The H-atom transfer reaction was studied for a series of imidazoline, imidazolidine, and pyrrolidine-based alkoxyamines containing either isobutyrate-2-yl or 1-phenylethyl alkyl fragments. The C-O bond homolysis rate constants and activation energies were determined by H-1 NMR product analysis as a function of temperature. Inter- and intramolecular H-atom transfer reactions were distinguished by examination of alkoxyamine thermolysis products in the absence and in the presence of a radical scavenger (thiophenol or deuterated styrene). A correlation between the structure of the nitroxyl fragment of alkoxyamine and the H-transfer reaction was found. The high steric demands of the substituents on the nitroxyl part of the isobutyrate-2-yl alkoxyamine decrease both types of reaction. For alkoxyamines containing the 1-phenylethoxyamines, neither inter- nor intramolecular H-atom transfer was observed. Controlled polymerization of methylmethacrylate initiated with imidazoline-based alkoxyamine was observed, although the polymer obtained was not living. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 6579-6595, 2009