Doubly thermoresponsive ABC brush-linear-linear triblock copolymer nanoparticles of poly[poly(ethylene glycol) methyl ether vinylphenyl]-block-poly(N-isopropylacrylamide)block- polystyrene [P(mPEGV)-b-PNIPAM-b-PS] containing two thermoresponsive blocks of poly[poly(ethylene glycol) methyl ether vinylphenyl] [P(mPEGV)] and poly(N-isopropylacrylamide) (PNIPAM) are prepared by macro-RAFT agent mediated dispersion polymerization. The P(mPEGV)-b-PNIPAM-b-PS nanoparticles exhibit two separate lower critical solution temperatures or phase-transition temperatures (PTTs) corresponding to the linear PNIPAM block and the brush P(mPEGV) block in water. Upon temperature increasing above the first and then the second PTT, the hydrodynamic diameter (D-h) of the triblock copolymer nanoparticles undergoes an initial shrinkage at the first PTT and the subsequent shrinkage at the second PTT. The effect of the chain length of the PNIPAM block on the thermoresponsive behavior of the triblock copolymer nanoparticles is investigated. It is found that, the longer chains of the thermoresponsive PNIPAM block, the greater contribution on the transmittance change of the aqueous dispersion of the triblock copolymer nanoparticles. (c) 2014 Wiley Periodicals, Inc.