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Microwave-Assisted Polymer Synthesis: Recent Developments in ... monomer based on unsaturated soy bean fatty acids was polymerized successfully under microwave irradiation, whereby the unsaturated sites were not affected by the polymerization of the 2-oxazoline ring. [76] The resulting polymers with unsaturated side-chains could be cross-linked by UV-irradiation. This soy-based 2-oxazoline monomer was also used for the microwaveassisted two-step synthesis of amphiphilic poly(2-ethyl- 2-oxazoline)-block-poly(2-soy-alkyl-2-oxazoline) block copolymers that were successfully applied for the preparation of cross-linked micelles. [77,78] Radical Polymerizations Scheme 7. Schematic representation of the different selectivity in the synthesis of (meth)acrylamides that was found by Ritter and coworkers using microwave and thermal heating. [79,80] Radical polymerization techniques are widely used in both industry and academia because of the broad range of possible monomers and the relatively simple polymerization procedures. Moreover, radical polymerizations are compatible with water and therefore emulsion and dispersion polymerization procedures that exclude the use of organic solvents can be applied. This section will discuss the recent developments in microwave-assisted free radical polymerizations, free radical polymerizations in emulsion as well as controlled radical polymerizations. The section on free radical polymerization will specifically address the issue of copolymerizing different monomers under microwave irradiation since specific microwave absorption may lead to changes in monomer reactivity or in transition states and thus in the reactivity ratios. Free-Radical Polymerizations The free-radical polymerization of vinylic monomers is one of the major processes for the industrial production of bulk polymers like polystyrene and poly(methyl methacrylate). The previous review on microwave-assisted polymerizations covered a wide range of investigations on the effect of microwave irradiation on free radical homopolymerizations. [7] However, in recent years only a few of such investigations were reported and the major focus has shifted to copolymerizations. Nevertheless, Ritter and coworkers have investigated the microwave-assisted direct synthesis and polymerization of a series of chiral (meth)acrylamides. [79,80] The direct synthesis of chiral meth(acrylamide) from (meth)acrylic acid and 1-phenylethylamine under microwave irradiation yielded the desired vinyl monomers, whereas the same reaction under thermal heating resulted mainly in the formation of the Michael addition product (Scheme 7). The preferred formation of the desired (meth)acrylamides under microwave irradiation could be rationalized by the (zwitter) ionic intermediates that lead to the formation of the meth(acrylamide)s. The direct synthesis of (meth)acrylamides under microwave irradiation represents a major improvement compared to the conventional methods that make use of acid chloride reagents and/or coupling agents. In addition, it was demonstrated that the synthesis and polymerization of 1-phenylethyl (meth)acrylamide can be performed simultaneously in a one-pot reaction under microwave irradiation. Similarly, Bezdushna and Ritter reported a microwave acceleration for the direct synthesis of N-phenylmaleimide from maleic anhydride and aniline based on specific microwave absorption of the ionic intermediates. [81] Moreover, the synthesis of N-(2-ethoxyethyl)maleimide from its corresponding maleic acid in acetic anhydride as reactive solvent was also investigated under microwave irradiation. [82] Although this reaction also proceeds via ionic intermediates, no acceleration was observed due to specific microwave absorption in this case. Fischer et al. investigated the free radical polymerization of N-alkylacrylamides with 3-mercaptopropionic acid as chain transfer agent in methanol with thermal heating at ambient pressure and under superheated conditions as well as under microwave irradiation. [83] The chain transfer polymerization could be accelerated from 5 to 1 h when going to superheated conditions with thermal heating. When changing the heat source to microwave irradiation, the polymerization was further accelerated down to several seconds. However, the microwave-assisted polymerizations were performed without solvent in a domestic microwave oven under power control. Therefore, it is not clear whether the acceleration is due to thermal effects or not. Nevertheless, the accelerated microwave polymerization procedure was incorporated into university education allowing synthesis, isolation and characterization of the polymers within a one-day laboratory session. [84] The copolymerization of monomers with significantly different microwave absorption characteristics is believed Macromol. Rapid Commun. 2007, 28, 368–386 ß 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.mrc-journal.de 379
