolecules 2005;38:3590–6. [92] Sanderson CT, Palmer BJ, Morgan A, Murphy M, Dluhy RA, Mize T, et al. Classical metallocenes as photoinitiators for the anionic polymerization of an alkyl 2-cyanoacrylate. Macromolecules 2002;35:9648–52. [93] Weyermann J, Lochmann D, Georgens C, Rais I, Kreuter J, Karas M, et al. Physicochemical chracterization of cationic polybutylcyanoacrylat-nanoparticles by fluorescence correlation spectroscopy. Eur J Pharm Biopharm 2004;58:25–35. [94] Mellon W, Rinaldi D, Bourgeat-Lami E, D’Agosto F. Block copolymers of gamma-methacryloxy propyl trimethoxysilane and methyl methacrylate by RAFT polymerization. A new class of polymeric precursors for the sol–gel process. Macromolecules 2005;38:1591–8. [95] Hanton SD, Owens KG, Chavez-Eng C, Hoberg AM, Derrick PJ. Updating evidence for cationization of polymers in the gas phase during MALDI. Int J Mass Spectrom 2005;11: 23–30. [96] Carlmark A, Malmstro¨m EE. ATRP of dendronized aliphatic macromonomers of generation one, two, and three. Macromolecules 2004;37:7491–6. [97] Favier A, Charreyre MT, Pichot C. A detailed kinetic study of the RAFT polymerization of a bi-substituted acrylamide derivative: influence of experimental parameters. Polymer 2004;45:8661–74. [98] Mori H, Walther A, Andre X, Lanzendorfer MG, Muller AHE. Synthesis of highly branched cationic polyelectrolytes via selfcondensing atom transfer radical copolymerization with 2(diethylamino)ethyl methacrylate. Macromolecules 2004;37: 2054–66. [99] Malkoch M, Carlmark A, Woldegiorgis A, Hult A, Malmstrom EE. Dendronized aliphatic polymers by a combination of ATRP and divergent growth. Macromolecules 2004;37:322–9. [100] Yang P, Chan BCK, Baird MC. Is FeEt2(2,2 0 -dipyridyl)2 a Ziegler catalyst for polymerization of the polar monomer acrylonitrile? Organometallics 2004;23:2752–61. [101] Saito R, Yamaguchi K. Synthesis of bimodal methacrylic acid oligomers by template polymerization. Macromolecules 2003; 36:9005–13. [102] Saito R, Kobayashi H. Synthesis of polymers by template polymerization. 2. Effects of solvent and polymerization temperature. Macromolecules 2002;35:7207–13. [103] Wakioka M, Baek K-Y, Ando T, Kamigaito M, Sawamoto M. Possibility of living radical polymerization of vinyl acetate catalyzed by iron(I) complex. Macromolecules 2002; 35:330–3.

347

[104] Lang W, Rimmer S. Synthesis of telechelic oligo(isobutyl vinyl ethers)s via alkylation of silyl enol ethers by the propagating chain end in an ab initio cationic polymerization. Macromol Rapid Comm 2002;22:194–8. [105] Lang W, Sarker PK, Rimmer S. Cationic polymerization of vinyl ethers in the presence of sylil enol ethers. Macromol Chem Phys 2004;205:1001–20. [106] Barboiu B, Percec V. Metal catalyzed living radical polymerization of acrylonitrile initiated with sulfonyl chlorides. Macromolecules 2001;34:8626–36. [107] Lang W, Rimmer S. Mass spectrometry analysis of chain end functionalization in ab initio cationic Polymerization. Macromol Symp 2002;184:311–23. [108] Switek KA, Bates FS, Hillmyer MA. Star polymer synthesis using hexafluoropropylene oxide as an efficient multifunctional coupling agent. Macromolecules 2004;37:6355–61. [109] Trimpin S, Eichhorn P, Rader HJ, Mullen K, Knepper TP. Recalcitrance of poly(vinylpyrrolidone): evidence through matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry. J Chromatogr A 2001;938:67–77. [110] Marie A, Alves S, Fornier F, Tabet JC. Fluorinated matrix approach for the characterization of hydrophobic perfluoropolyethers by matrix-assisted laser desorption/ionization time-offlight MS. Anal Chem 2004;75:1294–9. [111] Ji H, Sato N, Nakamura Y, Wan Y, Howell A, Thomas QA, et al. Characterization of polyisobutylene by matrix-assisted laser desorption ionization time-of-flight mass spectrometry. Macromolecules 2002;35:1196–9. [112] Nagy M, Orosz L, Keki S, Deak G, Herczegh P, Zsuga M. New types of telechelic polyisobutylenes, 1— Synthesis and characterization of the bis(alpha,beta-Dglucopyranosyl) polyisobutylene. Macromol Rapid Comm 2004;25:1073–7. [113] Nagy M, Keki S, Orosz L, Deak G, Herczegh P, Levai A, et al. Novel and simple synthesis of carboxyl-terminated polyisobutylenes. Macromolecules 2005;38:4043–6. [114] Binder WH, Kunz MJ, Kluger C, Hayn G, Saf R. Synthesis and analysis of telechelic polyisobutylenes for hydrogen-bonded supramolecular pseudo-block copolymers. Macromolecules 2004;37:1749–59. [115] Ameduri B, Ladavie`re C, Delolme F, Boutevin B. First MALDI-TOF mass spectrometry of vinylidene fluorid telomers endowed with low defect chaining. Macromolecules 2004;37: 7602–9. [116] Janiak C, Lange KCH, Marquardt P, Kruger R-P, Hanselmann R. Analyses of propene and 1-hexene oligomers from zirconocene/MAO catalyst—mechanistic implications by NMR, SEC, and MALDI-TOF MS. Macromol Chem Phys 2002;203:129–38. [117] Yalcin T, Wallace WE, Guttman CM, Li L. Metal powder substrate-assisted laser desorption/ionization mass spectrometry for polyethylene analysis. Anal Chem 2002;74: 4750–6. [118] Lin-Gibson S, Brunner L, Vanderhart DL, Bauer BJ, Fanconi BM, Guttman CM, et al. Optimizing the covalent method for the mass spectrometry of polyolefins. Macromolecules 2002;35:7149–56. [119] Lopez RG, Boisson C, D’Agosto F, Spitz R, Boisson F, Bertin D, et al. Synthesis and characterization of macroalkoxyamines based on polyethylene. Macromolecules 2004;37: 3540–2.

348


[120] Bauer BJ, Wallace WE, Fanconi BM, Guttman CM. Covalent cationization method for the analysis of polyethylene by mass spectrometry. Polymer 2001;42:9949–53. [121] Busch BB, Staiger CL, Stoddard JM, Shea KJ. Living polymerization of sulfur ylides synthesis of terminally functionalized and telechelic polymethylene. Macromolecules 2002;35:8330–7. [122] Chen R, Yalcin T, Wallace WE, Guttman CM, Li L. Laser desorption ionization and MALDI Time-of-Flight mass spectrometry for low molecular mass polyethylene analysis. J Am Soc Mass Spectrom 2001;12:1186–92. [123] Kawaoka AM, Marks TJ. Organolanthanide-catalyzed synthesis of phosphine-terminated polyethylenes. Scope and mechanism. J Am Chem Soc 2005;127:6311–24. [124] Macha SF, Limbach PA, Hanton SD, Owens KG. Silver cluster interferences in matrix-assisted laser desorption/ionization (MALDI) mass spectrometry of nonpolar polymers. J Am Soc Mass Spectrom 2001;12:732–43. [125] Quirk RP, Guo Y, Wesdemiotis C, Arnould MA. Investigation of ethylene oxide oligomerization during functionalization of poly(butadienyl)lithium using MALDI-TOF MS and H-1 NMR analyses. Polymer 2004;45:3423–8. [126] Li Z, Hillmyer MA, Lodge TP. Synthesis and characterization of triptych-ABC star Triblock copolymers. Macromolecules 2004;37:8933–40. [127] Wollyung KM, Wesdemiotis C, Nagy A, Kennedy JP. Synthesis and mass spectrometry characterization of centrally and terminally amine-functionalized polyisobutylenes. J Polym Sci Part A Polym Chem 2005;43:946–58. [128] Maziarz EP, Liu XM. A modified thermal deposition unit for gel-permeation chromatography with matrix-assisted laser desorption/ionization and electrospray ionization time-of-flight analysis. Eur J Mass Spectrom 2002;8:397–401. [129] Maziarz EP, Liu XM, Baker GA. Post-gel permeation chromatography polymer blend analysis from a raster deposited matrix-assisted laser desorption/ionization target. Rapid Commun Mass Spectrom 2003;17:2450–4. [130] Liu XM, Maziarz EP, Heiler DJ, Grobe GL. Comparative studies of poly(dimethyl siloxanes) using automated GPCMALDI-TOF MS and on-line GPC-ESI-TOF MS. J Am Soc Mass Spectrom 2003;14:195–202. [131] Montaudo MS, Puglisi C, Samperi F, Montaudo G. Application of size exclusion chromatography matrix-assisted laser desorption/ionization time-of-flight to the determination of molecular masses in polydisperse polymers. Rapid Commun Mass Spectrom 1998;12:519–28. [132] Henkensmeier D, Abele BC, Candussio A, Thiem J. Syntheis and characterisation of terminal carbohydrate modified poly(dimethylsiloxane)s. Macromol Chem Phys 2004;205: 1851–7. [133] Henkensmeier D, Abele BC, Candussio A, Thiem J. Synthesis, characterization and degradadbility of polyamides derived from aldaric acids and chain end functionalised polydimethylsiloxanes. Polymer 2004;45:7053–9. [134] Kricheldorf HR, Langanke D. Polylactones, 56. ABA triblock copolymers derived from 3-caprolactone or L-lactide and a central polysiloxane block. Macromol Biosci 2001;1: 364–9. [135] White BM, Watson WP, Barthelme EE, Beckham HW. Synthesis and efficient purification of cyclic poly(dimethylsiloxane). Macromolecules 2002;35:5345–8.

