Combinatorial Methods, Automated Synthesis and High‐Throughput ...

Review

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Summary: For speeding-up preparation as well as investigating new polymeric materials, combinatorial techniques, parallel experimentation, and high-throughput screening methods represent a very promising approach in polymer chemistry: a large variety of parameters can be screened simultaneously resulting in new structure–property relationships. As previously described, polymer chemistry seems to be perfectly suited for combinatorial approaches since it is relatively easy to vary many parameters during the synthesis, processing, blending, or compounding. Moreover, the development and application of high-throughput screening techniques for polymer properties can accelerate the development of new materials and can result in new structure– property relationships. Therefore, these screening tools, together with parallel preparation techniques, will significantly decrease the time to market of new products. Here we provide an update of our recent overview covering new developments in the field of combinatorial and parallel polymer synthesis and high-throughput screening.

Development of high-throughput screening techniques for polymer properties.

Combinatorial Methods, Automated Synthesis and High-Throughput Screening in Polymer Research: The Evolution Continues Michael A. R. Meier, Richard Hoogenboom, Ulrich S. Schubert* Laboratory of Macromolecular Chemistry and Nanoscience, Eindhoven University of Technology and Dutch Polymer Institute (DPI), P. O. Box 513, 5600 MB Eindhoven, The Netherlands Fax: (þ31) 40/2474186; E-mail: [email protected]

Received: September 30, 2003; Revised: October 31, 2003; Accepted: October 31, 2003; DOI: 10.1002/marc.200300147 Keywords: combinatorial chemistry; high-throughput screening; materials; polymers; structure-property relations

1. Introduction The successful implementation of combinatorial and highthroughput methods in pharmaceutical research during the last decades[1–4] triggered the introduction of these fast experimentation techniques in other fields. However, the first examples of combinatorial approaches in material research lead back more than hundred years to Thomas A. Edison[5] and the photochemist Ciamician,[6] who had already applied parallel and combinatorial methods to discover suitablefilament materials for the incandescent lamp and for the discovery of novel photoactive materials, respectively. The first examples of parallel equipment also date back about a century.[7,8] In Figure 1, an historical autoclave bearing twelve parallel reactors (left) and a parallel shaker for Macromol. Rapid Commun. 2004, 25, 21–33

six flasks (right) are depicted. Today, similar pressurized reactors are available for automated parallel synthesizer robots in which stirring is performed by vortexing; demonstrating that these approaches were far ahead of their time (see reference [9] for a recent overview on the available highthroughput equipment for polymer research). Nowadays, combinatorial and high-throughput experimentation is also widely applied in the fields of catalysis,[10–13] inorganic materials[10,14–21] and polymer research. In particular, the field of polymer research seems to be perfectly suited for parallel and combinatorial approaches due to the fact that many parameters can be varied during synthesis, processing, blending, formulation, and compounding. In addition, numerous important parameters have to be investigated such as molecular weight,

DOI: 10.1002/marc.200300147

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polydispersity, polymerization kinetics, viscosity, hardness, stiffness, and other application-specific properties (e.g., high-throughput screening techniques applicable to coatings research).[22] Recently, a feasibility study found that combinatorial and high-throughput methods will represent an indispensable tool in the future of polymer research (although it most likely will not replace conventional techniques).[23] As a result, design of experiments (DoE), data-handling, and data-mining approaches will be more and more important.[24,25] Last year, we provided an overview of the status of combinatorial and parallel polymer synthesis and the available high-throughput screening techniques.[26] However, after the publication of this review many new contributions have been reported, which makes it worthwhile to describe the evolution of the field during the last year. In this review we will describe the new developments in combinatorial and high-throughput synthesis and screening in polymer research.

