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 degreesC 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, 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 0.5k(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 kt' 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 degreesC, confirming that cross-termination is irreversible at this temperature.