[136] Liu XM, Maziarz EP, Heiler DJ. Characterization of implant device materials using size-exclusion chromatography with mass spectrometry and with triple detection. J Chromatogr A 2004;1034:125–31. [137] Barre`re M, Ganachaud F, Bendejacq D, Dourges M-A, Maitre C, He´mery P. Anionic polymerization of octamethylcyclotetrasiloxane in miniemulsion II. Molar mass analyses and mechanism scheme. Polymer 2001;42:7239–46. [138] Maziarz EP, Liu XM, Quinn ET, Lai YC, Ammon DM, Grobe GL. Detailed analysis of alpha,omega-bis(4-hydroxybutyl) poly(dimethylsiloxane) using GPC-MALDI TOF mass spectrometry. J Am Soc Mass Spectrom 2002;13: 170–6. [139] Chemlik J, Planeta J, Rehulka P, Chmelik J. Determination of molecular mass distribution of silicone oils by supercritical fluid chromatography, matrix-assisted laser desorption ionization time-of-flight mass spectrometry and their off-line combination. J Mass Spectrom 2001;36:760–70. [140] Liu XM, Maziarz EP, Quinn E, Lai YC. A perspective on relative quantitation of a polydisperse polymer using chromatography and mass spectrometry. Int J Mass Spectrom 2004; 238:227–33. [141] Yan W, Gardella Jr JA, Wood TD. Quantitative analysis of technical polymer mixtures by matrix assisted laser desorption/Ionization time of flight mass spectrometry. J Am Soc Mass Spectrom 2002;13:914–20. [142] He J, Nebioglu A, Zong Z, Soucek MD, Wollyung KM, Wesdemiotis C. Preparation of a siloxane acrylic functional siloxane colloid for UV-curable organic–inorganic hybrid films. Macromol Chem Phys 2005;206:732–43. [143] Tecklenburg RE, Wallace E, Chen H. Characterization of a [(O3/2SiMe)x(OSi(OH)ME)y(OSiMe2)z] silsesquioxane copolymer resin by mass spectrometry. Rapid Commun Mass Spectrom 2001;15:2176–85. [144] Williams RJJ, Erra-Balsells R, Ishikawa Y, Nonami H, Mauri AN, Riccardi CC. UV-MALDI-TOF and ESI-TOF mass spectrometry characterization of silsesquioxanes obtained by the hydrolytic condensation of (3-glycidoxypropyl)-trimethoxysilane in an epoxidized solvent. Macromol Chem Phys 2001;202:2425–33. [145] Bujalski DR, Chen H, Tecklenburg RE, Moyer ES, Zank GA, Su K. Compositional and structural analysis of a (PhSiO3/2)0.35(MeSiO3/2)0.40(Me2ViSiO1/2)0.25 resin. Macromolecules 2003;36:180–97. [146] Dvornic PR, Hartmann-Thompson C, Keinath SE, Hill EJ. Organic-inorganic polyamidoamine (PAMAM) dendrimerpolyhedral oligosilsesquioxane (POSS) nanohybrids. Macromolecules 2004;37:7818–31. [147] Fasce DP, Williams RJJ, Erra-Balsells R, Ishikawa Y, Nonami H. One-step synthesis of polyhedral silsesqioxanes bearing bulky substituents: UV-MALDI-TOF and ESI-TOF mass spectrometry characterization of reaction products. Macromolecules 2001;34:3534–9. [148] Eisenberg P, Erra-Balsells R, Ishikawa Y, Lucas JC, Nonami H, Williams RJJ. Silsesquioxanes derived from the bulk polycondensation of [3-(methacryloxy)propyl] trimethoxysilane with concentrated formic acid: evolution of molar mass distributions and fraction of intramolecular cycles. Macromolecules 2002;35:1160–74. [149] Chen H, He M, Pei J, He H. Quantitative analysis of synthetic polymers using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Anal Chem 2003;75:6531–5.

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357 [150] Mwelase SR, Bariyanga J. MALDI time-of-flight mass spectrometry and thermogravimetric analysis of Mg(II), Ca(II), Cu(II), Zn(II) and Pt(II) adducts with monomethoxypolyethylene glycol 5000. J Mol Struct 2002;608:235–44. [151] Song T, Dai S, Tam KC, Lee SY, Goh SH. Aggregation behavior of two-arm fullerene-containing poly(ethylene oxide) . Polymer 2003;44:2529–36. [152] Mincheva Z, Georgiev G, Kalcheva V. Molecular mass characteristics of (2-benzothiazolon-3-yl)acetic acid-telechelic poly(ethylene oxide)s determined by MALDI-TOF mass spectrometry and SEC. Macromol Chem Phys 2002;203: 538–43. [153] Jarroux N, Gue´gan P, Buchmann W, Tortajada J, Cheradame H. A new versatile radical addition on a,udimethacrylate poly(ethylene oxide). Macromol Chem Phys 2004;205:1206–17. [154] Schmalz H, Lanzendo¨rfer MG, Abetz V, Mu¨ller AHE. Anionic polymerization of ethylene oxide in the presence of the phosphazene base ButP4—Kinetic investigations using in-situ FT-NIR spectroscopy and MALDI-TOF MS. Macromol Chem Phys 2003;204:1056–71. [155] Saucy DA, Ude S, Lenggoro IW, de la Mora JF. Mass analysis of water-soluble polymers by mobility measurement of chargereduced ions generated by electrosprays. Anal Chem 2004;76: 1045–53. [156] Mincheva Z, Hadijeva P, Kalcheva V, Seraglia R, Traldi P, Przybylski M. Matrix-assisted laser desorption/ionization, fast atom bombardment and plasma desorption mass spectrometry of polyethylene glycol esters of (2-benzothiazolon-3-yl)acetic acid. J Mass Spectrom 2001;36:626–32. [157] Schmatloch S, van den Berg AMJ, Alexeev AS, Hofmeier H, Schubert US. Soluble high-molecular-mass poly(ethylene oxide)s via self-organization. Macromolecules 2003;36: 9943–9. [158] Kim T, Seo HJ, Choi JS, Jang HS, Baek J, Kim K, et al. PAMAM-PEG-PAMAM: novel triblock copolymer as a biocompatible and efficient gene delivery carrier. Biomacromolecules 2004;5:2487–92. [159] Luftmann H, Rabani G, Kraft A. MALDI-TOF mass spectrometry for detecting impurities in ester-end-functionalized poly(tetrahydrofuran), poly(propylene glycol), and poly(caprolactone) telechelics. Macromolecules 2003;36: 6316–24. [160] Chakraborty D, Rodriguez A, Chen EYX. Catalytic ringopening polymerization of propylene oxide by organoborane and aluminum Lewis acids. Macromolecules 2003;36: 5470–81. [161] Gobom J, Mueller M, Egelhofer V, Theiss D, Lehrach H, Nordhoff E. A calibration method that simplifies and improves accurate detrmination of peptide molecular masses by MALDITOF MS. Anal Chem 2002;74:3915–23. [162] Billouard C, Carlotti S, Desbois P, Deffieux A. ‘Controlled’ high-speed anionic polymerization of propylene oxide initiated by alkali metal alkoxide/trialkylaluminum systems. Macromolecules 2004;37:4038–43. [163] Chatti S, Bortolussi M, Loupy A, Blais JC, Bogdal D, Roger P. Synthesis of new polyethers derived from isoidide under microwave and classical heating. J Appl Polym Sci 2003;90: 1255–66. [164] Bednarek M, Kubisa P. Matrix-assisted laser desorption/ionization time-of-flight mass specrometry analysis of atom

[165]

[166]

[167]

[168]

[169]

[170]

[171]

[172]

[173]

[174] [175]

[176]

[177]

[178]

349

transfer radical polymerization macroinitiators derived from poly(3-ethyl-3-hydroxymethyloxetane. J Polym Sci Part A Polym Chem 2004;42:608–14. Chatti S, Bortolussi M, Loupy A, Blais JC, Bogdal D, Majdoub M. Efficient synthesis of polyethers from isosorbide by microwave-assisted phase transfer catalysis. Eur Polym J 2002;38:1851–61. Suzuki Y, Hiraoka S, Yokoyama A, Yokozawa T. Solvent effect on chain-growth polycondensation for aroamtic polyethers. J Polym Sci Part A Polym Chem 2004;42:1198–207. Bednarek M, Kubisa P. Chain-growth limiting reactions in the cationic polymerization of 3-ethyl-3-hydroxymethyloxethane. J Polym Sci Part A Polym Chem 2004;42:245–52. Malvagna P, Impallomeni G, Cozzolino R, Spina E, Garozzo D. New results on matrix-assisted laser desorption/ionization mass spectrometry of widely polydisperse hydrosoluble polymers. Rapid Commun Mass Spectrom 2002;16: 1599–603. Spriestersbach K-H, Rode K, Pasch H. Matrix-assisted laser desorption/ionization mass spectrometry of synthetic polymers. 7. Analysis of fatty alcohol ethoxylates by coupled liquid chromatography at the critical point of adsorption and MALDI-TOF mass spectrometry. Macromol Symp 2003;193: 129–41. Keil C, Esser E, Pasch H. Matrix-assisted laser desorption/Ionization mass spectrometry of synthetic Polymers, 5a. Analysis of poly(propylene oxide)s by coupled liquid chromatography at the critical point of adsorption and MALDI-TOF mass spectrometry. Macromol Mater Eng 2001;286:161–7. Ke´ki S, Szila´gyi LS, Dea´k G, Zsuga M. Effects of different alkali metal ions on the cationization of poly(ethylene glycol)s in matrix-assisted laser desorption/ionization mass spectrometry: a new selective parameter. J Mass Spectrom 2002;37:1074–80. Tezuka Y, Komiya R. Metathesis polymer cyclization with telechelic poly(THF) having allyl groups. Macromolecules 2002;35:8667–9. Meier MAR, Schubert US. Evaluation of a new multiple-layer spotting technique for matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry of synthetic polymers. Rapid Commun Mass Spectrom 2003;17:713–6. Hoteling AJ, Owens KG. Improved PSD and CID on a MALDI-TOFMS. J Am Soc Mass Spectrom 2004;15:523–35. Creaser CS, Reynolds JC, Hoteling AJ, Nichols WF, Owens KG. Atmospheric pressure matrix-assisted laser desorption/ionisation ion trap mass spectrometry of synthetic polymers: a comparison with vacuum matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry. Eur J Mass Spectrom 2003;9:33–4. Hoteling AJ, Kawaoka K, Goodberlet MC, Yu W-M, Owens KG. Optimization of matrix-assisted laser desorption/ionisation time-of-flight collision-induced dissociation using poly(ethylene glycol). Rapid Commun Mass Spectrom 2003; 17:1671–6. Rashidezadeh H, Wang Y, Guo B. Matrix effects on selectivities of poly(ethylene glycol)s for alkali metal ion complexation in matrix-assisted laser desoprtion/ionization. Rapid Commun Mass Spectrom 2000;14:439–43. Terrier P, Buchmann W, Cheguillaume G, Desmazie`res B, Tortajada J. Analysis of poly(oxyethylene) and poly(oxypropylene) triblock copolymers by MALDI-TOF mass spectrometry. Anal Chem 2005;77:3292–300.

