Mechanism and kinetics of iodide-mediated polymerization of styrene

The kinetics of the bulk polymerization of styrene in the presence of a model ω-polystyryl iodide (M_n ≈ 2000, and M_w/M_n ≈ 1.26) as mediator and benzoyl peroxide as initiator was studied. The rate of polymerization, R_p, was found to be independent of the iodide concentration, showing that the stationary concentration of polymer radicals, [P*], is determined by the balance of initiation and termination rates, as in the conventional (iodide-free) system. The pseudo-first-order activation rate constant k_act of the model iodide was determined as a function of BPO concentration and temperature (50-80°C) by both the GPC curve-resolution and polydispersity-analysis methods. The results showed that k_act is directly proportional to [P*], which means that degenerative transfer (active species-exchanging transfer) is the only important mechanism of activation in this system. The activation energy for the transfer rate constant k_ex was found to be 27.8 kJ mol^-1, somewhat smaller than the known activation energy for the styrene propagation rate constant k_p of 32.5 kJ mol^-1. This indicates that lowering, rather than raising, the reaction temperature will be more effective in preparing polystyrenes with a narrower polydispersity by the iodide-mediated polymerization. This is because the most important parameter for determining the polydispersity of a degenerative-transfer-type system is the k_ex/k_p ratio, as has been known for some time.