n-Butyl acrylate has been polymerized in the presence of an alkoxyamine initiator derived from the PROXYL nitroxide 2,2',5-trimethyl-5'-phenylpyrrolidinyl-1-oxyl. It was found that polymerizations were rapid, reaching almost complete conversion within 2 h; in contrast, no conversion was observed when TEMPO was used as the mediator. The addition of a small amount of free nitroxide resulted in slower polymerizations although an induction period, the length of which varied with excess nitroxide concentration, was observed. Size exclusion chromatography indicated that polymerization control was poor; M-n initially increased rapidly and then much more slowly, and polydispersities were found to be broad and to increase with conversion. Quantitative(13) C NMR spectroscopy revealed the resulting poly(n-butyl acrylate) to be branched. Despite the poor control, the PBA was able to act as a macroinitiator for the polymerization of styrene, yielding a block copolymer with a growth in Mn and a reduction of polydispersity with conversion. It is suggested that the polymerization of n-butyl acrylate in the presence of the substituted PROXYL derivative is living but not controlled.