Nitroxide-Mediated Radical Polymerization of Carbon Dioxide-Expanded Methyl Methacrylate

INTRODUCTION Nitroxide-mediated radical polymerization (NMP), one of the techniques for controlled/living radical polymerization, is based on the reversible deactivation of propagating radicals by a stable nitroxide radical. A significant drawback of NMP is that it is in general incompatible with alkyl methacrylates primarily due to the tendency of nitroxide radicals and the corresponding propagating radicals to undergo disproportionation generating a hydroxylamine and a dead chain with an unsaturated end group. This reaction may also proceed unimolecularly, although it has been shown for the present system that the bimolecular pathway is predominant. It was reported in 2007 that the nitroxide known as 2,2-diphenyl-3-phenylimino-2,3-dihydroindol-1-yloxyl nitroxide (DPAIO) can be successfully used for NMP of methyl methacrylate (MMA) because of the absence of such disproportionation reactions. However, NMP of alkyl methacrylates using common nitroxides such as 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) and N-tert-butyl-N-[1-diethylphosphono(2,2-dimethylpropyl)] (SG1) does proceed satisfactorily in the presence of a relatively small amount (approx. 8 mol %) of styrene, that is, as a copolymerization.