Highly branched poly(N-isopropylacrylamide) (PNIPAM) has been synthesized by a reversible addition-fragmentation chain transfer (RAFT) copolymerization of NIPAM and a vinyl contained trithiocarbonate RAFT agent. H-1-NMR measurements revealed that the degrees of branch (DB) are in the range of 0.032-0.105. Laser light scattering (LLS) measurements gave the hydrodynamic radii (Rh) of the polymers to be 3.6-5.7 nm with molecular weight in the range of 1.3 x 10(4) g/mo1-2.3 x 10(-4) g/mol. Highly branched PNIPAM with terminal thiol groups were obtained by aminolysis the polymers, and the product can be oxidized by air to form disulfide bonds (-S-S-) among chains and resulting in the formation of nanoparticle in aqueous solution. Interestingly, the nanoparticle in size of R-h (similar to) 80 nm showed a thermogelling behavior to form bulk hydrogel when the temperature was increased up to 25 degrees C due to the thermo-induced association of the PNIPAM chains among the nanoparticles. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 118: 3391-3399, 2010