Mechanism and kinetics of iodide-mediated polymerization of styrene

The kinetics of the bulk polymerization of styrene in the presence of a model omega-polystyryl iodide (M-n approximate to 2000, and M-w/M-n approximate to 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 degrees 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.