Mechanism and kinetics of the free radical ring-opening polymerization of eight-membered cyclic allylic disulfide monomers

Remote methyl substituents were found to have a significant effect on the free radical ring-opening polymerization of cyclic allylic eight-membered disulfide monomers. High concentrations of initiator were required to achieve reasonable polymerization rates for the monomer with a greater number of methyl substituents, 2,2,4-trimethyl-7-methylene-1,5-dithiacyclooctane (1b). Monomer 1b polymerized significantly slower than the analogous monomer containing only one 2-methyl substituent, 2-methyl-7-methylene-1,5-dithiacyclooctane (1a). Values of k_p/k_t^0.5 (k_p and k_t are are rate coefficients for propagation and termination, respectively) for both monomers were obtained at 30-120°C. Examination of the Arrhenius parameters for propagation revealed that the fragmentation step appeared to exert a greater influence on the overall propagation rate for 1a, Polymerization rates were influenced by depropagation at the relatively low temperatures of 50-60°C (1a) and 70-85°C (1b), and both monomers exhibited the same ceiling temperature of approximately 125°C at [M]₀ = 2 M in benzene.