Thioketone-mediated polymerization with dithiobenzoates: Proof for the existence of stable radical intermediates in RAFT polymerization

A novel dithioester control agent [dimethyltetrathioterephtalate (DMTTT)] is presented for the thioketone-mediated radical polymerization (TKMP) of n-butyl acrylate. The rate of polymerization is significantly decreased in the presence of DMTTT indicating formation of dormant radical species. During polymerization, molar masses increase linearly with monomer conversion with reasonably narrow initial molar mass distributions (PDI between 1.3 and 1.8), whereas the dispersity increases during the course of the polymerization due to irreversible termination of both propagating and dormant radicals. The present results thus highlight the possibility of a mixed mechanism operating in RAFT polymerization, which combines slow fragmentation (long-lived intermediates) and intermediate radical termination. Living characteristics are observed in acrylate radical polymerization in the presence of a dithiobenzoate possessing a nonleaving R group. These observations account for the presence of dormant radicals-which are analogous to RAFT intermediate radicals-and corroborate the compatibly between slow fragmentation and intermediate radical termination.