Low-Dispersity Polymers in <i>Ab Initio</i> Emulsion Polymerization: Improved MacroRAFT Agent Performance in Heterogeneous Media

Abstract

We demonstrate that the in-built monomer-feeding mechanism in an emulsion polymerization can be used to dramatically increase control (providing low molar mass dispersity (Đ) ≤1.15) over polymerizations mediated by reversible addition–fragmentation chain transfer (RAFT) agents with relatively low transfer constants (Ctr). An amphiphilic RAFT agent [RSC(═S)Z], based on a hydrophilic methacrylic R-group [Ċ(CH3)2CO2-PEG] and a hydrophobic Z group with Ctr ≈ 2, was used to mediate the polymerization of a range of methacrylate monomers under both heterogeneous and homogeneous conditions. Consistent with the low Ctr, batch miniemulsion or solution polymerizations did not provide polymers with low Đ. The issue of a low Ctr is overcome in an emulsion polymerization when the [monomer]/[RAFT agent] ratio at the locus of polymerization is substantially lower than the overall ratio, due to the presence of a discrete monomer droplet phase. The proposed mechanism is supported by a theoretical model. As a demonstration of the increased level of control achievable, the system has been exploited to generate methacrylate multiblock copolymers.