Elucidation of the Termination Mechanism of the Radical Polymerization of Isoprene

The termination mechanism in the radical polymerization of isoprene (Ip), a representative diene monomer, was investigated through product analyses formed by the reaction of polyisoprene (PIp) model radicals and structurally controlled PIp end radicals. The results revealed that the termination occurred exclusively by combination (Comb) with >97% selectivity, regardless of the conditions, even in highly viscous media. This finding contrasts sharply with the termination mechanism in styrene polymerization, in which increasing viscosity shifts the termination mechanism entirely from Comb to disproportionation (Disp). Therefore, an extremely high propensity for Comb selectivity in Ip polymerization is demonstrated. Additionally, since Ip is used as a coupling agent in radical coupling reactions, these results also provide valuable insights into synthesizing new polymer materials based on IP-derived polymer end radical species.