Mechanism and Kinetics of RAFT-Based Living Radical Polymerizations of Styrene and Methyl Methacrylate

The bulk polymerizations of styrene 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 k ex for a polystyryl dithioacetate (PSt−SCSCH 3 ) was 21.0 kJ mol -1, which is reasonable for a fast addition reaction.