Styrene is polymerized in the presence of 4-methacryloyloxy-2,2,6,6-tetramethylpiperidine-1-oxy. The polymerization rate is higher than that using 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxy as counterradical. However, the molecular weight does not increase linearly with monomer conversion and the ultimate polydispersity is rather high. After posttreating with ascorbic acid, the molecular weight decreases and a narrow polydispersity is obtained. A branched or hyperbranched structure was thus suggested.