R. Hoogenboom, U. S. Schubert to result in specific microwave effects resulting in changes in the reactivity ratios. Fellows has tried to address these speculated effects for the free radical copolymerizations of methyl methacrylate and styrene as well as butyl methacrylate with styrene or isoprene in toluene under microwave irradiation (monomode microwave reactor). [85] However, no changes in reactivity ratios were observed although more detailed studies were required for the copolymerization of butyl methacrylate and isoprene. The microwave-assisted polymerization procedure did accelerate the polymerizations by a factor of 1.7, which could be ascribed to an increase in radical flux. It was proposed that the increased radical flux under microwave irradiation is due to rapid orientation of the radicals that are formed from decomposition of the azoisobutyronitrile as depicted in Scheme 8. This orientation would reduce the number of direct terminations by recombination of the two radical fragments under microwave irradiation and thus cause a higher radical flux. In similar investigations, Greiner and coworkers investigated the free radical copolymerization of methyl methacrylate and styrene with different initiators in different solvents using both microwave (monomode microwave reactor) and thermal heating. [86] In contrast to the findings of Fellows, [85] the polymerizations in toluene revealed very similar polymerization rates for both heating methods, whereas the polymerizations in DMF were all accelerated under microwave irradiation. Nevertheless, regardless of the used solvent and initiator the reactivity of both monomers were not affected by the use of microwave irradiation. Agarwal et al. studied the copolymerization of 2,3,4,5,6- pentafluorostyrene and N-phenylmaleimide. [87] Comparison of microwave heating and thermal heating for this copolymerization revealed a higher initial polymerization rate and a lower final monomer conversion for the microwave- assisted procedure. The authors speculated that the lower final monomer conversion under microwave irradiation might be ascribed to an increased amount of diffusion-controlled termination reactions although no direct association was made with microwaves. The synthesized copolymers of 2,3,4,5,6-pentafluorostyrene and N-phenylmaleimide exhibited both high glass transition temperature as well as high hydrophobicity. The copolymerization of N,N-dimethylaminoethyl methacrylate with allylthiourea was performed under microwave irradiation (domestic microwave oven) by Lu et al., whereby both the influence of reaction time and microwave power on the copolymerization were studied. [88] Subsequently, copper was coordinated to this polymer by microwave irradiation of a solution of the copolymer with blue vitriod. This polymer-copper system was successfully applied as an heterogeneous catalyst for the polymerization of methyl methacrylate. Besides the free radical copolymerization of different monomers, several studies were reported in which vinylic polymers were grafted onto natural polymers under microwave irradiation using domestic microwave ovens. Sanghi and coworkers reported grafting of acrylonitrile [89] and acrylamide [90] onto guar gum under both thermal heating and microwave irradiation. Grafting with thermal heating was performed at 35 8C in the presence of redox initiating systems (potassium persulfate and ascorbic acid). Under microwave irradiation, grafting could be achieved in the absence of this initiating system at 97 8C, whereas control experiments with thermal heating at 100 8C without initiator did not show any grafting, indicating the presence of non-thermal microwave effects. Grafting of acrylamide onto the guar gum under microwave irradiation in the presence of the redox initiating system resulted in higher grafting efficiency. The same group also reported grafting of acrylonitrile, [91] acrylamide, [92] and methyl methacrylate [93] onto chitosan using microwave heating. Similar to the grafting on guar gum, it was found that radically grafting onto the chitosan could be achieved without any redox initiating system when applying microwave irradiation. Grafting of both acrylamide and methyl methacrylate was demonstrated to improve the solubility of the chitosan at neutral pH. Moreover, these grafted copolymers showed increased zinc(II) binding (methyl methacrylate and acrylamide) and/or calcium(II) binding (acrylamide) making them suitable candidates for Scheme 8. Schematic representation of the direct orientation of radicals that are formed from the decomposition of azoisobutyronitrile under microwave irradiation that was proposed to explain the higher radical flux observed for microwave-assisted polymerizations compared to thermal polymerizations. [85] 380 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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