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 <b>(1b)</b>. Monomer <b>1b</b> polymerized significantly slower than the analogous monomer containing only one 2-methyl substituent, 2-methyl-7-methylene-1,5-dithiacyclooctane <b>(1a)</b>. Values of <i>k</i><sub>p</sub>/<i>k</i><sub>t</sub><sup>0.5</sup> (<i>k</i><sub>p</sub> and <i>k</i><sub>t</sub> 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 <b>1a</b>. Polymerization rates were influenced by depropagation at the relatively low temperatures of 50−60 °C (<b>1a</b>) and 70−85 °C (<b>1b</b>), and both monomers exhibited the same ceiling temperature of approximately 125 °C at [M]<sub>0</sub> = 2 M in benzene.