Termination Mechanism of the Radical Polymerization of Acrylates

The termination mechanism of the radical polymerization of acrylates, namely the selectivity of disproportionation (Disp) and combination (Comb) between polymer end radicals, is unambiguously determined by the reaction of polyacrylate end radicals generated from corresponding "living" organotellurium ω-end polymer. While textbooks describe the occurrence of Comb, the reaction at 25 °C exclusively gives the Disp products. Ab initio molecular dynamics suggests that the products form by two pathways: The direct disproportionation reaction and a novel stepwise process that involves the initial formation of the C-O coupling product followed by intramolecular rearrangement. The termination at high temperature and low radical concentration increases the contribution of back-biting reaction giving mid-chain radicals, and complex reaction pathways of the mid-chain radicals are clarified for the first time. The termination mechanism of the radical polymerization of acrylate, a long-standing controversy, is unambiguously clarified by the reaction of structurally well-defined polyacrylate end radicals and subsequent product analyses. The exclusive disproportionation is observed at ambient temperature. The two reaction pathways giving the disproportionation products including a novel stepwise mechanism via O-C_ipso coupling are suggested by a computational study.