TITLE:
Elucidation of Acceleration Mechanisms by a Photosensitive Onium Salt for Nitroxide-Mediated Photocontrolled/Living Radical Polymerization
AUTHORS:
Eri Yoshida
KEYWORDS:
Photocontrolled/Living Radical Polymerization, Acceleration Mechanisms, Electron Transfer, (4-tert-Butylphenyl)Diphenylsulfonium Triflate, 4-Methoxy-2, 2, 6, 6-Tetramethylpiperidine-1-Oxyl, Oxoaminium Salt
JOURNAL NAME:
Open Journal of Polymer Chemistry,
Vol.4 No.3,
August
14,
2014
ABSTRACT:
The acceleration mechanisms by a photosensitive onium salt for the
nitroxide-mediated photocontrolled/living radical polymerization (photo-NMP)
were determined. The photo-NMP of methyl methacrylate was performed by
irradiation 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)diphenylsulfonium
triflate (tBuS) to produce
a polymer with a molecular weight distribution
as narrow as the polymerization in its absence. (±)-Camphor-10-sulfonic acid or 2-fluoro-1-methylpyridinium p-toluenesulfonate
had no effect on the polymerization speed, suggesting that tBuS did not serve as the photo-acid generator for the
photo-NMP. It was found that the acceleration of the polymerization was based
on the electron transfer from MTEMPO into tBuS
in the excited state to temporarily generate a free radical propagating chain
end and an oxoaminium salt (OAS), the one-electron oxidant of MTEMPO. This
electron transfer mechanism was verified on the basis of the fact that the
photo-NMP in the presence of tBuS
was still accelerated by triphenylamine, the electron transfer inhibitor, to
partly produce a polymer with an uncontrolled molecular weight. The formation
of an uncontrolled molecular weight polymer indicated the generation of a free
radical propagating chain end due to the deactivation of the OAS by the
triphenylamine. It was deduced that tBuS
served as the electron acceptor from MTEMPO in the excited state to temporarily
produce a free radical propagating chain end along with OAS, resulting in the
acceleration of the polymerization.