Single-Mode Microwave Ovens as New Reaction Devices: Accelerating the Living Polymerization of 2-Ethyl-2-Oxazoline Frank Wiesbrock, Richard Hoogenboom, Caroline H. Abeln, Ulrich S. Schubert* Laboratory of Macromolecular Chemistry and Nanoscience, Eindhoven University of Technology and Dutch Polymer Institute (DPI), P.O. Box 513, 5600 MB Eindhoven, The Netherlands E-mail: u.s.schubert@tue.nl Received: August 12, 2004; Revised: September 2, 2004; Accepted: September 3, 2004; DOI: 10.1002/marc.200400369 Keywords: activation energy; green chemistry; 2-oxazoline; ring-opening polymerization; single-mode microwave system Introduction Since their introduction at the beginning of the new millennium, single-mode microwave reactors have found their way into chemical laboratories all over the world. [1,2] Besides the impressive increase in reaction rates observable for a plethora of reactions, these single-mode systems allow for accurate control of temperature and pressure inside the reaction vial, rendering reproducibility and a facilitated scale-up of the reactions performed. [3] Furthermore, by the fast and direct heating of the reactants, numerous reactions have been reported to give higher yields and an improved purity of the desired products when carried out under microwave irradiation. As an additional consequence, reac- tions can be performed in reduced solvent amounts (green chemistry). Contrary to their well-established use in a steadily growing number of organic reactions, it is only very recen- tly that polymer chemists have discovered single-mode microwave systems as new reaction devices. The first sets of polymerization reactions have been performed under microwave irradiation. The corresponding findings show the advantages of microwave irradiation for the polymer synthesis, above all, the increased reaction rates and improved polymer properties. [4–6] Surprisingly, and in contrast to controlled radical polymerizations, [7] living ionic polymerizations that represent an important class of controlled polymerization techniques have not been inves- tigated so far. To obtain data for this important type of reaction, we chose the living cationic ring-opening poly- merization (CROP) of 2-ethyl-2-oxazoline as a first example (Scheme 1). [8] Its investigation began in 1966, [9] Summary: The ring-opening cationic polymerization of 2-ethyl-2-oxazoline was performed in a single-mode micro- wave reactor as the first example of a microwave-assisted living polymerization. The observed increase in reaction rates by a factor of 350 (6 h ! 1 min) in the range from 80 to 190 8C could be attributed solely to a temperature effect as was clearly shown by control experiments and the determined activation energy. Because of the homogenous microwave irradiation, the polymerization could be performed in bulk or with drastically reduced solvent ratios (green chemistry). Monomer conversion, represented by the ratio ln{[M 0 ]/[M t ]}, plotted against time for six temperatures in the range from 80 to 180 8C, and polymerization reaction vials, showing an increase in yellow color for those reactions performed (well) above and below 140 8C, indicating side reactions. Macromol. Rapid Commun. 2004, 25, 1895–1899 DOI: 10.1002/marc.200400369 ß 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Communication 1895