Cumyl dithiobenzoate mediated RAFT polymerization of styrene at high temperatures

Self-initiated reversible addition fragmentation chain transfer (RAFT) polymerizations of styrene at temperatures of 120, 150, and 180 degrees C, using cumyl dithiobenzoate (CDB) at concentrations between 5.0 x 10(-3) and 2.0 x 10(-2) mol L-1 as the RAFT agent were performed at 1000 bar. The increase of average molecular weight with monomer conversion, the shape of the molecular weight distributions, and polydispersity indices below 1.5 at monomer conversions up to about 50% indicate control of styrene bulk polymerization even at the high experimental temperatures. Neither a substantial decomposition of the RAFT agent nor a change in the overall polymerization process, e.g., by ionic side reactions, is observed. Polymerization rates are lower than in conventional styrene polymerization. The rate retardation effect increases with CDB concentration but is clearly reduced toward higher temperature. The lower retardation effect at high temperatures is assigned to a lower equilibrium concentration of the intermediate RAFT radical. The experimental rate data can be consistently described in terms of the concept of irreversible termination of the intermediate RAFT radical. On the other hand, the data are qualitatively and semiquantitatively inconsistent with the idea of slow fragmentation of intermediate radicals. The analysis of the kinetic data results in a reaction enthalpy of about 50 M mol(-1) for the beta-scission reaction of the intermediate RAFT radical.