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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 (kf0) for virtually similar RAFT systems of PSt[BOND]S[BOND]C · (Ph)[BOND]S[BOND]PSt. In this communication, we explain the estimates of kf ∼ 104/s and the choices of the rate constant in modeling based on experimental polymerization rate and radical concentration data. The use of kf ∼ 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

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