Rate Retardation in Reversible Addition−Fragmentation Chain Transfer (RAFT) Polymerization: Further Evidence for Cross-Termination Producing 3-Arm Star Chain

New pieces of experimental evidence were presented showing that the rate retardation in the polymerization of styrene with polystyryl dithiobenzoate (PSt-SCSPh) at 60 °C is caused by the irreversible cross-termination between the polystyryl radical (PSt • ) and the intermediate radical produced by the addition of PSt • to PSt-SCSPh. The polymerization rate R p decreased with an increase of [PSt-SCSPh] such that a plot of 1/ R p 2 vs [PSt-SCSPh] was linear, as the irreversible cross-termination mechanism demands. The cross-termination rate constant k t ‘ was estimated to be about 0.5 k t, where k t is the self-termination rate constant of PSt • . The formation of a 3-arm star, a possible product of cross-termination, was evidenced by a model experiment involving the analysis by GPC-MALLS (gel permeation chromatography equipped with a multiangle laser-light scattering detector). The concentration of the 3-arm star relative to that of the doubled molecular weight linear chain produced by self-termination of PSt • was proportional to [PSt-SCSPh], and the value of k t ‘ deduced by this model experiment well agreed with that determined by the kinetic (rate retardation) experiment mentioned above. The 3-arm star was perfectly stable in the time scale of the experiments at 60 °C, confirming that cross-termination is irreversible at this temperature.