Graphene nanosheets offer intriguing electronic, thermal and mechanical properties and are expected to find a variety of applications in high-performance nanocomposite materials. The great challenge of exfoliating and dispersing pristine graphite or graphene sheets in various solvents or matrices can be achieved by facilely and properly chemical functionalization of the carbon nanosheets. Here we reported an efficient way to functionalize graphene sheets with presynthesized polymer via a combination of atom transfer nitroxide radical coupling chemistry with the grafting-onto strategy, which enable us to functionalize graphene sheets with well-defined polymer synthesized via living radical polymerization. A radical scavenger species, 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), was firstly anchored onto -COOH groups on graphene oxide (GO) to afford TEMPO-functionalized graphene sheets (GS-TEMPO), meanwhile, the GO sheets were thermally reduced. Next, GS-TEMPO reacted with Br-terminated well-defined poly(N-isopropylacrylamide) (PNIPAM) homopolymer, which was presynthesized by SET-LRP, in the presence of CuBr/N,N,N',N',N ''-pentamethyldiethylenetriamineto form PNIPAM-graphene sheets (GS-PNIPAM) nanocomposite in which the polymers were covalently linked onto the graphene via the alkoxy-amine conjunction points. The PNIPAM-modified graphene sheets are easily dispersible in organic solvents and water, and a temperature-induced phase transition was founded in the water suspension of GS-PNIPAM. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 1582-1590, 2011