350


[179] Okuno S, Wada Y, Arakawa R. Quantitative analysis of polypropyleneglycol mixtures by desorption/ionization on porous silicon mass spectrometry. Int J Mass Spectrom 2005; 241:43–8. [180] Lewis WG, Shen ZX, Finn MG, Siuzdak G. Desorption/ionization on silicon (DIOS) mass spectrometry: background and applications. Int J Mass Spectrom 2003;226:107–16. [181] Chen H, He M. Quantitation of synthetic polymers using an internal standard by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. J Am Soc Mass Spectrom 2005;16:100–6. [182] Holder E, Marin V, Meier MAR, Schubert US. A novel lightemitting mixed-ligand iridium(III) complex with a terpyridinepoly (ethylene glycol) macroligand. Macromol Rapid Comm 2004;25:1491–6. [183] Shen ZX, Thomas JJ, Averbuj C, Broo KM, Engelhard M, Crowell JE, et al. Porous silicon as a versatile platform for laser desorption/ionization mass spectrometry. Anal Chem 2001;73: 612–9. [184] Mineo P, Scamporrino E, Vitalini D. Synthesis and characterization of uncharged water-soluble star polymers containing a porphyrin core. Macromol Rapid Commun 2002;23:681–7. [185] Micali N, Villari V, Mineo P, Vitalini D, Scamporrino E, Crupi V, et al. Aggregation phenomena in acqueous solutions of uncharged star polymers with a porphyrin core. J Phys Chem B 2003;107:5095–100. [186] Hasegawa Y, Miyauchi M, Takashima Y, Yamaguchi H, Harada A. Supramolecular polymers formed from betacyclodextrins dimer linked by poly(ethylene glycol) and guest dimers. Macromolecules 2005;38:3724–30. [187] Unal S, Lin Q, Mourey TH, Long TE. Tailoring the degree of branching: preparation of poly(ether ester)s via copolymerization of poly(ethylene glycol) oligomers (A2) and 1,3,5benzenetricarbonyl trichloride (B3). Macromolecules 2005; 38:3246–54. [188] O’Donnell PM, Brzezinska K, Powell D, Wagener KB. ‘Perfect comb’ ADMET graft copolymers. Macromolecules 2001;34:6845–9. [189] Wollyung KM, Xu K, Cochran M, Kasko AM, Mattice WL, Wesdemiotis C, et al. Synthesis and mass spectrometry studies of an amphiphilic polyether-based rotaxane that lacks an enthalpic driving force for threading. Macromolecules 2005; 38:2574–3786. [190] Kricheldorf HR, Vakhtangishvili L, Schwarz G, Kruger RP. Cyclic hyperbranched poly(ether ketone)s derived from 3,5bis(4-fluorobenzoyl)phenol. Macromolecules 2003;36:5551–8. [191] Zolotukhin MG, Colquhoun HM, Sestiaa LG, Williams DJ, Rueda DR, Flot D. Formation of crystalline macrocyclic phases during electrophilic precipitation-polycondensation syntheses of poly(arylene ether ketone)s. Polymer 2004;45: 783–90. [192] Kricheldorf HR, Bo¨hme S, Schwarz G, Kruger R-P, Schulz G. Macrocycles. 18. The role of cyclization in syntheses of poly(ether-sulfone)s. Macromolecules 2001;34:8886–93. [193] Hanton SD, Parees DM, Owens KG. MALDI PSD of low molecular weight ethoxylated polymers. Int J Mass Spectrom 2004;238:257–64. [194] Enjalbal C, Ribie`re P, Lamaty F, Yadav-Bhatnagar N, Martinez J, Aubagnac J-L. MALDI-TOF MS analysis of soluble PEG based multi-step synthetic reaction mixtures with automated detection of reaction failure. J Am Soc Mass Spectrom 2005;16:670–8.

[195] Lin-Gibson S, Bencherif S, Cooper JA, Wetzel SJ, Antonucci JM, Vogel BM, et al. Synthesis and characterization of PEG dimethacrylates and their hydrogels. Biomacromolecules 2004;5:280–7. [196] Rieger J, Bernaerts KV, Du Prez FE, Jerome R, Jerome C. Lactone end-capped poly(ethylene oxide) as a new building block for biomaterials. Macromolecules 2004;37:9738–45. [197] Lin-Gibson S, Bencherif S, Cooper JA, Wetzel SJ, Antonucci JM, Vogel BM, et al. Synthesis and characterization of PEG dimethacrylates and their hydrogels. Biomacromolecules 2004;5:1280–7. [198] Edson JB, Knauss DM. Thianthrene as an activating group for the synthesis of poly(aryl ether thianthrene)s by nucleophilic aromatic substitution. J Polym Sci Part A Polym Chem 2004; 42:6353–63. [199] Kricheldorf HR, Hobzova R, Vakhtangishvili L, Schwarz G. Multicyclic poly(ether ketone)s obtained by polycondensation of 2,6,4 0 -trifluorobenzophenone with various diphenols. Macromolecules 2005;38:4630–7. [200] Benomar SH, Clench MR, Allen DW. The analysis of alkylphenol ethoxysulphonate surfactants by high-performance liquid chromatography, liquid chromatography—electrospray ionisation MS and MALDI MS. Anal Chim Acta 2001;445: 255–67. [201] Suzuki Y, Hiraoka S, Yokoyama A, Yokozawa T. Chaingrowth polycondensation for aromatic polyethers with low polydispersities: living polymerization nature in polycondensation. Macromolecules 2003;36:4756–65. [202] Wang DA, Williams CG, Li QA, Sharma B, Elisseeff JH. Synthesis and characterization of a novel degradable phosphate-containing hydrogel. Biomaterials 2003;24:3969–80. [203] Jayakannan M, Van Dogen JLJ, Behera GC, Ramakrishnan S. SEC-MALDI-TOF spectral characterization of a hyperbranched polyether prepared via melt transetherification. J Polym Sci Part A Polym Chem 2002;40:4463–76. [204] Huang F, Gibson HW. Formation of a supramolecular hyperbranched polymer from self-organization of an AB2 monomer containing a crown ether and two paraquat moieties. J Am Chem Soc 2004;126:14738–9. [205] Kro´l P, Kro´l B, Dziwin´ski E. Study on the synthesis of brominated eposy resins. J Appl Polym Sci 2003;90: 3122–34. [206] Zhang DH, Hillmyer MA, Tolman W. A new synthetic route to poly[3-hydroxypropionic acid](P[3-HP]): ring-opening polymerization of 3-HP macrocyclic esters. Macromolecules 2004;37:8198–200. [207] Takahashi Y, Okajima S, Toshima K, Matsumura S. Lipasecatalyzed transformation of poly(lactic acid) into cyclic oligomers. Macromol Biosci 2004;4:346–53. [208] Pantiru M, Iojoiu C, Hamaide T, Delolme F. Influence of the chemical structure of transfer agents in coordinated anionic ring-opening polymerization: to one-step functional oligomerization of 3-caprolactone. Polym Int 2004;53:506–14. [209] Ke´ki S, Bodna´r I, Borda J, Dea´k G, Zsuga M. Melt polycondensation of D,L-lactic acid: MALDI-TOF MS investigation of the ring-chain equilibrium. J Phys Chem B 2001; 105:2833–6. [210] Williams JB, Chapman TM, Hercules DM. Matrix-assisted laser desorption/ionization mass spectrometry of discrete mass poly(butylene glutarate) oligomers. Anal Chem 2003;75: 3092–100.

