End-functionalized copolymers prepared by the addition-fragmentation chain-transfer method: Vinyl acetate/methacrylonitrile system

Copolymers of vinyl acetate and methacrylonitrile were prepared by free-radical polymerization in the presence of the chain-transfer agent (CTA) ethyl-α-(t-butanethiomethyl)acrylate. Molecular weight measurements showed that the chain-transfer constants increased with the vinyl acetate content of the comonomer mixture, ranging from 0.42 for methacrylonitrile to 6.3 for the copolymerization of a vinyl acetate-rich monomer mix (89/11). The bulk copolymer composition was not appreciably affected by the amount of CTA used in the copolymerization. The efficiency of the addition-fragmentation mechanism in producing specifically end-functionalized copolymers was investigated with ¹H NMR spectroscopy. Spectral peaks consistent with all the expected end groups were observed for all comonomer feeds. Peaks consistent with other end groups were also observed, and these were particularly prominent for copolymers made with lower CTA concentrations. At the highest concentrations used, quantitative measurements of end-group concentrations indicated that 70-80% of the end groups were those expected on the basis of the addition-fragmentation chain-transfer mechanism.