2. Parallel, Automated, and Combinatorial Polymer Synthesis The main goals for high-throughput and combinatorial polymer synthesis are: a) speeding-up the design of new materials, b) faster optimization of polymerization conditions, and c) elucidation of structure–property relationships. Several approaches have been applied to achieve these goals, ranging from a rather simple parallel manual synthesis to completely automated workflows with synthetic robots. The currently available (automated) parallel polymerization equipment seems to be suitable for all known polymerization techniques, whereby it should be mentioned that it is still difficult to handle high-viscosity systems. Up to now, many different synthetic polymerization methods have been used, ranging from polycondensation by free radical emulsion polymerization to various living polymerization techniques. The following sections will provide detailed information concerning the synthetic

Michael A. R. Meier was born in Ingolstadt (Germany) in 1975. He graduated in chemistry from the University of Regensburg (Germany) in 2002. His diploma thesis dealt with the fluorosensing of ammonium ions via molecular recognition in polymeric emulsion membranes and was carried out at the Institute of Analytical Chemistry, Chemo- & Biosensors at the University of Regensburg with Prof. O. Wolfbeis. In May 2002 he started his PhD thesis with Prof. U. S. Schubert at the Eindhoven University of Technology, The Netherlands, in the fields of combinatorial polymer research and supramolecular polymers.

Richard Hoogenboom was born in 1978 in Rotterdam (The Netherlands). In 2001, he obtained his MSc degree in chemical engineering at the Eindhoven University of Technology, where his undergraduate research concerning quadruple hydrogen bonding of the 2-ureido-4[1H]-pyrimidinone unit in water was performed in the group of Professor Bert Meijer (Eindhoven, The Netherlands). After a three month internship within the group of Prof. Andrew Holmes (Cambridge, United Kingdom), he started in November 2001 with his PhD work under supervision of Prof. Ulrich S. Schubert (Eindhoven, The Netherlands) focusing on supramolecular initiators for controlled polymerization techniques and automated parallel synthesis of well-defined polymers. Ulrich S. Schubert was born in Tu¨bingen in 1969. He studied chemistry at the Universities of Frankfurt and Bayreuth (both Germany) and the Virginia Commonwealth University, Richmond (USA). His PhD work was performed under the supervision of Professor Eisenbach (Bayreuth, Germany) and Professor Newkome (Florida, USA). In 1995 he obtained his doctorate with Prof. Eisenbach. After a postdoctoral training with Professor Lehn at the Universite´ Strasbourg (France) he moved to the Technische Universita¨t Mu¨nchen (Germany) to obtain his habilitation in 1999 (with Professor Nuyken). From 1999 to spring 2000 he held a temporal position as a professor at the Center for NanoScience at the Universita¨t Mu¨nchen (Germany). Since Summer 2000 he has been Full-Professor at the Eindhoven University of Technology (Chair for Macromolecular Chemistry and Nanoscience). Currently he is also a member of the management team of the Dutch Polymer Institute (DPI) and program manager of the technology area ‘‘High-throughput experimentation and combinatorial material research’’. His awards include the Bayerischen Habilitations-Fo¨rderpreis, the Habilitandenpreis of the GDCh (Makromolekulare Chemie), the Heisenberg-Stipendium of the DFG and the Dozenten-Stipendium of the Fonds der Chemischen Industrie. The major focus of his research interest relates to organic heterocyclic chemistry, supramolecular materials, combinatorial material research, nanoscience and tailor-made macromolecules. Macromol. Rapid Commun. 2004, 25, 21–33

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Figure 1. Historical parallel synthesis equipment: autoclave for 12 pressure reactions (left) and shaking device for 6 flasks (right) (from reference [7]).

parallel and combinatorial approaches in polymer research that have been reported in the last year.

2.1. Polycondensation As reported in the previous review, manual parallel polycondensation reactions of diphenols and diacids with small, systematic structural variations have been performed by Kohn and co-workers[27] and a similar approach was used by Langer et al. for the polycondensation of diacrylates and amines.[28] A higher degree of automation (automated dispensing of reagents) was reported for the synthesis of conjugated polymers from dihalogenated and diethynyl monomers using a Pd-catalyzed carbon–carbon coupling reaction by Lavastre et al.[29] and a fully automated parallel approach was applied for melt-polymerizations of bisphenol A (BPA) and diphenylcarbonate (DPC) by General Electric.[30] During the last year, Langer et al. reported the further exploration of possible structure– property relationships of the 140-membered degradable