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357 [211] Williams JB, Chapman TM, Hercules DM. Synthesis of discrete mass poly(butylene glutarate) oligomers. Macromolecules 2004;36:3898–908. [212] Young SK, Gemeinhardt GC, Sherman JW, Storey RF, Mauritz KA, Schiraldi DA, et al. Covalent and non-covalently coupled polyester-inorganic composite materials. Polymer 2002;43:6101–14. [213] Mize TH, Simonsick WJ, Amster IJ. Characterization of polyesters by matrix-assisted laser desorption/ionization and Fourier transform mass spectrometry. Eur J Mass Spectrom 2003;9:473–86. [214] Andres PR, Schubert US. Metallo-polymerization/-cyclization of a C16-bridged di-terpyridine ligand and iron(II) ions. Macromol Rapid Comm 2004;25:1371–5. [215] Ma H, Okuda J. Kinetics and mechanism of L-lactide polymerization by rare earth metal silylamido complexes: effect of alcohol addition. Macromolecules 2005;38:2665–73. [216] Pack JW, Kim SH, Park SY, Lee Y-W, Kim YM. Kinetic and mechanistic studies of L-lactide polymerization in supercritical chlorodifluoromethane. Macromolecules 2003; 36:8923–30. [217] Takashima Y, Osaki M, Harada A. Cyclodextrin-initiated polymerization of cyclic esters in bulk: formation of polyestertethered cyclodextrins. J Am Chem Soc 2004;126:13588–9. [218] Chikh L, Arnaud X, Guillermain C, Tessier M, Fradet A. Cyclizations in hyperbranched aliphatic polyesters and polyamides. Macromol Symp 2003;199:209–21. [219] Sepulchre M, Sepulchre M-O, Belleney J. Aliphatic-aromatic hyperbranched polyesters by polycondensation of potassium 3, 5-bis(bromoethyl)benzoate: formation of cyclic structure. Macromol Chem Phys 2003;204:1679–85. [220] Kricheldorf HR, Hachmann-Thiessen H, Schwarz G. Telechelic and star-shaped poly(L-lactide)s by means of bismuth(III) acetate as initiator. Biomacromolecules 2004;5:492–6. [221] Ming W, Lou X, van de Grampel RD, van Dongen JLJ, van der Linde R. Partial fluorination of hydroxyl end-capped oligoesters revealed by MALDI-TOF mass spectrometry. Macromolecules 2001;34:2389–93. [222] Storey RF, Brister LB, Sherman JW. Structural characterization of poly(3-caprolactone-b-isobutylene-b-3-caprolactone) block copolymers by MALDI-TOF mass spectrometry. J Macromol Sci Pure Appl Chem 2001;A38:107–22. [223] Kricheldorf HR, Hachmann-Thiessen H. Polylactones 60: comparison of reactivity of Bu3SnOEt, Bu2Sn(OEt)2, and BuSn(OEt)3 as initiators of epsilon–caprolactone. J Macromol Sci Pure Appl Chem 2004;A41:335–43. ¨ sterholm H, Sela¨ntaus M, Ja¨rvinen, Vainiotalo P. [224] Laine O, O Determination of cyclic polyester oligomers by gel permeation chromatography and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Rapid Commun Mass Spectrom 2001;15:1931–5. [225] Somogyi A, Boikova N, Padias AB, Hall Jr HK. Analysis of all aromatic polyesters by matrix-assisted laser desorption ionization time-of-flight mass spectrometry. Macromolecules 2005;38:4067–71. [226] Kricheldorf HR, Rabenstein M, Maskos M, Schmidt M. Macrocycles. 15. The role of cyclization in kinetically controlled polycondensations, 1. Polyester syntheses. Macromolecules 2001;34:713–22. [227] Kricheldorf HR, Chatti S, Schwarz G, Kruger RP. Macrocycles 27: cyclic aliphatic polyesters of isosorbide. J Polym Sci Part A Polym Chem 2003;41:3414–24.

351

[228] Kricheldorf HR, Hobzova R, Schwarz G, Schultz CL. Hyperbranched polyesters of 3,5-diacetoxybenzoic acid: a reinvestigation. J Polym Sci Polym Chem 2004;42: 3751–60. [229] Kricheldorf HR, Richter M, Schwarz G. Macrocycles. 19. Cyclization in the nematic phase? Polyesters derived from hydroquinone 4-hydroxybenzoate and aliphatic dicarboxylic acids. Macromolecules 2002;35:5449–53. [230] Kricheldorf HR, Petermann O. New polymer syntheses. 110. Ring-opening polycondensation of two cyclic monomerspolyesters from ethylene sulfite and cyclic anhydrides. Macromolecules 2001;34:8841–6. [231] Kricheldorf HR, Hobzova R, Schwarz G. Cyclic hyperbranched polyesters derived from 4,4-bis(4 0 -hydroxyphenyl) valeric acid. Polymer 2003;44:7361–8. [232] Hoteling AJ, Mourey TH, Owens KG. Importance of solubility in the sample preparation of poly(ethylene terephthalate) for MALDI TOFMS. Anal Chem 2005;77:750–6. [233] Lou X, van Dongen JLJ, Janssen HM, Lange RFM. Characterization of poly(butylene terephthalate) by size-exclusion chromatography and matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry. J Chromatogr A 2002; 976:145–54. [234] Heller M, Schubert US. Optically active supramolecular poly(L-lactide)s end-capped with terpyridine. Macromol Rapid Commun 2001;22:1358–63. [235] Persson PV, Schroder J, Wickholm K, Hedenstrom E, Iversen T. Selective organocatalytic ring-opening polymerization: a versatile route to carbohydrate-functionalized poly(3caprolactones). Macromolecules 2004;37:5889–93. [236] Baéz JE, Martıńez-Richa A, Marcos-Fernańdez A. One-step route to a-hydroxyl-u-(carboxyl acid) polilactones using catalysis by decamolybdate anion. Macromolecules 2005;38: 1599–608. [237] Guillaume SM, Schappacher M, Soum A. Polymerization of 3-caprolactone initiated by Nd(BH4)(3)(THF)(3): synthesis of hydroxytelechelic poly(3-caprolactone). Macromolecules 2003;36:54–60. [238] Yang J, Jia L, Yin LZ, Yu JY, Shi Z, Fang Q, et al. A novel approach to biodegradable block copolymers of epsilon-caprolactone and delta-valerolactone catalyzed by new aluminum metal complexes. Macromol Biosci 2004;4: 1092–104. [239] Roberts GE, Davis TP, Heuts JPA, Ball GE. Monomer substituent effects in catalytic chain transfer polymerization: tert-butyl methacrylate and dimethyl itaconate. Macromolecules 2002;35:9954–63. [240] Pasch H, Rode K. Use of MALDI mass-spectrometry for molar mass-sensitive detection in liquid-chromatography of polymers. J Chromatogr A 1995;699:21–9. [241] Danis PO, Karr DE. A facile sample preparation for the analysis of synthetic organic polymers by MALDI. Org Mass Spectrom 1993;28:923–5. [242] Arakawa R, Shimomae Y, Morikawa H, Ohara K, Okuno S. Mass spectrometric analysis of low molecular mass polyesters by laser desorption/ionization on porous silicon. J Mass Spectrom 2004;39:961–5. [243] Laine O, Laitinen T, Vainiotalo P. Characterization of polyesters prepared from three different phthalic acid isomers by CID-ESI-FT-ICR and PSD-MALDI-TOF mass spectrometry. Anal Chem 2002;74:4250–8.

352


[244] Parker D, Feast WJ. Synthesis, structure, and properties of core-terminated hyperbranched polyesters based on dimethyl 5-(2-hydroxyethoxy)isophthalate. Macromolecules 2001;34: 5792–8. [245] Colomines GL, Robin JJ, Tersac G. Study of the glycolysis of PET by oligoesters. Polymer 2005;46:3230–47. [246] Kricheldorf HR, Hachmann-Thiessen H. Telechelic and starshaped poly(3-caprolactone) functionalized with triethoxysilyl groups—new biodegradable coatings and adhesives. Macrom Chem Phys 2005;206:758–66. [247] Sepulchre M, Sepulchre M-O, Belleney J. Cycle formation in the polycondensation of potassium 3-bromomethyl-5-methylbenzoate. Macromol Chem Phys 2003;204:618–31. [248] Kricheldorf HR, Rost S. Spirocycles versus networks: polycondensations of Ge(OEt)4 with various aliphatic a,udiols. Macromolecules 2004;37:7955–9. [249] Ambrogi V, Giamberini M, Cerruti P, Pucci P, Menna N, Mascolo R, et al. Liquid crystalline elastomers based on diglycidyl terminated rigid monomers and aliphatic acids. Part 1. Synthesis and characterization. Polymer 2005;46:2105–21. [250] Park JS, Akiyama Y, Winnik FM, Kataoka K. Versatile synthesis of end-functionalized thermosensitive poly(2-isopropyl-2-oxazolines). Macromolecules 2004;37:6786–92. [251] Chisholm MH, Navarro-Llobet D, Zhou ZP. Poly(propylene carbonate). 1. More about poly(propylene carbonate) formed from the copolymerization of propylene oxide and carbon dioxide employing a zinc glutarate catalyst. Macromolecules 2002;35:6494–504. [252] Kricheldorf HR, Bo¨hme S, Schwarz G, Schultz CL. Polymers of carbonic acid, 31a Cyclic polycarbonates by hydrolytic polycondensation of bispenol-A bischloroformate. Macromol Rapid Commun 2002;23:803–8. [253] Kricheldorf HR, Bo¨hme S, Schwarz G, Schultz CL. Cyclic polycarbonates by polycondensation of bisphenol A with triphosgene. Macromolecules 2004;37:1742–8. [254] Puglisi C, Samperi F, Carroccio S, Montaudo G. Analysis of poly(bisphenol A carbonate) by size exclusion chromatography/matrix-assisted laser desorption/ionization. 1. End group and molar mass determination. Rapid Commun Mass Spectrom 1999;13:2260–7. [255] Puglisi C, Samperi F, Carroccio S, Montaudo G. Analysis of poly(bisphenol A carbonate) by size exclusion chromatography/matrix-assisted laser desorption/ionization. 2. Self-association due to phenol end groups. Rapid Commun Mass Spectrom 1999;13:2268–77. [256] Coulier L, Kaal ER, Hankmeier Th. Comprehensive twodimensional liquid chromatography and hyphenated liquid chromatography to study the degradation of poly(bisphenol A) carbonate. J Chromatogr A 2005;1070:79–87. [257] Libiszowski J, Kowalski A, Szymanski R, Duda A, Raquez JM, Degeé P, et al. Monomer-linear macromolecules-cyclic oligomers equilibria in the polymerization of 1,4-dioxane-2one. Macromolecules 2004;37:52–9. [258] Scamporrino E, Bazzano S, Vitalini D, Mineo P. Insertion of copper(II)/schiff-base complexes with NLO properties into commercial polycarbonates by thermal processes. Macromol Rapid Commun 2003;24:236–41. [259] Scamporrino E, Alicata R, Bazzano S, Vitalini D, Mineo P. New copoly(bisphenol-A)carbonates, having hydrophilic porphyrin units as end-groups, synthesized by melt-reacting processes. Macromol Symp 2004;218:21–8.

