Hydroxyl alkoxyamine (HMPAP) in DMF and in tert-butylbenzene with oxygen as a scavenger was studied with electron spin resonance spectroscopy. From kinetics studies performed at 120 degrees C, it was found that the rate constant of C-O bond homolysis was smaller in DMF than in tert-butylbenzene. The difference in the behavior could be related to the formation of an activated six-membered intermediate formed through intramolecular H-bonding in tertbutylbenzene and the disruption of H-bonding in the polar solvent DMF. This led to alteration of the equilibrium constant between the alkoxyamine and the corresponding nitroxide and C-centered radical in DMF and decreased initiating efficiency for the controlled /living radical polymerization using HMPAP as initiator. (c) 2006 Wiley Periodicals, Inc.