The micellar macro-RAFT agent-mediated dispersion polymerization of styrene in the methanol/water mixture is performed and synthesis of temperature-sensitive ABC triblock copolymer nanoparticles is investigated. The thermoresponsive diblock copolymer of poly(N,N-dimethylacrylamide)-block-poly[N-(4-vinylbenzyl)-N,N-diethylamine] trithiocarbonate forms micelles in the polymerization solvent at the polymerization temperature and, therefore, the dispersion RAFT polymerization undergoes as similarly as seeded dispersion polymerization with accelerated polymerization rate. With the progress of the RAFT polymerization, the molecular weight of the synthesized triblock copolymer of poly(N,N-dimethylacrylamide)-block-poly[N-(4-vinylbenzyl)-N, N-diethylamine]-b-polystyrene linearly increases with the monomer conversion, and the PDI values of the triblock copolymers are below 1.2. The disper-sion RAFT polymerization affords the in situ synthesis of the triblock copolymer nanoparticles, and the mean diameter of the triblock copolymer nanoparticles increases with the polymerization degree of the polystyrene block. The triblock copolymer nanoparticles contain a central thermoresponsive poly [N(4-vinylbenzyl)-N,N-diethylamine]block, and the soluble-to-insoluble phase-transition temperature is dependent on the methanol content in the methanol/water mixture. (C) 2014 Wiley Periodicals, Inc.