[260] Kricheldorf HR, Schwarz G, Bohme S, Schultz CL, Wehrmann R. Macrocycles, 22: Synthesis and characterization of cyclic poly(bisphenol A carbonate)s by interfacial polycondensation of bisphenol-A with diphosgene. Macromol Chem Phys 2003;204:1398–405. [261] Keki S, Torok J, Deak G, Zsuga M. Ring-opening oligomerization of propylene carbonate initiated by the bisphenol A/KHCO3 system: a matrix-assisted laser desorption/ionization mass spectrometric study of the oligomers formed. Macromolecules 2001;34:6850–7. [262] Kricheldorf HR, Masri MA, Schwarz G. Cyclic polyamide-6 by thermal polycondensation of 3-caprolactam and 3-aminocaproic acid. Macromolecules 2003;36:8648–51. [263] Kricheldorf HR, Bo¨hme S, Schwarz G. Macrocycles. 17. The role of cyclization in kinetically controlled polycondensations, 2. Polyamides. Macromolecules 2001;34:8879–85. [264] Gibson HW, Nagvekar DS, Yamaguchi N, Bhattacharjee S, Wang H, Vergne MJ, et al. Polyamide pseudorotaxanes, and catenanes based on bis(5-carboxy-1,3-phenylene)-(3xC2)crown-x ethers. Macromolecules 2004;37:7514–29. [265] Murgasova R, Hercules DM. Matrix-assisted laser desorption/ionization collision-induced dissociation of linear single oligomers of nylon-6. J Mass Spectrom 2001;36:1098–107. [266] Shan L, Murgasova R, Hercules DM, Houalla M. Electrospray and matrix-assisted laser desorption/ionization mass spectral characterization of linear single nylon-6 oligomers. J Mass Spectrom 2001;36:140–4. [267] Fan SC, Schwarz G, Kricheldorf HR. Cyclic aromatic polyamides via the triphenylphosphite method. J Macromol Sci Pure Appl Chem 2004;A41:779–90. [268] Gies AP, Nonidez WK, Anthamatten M, Cook RC, Mays JW. Characterization of insoluble polyimide oligomers by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Rapid Commun Mass Spectrom 2002;16: 1903–10. [269] Weidner SM, Just U, Wittke W, Ritting F, Gruber F, Friedrich JF. Analysis of modified polyamide 6,6 using coupled liquid chromatography and MALDI-TOF-mass spectrometry. Int J Mass Spectrom 2004;238:235–44. [270] Gies AP, Nonidez WK. A technique for obtaining matrixassisted laser desorption/ionization time-of-flight mass spectra of poorly soluble and insoluble aromatic polyamides. Anal Chem 2004;76:1991–7. [271] Murgasova R, Hercules DM, Edman JR. Characterization of polyimides by combining spectrometry and selective reaction. Macromolecules 2004;37:5732–40. [272] Fournier I, Marie A, Lesage D, Bolbach G, Fournier F, Tabet JC. Post-source decay time-of-flight study of fragmentation mechanisms of protonated synthetic polymers under matrix-assisted laser desorption/ionization conditions. Rapid Commun Mass Spectrom 2002;16:696–704. [273] Puglisi C, Samperi F, Alicata R, Montaudo G. End-groupsdependent MALDI spectra of polymer mixtures. Macromolecules 2002;35:3000–7. [274] Alicata R, Montaudo G, Puglisi C, Samperi F. Influence of chain end-groups on the matrix assisted laser desorption/ionization spectra of polymer blends. Rapid Commun Mass Spectrom 2002;16:1–13. [275] Kanoh S, Nishimura T, Mitta Y, Ueyama A, Motoi M, Tanaka T, et al. Controlled precipitation polymerization of phthalimidomethyloxirane in cationic isomerization ring-opening manner. Macromolecules 2002;35:651–7.

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357 [276] Johnson DM, Reybuck SE, Lawton RG, Rasmussen PG. Condensation of DAMN with conjugated aldehydes and polymerizations of the corresponding imines. Macromolecules 2005;38:3615–21. [277] Jordan R, Martin K, Rader HJ, Unger KK. Lipopolymers for surface functionalizations. 1. Synthesis and characterization of terminal functionalized poly(N-propionylethylenimine)s. Macromolecules 2001;34:8858–65. [278] Shaikh AA, Schwarz G, Kricheldorf HR. Macrocycles. 23. Odd-even effect in the cyclization of poly(ester imide)s derived from catechols. Polymer 2003;44:2221–30. [279] Mengerink Y, Peters R, deKoster CG, van der Wal S, Calessen HA, Cramers CA. Separation and quantification of the linear and cyclic structures of polyamide-6 at the critical point of adsorption. J Chromatogr A 2001;914:131–45. [280] Kricheldorf HR, VonLossow C, Schwarz G. Primary amine and solvent-induced polymerizations of L- or D,L-phenylalanine N-carboxyanhydride. Macromol Chem Phys 2005; 206:282–90. [281] Peterson J, Allikmaa V, Subbi J, Pehk T, Lopp M. Structural deviations in poly(amidoamine) dendrimers: a MALDI-TOF MS analysis. Eur Polym J 2003;39:33–42. [282] Aulenta F, Drew MGB, Foster A, Hayes W, Rannard S, Thornthwaite DW, et al. Synthesis and characterization of fluorescent poly(aromatic amide) dendrimers. J Org Chem 2005;70:63–78. [283] Clausnitzer C, Voit B, Komber H, Voigt D. Poly(ether amide) dendrimers via nucleophilic ring-opening addition reactions of phenol groups toward oxazolines: synthesis and characterization. Macromolecules 2003;36:7065–74. [284] Takihana Y, Shiotsuki M, Sanda F, Masuda T. Synthesis and properties of carbazole-containig poly(arylenethynylenes) and poly(aryleneimines). Macromolecules 2004;37:7578–83. [285] Peetz R, Strachota A, Thorn-Csańyi E. Homologous series of 2, 5-diheptyloxy-p-phenylene vinylene (DhepO-PV) oligomers with vinyl or 1-butenyl end groups: syntheis, isolation, and microstructure. Macromol Chem Phys 2003;204:1439–50. [286] Liu JS, Loewe RS, McCullough RD. Employing MALDI-MS on poly(alkylthiophenes): analysis of molecular weights, molecular weight distributions, end-group structures, and end-group modifications. Macromolecules 1999;32:5777–85. [287] Welsh DM, Kloeppner LJ, Madrigal L, Pinto MR, Thompson BC, Schanze KS, et al. Regiosymmetric dibutylsubstituted poly(3,4-propylenedioxythiophene)s as highly electron-rich electroactive and luminescent polymers. Macromolecules 2002;35:6517–25. [288] Reeves BD, Grenier CRG, Argun AA, Cirpan A, McCarley TD, Reynolds JR. Spray coatable electrochromic dioxythiophene polymers with high coloration efficiencies. Macromolecules 2004;37:7559–69. [289] Coppo P, Cupertino DC, Yeates SG, Turner ML. Synthetic routes to solution-processable polycyclopentadithiophenes. Macromolecules 2003;36:2705–11. [290] Nilsson KPR, Olsson JDM, Konradsson P, Inganas O. Enantiomeric substituents determine the chirality of luminescent conjugated polythiophenes. Macromolecules 2004;37: 6316–21. [291] McCarley TD, Noble IV CO, DuBois Jr CJ, McCarley RL. MALDI-MS evaluation of poly(3-hexylthiophene) synthesized by chemical oxidation with FeCl3. Macromolecules 2001;34: 7999–8004.

