Mechanism and kinetics of RAFT-based living radical polymerizations of styrène and methyl methacrylate

The bulk polymerizations of styrène and methyl methacrylate in the presence of model polymer-dithiocarbonate adducts as mediators and benzoyl peroxide (BPO) as a conventional initiator were kinetically studied. The polymerization rate, and hence the concentration of polymer radical P, was proportional to [BPO]^1/2. The pseudo-first-order activation rate constants k_act were determined by the GPC peak-resolution and the polydispersity-analysis methods. The results showed that k_act is directly proportional to [P'], indicating that reversible addition-fragmentation chain transfer (RAFT) is the only important mechanism of activation. The magnitude of the exchange rate constant k_ex (= k_act/[P]) was strongly dependent on both the structures of the dithiocarbonate group and the polymer. The k_ex values for the three RAFT systems examined in this work were all very large, which explains why these systems can provide low-polydispersity polymers from an early stage of polymerization. The activation energy of &ex for a polystyryl dithioacetate (PSt-SCSCHa) was 21.0 kJ mol^-1, which is reasonable for a fast addition reaction.