We report the facile atom transfer radical polymerization (ATRP) of a commercially available hydrophilic monomer, methoxy-capped oligo(ethylene glycol) methacrylate (OEGMA), under remarkably mild conditions. Various bromide-based initiators, in conjunction with a copper-based catalyst, allow rapid homopolymerization of OEGMA in water at 20 degrees C. Good living character was achieved with two ligands, namely 2,2'-bipyridine (bpy) and 1,1,4,7,10,10-hexamethyltriethylenetetramine (HMTETA). Kinetic studies confirm that, for bpy, 90% conversion is typically achieved within 20 min, and the polymerization is first; order even up to very high conversions. Molecular weight increases linearly with conversion and is close to the target molecular weight in all cases. Polydispersities remain narrow (M-w/M-n = 1.15-1.30) throughout the polymerization. It is possible to lower the copper catalyst by a factor of 10 (i.e., [Cu(I)]/[initiator] = 0.10) without significant loss of control over the polymerization, which is good evidence for the highly active nature of the copper catalyst. To investigate the role played by the water in promoting rapid ATRP, selected bulk polymerizations were also carried out. With the bpy ligand it was found that the polymerization was living but slow: 90% conversion required 15 h at 20 degrees C even though the monomer, initiator, and catalyst concentrations were significantly higher in the bulk than in aqueous ATRP. Remarkably, the polydispersity of the final polymer isolated from this slow bulk polymerization was relatively high at 1.30, whereas typical final polydispersities for aqueous ATRP are 1.20-1.25. There are several factors that may account for the unexpectedly rapid, yet controlled, aqueous ATRP of OEGMA at 20 degrees C. The highly polar nature of water is believed to promote the formation of a very active mononuclear [Cu(bpy)(2)](+) catalyst. Furthermore, since OEGMA is a sterically congested monomer, its termination rate constant is several orders of magnitude lower than that of conventional methacrylates, which is expected to lead to improved living character. A third possibility is that the amphiphilic nature of the OEGMA monomer may lead to micellar polymerization, but no evidence was found for monomer micelles prior to ATRP. Finally, the cationic copper catalyst is easily removed from the aqueous reaction solution using silica chromatography, and the nonionic OEGMA homopolymer is readily isolated as a colorless solid.