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 (D) <= 1.15) over polymerizations mediated by reversible addition-fragmentation chain transfer (RAFT) agents with relatively low transfer constants (C-tr). An amphiphilic RAFT agent [RSC(= S)Z], based on a hydrophilic methacrylic R-group [(C) over dot(CH3)(2)CO2-PEG] and a hydrophobic Z group with C-tr approximate to 2, was used to mediate the polymerization of a range of methacrylate monomers under both heterogeneous and homogeneous conditions. Consistent with the low C-tr, batch miniemulsion or solution polymerizations did not provide polymers with low D. The issue of a low C-tr 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.