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Microwave-Assisted Polymer Synthesis: Recent Developments in ... polymers improved significantly compared to conventional poly(urethanes). Polymerizations via C–C Coupling Reactions C–C coupling procedures are often applied for step-growth polymerization resulting in conjugated polymers that can be applied in, e.g., light-emitting diodes, solar cells, and organic field effect transistors. [34–36] These C–C coupling procedures are often based on metal catalysis which might benefit from microwave irradiation by specific absorption of the metal ions. Khan and Hecht investigated the palladium-catalyzed synthesis of poly(m-phenyleneethynylene)s under both thermal heating and microwave heating (multimode microwave reactor). [37] A novel synthetic method was developed in which both deprotection and polymerization of AB 0 monomers or AA and BB 0 monomers were performed simultaneously resulting in defect-free polymer structures as depicted in Scheme 4(a). It was found that the microwave-assisted polymerization was comparable with thermal heating in toluene, which was ascribed to the low dipole moment of the solvent. When the solvent was changed to acetonitrile, the polymerization under microwave irradiation resulted in higher polymerization rates as well as higher molecular weight polymers, whereby the microwave-polymerizations were performed under different conditions. Tierney and coworkers investigated the synthesis of polythiophenes via Stille-type cross-coupling [Scheme 4(b)] in a monomode microwave reactor. [38] Soluble semi-conducting polythiophenes were obtained using both thermal and microwave heating. However, it was claimed that the microwave-assisted Scheme 4. Microwave-assisted polymerizations via C–C coupling procedures. (a) Palladium-catalyzed synthesis of poly(m-phenyleneethynylene)s; (b) Stille-type synthesis of polythiophene; (c) synthesis of poly(phenylenevinylene) via Heck coupling; (d) Ni(0)-mediated copolymerization of dibromofluorene with a dichloro platinum-salen complex. Macromol. Rapid Commun. 2007, 28, 368–386 ß 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.mrc-journal.de 373
R. Hoogenboom, U. S. Schubert polymerization method yielded higher molecular weight polymers and slightly lower polydispersity indices, whereby the direct comparison was performed at different polymerization temperatures. The Suzuki C–C coupling method was investigated under microwave irradiation (monomode microwave reactor) by Scherf et al. [39] Three differently substituted naphthalene boronic ester monomers were coupled to 4,4 0 -didecyl-2 0 ,5 0 - dibromoterephthalophenone using a palladium catalyst. The polymerization of the less sterically hindered 2,6-diboronic ester naphthalene under both microwave irradiation and thermal heating gave similar results. However, the more sterically hindered 1,5-diboronic ester naphthalenes could not be polymerized using conventional heating, but the use of microwave irradiation allowed their polymerization. All resulting conjugated polymers served as precursors for the formation of ladder-type polymers, which have desirable optical and emitting properties. Another palladium-catalyzed C–C coupling procedure, namely the Heck reaction, was studied for the formation of conjugated polymers under both microwave (monomode microwave reactor) and thermal heating by Ritter and coworkers. [40] The copolymerization of divinylbenzene and 1,4-diiodo-2,5- dibutoxybenzene [Scheme 4(c)] was studied in refluxing dioxane to ensure the same polymerization temperature in both open vessel reactions. This direct comparison revealed a slight acceleration of the polymerization under microwave irradiation. In addition, the molecular weight of the resulting poly(2,5-dibutoxy-1, 4-phenylenevinylene) was also a little higher when microwave irradiation was applied as heat source. The previous examples were all based on C–C coupling reactions that require two different functional groups. On the contrary, Ni(0)-catalyzed coupling reactions can be used for homocoupling reactions. Yamamoto and coworkers used a Ni(0) catalyzed polymerization procedure for the synthesis of poly(pyrazine-2,5-diyl) starting from 2,5-dibromopyrazine. [41] The thermal polymerization required 2 d polymerization time, while the microwaveassisted procedure was finished within 10 min. Unfortunately, no temperature was given for the microwave procedure and, thus, it is not clear whether the observed acceleration is due to thermal or microwave effects. Nevertheless, the polymer obtained under microwave irradiation had a higher molecular weight. Carter et al. reported the use of a Ni(0) catalyzed C–C coupling for the synthesis of poly(biphenylmethylene)s starting from bistriflate monomers under microwave irradiation (monomode microwave reactor). [42] It was demonstrated that the polymerization could be performed using both thermal and microwave heating, whereby no significant differences were observed in the resulting polymers. Nevertheless, the microwave-assisted polymerizations were performed for only 10 min at 200 8C, whereas the conventional polymerizations were performed for 16–24 h at 80 8C. Surprisingly, endcapping the polymerization by the presence of 4-bromostyrene could also be performed at 200 8C under microwave heating without coupling or degradation of the vinyl groups. The Ni(0)-mediated polymerization procedure was also applied by Scherf and coworkers for the synthesis of polyfluorenes with electrophosphorescent platinum-salen chromophores [Scheme 4(d)]. [43] The polymerization required 3 d under thermal heating in tetrahydrofuran (THF) at 80 8C and could be accelerated down to 12 min under microwave irradiation at 115 or 220 8C in THF of a mixture of N,N-dimethylformamide (DMF) and toluene, respectively. The resulting copolymers revealed high electroluminescence efficiencies due to energy transfer from the polyfluorene to the salen complex. Other Step-Growth Polymerizations Besides poly(amide-imide)s and conjugated polymers, several other polymers have been prepared using stepgrowth polymerization mechanisms under microwave irradiation in the last couple of years. The synthesis of biodegradable aliphatic polyesters was investigated under microwave irradiation (monomode microwave reactor) by Nagahata and coworkers. [44] Direct polycondensation of succinic acid and butanediol was investigated in the presence of a stannyl catalyst [Scheme 5(a)]. The polymerization conditions were investigated in detail, whereby the (absence of) solvent, catalyst concentration, polymerization temperature, stoichiometry, and reaction time were varied. The optimal polymerization conditions were also tested in a conventionally heated reaction demonstrating a much lower polymerization rate compared to the microwave-assisted polymerization method. The authors speculate that the ten-fold acceleration under microwave irradiation might be due to specific microwave absorption by the released water molecules leading to quicker evaporation of the water and thus a shift in equilibrium towards the polymer. Chatti et al. investigated the synthesis of poly(ether-ester)s from an isosorbide-based aliphatic diol and two different diacid chlorides under microwave irradiation (monomode microwave reactor). [45] The used isosorbide-based material is an interesting building block for polymer structures because it represents a renewable resource. The bulk polymerizations revealed a remarkable acceleration under microwave irradiation compared to thermal heating under the same conditions that was ascribed to the enhanced polarity of the transition state during the ester formation. In addition, it was found that the microwave-assisted polymerization procedure gave less degradation at longer reaction times (8 h) compared to conventional heating. The synthesis of 374 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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