353

[292] Quirk RP, You FX, Wesdemiotis C, Arnould MA. Anionic synthesis and characterization of omega-hydroxyl-functionalized poly(1,3-cyclohexadiene). Macromolecules 2004;37: 1234–42. [293] Babudri F, Colangiuli D, Farinola GM, Naso F. A general strategy for the synthesis of coniugated polymers based upon the palladium-catalysed cross-coupling of grignard reagents with unsaturated halides. Eur J Org Chem 2002;2785–91. [294] Chen K, Liang ZA, Meng YZ, Hay AS. Synthesis and ringopening polymerization of co-cyclic(aromatic aliphatic disulfide) oligomers. Polym Adv Technol 2003;14:719–28. [295] Liang ZA, Meng YZ, Li L, Du XS, Hay AS. Novel synthesis of macrocyclic aromatic disulfide oligomers by cyclodepolymerization of aromatic disulfide polymers. Macromolecules 2004; 37:5837–40. [296] Chen K, Meng YZ, Tjong SC, Hay AS. One-step synthesis and ring-opening polymerization of novel macrocyclic(arylene multisulfide) oligomers. J Appl Polym Sci 2004;91:735–41. [297] Quirk RP, Gomochak DL, Bhatia RS, Wesdemiotis C, Arnould MA, Wollyung K. Synthesis of diene-functionalized macromonomers via functionalization with hexa-1,3,5-triene. Macromol Chem Phys 2003;204:2183–96. [298] Dolan AR, Wood TD. Analysis of polyaniline oligomers by laser desorption ionization and solventless MALDI. J Am Soc Mass Spectrom 2004;15:893–9. [299] Neubert H, Knights KA, de Mighel YR, Cowan DA. MALDITOF post-source decay investigation of alkali metal adducts of apolar polypentylresorcinol dendrimers. Macromolecules 2003;36:8297–303. [300] Trimpin S, Grimsdale AC, Ra¨der J, Mu¨llen K. Characterization of an insoluble poly(9,9-diphenyl-2,7-fluorene) by solvent-free sample preparation for MALDI-TOF mass ppectrometry. Anal Chem 2002;74:3777–82. [301] Colquhoun HM, Lewis DF, Hodge P, Ben-Haida A, Williams DJ, Baxter I. Ring-chain interconversion in highperformance polymer systems. 1. [Poly(oxy-4,4 0 -biphenyleneoxy-1,4-phenylenesulfonyl-1,4-phenylene)](Radel-R). Macromolecules 2002;35:6875–82. [302] Bo¨hme F, Kunert C, Komber H, Voigt D, Friedel P, Khodja M, et al. Polymeric and macrocyclic ureas based on metasubstituted aromatic diamines. Macromolecules 2002;35: 4233–7. [303] Eastmond GC, Paprotny J. The influence of multiple orthosubstituted phenylenes on the nature of poly(ether imide)s. Polymer 2004;45:1073–8. [304] Przybilla L, Brand J-D, Yoshimura K, Ra¨der J, Mu¨llen K. MALDI-TOF mass spectrometry of insoluble giant polycyclic aromatic hydrocarbons by a new method of sample preparation. Anal Chem 2000;72:4591–7. [305] Chen H, He M, Pei J, Liu B. End-group analysis of blue lightemitting polymers using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Anal Chem 2002;74: 6252–8. [306] Xu P, Kumar J, Samuelson L, Cholli AS. Monitoring the Enzymatic polymerization of 4-phenylphenol by matrixassisted laser desorption ionization time-of-flight mass spectrometry: a novel approach. Biomacromolecules 2002;3: 889–93. [307] Simpson CD, Mattersteig G, Martin K, Gherghel L, Bauer RE, Rader HJ, et al. Nanosized molecular propellers by cyclodehydrogenation of polyphenylene dendrimers. J Am Chem Soc 2004;126:3139–47.

354


[308] Kricheldorf HR, Hobzova R, Schwarz G, Vakhtangishvili L. Multicyclic poly(benzonitrile ether)s based on 1,1,1-tris(4hydroxyphenyl)ethane and isomeric difluorobenzonitriles. Macromolecules 2005;38:1736–43. [309] Yoshimura K, Przybilla L, Ito S, Brand JD, Wehmeir M, Ra¨der HJ, et al. Characterization of large synthetic polycyclic aromatic hydrocarbons by MALDI- and LD-TOF mass spectrometry. Macromol Chem Phys 2001;202:215–22. [310] Ostrauskaite J, Strohriegl P. Formation of macrocycles in the synthesis of poly(N-(2-ethylhexyl)carbazol-3,6-diyl). Macromol Chem Phys 2003;204:1713–8. [311] Chen H, He M, Cao X, Zhou X, Pei J. Efficient matrix-assisted laser desorption/ionization method for fully p-conjugated dendrimers. Rapid Commun Mass Spectrom 2004;18:367–70. [312] Morrison DA, Eadie L, Davis TP. Catalytic chain transfer isomerization reactions involving 2-phenylallyl alcohol. Macromolecules 2001;34:7967–72. [313] Willemse RXE, Staal BBP, Donkers EHD, van Herk AM. Copolymer fingerprints of polystyrene-block-polyisoprene by MALDI-ToF-MS. Macromolecules 2004;37:5717–23. [314] Polce MJ, Klein DJ, Harris FW, Modarelli DA, Wesdemiotis C. Structural characterization of quinoxaline homopolymers and quinoxaline/ether sulfone copolymers by MALDI mass spectrometry. Anal Chem 2001;73:1948–58. [315] Cox JF, Johnston MV, Qian K, Peiffer DG. Compositional analysis of isobutylene/p-methylstyrene copolymers by matrixassisted laser desorption/ionization mass spectrometry. J Am Soc Mass Spectrom 2004;15:681–8. [316] Montaudo G, Montaudob MS, Puglisi C, Samperi F. Structural characterization of multicomponent copolyesters by mass spectrometry. Macromolecules 1998;31:8666–76. [317] Montaudo G, Puglisi C, Samperi F. Transreactions in condensation polymers. In: Fakirov S, editor. Weinheim (D): Wiley-VCH; 1999 [chapter 4]. [318] Samperi F, Montaudo MS, Puglisi C, Di Giorgi S, Montaudo G. Structural characterization of copolyamides synthesized via the facile blending of polyamides. Macromolecules 2004;37: 6449–59. [319] Impallomeni G, Giuffrida M, Barbuzzi T, Musumarra G, Ballistreri A. Acid catalyzed transesterification as a route to poly(3-hydroxybutyrate-co-3-caprolactone) copolymers from their homopolymers. Biomacromolecules 2002;3:835–43. [320] Tillier D, Lefebvre H, Tessier M, Blais J-C, Fradet A. High temperature bulk reaction between poly(ethylene terephthalate) and lactones: 1H NMR and SEC/MALDI-TOF MS study. Macromol Chem Phys 2004;205:581–92. [321] Puglisi C, Samperi F, Di Giorgi S, Montaudo G. Exchange reactions occurring through active chain ends. MALDI-TOF characterization of copolymers from Nylon 6,6 and Nylon 6, 10. Macromolecules 2003;36:1098–107. [322] Samperi F, Puglisi C, Alicata R, Montaudo G. Essential role of chain ends in the nylon6/poly(ethylene terephthalate exchange. J Polym Sci Polym Chem 2003;41:2778–93. [323] Samperi F, Montaudo MS, Puglisi C, Alicata R, Montaudo G. Essential role of chain ends in the Ny6/PBT exchange. A combined NMR and MALDI approach. Macromolecules 2003; 36:7143–54. [324] Viala S, Tauer K, Antonietti K, Kru¨ger R-P, Bremser W. Structural control in radical polymerization with 1,1-diphenylethylene. 1. Copolymerization of 1,1-diphenylethylene with methyl methacrylate. Polymer 2002;43:7231–41.

[325] Montaudo MS. Determination of the compositional distribution and compositional drift in styrene/maleic anhydride copolymers. Macromolecules 2001;34:2792–7. [326] Montaudo MS. Mass Spectra of copolymers which displays compositional drift or sequence constraints. J Am Soc Mass Spectrom 2004;15:374–84. [327] Venkatesh R, Harrisson S, Haddleton DM, Klumperman B. Olefin copolymerization via controlled radical polymerization: copolymerization of acrylate and 1-octene. Macromolecules 2004;37:4406–16. [328] Kaufman JM, Jaber AJ, Stump MJ, Simonsick Jr WJ, Wilkins CL. Int J Mass Spectrom 2004;234:153–60. [329] Chevallier P, Soutif J-C, Brosse J-C, Brunelle A. Poly(amide ester)s from 2,6-pyridinedicarboxylic acid and ethanolamine derivatives: identification of macrocycles by matrix-assisted laser desorption/ionization mass spectrometry. Rapid Commun Mass Spectrom 2002;16:1476–84. [330] Meier MAR, Lohmeijer BGG, Schubert US. Characterization of defined metal-containing supramolecular block copolymers. Macromol Rapid Commun 2003;24:852–7. [331] Jayakannan M, van Dongen JLJ, Janssen RAJ. Mechanistic aspects of the suzuki polycondensation of thiophenebisboronic derivatives and diiodobenzenes analyzed by MALDI-TOF mass spectrometry. Macromolecules 2001;34:5386–93. [332] Venkatesh R, Vergouwen F, Klumperman B. Copolymerization of allyl butyl ether with acrylates via controlled radical Polymerization. J Polym Sci Part A Polym Chem 2004;42: 3271–84. [333] Babudri F, Cardone A, Farinola GM, Naso F, Cassano T, Chiavarone L, et al. Synthesis and optical properties of a copolymer of tetrafluoro-and dialkoxy-substituted poly(pphenylenevinylene) with a high percentage of fluorinated units. Macromol Chem Phys 2003;204:1621–7. [334] Baker ES, Gidden J, Simonsick WJ, Grady MC, Bowers MT. Sequence dependent conformations of glycidyl methacrylate/butyl methacrylate copolymers in the gas phase. Int J Mass Spectrom 2004;238:279–86. [335] Jackson AT, Scrivens JH, Williams JP, Baker ES, Gidden J, Bowers MT. Microstructural and conformational studies of polyether copolymers. Int J Mass Spectrom 2004;238:287–97. [336] Montaudo MS, Adamus G, Kowalczuk M. Bivariate distribution in copolymers: a new model. J Polym Sci Part A Polym Chem 2002;40:2442–8. [337] Montaudo MS. Copolymer characterization by SEC-NMR and SEC-MALDI. Polymer 2002;43:1587–97. [338] Bechtold K, Hillmyer MA, Tolman WB. Perfectly alternating copolymer or lactic acid and ethylene oxide as a plasticizing agent for polylactide. Macromolecules 2001;34:8641–8. [339] Zhang DH, Xu JY, Alcazar-Roman L, Greenman L, Cramer CJ, Hillmyer MA, et al. Isotactic polymers with alternating lactic acid and oxetane subunits from the endoentropic polymerization of a 14-membered ring. Macromolecules 2004;37:5274–81. [340] Ke´ki S, Bodna´r I, Borda J, Dea´k G, Batta G, Zsuga M. Matrixassisted laser desorption/ionization mass spectrometry study of the oligomers formed from lactic acid and diphenylmethane diisocyanate. Macromolecules 2001;34:7288–93. [341] Milani B, Scarel A, Durand J, Mestroni G, Seraglia R, Carfagna C, et al. MALDI-TOF mass spectrometry in the study of CO/Aromatic olefins terpolymers. Macromolecules 2003; 36:6295–7.