polymer library previously synthesized from seven diacrylates and twenty amines.[28,31] After exclusion of the nonwater soluble and not sufficient DNA interacting polymers (determined using an agarose gel electrophoresis retardation assay), the remaining 56 polymers were tested for in vitro transfection and cellular uptake (Figure 2) leading to the identification of two novel polymers that revealed transfection levels similar to leading commercially available lipid-based reagents. However, no definitive structure–property relationships could be derived, because the library was too small, not diverse enough, and the molecular weights were too different (1 000 to 50 000 Da). Meredith et. al evaluated combinatorial libraries of segmented poly(urethane urea) (SPUU) elastomers with gradients in curing temperature utilizing a high-throughput impact and stain apparatus.[32] Investigated were SPUUs consisting of toluene–diisocyanate end-capped poly(tetramethylene glycol) prepolymer and trimethylene glycol dip-aminobenzoate curative at curing temperatures ranging from 70 to 130 8C. Impact results of these libraries could be

Figure 2. Transfection (left) and cellular uptake (right) data as a function of structure (from reference [31]). Macromol. Rapid Commun. 2004, 25, 21–33

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correlated to the morphology and degree of intra-urea hydrogen bonding. The optimum strength in mechanical properties was found to result from a cure temperature of 94 8C. General Electric described a methodology for the automated high-throughput multiparameter optimization of polymerization conditions, which was applied to the melt polymerization of bisphenol A (BPA) and diphenylcarbonate (DPC) with sodium hydroxide as catalyst.[33] The most appropriate process conditions should give the largest differentiation between materials as a function of catalyst concentration and the best reproducibility. For this optimization, the reaction volumes, ratio of monomers, amount of catalyst, flow rate of inert gas, and dwell time were varied resulting in five 96-microreactor arrays. The different polymerizations were screened with fluorescence spectroscopy, whereby principal component analysis (PCA) was utilized to extract the desired descriptors from the spectra. Figure 3 shows the reflected light image obtained from a 96microreactor array (left) and normalized fluorescence spectra of the polymers made under varying conditions (right). After the complete screening and data analysis, the optimized process parameters resulted, indeed, in an improved discrimination between materials synthesized with different amounts of catalyst and in better reproducibility. In addition, the homogeneity of the polymers in the microreactors was also improved.

2.2. Radical Polymerization 2.2.1. Free Radical Polymerization After the previously reported automated parallel suspension polymerization of styrene, divinylbenzene, and vinylbenzylchloride by Bradley and co-workers,[34] the first example of automated parallel emulsion polymerization

has been reported by Schubert, Van Herk and co-workers.[35] Emulsion polymerizations were performed in five parallel reactors utilizing well-defined systems of styrene and vinyl acetate, whereby the vortex speed was identified as an important parameter to obtain suitable emulsions. After optimization of the vortex speed, the results obtained for the emulsion polymerizations in the automated synthesizer were comparable with results from ‘‘classical’’ stirred batch reactors regarding the particle sizes of the obtained polystyrene particles. However, a limitation regarding solid content was observed in these experiments. Figure 4 shows an automated emulsion polymerization performed in a Chemspeed ASW2000 robot (left) and the resulting stable emulsions (right). Parallel free radical polymerization is often used for the synthesis of molecularly imprinted polymer (MIP) libraries as recently reviewed by Batra and Shea.[36] Here we will give a brief overview of the contributions that were not included in last years review. The first example of the semiautomated synthesis of a molecularly imprinted polymer library has been reported by Takeuchi et al.[37] Methacrylic acid, 2-(trifluoromethyl)acrylic acid, ethylene glycol dimethacrylate (as cross-linker), and azobisisobutyronitrile (initiator) were dispensed in glass vials utilizing a computer controlled liquid dispenser. Subsequently, the mixtures were polymerized under UV light for 12 h in a thermostatic water bath. An instant first screening method was developed, whereby the MIPs were incubated without any washing steps after the polymerization. The amount of released template was determined by performing reversedphase high-pressure liquid chromatography on the supernatants. The preliminary results from this prescreening (depicted in Figure 5) showed similar trends as the regular screening. However, differences in the amounts of released templates were observed due to the use of different solvents

