Kinetic model of reversible addition-fragmentation chain transfer polymerization in microemulsions

A simplified kinetic model for RAFT microemulsion polymerization has been developed to facilitate the investigation of the effects of slow fragmentation of the intermediate macroRAFT radical, termination reactions, and diffusion rate of the chain transfer agent to the locus of polymerization on the control of the polymerization and the rate of monomer conversion. This simplified model captures the experimentally observed decrease in the rate of polymerization, and the shift of the rate maximum to conversions less than the 39% conversion predicted by the Morgan model for uncontrolled microemulsion polymerizations. The model shows that the short, but finite, lifetime of the intermediate macro-RAFT radical (1.3 × 10^-4-1.3 × 10^-2 s) causes the observed rate retardation in RAFT microemulsion polymerizations of butyl acrylate with the chain transfer agent methyl-2-(0ethylxanthyDpropionate. The calculated magnitude of the fragmentation rate constant (k_f = 4.0 × 10¹-4.0 × 10³ s^-1) is greater than the literature values for bulk RAFT polymerizations that only consider slow fragmentation of the macro-RAFT radical and not termination (k^f= 10^-2 S^-1). This is consistent with the finding that slow fragmentation promotes biradical termination in RAFT microemulsion polymerizations.