EPR Measurement of Fragmentation Kinetics in Dithiobenzoate-Mediated RAFT Polymerization

The kinetics of reversible addition fragmentation chain transfer (RAFT) polymerization of butyl acrylate with ethyl S-thiobenzoyl-2-thiopropionate (ETTP) as the RAFT agent has been studied. The concentrations of propagating (P-center dot) and intermediate radicals (INT) were measured via highly time-resolved EPR spectroscopy after initiation by a laser single pulse. Predici simulation of experimental data results in rate coefficients for -40 degrees C of k(ad) = (1.4 +/- 0 4) x 10(6)L mol(-1) s(-1), k(beta) = (4.7 +/- 1.5) s(-1), and k(t)(cross) = 0.25 x k(upsilon), where k(t) is the rate coefficient for termination of two P-center dot species. Fast fragmentation of the intermediate radical is thus observed at this low temperature. Measuring the ratio of INT center dot to P-center dot concentrations during stationary polymerization at -40 and -70 degrees C yields K-eq = k(ad)/k(beta) values of (2.3 +/- 0.6) x 10(5) and 75 +/- 15 L mol(-1), respectively, which correspond to an apparent activation energy difference, E-A(k(ad)/k(beta)), of -49.5 kJ mol(-1).