Derivatives of 2,2,5,5-tetraalkylimidazolidin-4-one-1-oxyl have been found to offer significant advantages over many of the nitroxides previously employed in nitroxide-mediated polymerization: homopolymers [e.g., polystyrene and poly(alkyl acrylate)], statistical copolymers [e.g., poly(styrene-co-acrylonitrile)], and block copolymers [e.g., poly(4-methylstyrene)-block-polystyrene and poly(n-butyl acrylate)-block-polystyrene] can be synthesized with controlled molecular weight, narrow molecular weight distribution, and defined end group functionality. The effect of substituents a to the nitroxide nitrogen and at the transannular nitrogen on the outcome of polymerization is explored. Appropriate selection of these substituents provides polymers of lower polydispersities than, for example, 2,2,6,6-tetramethylpiperidin-2-oxyl and derivatives. The nitroxides are synthesized from readily available precursors by a simple experimental route. Moreover, they are subject to fewer side reactions. In particular, disproportionation between the propagating radical with nitroxide appears to be of lesser significance.