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Microwave-Assisted Polymer Synthesis: Recent Developments in ... controlled radical polymerization techniques (1.18), were obtained with microwave irradiation. Yi et al. reported the surfactant-free emulsion copolymerization of styrene and N-isopropylacrylamide using microwave heating. [107,108] The resulting polymer particles were smaller and more homogeneous compared to particles made with thermal heating. It was demonstrated that these particles were thermoresponsive and their size decreased when the temperature was increased. Similarly, Zhang et al. synthesized thermosensitive poly(methyl methacrylate) coated poly(N-isopropylacrylamide) particles using a microwaveassisted two-step emulsion polymerization. [109] The last reported example of a water-based free radical polymerization under microwave irradiation is the dispersion polymerization of styrene using poly(N-vinylpyrrolidone) as stabilizer and azoisobutyronitrile as initiator was reported by Xu et al. [110] The polystyrene particles obtained with microwave irradiation were smaller, more uniform and more stable than those prepared with thermal heating as illustrated by Figure 7. Controlled Radical Polymerizations In contrast to free radical polymerizations, the occurrence of radical termination reactions is suppressed in controlled radical polymerization techniques. The suppression of termination reactions is achieved by lowering the concentration of free radicals by a reversible radical termination with metals [atom-transfer radical polymerization (ATRP)] or nitroxides [nitroxide-mediated radical polymerization (NMP)], or by a reversible chain transfer mechanism [reversible addition fragmentation chain-transfer polymerization (RAFT)]. Zhu and coworkers investigated the ATRP of styrene under pulsed microwave irradiation. [111] Controlled polymerization of styrene with pulsed microwave irradiation revealed polymerization rates that were three times larger than those for thermal heating, suggesting that the controlled nature of the polymerization was not affected as was demonstrated by 1 H NMR spectroscopy and chain extension experiments. Similarly, Hou et al. reported that the iron(II)-catalyzed ATRP of acrylonitrile could be accelerated under microwave irradiation. The Ru(II)-catalyzed ATRP of MMA was also enhanced by the use of microwave irradiation (monomode microwave reactor). [112] Demonceau and coworkers demonstrated acceleration of the methyl methacrylate polymerization under microwave irradiation at temperatures in the range of 100–140 8C and a deceleration at temperatures above 150 8C. In addition, the molecular weight distributions were found to be broader when using microwave irradiation regardless of the polymerization temperature. Further studies to elucidate the underlying principles of these investigations are in progress. In contrast to the previously described accelerations of ATRP reactions, Zhang and Schubert reported that the ATRP of methyl methacrylate with both microwave (monomode microwave reactor) and thermal heating revealed comparable polymerization rates. [113] In addition, increasing the polymerization temperature under microwave irradiation led to uncontrolled polymerizations due to higher radical concentrations. Wu et al. used microwave irradiation for the further end-functionalization of telechelic bromopolystyrene with C 60 fullerenes. [114] The use of microwave irradiation significantly increased the rate of fullerene coupling without changing the structure and properties of the resulting polymers. The NMP of methyl acrylate and tert-butyl acrylate in dioxane was investigated by Schubert and coworkers in a monomode microwave reactor. [115] Comparative studies with both microwave heating and thermal heating revealed similar polymerization kinetics. Nevertheless, at high monomer conversion above 90% the microwave-assisted polymerization procedure resulted in well-defined polymers while the thermal polymerization process showed increasing molecular weight distributions. This observation was attributed to Figure 7. Transmission electron microscope images of polystyrene particles obtained under similar conditions with microwave (left) and thermal (right) heating (reprinted with permission from ref. [110] ). Macromol. Rapid Commun. 2007, 28, 368–386 ß 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.mrc-journal.de 383