Figure 3. Reflected light image (left) and fluorescence spectra (right) obtained from a 96-microreactor array with solid polymeric materials (from reference [33]). Macromol. Rapid Commun. 2004, 25, 21–33

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Figure 4. Automated emulsion polymerization (left) together with the resulting stable emulsions (right).

for the different screening methods. Later on, the synthesis of MIPs was extended to find an atrazine decomposing polymer by creating a 100-membered library using combinations of methacrylic acid with 2-(dimethylamino)ethyl methacrylate, monoacryloxy ethyl phosphate, itaconic acid, 2-sulfoethyl methacrylate, or 2-(trifluoromethyl)acrylic acid.[38] It was found that only random copolymers from methacrylic acid and 2-sulfoethyl methacrylate exhibited atrazine decomposition activity. Takeuchi et al. also reported an improved method for the synthesis and screening of MIPs, whereby the polymerizations were

performed in 96-well microtiter plates and the screening was performed utilizing a fluorescence plate reader.[39] This approach decreases the preparation and evaluation time of MIPs significantly compared to the previously used methods. Recently, another manually synthesized MIP library was reported by Kempe and co-workers.[40] It was stated that the system used was unsuitable for automation due to solubility difficulties and instability of the template. The libraries were screened for binding of penicillin G by a batch-wise radioactive assay resulting in a selective MIP for this antibiotic. In addition to the synthesis of MIP libraries,

Figure 5. Amounts of released template from a molecularly imprinted polymer library obtained using an instant screening method (from reference [37]). Macromol. Rapid Commun. 2004, 25, 21–33

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the computational design of molecularly imprinted polymers has been also developed by Turner and co-workers.[41,42] A virtual library of functional monomers was designed and screened against a certain template using molecular modeling. The monomers giving the highest affinity were used in a simulated annealing (molecular dynamics) process resulting in an optimal stoichiometric ratio. The predicted MIP was synthesized and it revealed comparable affinity and sensitivity to polyclonal antibodies, whereby superior chemical and thermal stability was obtained.

2.2.2. Controlled Radical Polymerization As described previously,[26] automated synthesis robots have been successfully applied to various controlled radical polymerization techniques like reversible addition fragmentation transfer (RAFT/MADIX) polymerization,[43,44] atom transfer radical polymerization (ATRP)[45,46] and nitroxide-mediated polymerizations.[47] Recently, Schubert and co-workers reported automated parallel investigations of new reaction conditions and catalysts for ATRP.[48] The homogeneous ATRP of methyl methacrylate (MMA) mediated by CuBr/NHPMI was investigated regarding the influence of both initiator and solvent as depicted in Figure 6. Linear first-order kinetics with similar polymerization rates were obtained for ethyl 2-bromoisobutyrate (1-bromoethyl)benzene, and p-toluenesulfonyl chloride as initiators. In contrary, the molecular weight and polydispersities (PDI) of the obtained polymers were significantly influenced by the specific initiators used. Moreover, it was found that the polymerization rate increased dramatically when changing the solvent from toluene or p-xylene to nbutylbenzene resulting in increased radical termination reactions.

workers[49] and automated parallel living cationic ring opening polymerization of 2-ethyl-2-oxazoline as described by our group[50,51] have been reported previously. In the last year, we reported the general high-throughput approach as it is applied to the 2-oxazoline research.[52] In addition, preliminary results on a systematic kinetic screening of combinations of four different monomers, four initiators, four different monomer initiator ratios, and two temperatures (80 and 100 8C) were reported.[53] Figure 7 depicts an example of the kinetic investigation of the polymerization of 2-methyl-2-oxazoline with different initiators (benzyl bromide (BB), methyl tosylate (MeOTs), methyl triflate (MeOTf), and methyl iodide (MeI)) at both 80 and 100 8C. The automated synthesizer provides a platform for highly comparable kinetic investigations with continuous monitoring of the polymerization kinetics throughout the complete reaction time (16–20 h) without long intervals during nights or weekends.

