Living Cationic Polymerizations Utilizing an Automated Synthesizer ...

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Macromol. Rapid Commun. 2003, 24, 92–97

Communication: Living cationic ring-opening polymerizations of 2-ethyl-2-oxazoline and purification of the resulting polymers were performed utilizing an automated synthesizer. Eight polymers (500 mg scale) as well as 40 polymers (150 mg scale) were synthesized in parallel to investigate the reproducibility and the living character of the polymerizations. The poly(2-ethyl-2-oxazoline)s obtained such were characterized by means of 1H NMR spectroscopy, MALDI-TOF mass spectrometry and online gel permeation chromatography.

Living Cationic Polymerizations Utilizing an Automated Synthesizer: High-Throughput Synthesis of Polyoxazolines Richard Hoogenboom, Martin W. M. Fijten, Michael A. R. Meier, Ulrich S. Schubert* Laboratory of Macromolecular Chemistry and Nanoscience, Eindhoven University of Technology and Dutch Polymer Institute (DPI), PO Box 513, 5600 MB Eindhoven, The Netherlands Fax: þ31-(0)40-2474186; E-mail: [email protected]

Keywords: automated synthesis; combinatorial materials research; high-throughput experimentation; living cationic polymerization; parallel chemistry; poly(2-oxazoline)s

Introduction Combinatorial and high-throughput methods have had an enormous impact in the field of pharmaceutical research in the last decade.[1–3] A large number of potentially active compounds has been synthesized and screened automatically leading to faster drug discovery. Combinatorial methods were also introduced to materials research during the last decade. Combinatorial and parallel synthesis in combination with fast screening techniques have been applied to the field of catalysis,[4,5] to the preparation of luminescent materials[6,7] and, very recently, to selected areas of polymer research.[8,9] Polymer chemistry is very well suitable for combinatorial approaches due to the broad range of parameters that can be varied systematically, e.g., monomers, initiators, catalysts, controlled or not controlled conditions, targeted molecular weights, and block, graft or star-like structures. The area of living and controlled polymerization techniques is of high interest in order to produce polymers with well-defined structures, which can be further utilized to obtain nanomaterials, such as micelles[10,11] or novel ordered architectures.[8,12,13] In addition, functional groups can easily be introduced via (protected) funcMacromol. Rapid Commun. 2003, 24, No. 1

tional initiators or end-capping with suitable termination agents. To apply combinatorial and high-throughput chemistry to a controlled living polymerization technique (for nitroxide-mediated polymerizations, cf. ref.[8]), the cationic ring-opening polymerization of 2-ethyl-2-oxazoline was chosen (Scheme 1). Polymerization is initiated by a strong electrophile that attacks the endocyclic nitrogen atom of 2-ethyl-oxazoline to form an oxazolinium ring.[14–16] The C–O bond in this oxazolinium ring is weakened and propagation occurs by nucleophilic attack of the next monomer on this carbon atom. Polymerization can be terminated by the addition of a strong nucleophile. The living polymerization of 2-substituted 2-oxazolines provides a broad range of possibilities in order to design functional polymers by variation of initiator, end-capper and monomer(s). Depending on the utilized monomers, also amphiphilic polymers can be obtained.[17–19] The synthesis of polymers with different lengths and, therefore, different molar masses can be performed in order to study the living character of the polymerization process and to prepare telechelics and block copolymers for various applications.[16,20] These numerous variation parameters make the system well suited for an

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Living Cationic Polymerizations Utilizing an Automated Synthesizer: . . .

Scheme 1. Reaction mechanism of the living cationic ringopening polymerization of 2-ethyl-2-oxazoline.

automated and parallel approach. In this contribution we report the cationic ring-opening polymerization of 2-ethyl2-oxazoline utilizing an automated synthesizer, whereby the living character and the reproducibility of the automated polymerization was investigated in detail. For this purpose, a well-known model system with benzyl bromide as an initiator and piperidine as a terminating agent was used, and the ratio of monomer to initiator was varied.

Experimental Part Materials Solvents were purchased from Biosolve Ltd. Acetonitrile was dried over molecular sieves (3 A˚) and diethyl ether was used without further purification. 2-Ethyl-2-oxazoline (Aldrich Chemical Co.), benzyl bromide (Acros Organics) and piperidine (Merck-Schuchardt) were distilled over P2O5 and stored under argon. Instruments Reactions were carried out on a Chemspeed ASW2000 automated synthesizer. Two different polymerization setups were used: (i) Eight parallel reactions utilizing one reactor block with 16 reactor vessels of 13 mL each for the polymerizations and two reactor blocks containing 4 reaction vessels of 75 mL each for the precipitation step. (ii) 40 parallel reactions utilizing five reactor blocks with 16 reaction vessels of 13 mL each (40 polymerizations and 40 precipitation vessels). All reaction vessels were equipped with heating jacket and cold-finger reflux condenser. The ASW2000 was connected to a Huber Unistat 390W cryostat. 1H NMR spectra were recorded on a Varian AM-400 spectrometer or a Varian Gemini 300 spectrometer. MALDI-TOF-MS was performed on a Voyager-DETM PRO BiospectrometryTM workstation (Applied Biosystems) in linear operation mode. All spectra were obtained in positive ion mode and ionization was performed with a 337 nm pulsed nitrogen laser. All data was processed using the Data ExplorerTM software package (Applied Biosystems). Gel permeation chromatography (GPC) was measured online on a Shimadzu system with an SCL-10A

