The photoinduced solution polymerization of 4-methacryloyl-1,2,2,6,6-pentamethyl-piperidinyl (MPMP), used as a reactive hindered amine piperidinol derivative, was performed. The obtained MPMP homopolymer had a very narrow molecular weight distribution (1.06-1.39) according to gel permeation chromatography. The number-average and weight-average molecular weights increased linearly with the monomer conversion, this being characteristic of controlled/living free-radical polymerizations. Electron spin resonance signals were detected in the MPMP homopolymer and in a polymer mixture solution, and they were assigned to nitroxide radicals, which were bound to the polymer chains and persisted at a level of 10(-9) mol/L during the polymerization. Instead of the addition of mediated nitroxide radicals such as 2,2,6,6-tetramethyl-piperidinyl-1-oxy (TEMPO), those radicals (>N-O .) were formed in situ during the photopolymerization of MPMP, and so the reaction mechanism was understood as being similar to that of TEMPO-mediated controlled/living free-radical polymerization. (C) 2004 Wiley Periodicals, Inc.