2.4. Polyolefins Both Mu¨lhaupt and Symyx have applied fully automated synthesis and screening methods for polyolefin catalyst research. Recently, different high-throughput approaches for polyolefin research were reported by Avantium,[54] Mu¨lhaupt and co-workers,[55] as well as Symyx.[56] In addition, Symyx and Dow described the discovery and development of novel polyolefin catalysts using high-throughput screening techniques, whereby primary, secondary and tertiary screening methods were used (Figure 8).[57] For the primary screening, 384 microscale polymerizations were performed, resulting in ten interesting oct-1-ene polymerization catalysts. Around those hits a second 96-membered catalyst library was created, which was screened for ethylene/oct-1-ene copolymerizations on a larger scale

2.3. Ring Opening Polymerization Automated parallel controlled ring opening polymerizations of lactides and lactones from Hedrick and co-

Figure 6. Schematic representation of the different ATRP systems that were investigated utilizing an automated synthesizer (from reference [48]). Macromol. Rapid Commun. 2004, 25, 21–33

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Figure 7. Kinetic results obtained with the automated synthesizer for the polymerization of 2-methyl-2-oxazoline with different initiators at both 80 and 100 8C (from reference [53]). ß 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Figure 8. High-throughput discovery workflow as was utilized by Symyx and Dow for the discovery of new polyolefin catalysts (from reference [57]).

(15 mL) and at a higher temperature (130 8C) to meet industrial production processes more closely. From this secondary screening, two catalysts were selected for further investigation using 3.8 litre autoclaves. The results obtained from this large-scale testing correlated well with the results obtained during the secondary screening and thus validated the screening method. In addition, the novel catalysts perform at a level comparable to commercially relevant catalyst systems for linear low-density polyethylene (LLDPE).

3. Combinatorial Polymer Libraries Several methods for the preparation of continuous thin-film polymer libraries with gradients in thickness, composition, temperature, and others are known from the literature[58–65] and have been reviewed recently.[26] Examples include the preparation of polymer film thickness libraries with a velocity-gradient-knife-edge coating device[60,61] and the application of a microextruder equipped with two feeders for the preparation of composition libraries in one dimension.[65] An overview of the possibilities to prepare thin film libraries of polymers was recently published by Meredith et al.[66] Moreover, NIST developed a robust platform for testing multi-component complex fluids by modifying common microfluidic fabrication techniques allowing the preparation of new types of combinatorial libraries.[67] New measuring methods, like, for example, simple multicapillary viscometers, were utilized for the first demonstrations of the described technology in the field of emulsions and polymer blends. Two methods for the preparation of surface energy libraries on Si surfaces were recently Macromol. Rapid Commun. 2004, 25, 21–33

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introduced.[68,69] The methods include: i) buffered oxide etching followed by a gradient Piranha etching (H2SO4/ H2O2/H2O); and ii) chlorosilane monolayer formation followed by an UV exposure with a gradient in radiation. For the first method, commercially available silicon wafers were dipped in an aqueous HF/NH4F buffered oxide etch and subsequently dipped in a 40% NH4F volume fraction aqueous solution to obtain a hydrophobic SiH-terminated substrate. Afterwards, this substrate was gradually immersed into a Piranha solution at controlled rates utilizing a motion stage. After washing the substrate, a gradient in the ‘‘degree of hydrophilicity’’ was obtained arising from the different exposure times of the substrate to the Piranha solution. The second method utilized precleaned Si substrates, which were first treated with chlorosilane and afterwards exposed to a gradient in UV irradiation under an ozone atmosphere to obtain a gradient from a hydrophilic to a hydrophobic surface. For both methods the change in polarity on the respective surface was validated by contact angle measurements. A new development in that direction is the application of ink-jet printing devices for the preparation of polymer micro-arrays. An overview of the possibilities of ink-jet printing in polymer science has been published very recently.[70] Ink-jet printing is a familiar technique for transferring electronic data to paper or overhead transparencies. However, a trend can be observed to turn ink-jet printing into a tool for micro-deposition of minute quantities of material, in particular polymers from solution. As recently discussed in the literature,[70] ink-jet printing offers entirely new possibilities in combinatorial materials research, where it may bridge the gap between parallel synthesis and automated characterization through the preparation of polymer micro-arrays. A schematic representation of a combinatorial polymer micro-array together with images of the drop formation from an ink-jet dispenser is shown in Figure 9. Furthermore, it has been demonstrated that ink-jet printing allows the automated preparation of matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry (MALDI-TOF MS) samples and therefore is a valuable tool for high-throughput screening (HTS) in combinatorial polymer research.[71] This will be discussed in detail in Section 4.1. An up-to-date review of these described techniques in the field of coatings is also available (see reference [22]).

