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 kact were determined by the GPC peak-resolution and the polydispersity-analysis methods. The results showed that kact 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 kex (= kact/[P]) was strongly dependent on both the structures of the dithiocarbonate group and the polymer. The kex 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.