RAFT Emulsion Polymerization for (Multi)block Copolymer Synthesis: Overcoming the Constraints of Monomer Order

Abstract

Synthesis of (multi)block copolymers using reversible addition–fragmentation chain transfer (RAFT) polymerization generally suffers from the limitation that the order of the blocks must be considered. Herein, syntheses of block copolymers by RAFT polymerization using trithiocarbonate RAFT species are conducted as solution, miniemulsion, and emulsion polymerizations to demonstrate that the issue of monomer order for styrene and butyl methacrylate can be largely overcome in emulsion polymerization under carefully chosen conditions. The presence of monomer droplets in emulsion polymerization—in addition to polymer particles that constitute the locus of polymerization—leads to a reduction in the ratio of RAFT end groups to monomer at the locus of polymerization. Consequently, fragmentation of the RAFT adduct radical in the “backward” (“wrong”) direction is associated with fewer monomer additions, thus minimizing the impact of this undesired kinetic event. It is demonstrated that RAFT emulsion polymerization can be exploited to prepare an alternating pentablock copolymer composed of methacrylates (with 10 mol % styrene) and styrene without consideration of monomer order, thereby significantly broadening the scope of RAFT polymerization for multiblock copolymer synthesis.