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Microwave-Assisted Polymer Synthesis: Recent Developments in ... reaction mixture, whereby the use of good microwave absorbing solvents results in very fast heating. [5] Besides the advantages of fast and homogeneous heating as well as the possible high-temperature chemistry, non-thermal microwave effects due to specific heating of polar intermediates have been observed, e.g., leading to modified Richard Hoogenboom was born in 1978 in Rotterdam (The Netherlands). In 2001 he obtained his M.Sc. degree in chemical engineering at the Eindhoven University of Technology, whereby his undergraduate research was performed in the group of Bert Meijer (Eindhoven, The Netherlands). During the studies, he performed an internship within the group of Andrew Holmes (Cambridge, United Kingdom). In 2005, he obtained his Ph.D. under supervision of Ulrich S. Schubert (Eindhoven, The Netherlands) focusing on supramolecular initiators for controlled polymerization techniques, automated parallel synthesis of well-defined polymers and microwave irradiation in polymer chemistry. Currently, he is working as project leader for the Dutch Polymer Institute (DPI) with a major focus on the use of high-throughput experimentation and microwave irradiation for living/controlled polymerization techniques. Ulrich S. Schubert was born in Tübingen in 1969. He studied chemistry at the Universities of Frankfurt and Bayreuth (both Germany) and the Virginia Commonwealth University, Richmond (USA). His Ph.D. 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 Université Strasbourg (France) he moved to the Technische Universität München (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 Universität München (Germany). Since Summer 2000 he is Full- Professor at the Eindhoven University of Technology (Chair for Macromolecular Chemistry and Nanoscience). From 2003 on he is member of the management team of the Dutch Polymer Institute. His awards include the Bayerischen Habilitations-Förderpreis, the Habilitandenpreis of the GDCh (Makromolekulare Chemie), the Heisenberg-Stipendium of the DFG, the Dozenten- Stipendium of the Fonds der Chemischen Industrie and a VICI award of NWO. The major focus of the research interest of his relates to organic heterocyclic chemistry, supramolecular materials, combinatorial material research, nanoscience and tailor-made macromolecules. selectivities and enabling reactions that cannot be performed with thermal heating. [6] These non-thermal microwave effects are thought to arise from specific microwave absorption by polar components of a reaction making them more reactive under microwave irradiation when compared to thermal heating. The use of microwave irradiation in polymer chemistry is an emerging field of research that we reviewed in 2004. [7] Up to that moment, many investigations of microwave-assisted polymerizations were conducted in domestic microwave ovens making the reproducibility and safety of the experiments doubtful due to insufficient temperature control. However, the development of commercial microwave synthesizers with excellent temperature control significantly improved the reliability of the reported microwave-assisted polymerizations. As a result, the number of publications on microwave-assisted polymerizations per year has shown a rapid expansion (Figure 1). [8] In fact, a similar figure in the previous review showed maximum 90 publications per year, which almost doubled in the last two years. In the current review, the progress in the field of microwave-assisted polymer synthesis since the previous review will be discussed with a main focus on step-growth polymerizations, ring-opening polymerization, and (controlled) radical polymerizations. Special attention will be given to the occurrence of non-thermal microwave effects, which is still a controversial topic. Step-Growth Polymerization Step-growth polymerizations are based on the coupling of two multifunctional, mostly bifunctional, monomers. The resulting coupled product also contains the functional groups and thus reacts in the same manner as the Figure 1. Number of publications on microwave-assisted polymerizations per year. [8] Macromol. Rapid Commun. 2007, 28, 368–386 ß 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.mrc-journal.de 369
R. Hoogenboom, U. S. Schubert monomer eventually leading to polymeric materials. The most studied step-growth polymerization methods are better known as polycondensations due to the release of water during the coupling reactions. In this section, microwave-assisted step-growth polymerizations are discussed starting with polyamides, polyimides, and poly(amide-imide)s, which are the most studied class of step-growth polymerizations under microwave irradiation. Nevertheless, polymerization via C–C coupling reactions resulting in conjugated polymers is becoming more popular under microwave irradiation and will be discussed as well. The final part of this section deals with other step-growth polymerization reactions including the synthesis of polyethers, polyesters, and polyurethanes. Polyamides, Polyimides, and Poly(amide-imide)s Linear aromatic polyamides, polyimides, and poly(amideimide)s exhibit excellent thermal, mechanical, and chemical stabilities. As a result, these materials are often used in high-performance applications. However, the rigid structure of these materials makes them hardly soluble in organic solvents and, therefore, the use of hightemperature microwave-assisted polymerization procedures might be advantageous. The use of microwave irradiation for the synthesis of poly(aspartic acid) starting from maleic anhydride was investigated by Pielichowski et al. [9] This multistep procedure includes hydrolysis of the maleic anhydride, condensation with ammonium hydroxide followed by polycondensation resulting in poly(anhydroaspartic acid). Subsequent room temperature hydrolysis yielded the desired poly(aspartic acid). It was claimed that the use of microwave irradiation (multimode microwave reactor) accelerated the process by a factor of ten without influencing the yield. Faghihi and Hagibeygi have used a microwave-assisted polymerization method (domestic microwave) for the synthesis of polyamides containing azo-benzene moieties. [10] 4,4 0 -Azobenzoyl chloride was reacted with eight different 5,5 0 -disubstituted hydantoin derivatives as depicted in Scheme 1(a). The hydantoin moieties improved the solubility of the resulting polyamides while the high thermal stability of the material was retained. The authors claimed higher yields and efficiencies for the microwave-assisted polymerization procedure in o-cresol compared to a standard polymerization method with conventional heating in N,Ndimethylacetamide (DMAc) or bulk. In a similar work, Loupy et al. reported the microwave-assisted synthesis (monomode microwave reactor) of chiral polyamides by the step-growth polymerization of diphenylaminoisosorbide with several diacyl chlorides as depicted in Scheme 1(b). [11] Microwave-assisted polymerizations in the presence of N-methylpyrrolidone led to faster polymerizations and higher molecular weight products when compared to standard polymerization methods (however, no direct comparison between microwave and thermal heating was reported). The N-methylpyrrolidone in the microwave polymerization procedure was required to induce effective homogeneous heating of the monomers and the formed polymers. Lu and coworkers used microwave irradiation for the step-growth polymerization of Scheme 1. Polyamides (a and b) and polyimide (c) prepared via microwave-assisted step-growth polymerizations: (a) Polyamide synthesis from 4,4 0 -azobenzoyl chloride and substituted hydantoins; (b) preparation of polyamides from diphenylaminoisosorbide with several diacyl chloride; (c) polyimide synthesis from benzophenone tetracarboxylic dianhydride and p-phenylene diisocyanate. 370 Macromol. Rapid Commun. 2007, 28, 368–386 ß 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/marc.200600749
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