RAFT dispersion copolymerization was carried out using methyl methacrylate (MMA) and tripropylene glycol diacrylate (TPGDA) as comonomers and ethanol/water as a mixed solvent and S-1 dodecyl-S′-(α

α-dimethyl-α″-aceticacid)trithiocarbonate as a chain transfer agent to inhibit the occurrence of gelation.The long nucleation period and gelation were observed in onestep

batch dispersion polymerizations.These problems can be overcomed by a two-stage method — half of MMA

solvent and all RAFT agent and TPGDA were added to the system after the end of the nucleation stage.In this way

coagulum was avoided and the particle size was controlled.The morphology of PMMA particles was observed by optical microscope.Branched poly(methyl methacrylate)(MMA) particles with an average diameter within 1.9～2.7 μm and size distribution of 1.12～1.24 were prepared.Molecular weight

molecular weight distribution

compositions and structure of copolymers were investigated by GPC and 1H-NMR characterization.The GPC curves showed a bimodal distribution

indicating that MMA homopolymer was formed during the nucleation stage and MMA and TPGDA branched copolymer was formed after the nucleation stage as proved by 1H-NMR.TPGDA fraction in the copolymer was determined to be lower than that in the initial monomer system.In addition

the intrinsic viscosity of the copolymer decreased with conversion

and the Mark-Houwink exponent α of copolymer reduced from 0.643 to 0.548

which further confirmed the branched structure of the copolymer.