system controller, LC-10AD pump, RID-6A refractive index detector and a PLgel 5 mm Mixed-D column. 1% triethylamine in THF was used as an eluent at a flow rate of 1 mL/min and molecular weights were calculated against poly(methyl methacrylate) standards. Optimized GPC traces were obtained on a Waters system with a 1515 pump, a 2414 refractive index detector and a Waters Styragel HT4 column utilizing 4% triethylamine in chloroform as eluent (flow rate of 2 mL/min) with the column oven set to 40 8C. The molecular weights were determined against polystyrene standards. GC measurements were performed on an Interscience Trace GC with a Trace Column RTX-5 connected to a PAL autosampler.

8 Parallel Polymerizations (500 mg Scale) Reaction vessels were heated to 120 8C, evaporated in vacuo for 15 min and subsequently filled with argon. This procedure was repeated four times to perform the reactions under an inert atmosphere (1.1 bar argon pressure in the reaction blocks and 1.5 bar argon pressure flushing through the glovebox of the automated synthesizer). Solutions of 2-ethyl-2-oxazoline (500 mg, 5.03 mmol) in 3 mL acetonitrile and solutions of benzyl bromide (varying amounts) in 1 mL acetonitrile were transferred into the 13 mL reaction vessels, while vortexing at 600 rpm. The mixtures were heated to 80 8C and vortexed at 600 rpm for 24 h with the reflux condensers set at 5 8C. Subsequently, solutions of piperidine (5 equivalents with respect to initiator) in 1 mL acetonitrile were added to terminate the reactions and the mixtures were vortexed (600 rpm) another 4 h at 80 8C under reflux. Solvents were removed at 40 8C under reduced pressure (10 mbar) and, subsequently, 2.0 mL dichloromethane were dispensed into the reaction vessels at room temperature. To completely dissolve the polymers, the reactors were vortexed (600 rpm) for 10 min. These polymer solutions were then precipitated by adding them dropwise to 50 mL diethyl ether (Et2O), which was dispensed in 75 mL reaction vessels. For this precipitation the reaction vessels were cooled to 20 8C and the vortex speed was set at 400 rpm. After vortexing for 30 min at 20 8C, the vortex was switched off. Ten minutes later (after sedimentation of the polymers), 40 mL Et2O were removed from each vessel (needle) and the remaining Et2O was evaporated under reduced pressure (40 8C, 10 mbar, 400 rpm). The polymers were washed two times by adding Et2O (50 mL) into each reaction vessels and removing it again utilizing the needle (45 mL) and by evaporation. The polymers were dissolved in 3.5 mL dichloromethane by vortexing (400 rpm) the solvent for 10 min in the reaction vessels and these solutions were transferred into 8 mL vials. The vials were taken out of the automated synthesizer and the solvent was evaporated under a stream of air and finally the polyoxazolines were dried in a vacuum oven (40 8C, 102 mbar) to yield white solids. Poly(2-ethyl-2-oxazoline) with [M]/[I] ¼ 20: 1 H NMR (400 MHz, CD2Cl2): d ¼ 7.35–7.1 (m, Ar, 7H), 4.42 (m, Ar–CH2, 2H), 3.40 (br, N–CH2–CH2–N, 87H), 3.09 (t, CH2–N(CH2)2, 4H), 2.30 (br, N–CO–CH2, 50H), 1.82 (quintet, CH2–N(CH2–CH2)2, 4H), 1.65 (quintet, N–CH2–CH2–CH2, 2H), 1.03 (br, CH3, 65H).