4. High-Throughput Characterization of Polymers As pointed out in the last review,[26] high-throughput screening possibilities for synthetic polymers were far less developed than the corresponding parallel and automated synthetic procedures. A change in this trend could be observed over the last twelve months, since most publications concerning combinatorial polymer research focused ß 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Figure 9. Left: Ink-jet printing of a polymer solution. Pictures were captured at different times to show the droplet formation. Right: Schematic representation of an ink-jet printed polymer micro-array.

on fast and accurate characterization of polymer properties. The following sections will provide a short summary of existing high-throughput screening techniques and the recent new developments.

4.1. Screening of Molecular Weight It has been demonstrated that gel permeation chromatography (GPC), representing the standard molecularweight-determination method for macromolecules, can be accelerated by the application of several methodologies: high-speed columns, parallelization and/or flow-injection analysis (FIA),[72–76] as it was already shown in the previous review.[26] All of the mentioned strategies are commercially available and therefore easy to implement into the workflow of combinatorial material research. Typical analysis times for one GPC run utilizing this equipment can be less than three minutes, which represents a significant improvement compared to conventional analysis times of up to 30 min. Nevertheless, it should be mentioned that this gain in speed is accompanied by a loss in accuracy. MALDI-TOF MS, allowing the absolute determination of molecular weights, molecular-weight distributions, as well as end group analysis of macromolecules, was recently

further optimized for fast and easy sample preparation[77] and afterwards integrated into the workflow of combinatorial material research within our own research group.[71,78] The optimization for faster sample preparation included the introduction of a multiple-layer spotting sample preparation technique for MALDI-TOF MS of synthetic polymers,[77] which saves time on the one hand, and provides improved analytical results on the other. This sample preparation technique allowed the analysis of highmolecular-weight poly(ethylene glycol)s for the first time. Furthermore, it was easy to integrate into a commercially available synthesizer allowing automated sample preparation and the online monitoring of polymerization reactions as it was shown for the living cationic ring opening polymerization of 2-ethyl-2-oxazoline.[78] Furthermore, our group has recently shown that ink-jet printing is a very valuable tool for the automated MALDI-TOF MS sample preparation of synthetic polymers.[71] Improved analytical results were obtained for poly(ethylene glycol)s as well as poly(methyl methacrylate)s. Moreover, this lead to a further miniaturization of the sample preparation allowing the positioning of at least 400 times more samples onto a MALDI target compared to conventional sample preparation. Figure 10 provides an overview of the miniaturization

Figure 10. Comparison of different multiple-layer sample preparation techniques for MALDI-TOF MS as utilized in combinatorial polymer research. Macromol. Rapid Commun. 2004, 25, 21–33

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and automation of the described MALDI-TOF MS sample preparation techniques.

