Elucidation of Acceleration Mechanisms by a Photosensitive Onium Salt for Nitroxide-Mediated Photocontrolled/Living Radical Polymerization

Abstract

The acceleration mechanisms by a photosensitive onium salt for thenitroxide-mediated photocontrolled/living radical polymerization (photo-NMP)were determined. The photo-NMP of methyl methacrylate was performed byirradiation at room temperature using 4-methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl(MTEMPO) as the mediator and (2RS, 2’RS)-azobis(4-methoxy-2,4-dimethylvaleronitrile)as the initiator. The polymerization was accelerated in the presence of (4-tertbutylphenyl)diphenylsulfoniumtriflate (tBuS) to producea polymer with a molecular weight distributionas narrow as the polymerization in its absence. (±)-Camphor-10-sulfonic acid or 2-fluoro-1-methylpyridinium p-toluenesulfonatehad no effect on the polymerization speed, suggesting that tBuS did not serve as the photo-acid generator for thephoto-NMP. It was found that the acceleration of the polymerization was basedon the electron transfer from MTEMPO into tBuSin the excited state to temporarily generate a free radical propagating chainend and an oxoaminium salt (OAS), the one-electron oxidant of MTEMPO. Thiselectron transfer mechanism was verified on the basis of the fact that thephoto-NMP in the presence of tBuSwas still accelerated by triphenylamine, the electron transfer inhibitor, topartly produce a polymer with an uncontrolled molecular weight. The formationof an uncontrolled molecular weight polymer indicated the generation of a freeradical propagating chain end due to the deactivation of the OAS by thetriphenylamine. It was deduced that tBuSserved as the electron acceptor from MTEMPO in the excited state to temporarilyproduce a free radical propagating chain end along with OAS, resulting in theacceleration of the polymerization.