RAFT ab initio emulsion copolymerization of gamma-methyl-alpha-methylene-gamma-butyrolactone (MeMBL) and styrene with various monomer ratios was investigated with PAA(20)-PS5 trithiocarbonate being mediator and surfactant at 70 degrees C. The polymerization was fully controlled as evidenced by stable latexes, well predicted molecular weights and low PDIs about 1.2 when [MeMBL]:[St] <= 1:2. However, in the case of [MeMBL]:[St] = 1:1, the molecular weight became out of control due to gel effect of RAFT reactions. The relatively water-soluble MeMBL was found to dramatically influence the polymerization kinetics and copolymer properties, as evidenced by (I) No inhibition period was observed; (2) The copolymer compositions in the early stage of polymerization were much higher than those predicted by the Mayo-Lewis equation in the case of [MeMBL]:[St] = 1:2, leading to the formation of linear gradient copolymer. (3) T(g)s of the copolymers from emulsion copolymerization were higher than those from bulk copolymerization. It was concluded that we could synthesize the gradient copolymer with preset molecular weight and low PDI in RAFT emulsion polymerization, T-g of which should be much higher than 100 degrees C. (C) 2013 Elsevier Ltd. All rights reserved.