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 the 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.