The cumyl dithiobenzoate (CDB)-mediated reversible addition fragmentation chain transfer (RAFT) polymerization of styrene at 30 degrees C is studied via both kinetic experiments and high-level ab initio molecular orbital calculations. The kinetic data clearly indicate the delayed onset of steady-state behavior. Such an observation is consistent with the slow fragmentation model for the RAFT process, but cannot be reconciled with the cross-termination model. The comprehensive failure of the cross-termination model is quantitatively demonstrated in a detailed kinetic analysis, in which the independent influences of the pre-equilibria and main equilibria and the possible chain length dependence of cross-termination are fully taken into account. In contrast, the slow fragmentation model can describe the data, provided the main equilibrium has a large fragmentation constant of at least 8.9 x 10(6) L mol(-1). Such a high equilibrium constant (for both equilibria) is consistent with high-level ab initio quantum chemical calculations (K = 7.3 x 10(6) L mol(-1)) and thus appears to be physically realistic. Given that the addition rate coefficient for macroradicals to (polymeric) RAFT agent is 4 x 10(6) L mol(-1) s(-1), this implies that the lifetime of the RAFT adduct radicals is close to 2.5 s. Since the radical is also kinetically stable to termination, it can thus function as a radical sink in its own right.

The reversible addition−fragmentation chain transferchain length dependenttermination (RAFT-CLD-T) method is employed to map out the chain length dependence of the termination rate coefficient in methyl acrylate (MA) bulk free radical polymerizations at 80 °C. Methoxycarbonylethyl phenyldithioacetate (MCEPDA)a novel RAFT agent carrying a methyl acryl leaving groupis identified as suitable for the RAFT-CLD-T method applied to methyl acrylate, as interfering inhibition and rate retardation effects are avoided. The chain length dependency of the termination rate coefficient was constructed in a stepwise fashion since the MA/MCEPDA system displays hybrid behavior (between conventional and living free radical polymerization), resulting in initial high molecular weight polymers formed at low RAFT agent concentrations. The chain length dependency of k t in the MA system for chain lengths, i, ranging from 5 to 800 at 80 °C may be described by a value for α of 0.36 ± 0.05 (where α is the slope of the associated log k t i , i vs log i plot). An alternative RAFT agent, dimethoxycarbonylethyl trithiocarbonate (DMCETC), may not be as ideally applicable to map CLD dependent k t i , i in MA polymerizations. Since the leaving group of both RAFT reagents is identical to the propagating methyl acrylate radical, the addition rate coefficient of the methyl acrylate propagating radicals to the initial and polymeric RAFT agent, k β, was determined and found to be close to 1.4 × 106 L mol-1 s-1 for MCEPDA and 2.1 × 106 L mol-1 s-1 for DMCETC at 60 °C.

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