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40 Parallel Polymerizations (150 mg Scale) The same procedure as for the 500 mg scale polymerization was utilized. Solutions of 2-ethyl-2-oxazoline (150 mg, 1.51 mmol) in 1 mL acetonitrile and solutions of benzyl bromide (varying amounts) in 1 mL acetonitrile were transferred to the 13 mL reaction vessels while vortexing (600 rpm). After 24 h reaction time (80 8C, 600 rpm), the polymerizations were terminated by the addition of piperidine (5 equivalents with respect to initiator) in 1 mL acetonitrile and stirred for another 4 h at 80 8C. Subsequently, the solvent was evaporated (40 8C, 10 mbar) and the polymers were dissolved by adding dichloromethane (700 mL) and stirring the vessels for 10 min. The dissolved polymers were precipitated by dropwise addition into 8.8 mL diethyl ether (13 mL reaction vessels, 20 8C, 600 rpm). After 30 min of vortexing (600 rpm) and 10 min waiting without vortexing, 7.0 mL Et2O were removed with a needle and the remaining solvent was evaporated (40 8C, 10 mbar, 600 rpm). The polymers were washed two times with 8.8 mL Et2O. The poly(2-ethyl-2-oxazoline)s were dissolved by the addition of dichloromethane (2.0 mL), and for the following steps the same procedure was utilized as for the 500 mg experiments.

Results and Discussion Living cationic polymerizations of 2-ethyl-2-oxazoline were performed on a Chemspeed ASW 2000 automated synthesizer (Figure 1a–c) in order to investigate the living

Figure 1. (a) Chemspeed ASW2000 automated synthesizer, (b) close-up of the 80 parallel reactors of ASW2000, and (c) close-up of the precipitation of synthesized polymers.

character and the reproducibility of the polymerizations in the robot system. This commercially available automated synthesizer was equipped with glass reactors (in our case: 13 mL and 75 mL) and cold-finger reflux condensers. Furthermore, vacuum could be applied to the reactors (down to 1 mbar). For an inert atmosphere, in both the reactors and the glovebox on top of the robotic system argon overpressure was applied. Stirring of the reactions was performed by vortexing the reactor blocks. As already discussed in the introduction, the living cationic polymerization of 2-substituted 2-oxazoline is well suited for a parallel approach due to the multiple parameters that can be varied.[14–16] However, before screening different parameters, the reproducibility and living character of the polymerizations in the automated synthesizer had to be investigated in detail. Therefore, we studied a model polymerization of 2-ethyl-2oxazoline initiated with benzyl bromide and piperidine as a terminating agent, whereby the molecular weights of the polymers were varied from 1 000 to 10 000 Da. Eight parallel polymerizations at a 500 mg scale as well as 40 parallel polymerizations at a 150 mg scale were performed successfully, including automated precipitation of the synthesized polymers in diethyl ether (Figure 1c) and automated transfer of the polymers into vials. The resulting poly(2-ethyl-2-oxazoline)s were obtained in yields ranging from 55% to 95% (Figure 2). Up to a monomer/initiator ratio of 80 the yields were comparable to ‘‘traditionally’’ synthesized polymers with a monomer/initiator ratio of 50. Nevertheless, the yields of the automated polymerizations were decreasing towards higher molecular weights. GC measurements after 24 h reaction time revealed complete conversion for polymerizations with monomer/initiator

Figure 2. Yields of both the 8 and 40 poly(2-ethyl-2-oxazoline)s synthesized in parallel compared to yields of ‘‘classically’’ synthesized polymers with [M]/[I] ¼ 50 (dotted lines).


ratios of both 50 and 100, thus showing that the lower yields are not due to insufficient reaction times. The gradual decrease in yield towards higher-molecular-weight polymers is probably due to the loss of material during precipitation and transfer procedures since the vessels were not rinsed twice as in the normal lab procedure. Figure 2 reveals that two out of the 48 poly(2-ethyl-2-oxazoline)s were obtained in low yields of around 30%. This discrepancy is most likely due to the loss of polymer during work-up, because further characterization by means of NMR spectroscopy, GPC and MALDI-TOF-MS revealed no irregularities. The poly(2-ethyl-2-oxazoline)s synthesized in parallel were characterized by means of 1H NMR spectroscopy in order to determine the number-average molecular weights (M n) by careful integration of the signals assigned to initiator protons (d ¼ 7.3 ppm) and backbone protons of the polymers (d ¼ 1.0 ppm, 2.3 ppm and 3.5 ppm). Since the resonances of the benzyl bromide protons overlapped with the signal of the standard solvent, deuterated chloroform, the spectra were recorded in deuterated dichloromethane. Plotting the M n values against monomer/initiator ([M]/[I]) ratios (Figure 3) illustrates the linear relationship between M n and [M]/[I]. The observed M n values are within ten percent of the calculated molecular weights of the polymers. These 1H NMR spectroscopic results indicate that the polymers are synthesized in a controlled (living) way. In order to further investigate the polymers, MALDI-TOF-MS was performed on the poly(2-ethyl-2-oxazoline)s synthesized in parallel. Mixing the polymers with the matrix and subsequent spotting on the MALDI target was carried out automatically with the synthesis robot.[21] In a first attempt, the polymer solutions in dichloromethane were mixed with dithranol. However, with this matrix the polymers could only be investigated up to M ns of 4 000 Da. To improve the ionization/desorption process, the polymers were mixed