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357 [342] Cox FJ, Qian K, Patil AO, Johnston MV. Microstructure and composition of ethylene-carbon monoxide copolymers by matrix-assisted laser desorption/ionization mass spectrometry. Macromolecules 2003;36:8544–50. [343] Venkatesh R, Klumperman B. Olefin copolymerization via controlled radical polymerization: copolymerization of methyl methacrylate and 1-octene. Macromolecules 2004;37:1226–33. [344] Cai Y, Tang Y, Armes SP. Direct synthesis and stimulusresponsive micellization of y-shaped hydrophilic block copolymers. Macromolecules 2004;37:9728–37. [345] Przybilla L, Francke V, Ra¨der HJ, Mu¨llen K. Block length determination of a poly(ethylene oxide)-b-poly(p-phenylene ethynylene) diblock copolymer by means of MALDI-TOF mass spectrometry combined with fragment-ion analysis. Macromolecules 2001;34:4401–5. [346] Chen R, Tseng AM, Uhing M, Li L. Application of an integrated matrix-assisted laser desorption/ionization time-offlight mass spectrometry and tandem mass spectrometry approach to characterizing complex polyol mixtures. J Am Soc Mass Spectrom 2001;12:55–60. [347] Arnould MA, Wesdemiotis C, Geiger RJ, Park ME, Buehner RW, Vanderrorst D. Structural characterization of polyester copolymers by MALDI mass spectrometry. Prog Org Coat 2002;45:305–12. [348] Dhanabalan A, van Dongen JLJ, van Duren JKJ, Janssen HM, van Hal PA, Janssen RAJ. Synthesis, characterization, and electrooptical properties of a new alternating N-dodecylpyrrole-benzothiadiazole copolymer. Macromolecules 2001; 34:2495–501. [349] Scherman OA, Rutenberg IM, Grubbs RH. Direct synthesis of soluble, end-functionalized polyenes and polyacetylene block copolymers. J Am Chem Soc 2003;125:8515–22. [350] Berlin A, Zotti G, Zecchin S, Schiavon G. Thiophene/cyclopentadiene regular copolymers from electrochemical oxidation of dithienylcyclopentadienes. Macromol Chem Phys 2002;203: 1228–37. [351] Baek JB, Harris FW. Fluorine- and hydroxyl-terminated hyperbranched poly(phenylquinoxalines) (PPQs) from copolymerization of self-polymerizable AB and AB2, BA, and BA2 monomers. Macromolecules 2005;38:1131–40. [352] Salhi S, Tessier M, Blais J-C, El Gharbi R, Fradet A. Synthesis of aliphatic–aromatic copolyesters by a high temperature bulk reaction between poly(ethylene terephthalate) cyclodi(ethylene succinate). Macromol Chem Phys 2004;205:2391–7. [353] Chen H, He M, Wan X, Yang L, He H. Matrix-assisted laser desorption/ionization study of cationization of PEO–PPP rodcoil diblock polymers. Rapid Commun Mass Spectrom 2003; 17:177–82. [354] Lee H, Chang T, Lee D, Shim MS, Ji H, Nonidez WK, et al. Chraracterization of poly(L-lactide)-block-poly(ethylene oxide)-block-poly(L-lactide) triblock copolymer by liquid chromatography at the critical condition and by MALDITOF mass spectrometry. Anal Chem 2001;73:126–32. [355] Runyon JR, Barnes DE, Rudd JF, Tung LH. Multiple detectors for molecular weight and compositional analysis of copolymers by gel permeation chromatography. J Appl Polym Sci 1969;13:2359–66. [356] Ihara E, Kurokawa A, Koda T, Muraki T, Itoh T, Inoue K. Benzyne as a monomer for polymerization: alternating copolymerization of benzyne and pyridine to give novel polymers with o-phenylene and 2,3-dihydropyridine units in the main chain. Macromolecules 2005;38:2167–72.

355

[357] Rajot I, Bone S, Graillat C, Hamaide Th. Nonionic nanoparticles by miniemulsion polymerization of vinyl acetate with oligocaprolactone macromonomer or miglyol as hydrophobe. Application to the encapsulation of indomethacin. Macromolecules 2003;36:7484–90. [358] Bernaerts KV, Schacht EH, Goethals EJ, Du Prez FE. Synthesis of poly(tetrahydrofuran)-b-polystyrene block copolymers from dual initiators for cationic ring-opening polymerization and atom transfer radical polymerization. J Polym Sci Part A Polym Chem 2003;41:3206–17. [359] Elyashiv-Barad S, Greinert N, Sen A. Copolymerization of methyl acrylate with norbornene derivatives by atom transfer radical polymerization. Macromolecules 2002;35:7521–6. [360] Maier S, Loontjens T, Scholtens B, Mulhaupt R. Isocyanatefree route to caprolactam-blocked oligomeric isocyanates via carbonylbiscaprolactam- (CBC-) mediated end group conversion. Macromolecules 2003;36:4727–34. [361] Ma Q, Wooley KL. The preparation of t-butyl acrylate, methyl acrylate, and styrene block copolymers by atom transfer radical polymerization: precursor to amphiphilic and hydrophilic block copolymers and conversion to complex nanostructured materials. J Polym Sci Part A Polym Chem 2000;38: 4805–20. [362] Rizzarelli P, Puglisi C, Montaudo G. Sequence determination in aliphatic poly(ester amide)s by PSD-MALDI/TOF/TOFMS/MS. Rapid Commun Mass Spectrom, 2005;19:2407–18. [363] Zhang C, Ochiai B, Endo T. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry study on copolymers obtained by the alternating copolymerization of bis (glactone) and epoxide with potassium tert-butoxide. J Polym Sci Part A Polym Chem 2005;43:2643–9. [364] Mucke A, Rieger B. Novel linear and branched poly(1,4ketone)-b-polyalcohol block structures through control of the catalyst initiation mechanism. Macromolecules 2002;35: 2865–7. [365] Shi SDH, Hendrickson CL, Marshall AG, Simonsick WJ, Aaserud DJ. Identification, composition, and asymmetric formation mechanism of glycidyl methacrylate butyl methacrylate copolymers up to 7000 Da from electrospray ionization ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry. Anal Chem 1998;70:3220–6. [366] Puglisi C, Samperi F, Di Giorgi S, Montaudo G. MALDI-TOF characterisation of thermally generated gel from nylon 66. Polym Degrad Stab 2002;78:369–78. [367] Murgasova R, Brantley EL, Hercules DM, Nefzger H. Characterization of polyester–polyurethane soft and hard blocks by a combination of MALDI, SEC, and chemical degradation. Macromolecules 2002;35:8338–45. [368] Samperi F, Puglisi C, Alicata R, Montaudo G. Thermal degradation of poly(ethylene terephthalate) at the processing temperature. Polym Degrad Stab 2004;83:3–10. [369] Samperi F, Puglisi C, Alicata R, Montaudo G. Thermal degradation of poly(butylene terephthalate) at the processing temperature. Polym Degrad Stab 2004;83:11–17. [370] Lattimer RP. Pyrolysis mass spectrometry of acrylic acid polymers. J Anal Appl Pyrolysis 2003;68–69:3–14. [371] Lattimer RP. Mass spectral analysis of low-temperature pyrolysis products from poly(tetrahydrofuran). J Anal Appl Pyrolysis 2001;57:57–76. [372] Lattimer RP, Polce MJ, Wesdemiotis C. MALDI-MS analysis of pyrolysis products from a segmented polyurethane. J Anal Appl Pyrolysis 1998;48:1–15.

356


[373] Gallet G, Carroccio S, Rizzarelli P, Karlsson S. Thermal degradation of poly(ethylene oxide–propylene oxide–ethylene oxide) triblock copolymer: comparative study by SEC/NMR, SEC/MALDI-TOF-MS and SPME/GC-MS. Polymer 2002;43: 1081–94. [374] Audouin F, Nuffer R, Mathis C. Thermal stability of the fullerene-chain link in 6-arm PS stars with a C60 core. J Polym Sci Part A Polym Chem 2004;42:4820–9. [375] Apicella B, Ciajolo A, Milian M, Galmes C, Herod AA, Kandiyoti R. Oligomeric carbon and siloxane series observed by matrix-assisted laser desorption/ionisation and laser desorption/ionisation mass spectrometry during the analysis of soot formed in fuel-rich flames. Rapid Commun Mass Spectrom 2004;18:331–8. [376] Puglisi C, Samperi F, Carroccio S, Montaudo G. MALDI-TOF investigation of polymer degradation pyrolysis of poly(bisphenol A carbonate). Macromolecules 1999;26:8821–8. [377] Carroccio S, Puglisi C, Montaudo G. Mechanism of thermal oxidation of poly(bisphenol A carbonate). Macromolecules 2002;35:4297–305. [378] Montaudo G, Carroccio S, Puglisi C. Thermal oxidation of poly(bisphenol A carbonate) investigated by SEC/MALDI. Polym Degrad Stab 2002;77:137–46. [379] Weidner S, Ku¨hn G, Friedrich J, Schro¨der H. Plasmaoxidative and chemical degradation of poly(ethylene terephthalate) studied by matrix-assisted laser/desorption ionization mass spectrometry. Rapid Commun Mass Spectrom 1996;10:40–6. [380] Weidner S, Ku¨hn G, Werthmann B, Schroeder H, Just U, Borowski R, et al. A new approach of characterizing the hydrolytic degradation of poly(ethylene terephthalate) by MALDI-MS. J Polym Sci A Polym Chem 1997;35:2183–92. [381] Lattimer RP. Mass spectral analysis of low-temperature pyrolysis products from poly(ethylene glycol). J Anal Appl Pyrolysis 2000;56:61–78. [382] Carroccio S, Puglisi C, Montaudo G. New vistas in the photooxidation of nylon 6. Macromolecules 2003;36:7499–507. [383] Carroccio S, Puglisi C, Montaudo G. MALDI investigation of the photooxidation of nylon-66. Macromolecules 2004;37: 6037–49. [384] Carroccio S, Rizzarelli P, Puglisi C, Montaudo G. MALDI investigation of photooxidation in aliphatic polyesters: poly(butylene succinate). Macromolecules 2004;37:6576–86. [385] Carroccio S, Puglisi C, Montaudo G. Photo-oxidation products of polyetherimide ULTEM determined by MALDI-TOF- MS. Kinetics and mechanisms. Polym Degrad Stabil 2003;80: 459–76. [386] Carroccio S, Puglisi C, Montaudo G. Comparison of photo and thermal oxidation processes in polyetherimide. Macromolecules 2005;38:6863–70. [387] Carroccio S, Puglisi C, Montaudo G. 1. New vistas in polymer degradation. Thermal oxidation processes in poly(ether imide). Macromolecules 2005;38:6849–62. [388] Sato H, Kiyono Y, Ohtani H, Tsuge S, Aoi H, Aoi K. Evaluation of biodegradation behavior of poly(3 caprolactone) with controlled terminal structure by pyrolysis-gas-chromatography and matrix-assisted laser desorption/ionization mass spectrometry. J Anal Appl Pyrolysis 2003;68–69:37–49. [389] Sato H, Shibata A, Wang Y, Yoshikawa H, Tamura H. Characterization of biodegradation intermediates of non-ionic surfactants by matrix-assisted laser desorption/ionization-mass

