The pyridyl alkoxyamine, which is composed of the 1-phenylethyl radical and a pyridyl nitroxide fragments, displays protonation-controlled C?ON bond homolysis. Its dissociation rate constant kd value is approximately halved at 100 degrees C in tert-butyl benzene when it is protonated by one equivalent of trifluoroacetic acid. Moreover, the bulk polymerization of styrene at 125 degrees C is performed with a good control over the molecular weight and the dispersity when initiated with this alkoxyamine under its basic and acidic forms but the protonation has induced a strong decreased polymerization rate. In contrast, in the case of n-butyl acrylate, the control over the polymerization is lost for the protonated pyridyl alkoxyamine because the pyridyl nitroxide is less thermally stable under its acidic form. (C) 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012