R. Hoogenboom, U. S. Schubert the more homogeneous heating under microwave irradiation that becomes more apparent at high conversions due to the increased viscosity of the polymerization mixture. In contrast, Zhu et al. observed a clear acceleration of the NMP of styrene in bulk under microwave irradiation (monomode microwave reactor). [116] The rate of polymerization increased with increasing microwave power while the control over the polymerization was maintained. The microwave-assisted RAFT polymerization of both styrene and methyl methacrylate was investigated by Perrier and coworkers. [117] For both monomers an increase in polymerization rate was observed when microwave irradiation was applied as heat source. Moreover, linear first-order kinetics were observed and a linear increase in molecular weight with conversion demonstrating a good control over the polymerizations under microwave irradiation was also observed. Zhu et al. reported similar observations for the RAFT polymerization of styrene using a domestic microwave oven. [118] microwave irradiation that were observed in recent years. In addition, several open questions such as specific microwave absorption in copolymerizations as well as upscaling issues have to be addressed in the coming years. Nevertheless, it is expected that the use of microwave irradiation will evolve from a research topic into a common research tool in polymer science. Whether microwave-assisted polymerization procedures will be incorporated into future commercial processes is still uncertain and would only be feasible for processes that show a clear (economical) advantage over thermal heating. Acknowledgements: This study was supported by the Dutch Polymer Institute (DPI) and the Fonds der Chemischen Industrie. Received: October 30, 2006; Accepted: December 8, 2006; DOI: 10.1002/marc.200600749 Keywords: microwave irradiation; polymer; radical polymerization; ring-opening polymerization; step-growth polymerization Summary and Outlook The use of microwave irradiation in polymer chemistry is a rapidly expanding field of research. In the last two years, the number of publications per year has doubled. The main reason for this increased interest in the use of microwave irradiation is the often observed acceleration of polymerizations when performed with microwave irradiation. Nevertheless, in many cases the acceleration is due to increased polymerization temperatures compared to thermal heating, whereby it should be noted that these increased temperatures are much easier accessible when using microwave irradiation. In addition to the increased temperatures, many of the observed advantages of microwave heating can be ascribed to the fast and homogeneous heating that can prevent thermal decomposition and/or other side reactions caused by local overheating over the polymerization mixtures with thermal heating. Besides these improvements that can be ascribed to thermal effects under microwave irradiation, several examples have been reported in which non-thermal microwave effects were observed. These non-thermal microwave effects can be rationalized by specific microwave absorption of polar intermediates and or reagents making them more reactive. However, the observation of such effects seems to depend on the choice of microwave reactors (domestic, monomode, or multimode microwaves) as well as the choice of polymerization conditions. In addition, attempts to change monomer reactivity and reactivity ratios in copolymerizations of monomers with different polarities have not succeeded so far. The field of microwave-assisted polymerizations is believed to continue its rapid expansion in the coming years inspired by the large number of beneficial effects of [1] B. L. Hayes, ‘‘Microwave Synthesis: Chemistry at the Speed of Light’’, CEM Publishing, Matthews 2002. [2] C. O. Kappe, Angew. Chem. Int. Ed. 2004, 43, 6250. [3] J. P. Tierney, P. Lidstrom, ‘‘Microwave Assisted Organic Chemistry’’, Taylor & Francis Group, Abingdon 2004. [4] C. O. Kappe, A. Stadler, ‘‘Microwaves in Organic and Medicinal Chemistry’’, Wiley VCH, Weinheim 2005. [5] C. Gabriel, S. Gabriel, E. H. Grant, B. S. J. Halstead, D. M. P. Mingos, Chem. Soc. Rev. 1998, 27, 213. [6] A. d. l. Hoz, A. Diaz-Ortiz, A. Moreno, Chem. Soc. Rev. 2005,34, 164. [7] F. Wiesbrock, R. Hoogenboom, U. S. Schubert, Macromol. Rapid Commun. 2004, 25, 1739. [8] Results of a search for ‘‘Microwave and Polymerization’’ in the SciFinder database on October 23, 2006. [9] J. Pielichowski, E. Dziki, J. Polaczek, Polym. J. Chem. Technol. 2003, 5, 3. [10] K. Faghihi, M. Hagibeygi, Eur. Polym. J. 2003, 39, 2307. [11] A. A. Caouthar, A. Loupy, M. Bortolussi, J.-C. Blais, L. Dubreucq, A. Meddour, J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 6480. [12] N. Li, J. Lu, S. Yao, Macromol. Chem. Phys. 2005, 206, 559. [13] H. Yeganeh, B. Tamami, I. Ghazi, Eur. Polym. J. 2004,40, 2059. [14] K. Faghihi, Chin. J. Polym. Sci. 2004, 22, 343. [15] K. Faghihi, K. Zamani, A. Mirsamie, S. Mallakpour, Polym. Int. 2004, 53, 1226. [16] K. Faghihi, A. Mirsamie, Chin. J. Polym. Sci. 2005, 23, 63. [17] K. Faghihi, M. Hajibeygi, J. Appl. Polym. Sci. 2004, 92, 3447. [18] S. Mallakpour, E. Kowsari, J. Appl. Polym. Sci. 2005, 96, 435. [19] S. Mallakpour, E. Kowsari, Polym. Eng. Sci. 2006, 46, 558. [20] S. Mallakpour, E. Kowsari, Iran. Polym. J. 2005, 14, 799. [21] S. Mallakpour, M. H. Shahmohammadi, Iran. Polym. J. 2005, 14, 473. [22] S. Mallakpour, E. Kowsari, Polym. Adv. Technol. 2005, 16, 466. [23] S. Mallakpour, E. Kowsari, Iran. Polym. J. 2006, 15, 239. 384 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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