ESR study of radicals in conventional radical polymerizations using model radicals generated from polymers prepared by atom transfer radical polymerization

Radical precursors of poly (meth) acrylates with given chain lengths were prepared by atom transfer radical polymerization (ATRP) technique. Model radicals with given chain lengths were generated by reaction of the precursors with organotin compound. The radicals were observed by electron spin resonance (ESR) spectroscopy. ESR spectra of methacrylate model radicals with degrees of polymerizations (P-n) of 30, 50, and 100 were measured. On comparison of these spectra, clear chain length dependence was observed and a detection of polymeric propagating radicals was clearly shown in the conventional radical polymerization. ESR spectra of model radicals of acrylates did not display chain-length dependence. The spectra suggested a formation of significant amounts of mid-chain radicals from acrylate propagating radicals via 1,5-hydrogen shift. ESR spectra of dimeric model radical of acrylate clearly provided experimental evidence of the 1,5-hydrogen shift. Dimers of various (meth)acrylate units: methyl acrylate (MA), tert-butyl acrylate (tBA), and methyl methacrylate (MMA), were prepared by ATRA. These dimers provided radicals mimicking the active species in a radical copolymerization of MA, MMA and tBA. Well-resolved ESR spectra of the dimeric radicals confirm electronic and steric effects of the penultimate unit on the propagating radical.