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Microwave-Assisted Polymer Synthesis: Recent Developments in ... benzoguanamine and pyromellitic anhydride resulting in the formation of the p-p conjugated poly(amic acid). [12] Although the polymer was synthesized under microwave irradiation, no comparison was made to thermal heating. The side chains of the resulting poly(amic acid) were further functionalized using both azo as well as isocyanate coupling procedures at moderate temperatures (no microwave irradiation) to study the influence on the fluorescence and non-linear optical properties of the materials. Direct synthesis of polyimides via the step-growth polymerization of isocyanates and anhydrides under microwave irradiation was reported by Yeganeh et al. [13] The polymerizations were performed in a closed Teflon mold inside a domestic microwave oven. The feasibility of this approach was first demonstrated by the model reaction of phthalic anhydride with p-phenylene diisocyanate followed by the model polymerization of benzophenone tetracarboxylic dianhydride and p-phenylene diisocyanate [Scheme 1(c)]. The effects of microwave power, solvent amount, reaction time, and catalyst were optimized for this model polymerization reaction. Subsequently, the optimal microwave-assisted polymerization procedure was applied for the synthesis of novel polyimides. Most of the recent investigations on microwave-assisted polycondensations have been performed on the synthesis of poly(amide-imides). The reported polymerizations were all performed using domestic microwave ovens. The monomers were placed and ground in a porcelain dish. After the addition of a small amount of o-cresol, the mixture was ground again followed by microwave heating. The microwave-assisted synthesis of a series of optically active poly(amide-imides), in which the chirality resulted from the incorporation of amino acids, was reported by Faghihi et al. [14–16] N,N 0 -(pyromellitoyl)-bis(amino acid chloride)s were reacted with eight hydantoin derivatives as depicted in Scheme 2(a). The polycondensations proceeded rapidly compared to the conventional polymerization method under thermal heating and was completed within 10 min. Nevertheless, the occurrence of non-thermal microwave effects was not investigated and no direct comparison between the different heat sources was made. To provide the optical activity, L-leucine, L-valine, or L-alanine were incorporated as amino acids. The resulting poly(amide-imide)s might, e.g., be suitable as column material for the separation of enantiomeric mixtures. Similarly, Faghihi and Hajibeygi reported the polycondensation reaction of N,N 0 -(4,4 0 - diphenyl ether) bistrimellitimide diacid chloride with the same hydantoin derivatives under microwave irradiation. [17] The increased solubility of the resulting poly(amide-imide)s could be the basis for a novel class of processable high-performance plastics. Mallakpour and Kowsari investigated the synthesis of optically active poly(amide-imide)s by the polycondensation of N,N 0 -(4, 4 0 -oxydiphthaloyl)-bis(amino acid chloride)s (amino acids used are L-leucine, L-valine, and L-isoleucine) and aromatic diamines under microwave irradiation [Scheme 2(b)]. [18–20] It was demonstrated that comparable polymers can be obtained under both microwave and thermal heating (different procedures), although shorter reaction times were required when using microwave irradiation. The resulting optically active polymers were readily soluble in organic solvents and exhibited good thermal stability. Similarly, Mallakpour and Shahmohammadi replaced the central N,N 0 -(4,4 0 -oxydiphthaloyl) group by Scheme 2. Poly(amide-imide)s that were synthesized under microwave irradiation: (a) Reaction of N,N 0 -(pyromellitoyl)-bis(amino acid chloride)s with hydantoin; (b) copolymerization of N,N 0 -(4,4 0 -oxydiphthaloyl)-bis(amino acid chloride)s with aromatic diamines; (c) synthesis of flame-retardant polymers based on N,N 0 -(3,3 0 -diphenylphenylphosphine oxide) bistrimellitimide and aromatic diamines. Macromol. Rapid Commun. 2007, 28, 368–386 ß 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.mrc-journal.de 371
R. Hoogenboom, U. S. Schubert N,N 0 -(pyromellitoyl), [21] N,N 0 -(4,4 0 -sulfone-diphthaloyl), [22] or N,N 0 -(4,4 0 -hexafluoroisopropylidenediphthaloyl) [23] groups resulting in novel classes of optically active poly(amideimide)s, whereby the latter synthesis was also performed in the presence of ionic liquids to improve the microwave absorption. More recently, Faghihi and Zamani reported the microwave-assisted synthesis of phosphine-containing poly(amide-imide)s that exhibit flame-retardant properties. [24] N,N 0 -(3,3 0 -diphenylphenyl phosphine oxide) bistrimellitimide diacid chloride and a variety of aromatic diamines were polymerized resulting in processable polymers with excellent thermal stability as well as flame retardancy [Scheme 2(c)]. In related research, Faghihi and coworkers prepared poly(amide-imide)s under microwave irradiation from N,N 0 -(4,4 0 -diphenyl ether) bis(trimellitimido) [25] or N,N 0 -(pyromellitoyl)-bis-L-alanine [26] diacid chlorides with a variety of tetrahydropyrimidinones and tetrahydro-2- thioxopyrimidines. All previously discussed examples of poly(amideimide)s were based on symmetrical monomer units. In contrast, Mallakpour and Kowsari reported the synthesis of non-symmetrical acid chlorides derived from epiclon and L-isoleucine, [27] L-methionine, [28] or L-valine. [29] These optically active acid chlorides were subsequently polymerized with different aromatic diamines under microwave irradiation as depicted in Scheme 3(a). The combination of both aromatic and aliphatic spacers in poly(amide-imide)s was investigated by Mallakpour and Rafiemanzelat. [30] An asymmetric optically active diacid based on N-trimellitylimido-L-valine was reacted with different (aliphatic) diisocyanates resulting in the corresponding optically active poly(amide-imide)s as depicted in Scheme 3(b). The resulting polymers were used to study structure-property relationships and it was found that the incorporation of flexible spacer units improved the solubility of the materials but did not negatively influence the thermal stability. A similar N-trimellitylimido-Lleucine diacid chloride was reacted with eight different hydantoin derivatives yielding the corresponding optically active poly(amide-imide)s. [31] The L-valinebased asymmetric diacid central unit was used for the microwave-assisted preparation of poly(amide-imideurethane)s. [32,33] The diacid and poly(ethylene glycol diol)s were copolymerized using 4,4 0 -methylene-bis(4-phenylisocyanate) as coupling agent in a one-step [32] or twostep [33] procedure [Scheme 3(c)]. For the two-step procedure, the diisocyanate was first reacted with the diol or the diacid and subsequently with the other reagent. It was found that the properties of the polymers could be influenced by changes in catalyst, microwave power, irradiation time as well as the molecular weight of the utilized poly(ethylene glycol). The resulting thermoplastic poly(amide-imide-urethane)s revealed good thermal stability and phase mixing. The thermal stability of these Scheme 3. Synthesis of poly(amide-imide)s containing asymmetric building blocks: (a) Reaction of a non-symmetrical acid chloride derived from epiclon and amino acids with aromatic diamines; (b) Copolymerization of N-trimellitylimido-L-valine diacid with diisocyanates; (c) synthesis of poly(amide-imide-urethane)s based on N-trimellitylimido-L-valine diacid, poly(ethylene glycol) and 4, 4 0 -methylene- bis(4- phenylisocyanate). 372 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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