Poly(vinyl ester) Block Copolymers Synthesized by Reversible Addition-Fragmentation Chain Transfer Polymerizations

Homopolymerizations and block copolymerizations of vinyl acetate (VAc), vinyl pivalate (VPv), and vinyl benzoate (VBz) by reversible addition-fragmentation chain transfer (RAFT) polymerization have been studied. Polymerizations of VAc initiated with 2,2'-azobis(isobutyronitrile) (AIBN) at 60 degrees C using two different xanthate RAFT agents C2H5OC(=S)SR (R = -CH(CH3)CO2C2H5 (1) and -CH(CH3)-O2CC(CH3)(3) (2)) were examined to elucidate the dependence of the polydispersities of the resulting polymers on the RAFT agent leaving group R. RAFT agent 2, in which the leaving R-group mimics a growing vinyl ester polymer chain, consistently yields poly(vinyl acetates) having broader polydispersities than those synthesized using 1 (M-n = 3.6-14 kg/mol and M-w/M-n = 1.15-1.33). While VPv exhibits similar controlled polymerization behavior to VAc, RAFT homopolymerizations of VBz mediated by 1 indicate this electron deficient vinyl ester requires higher temperatures to effect controlled polymerizations to yield polymers having M-n = 4-14 kg/mol and M-w/M-n = 1.29-1.53. Chain extension reactions from xanthate-terminated vinyl ester homopolymers with VAc, VPv, and VBz proceed with variable efficiencies to furnish block copolymers that microphase separate in the melt state as determined by small-angle X-ray scattering.