Mechanism and kinetics of organostibine-mediated living radical polymerization of styrene

The polymerization of styrene with a polystyrene-dimethylstibanyl (PS-SbMe₂) adduct as a mediator and azobis(isobutyronitrile) (AIBN) as a conventional radical initiator was kinetically studied. PS-SbMe₂ had no detectable effect on the polymerization rate R_p, which was virtually the same as that of the conventional (stibanyl-free) system. The pseudo-first-order activation rate constant k_act was proportional to R_p, meaning that degenerative (exchange) chain transfer is the only important activation mechanism, in the examined range of temperature (40-100°C). The exchange rate constant k_ex was sufficiently large at these temperatures, explaining why this polymerization can afford low-polydispersity polymers. The k_ex was about twice as large as that for polystyrene-methyltellanyl (PS-TeMe). This rationalizes the better polydispersity controllability in the PS-SbMe₂ system than in the PS-TeMe system. The activation energy of k_ex was 22.6 kJ mol ^-1, which is smaller than that (27.8 kJ mol^-1) for polystyrene-iodide (PS-I) and slightly larger than that (21.0 kJ mol ^-1) for polystyrene-dithioacetate (PS-SCSMe).