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_P² vs [PSt-SCSPh] was linear, as the irreversible cross-termination mechanism demands, The cross-termination rate constant k _t′ was estimated to be about 0.5k_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.