with both dithranol and NaI before spotting. In this way the poly(2-ethyl-2-oxazoline)s could be characterized up to a molecular weight of 7 000 Da. The difference between the peaks was found to be 99 mass units, which corresponds to the mass of an 2-ethyl-2-oxazoline monomer unit. Endgroup analysis confirmed that the polymers were initiated with benzyl bromide and terminated with piperidine. Successful termination of the living polymer chains with piperidine also demonstrated the living character of the polymerizations performed in the automated synthesizer. Figure 4 shows the M n values determined by means of MALDI-TOF-MS measurements plotted against the [M]/ [I] ratios. Again, a linear relationship between M ns and [M]/ [I] ratios was obtained. Experimental values were within five percent of the calculated ones and the polydispersity indexes (PDIs) calculated from MALDI spectra are ranging from 1.03 to 1.14. All results obtained from MALDI-TOFMS experiments support a controlled polymerization mechanism for the polymerization of 2-ethyl-2-oxazoline in the automated synthesizer. Finally, online GPC measurements (15 min per run) were performed. Up to an [M]/[I] ratio of 40, the experimental molecular weights correspond with the theoretical values, and narrow polydispersities were obtained. However, higher-molecular-weight polymers revealed significant tailing of the GPC signals, which is caused by interactions of the nitrogen atoms in the backbone with the column material (see also ref.[22]). Several attempts were performed to eliminate these interactions utilizing offline GPC and changing the eluent (THF or chloroform with or without triethylamine), temperature of the column oven, and flow speed. Best results were obtained with chloroform containing 4% of triethylamine, the column oven set to 40 8C and a flow speed of 2 mL/min. However, optimizing GPC conditions could reduce the tailing but not completely

Figure 3. M n, determined by means of 1H NMR spectroscopy, plotted against [M]/[I] ratio, together with the calculated molecular weights of the poly(2-ethyl-2-oxazoline)s.

Figure 4. M n and PDIs obtained by means of MALDI-TOF-MS plotted against [M]/[I] ratio.

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These results clearly demonstrate the feasibility to polymerize 2-ethyl-2-oxazoline in a (reproducible) living manner utilizing an automated synthesizer. Therefore, an acceleration of possible polymerizations by a factor of approximately 20 compared to ‘‘conventional’’ polymerization reactions done by a single person can be reached easily. In future research, the automated synthesizer will be utilized to polymerize different oxazoline monomers using functional initiators and functional end-cappers. High reproducibility of the parallel reactions in combination with the large number of possible experiments will allow a detailed investigation and understanding of the polymerization processes. Figure 5. M w and PDIs obtained by means of GPC for the poly(2-ethyl-2-oxazoline)s synthesized in parallel.

eliminate it. The Mw values obtained under these optimized GPC conditions increased linearly with [M]/[I] and were close to the calculated values (Figure 5), but the PDIs increased from 1.11 (1 000 Da) to 1.37 (10 000 Da), which is much higher than the values obtained by means of MALDI-TOF-MS. These high PDIs are probably due to the remaining interactions between poly(2-ethyl-2-oxazoline)s and the column material. GPC data also suggest a controlled (living) polymerization of 2-ethyl-2-oxazoline due to the linear increase in M w with [M]/[I] and the rather low polydispersities (< 1.27 for the 6 000 Da polymer).

Conclusions Poly(2-ethyl-2-oxazoline)s were successfully synthesized and purified utilizing an automated synthesizer. The yields obtained such were comparable with those obtained in ‘classical’ polymerizations in the laboratory up to a molecular weight of 8 000 Da. The yields however gradually decreased towards higher molecular weights, and the yields of poly(2-ethyl-2-oxazoline)s with a molecular weight of 10 000 Da were lower than those of ‘classical’ polymerizations. This is probably due to loss of polymer during precipitation and transfer. 1H NMR spectroscopy and MALDI-TOF-MS revealed number-average molecular weights of the obtained polymers, which are increasing linearly with increasing [M]/[I] ratios (within ten percent of the theoretical values). Polydispersities obtained from MALDI-TOF-MS were ranging form 1.02 to 1.14. Endgroup analysis with MALDI-TOF-MS revealed successful end-capping of the poly(2-ethyl-2-oxazoline)s with piperidine, proving the living character of the polymerizations. Optimized GPC characterization revealed a linear increase of M w with the [M]/[I] ratio. Rather high PDIs (ranging from 1.11 to 1.37) were obtained, but nevertheless, GPC results suggest a controlled polymerization.

Acknowledgement: The authors would like to thank NWO, DPI and the Fonds der Chemischen Industrie for financial support and Chemspeed Ltd. for collaboration.

Received: September 13, 2002 Revised: November 12, 2002 Accepted: November 15, 2002

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