Temperature-responsive self-assembly of star block copolymers with poly(ionic liquid) segments

Abstract

Novel star block copolymers containing poly(N-vinylimidazolium salt) as a poly(ionic liquid) segment and poly(N-isopropylacrylamide) (poly(NIPAAm)) as a thermoresponsive segment were synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization. Two R-designed tetrafunctional chain transfer agents (CTAs), including a xanthate-type CTA and a dithiocarbamate-type CTA, were compared for the polymerization of 1-ethyl-3-vinylimidazolium bromide (VEI-Br), which is an ionic liquid-type monomer. The dithiocarbamate-type tetrafunctional CTA was the most efficient CTA for the controlled synthesis of four-armed poly(VEI-Br) stars with low polydispersity values and controlled molecular weights. Star block copolymers with inner thermoresponsive segments connected to their core were synthesized by the RAFT polymerization of VEI-Br, using poly(NIPAAm) stars. In contrast, the RAFT polymerization of NIPAAm using poly(VEI-Br) stars afforded star block copolymers, with block arms consisting of outer block copolymer segments of thermoresponsive poly(NIPAAm). Thermally induced phase separation behavior and assembled structures of star block copolymers were studied in aqueous solution. Four-arm star block copolymers comprising poly(N-vinylimidazolium salt) as a poly(ionic liquid) segment and poly(NIPAAm) as a thermoresponsive segment were synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization. Stimuli-responsive properties and temperature-responsive self-assembly process of these star block copolymers were studied in aqueous solution. Chain architectures, the sequence and comonomer composition of the diblock arms, and salt concentration had a significant effect on their thermally induced phase separation behavior and assembled structures.