A difference of six orders of magnitude: A reply to “the magnitude of the fragmentation rate coefficient”

Abstract There has been an ongoing debate regarding the mechanism that causes rate retardation phenomena observed in some reversible addition‐fragmentation transfer (RAFT) polymerization systems. Some attribute the retardation to slow fragmentation of adduct radicals, others attribute it to fast fragmentation coupled with cross‐termination between propagating and adduct radicals. There exists a difference of six orders of magnitude (10 −2 versus 10 4 /s) in the reported values of the fragmentation rate constant ( k f0 ) for virtually similar RAFT systems of PStSC · (Ph)SPSt. In this communication, we explain the estimates of k f ∼ 10 4 /s and the choices of the rate constant in modeling based on experimental polymerization rate and radical concentration data. The use of k f ∼ 10 −2 /s in the model results in a calculated adduct radical concentration level of 10 −4 to 10 −3 mol/L, which appears to directly contradict the reported electron spin resonance (ESR) data in the range of <10 −6 mol/L. We hope that this open discussion can stimulate more effort to resolve this outstanding difference. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2833–2839, 2003