Amphiphilic homopolymers of the hydroxy-functional macromonomer poly(ethylene glycol) monomethacrylate (PEGMA) and its copolymers with methyl methacrylate (MMA) were prepared by atom transfer radical polymerization. Commercially available PEGMA (Aldrich) contains nonfunctional poly(ethylene glycol) (PEG), monofunctional PEGMA, and difunctional poly(ethylene glycol) dimethacrylate (PEGDMA) in a 1:3:1 ratio as analyzed by HPLC. A solvent extraction procedure yielded a PEGMA enriched mixture (PEG:PEG-MA:PEGDMA = 5:92:3) with PEGMA M. values of 480 and 410 Da based on HPLC and H-1 NMR, respectively. ATRP homopolymerization of this purified PEGMA in the hydroxyl-bearing solvents, cyclohexanol and ethanol (epsilon = 16.4 and 25.3, respectively), yielded well-defined homopolymers (M-w/M-0 < 1.1). On the other hand, ATRP copolymerizations with MMA (PEGMA content = 10, 18, 24, 30, and 40 mol %) yielded best results in non-hydrogen-bonding diphenyl ether (epsilon = 3.37). The homopolymer and copolymers containing more than 24 mol % (57 wt %) PEGMA were water-soluble and exhibited sharp lower critical solution temperatures (LCST) that increased with increasing PEGMA content as expected. Unlike similar PEGMA-based amphiphilic copolymers prepared by conventional free radical polymerizations, the ATR polymerizations reported here proceeded to high conversions (60-100%) and yielded well-defined polymers (M-w/M-n = 1.1-1.15) with no gel fraction that remained linear and water-soluble after storage in ambient conditions for several months.