4.2. Optical Screening Methods Optical screening methods are excellently suited for the high-throughput evaluation of certain polymer properties and/or their chemical composition, since it is relatively easy to miniaturize and parallelize these methods by utilizing imaging techniques or optical fibers. Furthermore, these techniques can be used for online monitoring applications since the optical signal can be read out continuously. Another important point, which allows easy integration of the mentioned techniques into combinatorial workflows, is the commercial availability of reader technologies for absorbance and fluorescence, as well as for infrared spectroscopy (for example see references [79,80]). As already described previously,[26] FT-IR and attenuated total reflection FT-IR (ATR FT-IR) spectroscopy are useful tools for the monitoring of monomer consumptions and for high-throughput evaluation of copolymer compositions. Near infrared (NIR) spectroscopy was also utilized for the composition determination of olefin copolymers (ethene/hex-1-ene and ethane/propene) in a high sample throughput mode.[81] It was stated that NIR spectroscopy can significantly improve the performance of rapid on-line polymer analysis if it is applied in conjunction with mid infrared (MIR) spectroscopy.

Potyrailo et al. demonstrated that fluorescence spectroscopy is a versatile tool in combinatorial polymer research.[33,82] Utilizing a 96-microreactor array for the preparation of bisphenol A polycarbonates, properties of interest such as molecular weight, the amount of branching, or catalyst selectivity could be correlated to fluorescence features. Therefore, a CCD-based spectrofluorometer and/ or a fluorescence scanning system were utilized.[82] Both fluorescence screening tools revealed good correlation to each other and to conventional characterization techniques. It was, for instance, possible to correlate the fluorescence (excitation 280 nm, emission 305 nm) of the solid polymer samples with their number-average molecular weight obtained by GPC. Furthermore, the selectivity of catalysts could be correlated to the ratio of fluorescence intensities (I400/I500) at 340 nm excitation. Figure 11 shows fluorescence imaging results from the microreactor array after the polymerization, as well as emission spectra of the polymers, and a validation of the two applied measuring techniques.

4.3. Screening of Morphology and Physical Properties Several methodologies have been applied to access physical and/or morphological data of polymers in an automated, parallel, and/or high-throughput approach such as the correlation of these properties with optical data[83] or the development of special equipment for faster analysis.[84] The evaluation of films (also coatings) in that context seems

Figure 11. Determination of selectivity of melt-polymerization catalysts in combinatorial 96-microreactor arrays by fluorescence. Fluorescence images of microreactors through (A) 400- and (B) 500-nm interference filters. (C) Emission spectra from polymers in the highlighted column of microreactors. (D) Correlation of I400/I500 values from serial spectroscopic analysis and parallel two-wavelength fluorescence imaging (from reference [82]). Macromol. Rapid Commun. 2004, 25, 21–33

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to be of special interest since most new results deal with the parallel and fast investigation of polymer films in order to obtain adhesion, crystallization, or dewetting information.[68,69,85 –90] Methods for the preparation of polymer thin-film libraries are described in Section 3 as well as in last years review.[26] Grunlan et al. described a method for the combinatorial screening of the moisture vapor transition rate (MVTR).[88] A mixture of Nafion (a sulfonic acid-substituted fluoro polymer) and crystal violet has been used as a moisture sensitive film, which was coated onto a poly(ethylene terephthalate) (PET) support film. After both sides of the PET support were laminated with a transfer adhesive, the sensor was laminated onto a glass substrate. This substrate was then covered with barrier films of interest and the slopes of absorbance as a function of time were converted to moisture vapor transition rate values using a series of known films as references. This procedure was transferred to a combinatorial MVTR assessment. It was shown that 20 emulsion based poly(vinylidene chloride) films (for the film/sensor preparation see reference [88]) could be assessed simultaneously. Ashley et al. studied the dewetting of polystyrene (PS)[68,69] and poly(D-lactic acid) (PDLA)[69] films on chemically modified gradient energy surfaces (for the preparation of these gradient energy surfaces on Si surfaces see Section 3). A film thickness gradient of PS or a temperature gradient film of PDLA were prepared orthogonal to the surface energy gradient and were subsequently studied optically and evaluated by an automated batch program. In conclusion, the authors state that the presence of chemical wettability gradients on Si substrates destabilized both PS and PDLA films in opposing trends with surface energy. The crystallization behavior of isotactic polystyrene (iPS) films was studied in a combinatorial manner with an optical microscope.[85,86] Figure 12 shows selected optical microscope images of the iPS crystallization kinetics. The maximum radius of the spherulites, r, was measured using custom-made graphic software. The temperature dependence of the crystallization growth rate G obtained from high-throughput measurements is plotted in Figure 13, together with data from other references for comparison. A good agreement between the different methods was found. All data presented by the authors[85] could be accomplished in less than two days, including the microscopy pictures. Furthermore, it was shown that adhesion could be evaluated by means of high-throughput methodologies.[87,89,90] Therefore, a high-throughput test was developed on the basis of automated crosshatching after which adhesion loss was induced by exposing the crosshatched films to boiling water followed by rapid freezing and/or by tap pulling of the delaminated coating elements. Subsequently, the adhesion loss was determined by automated imaging and the data were evaluated utilizing imaging analysis algorithms. Figure 14 shows a crosshatched 48-membered coating Macromol. Rapid Commun. 2004, 25, 21–33

