A difference of six orders of magnitude: A reply to "The magnitude of the fragmentation rate coefficient"

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 10⁴/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 ∼ 10⁴/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.