n-Butyl methacrylate has been polymerized in bulk at 130 degrees C in the presence of given amounts of a nitroxide stable free radical (TEMPO = 2,2,6,6-tetramethylpiperidine-N-oxyl) using either a low molar mass alkoxyamine initiator (2,2,6,6-tetramethyl-1-(1-phenethyloxy)piperidine) or a TEMPO-capped polystyrene macroinitiator. Complete consumption of both initiators was always observed. In contrast, very low final monomer conversions were found. Proton NMR spectroscopy and MALDI-TOF mass spectrometry were used for investigation of the polymer structure. They showed that the formed poly(n-butyl methacrylate) had the attached initiator at one end and that a block copolymer was synthesized when the polystyrene macroinitiator was used. The other terminal functionality of the polymer was not a TEMPO-based alkoxyamine but a methylene unsaturation exclusively. Particularly, no saturated polymer which would also be formed by conventional disproportionation reaction between two propagating radicals could be detected. From this result, it was concluded that the main chain-breaking event is the beta-hydrogen transfer from a propagating radical to TEMPO (also called disproportionation reaction). The initial concentration of added TEMPO was shown to directly influence the poly(n-butyl methacrylate) block length independently of the initial concentration of alkoxyamine: the larger the concentration of TEMPO, the shorter the block length. The rate constant of disproportionation was calculated to be k(dis) = 1.4 x 10(6) L mol(-1) s(-1) at 130 degrees C.