Methyl methacrylate (MMA) was polymerized in bulk at 70 T in the presence of an alkoxyamine initiator with low dissociation temperature (the so-called BlocBuilder (TM)) and increasing amounts of free N-tert-butyl-N-(1-diethylphosphono-2,2-dimethylpropyl) nitroxide (SG1). Low final monomer conversions were reached, indicating a loss in radical activity due to side reactions such as irreversible homoterminations between the propagating radicals and beta-hydrogen transfer (also called disproportionation) from a propagating radical to a free-SG1 nitroxide. Proton NMR and MALDI-TOF mass spectrometry were used to analyze the polymer chain-ends and to clearly identify the main mechanism of irreversible termination. In particular, it was shown that all polymer chains were terminated by an alkene function in the presence of a large excess of free SG1, meaning that beta-hydrogen transfer from PMMA propagating radicals to the nitroxide SG1 was the major chain-stopping event. On the other hand, for a low excess of free SG1, the two termination modes coexisted. Kinetic modeling was then performed using the PREDICI software, and the rate constant of beta-hydrogen transfer, k(beta Htr), was estimated to be 1.69 x 10(3) L mol(-1) s(-1) at 70 degrees C. (C) 2008 Wiley Periodicals, Inc.