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Figure 12. Optical microscope images of isotactic polystyrene (iPS) crystallization kinetics at T ¼ 169 8C, film height ¼ 60 nm: (a) t ¼ 31 min, (b) t ¼ 46 min, (c) t ¼ 81 min, (d) t ¼ 256 min. The scale bar is 50 mm (from reference [85]).

library and a typical result of automated adhesion loss analysis. Finally, up-scaling revealed that the combinatorial leads resulted in new coatings with excellent adhesion behavior compared to standard materials. Moreover, it was shown that the adhesion of thin polymer films could be

Figure 13. The semi-logarithmic plot of growth rate, G, of isotactic polystyrene (iPS) films as a function of temperature. A representative sample of data obtained from high-throughout measurements at h ¼ 55 nm (*) are compared with literature values (from reference [85]). ß 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Figure 14. Left: An array of coatings after crosshatching. Right: Typical results of an adhesion analysis of a 48-element coating array (from reference [87]).

evaluated by utilizing a multilens combinatorial adhesion test (MCAT).[89,90] The general methodology of these tests was described in last years review.[26] Other high-throughput methods were developed for testing weathering conditions[83] as well as fast differential scanning calorimetry (DSC).[84] Weathering conditions were tested utilizing three types of aromatic polymers (polycarbonate, poly(butylenes terephthalate), and their 45/55 wt.-% blend) in combination with two types of pigments (rutile and carbon black).[83] Eleven different weathering conditions were evaluated by determining the amount of UV-induced degradation in the arrays of polymeric materials utilizing fluorescence imaging and spectroscopy. Applying this technique, the time of analysis of weathering conditions could be decreased by at least a factor of 150 if compared to conventional determinations of color change and gloss loss. In addition, a robust ranking of material performance could be provided. The acceleration of DSC analysis was established by development of an automated large sample array DSC (LSA-DSC or LSA).[84] The LSA can measure in temperature scanning (DSC) as well as isothermal (IMC) modes. The instrument can hold up to 100 samples and is capable of scanning at 0 to 2 8C
min1 within a temperature range of 10 to 200 8C including real time data analysis. By utilizing this instrument (additionally to a newly developed formulating procedure), 275 epoxy formulations could be evaluated by means of cure kinetics, whereby 270 manual labor hours could be avoided.

5. Conclusion The evolution of combinatorial polymer chemistry is still continuing and the field is still in a maturing process. Nevertheless, several examples of successful applications of parallel and automated synthetic procedures have been reported together with the introduction of new high-throughput screening techniques and extended data analysis within the last twelve months. Especially on the screening side, new developments might have a huge impact on the future of the Macromol. Rapid Commun. 2004, 25, 21–33

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field. Several new screening methods have been developed that dramatically decrease the time of analysis but do not necessarily decrease the quality of the data. It was shown for several examples that high-throughput data correlates very well with conventional measurement methods and furthermore, that screening data can even be of better quality than the data obtained by conventional methods. In conclusion, combinatorial techniques in material sciences represent a promising and still young approach, which might revolutionize the research in the area.

Acknowledgement: The authors thank the NWO, DPI and the Fonds der Chemischen Industrie for the financial support.

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