[390]

[391]

[392]

[393]

[394] [395]

[396]

[397]

[398]

[399]

[400]

[401] [402]

[403]

[404]

spectrometry. 1. Bacterial biodegradation of octylphenol polyethoxylate under aerobic conditions. Polym Degrad Stab 2001;74:69–75. Taguchi Y, Ishida Y, Ohtani H, Matsubara H. Direct analysis of an oligomeric hindered amine light stabilizer in polypropylene materials by MALDI-MS using a solid sampling technique to study its photostabilizing action. Anal Chem 2004;76:697–703. Zoller DL, Johnston MV. Microstructure of butadiene copolymers determined by ozonolysis/MALDI mass spectrometry. Macromolecules 2000;33:1664–70. Bankova M, Kumar A, Impallomeni G, Ballistreri A, Gross RA. Mass-selective lipase-catalyzed poly(3-caprolactone) transesteriication reactions. Macromolecules 2002;35: 6858–66. Cheng G, Boker A, Zhang M, Krausch G, Muller AHE. Amphiphilic cylindrical core-shell brushes via a ‘grafting from’ process using ATRP. Macromolecules 2001;34:6883–8. Whittal RM, Li L. High-resolution MALDI in a linear time-offlight mass-spectrometer. Anal Chem 1995;67:1950–4. Juhasz P, Roskey MT, Smirnov IP, Haff LA, Vestal ML, Martin SA. Applications of delayed extraction MALDI-timeof-flight mass spectrometry to oligonucleotide analysis. Anal Chem 1996;68:941–6. Whittal RM, Li L, Lee S, Winnik MA. Characterization of pyrene end-labeled poly(ethylene glycol) by high resolution MALDI time-of-flight mass spectrometry. Macromol Rapid Comm 1996;17:59–64. Lee S, Winnik MA, Whittal RM, Li L. Synthesis of symmetric fluorescently labeled poly(ethylene glycols) using phosphoramidites of pyrenebutanol and their characterization by MALDI mass spectrometry. Macromolecules 1996;29: 3060–72. Jackson AT, Yates HT, Lindsay CI, Didier Y, Segal JA, Scrivens JH, et al. Utilizing time-lag focusing MALDI mass spectrometry for the end group analysis of synthetic polymers. Rapid Commun Mass Spectrom 1997;11:520–6. Jackson AT, Yates HT, MacDonald WA, Scrivens JH, Critchley G, Brown J, et al. Time-lag focusing and cation attachment in the analysis of synthetic polymers by MALDItime-of-flight mass spectrometry. J Am Soc Mass Spectrom 1997;8:132–9. Erra-Balsells R, Nonami H. UV-matrix-assisted laser desorption/ionization time-of-flight mass spectrometry analysis of synthetic polymers by using nor-harmane as matrix. Arkivoc 2003;517–37. Zhang J, Zenobi R. Matrix-dependent cationization in MALDI mass spectrometry. J Mass Spectrm 2004;39:808–16. Bauer BJ, Guttman CM, Liu DW, Blair WR. Tri-alphanaphthylbenzene as a crystalline or glassy matrix for matrixassisted laser desorption/ionization: a model system for the study of effects of dispersion of polymer samples at a molecular level. Rapid Commun Mass Spectrom 2002;16: 1192–8. Soltzberg LJ, Patel P. Small molecule matrix-assisted laser desorption/ionization time-of-flight mass spectrometry using a polymer matrix. Rapid Commun Mass Spectrom 2004;18: 1455–8. Guo Z, Zhang Q, Zou H, Ni J. A method for the analysis of low-mass molecules by MALDI-TOF mass spectrometry. Anal Chem 2002;74:1637–41.

G. Montaudo et al. / Prog. Polym. Sci. 31 (2006) 277–357 [405] deVries MS, Hunziker HE. Polymer characterization by laser desorption with multiphoton ionization of end-group chromophores. Appl Surf Sci 1996;106:466–72. [406] Vitalini D, Mineo P, Scamporrino E. Effect of combined changes in delayed extraction time and potential gradient on the mass resolution and ion discrimination in the analysis of polydisperse polymers and polymer blends by delayed extraction matrix-assisted laser desorption/ionization time-offlight mass spectrometry. Rapid Commun Mass Spectrom 1999;13:2511–7. [407] Brown RS, Weil DA, Wilkins CL. Laser desorption-Fourier transform mass spectrometry for the characterization of polymers. Macromolecules 1986;19:1255–60. [408] Montaudo G, Puglisi C, Samperi F. Characterization of polymers by matrix-assisted laser desorption/ionization timeof-flight mass spectrometry: molecular weight estimates in samples of varying polydispersity. Rapid Commun Mass Spectrom 1995;9:453–60. [409] Ulmer L, Mattay J, Torres-Garcia HG, Luftmann H. The use of 2-[(2E)-3-(4-tert-butylphenyl)-2-methylprop-2-enylidene] malononitrile as a matrix for MALDI mass spectrometry. Eur J Mass Spectrom 2000;6:49–52. [410] Montaudo G, Scamporrino E, Vitalini D, Mineo P. Novel procedure for molecular weight averages measurement of polydisperse polymers directly from matrix-assisted laser desorption/ionization time-of-flight mass spectra. Rapid Commun Mass Spectrom 1996;12:1551–9. [411] Scamporrino E, Maravigna P, Vitalini D, Mineo P. A new procedure for quantitative correction of matrix-assisted laser desorption/ionization time-of-flight mass spectrometric responce. Rapid Commun Mass Spectrom 1998;12:646–50. [412] Mourey TH, Hoteling AJ, Balke ST, Owens KG. Molar mass distributions of polymers from size exclusion chromatography and matrix-assisted laser desorption ionization time-of-flight mass spectrometry. J Appl Polym Sci 2005;97:627–39. [413] Murgasova R, Hercules DM. Polymer characterization by combining liquid chromatography with MALDI and ESI mass spectrometry. Anal Bioanal Chem 2002;373:481–9.

357

[414] Ericson C, Phung QT, Horn DM, Peters EC, Fitchett JR, Ficarro SB, et al. An automated noncontact deposition interface for liquid chromatography matrix-assisted laser desorption/ionization mass spectrometry. Anal Chem 2003; 75:2309–15. [415] Zhang BY, McDonald C, Li L. Combining liquid chromatography with MALDI mass spectrometry using a heated droplet interface. Anal Chem 2004;76:992–1001. [416] Mukhopadhyay R. The automated union of LC and MALDI MS. Anal Chem 2005;76:150A–12. [417] Wright PV, Beevers MS. In: Semlyen JA, editor. Cyclic polymers. Amsterdam: Elsevier; 1986. p. 109. [418] Flory P, In: Principles of polymer chemistry. Ithaca, NY: Cornell University Press; 1971. [419] Elias HG, In: Macromolecules. New York: Plenum Press; 1984. [420] Glockner G. Chromatographic cross fractionation. In: Comprehensive polymer science, Allen G. Bevington J, editors. vol. 1. Oxford: Pergamon; 1989 [Chapter 6]. [421] Gores F, Kilz P. Chromatography of polymers. In: Provdes T, editor. ACS symposium series 521. Washington: ACS Publishing; 1993. [422] Chapman JR. Practical organic mass spectrometry: a guide for chemical and biochemical analysis. 2nd ed. NY: Wiley; 1993. [423] Loboda AV, Krutchinsky AN, Bromirski M, Ens W, Standing KG. A tandem quadrupole/time-of-flight mass spectrometer with a matrix-assisted laser desorption/ionisation source: design and performance. Rapid Commun Mass Spectrom 2000;14:1047–57. [424] Sleno L, Volmer DA. Ion activation methods for tandem mass spectrometry. J Mass Spectrom 2004;39:1091–112. [425] Muscat D, Henderickx H, Kwakkenbos G, van Benthem R, de Koster CG. In-source decay of hyperbranched polyesteramides in matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry. J Am Soc Mass Spectrom 2000;11:218–27. [426] Montando MS, Puglisi C, Samperi F, Montando G. Molar mass distributions and hydrodynamic interactions in random copolyesters investigated by SEC/MALDI. Macromolecules 1998;31:3839–45.