Studying the mechanism of thioketone-mediated polymerization via electrospray ionization mass spectrometry

The polymeric product spectrum generated in thioketone-mediated free radical polymerization (TKMP) was analyzed via electrospray ionization mass spectrometry. Poly(n-butyl acrylate) samples were synthesized in the presence of the (commercially available) thioketone 4,4-bis(dimethylamino)thiobenzophenone under variable reaction conditions in toluene solution at 80 °C. To unambiguously assign the mass spectra, the samples are prepared under variation of the monomer (going from n-butyl acrylate to ethyl acrylate) as well as by employing variable thermally decomposing initiators [i.e., 2,2'- azoisobutyronitrile and azobis(cyclohexanecarbonitrile)]. In all mass spectra, significant amounts of the expected cross-termination product, formed via bimolecular termination of propagating macroradicals with the dormant thioketone radical adduct (consisting of a propagating chain and the mediating thioketone) alongside conventional termination products can be identified. As the study was carried out on acrylate polymers, acrylate-specific reaction products arising from intramolecular transfer reactions followed by β-seission of the generated mid-chain radicals are also identified in the mass spectra. In addition, a species congruent with the dormant thioketone radical adduct itself (oxidized to its cationic state) was identified. Products that could potentially be formed via a chain transfer mechanism cannot be identified. The results presented here thus support the earlier suggested TKMP mechanism involving a highly stabilized adduct radical which undergoes significant cross-termination reactions.