A new cyclic nitroxide I and the corresponding alkoxyamines 9 and 10 were synthesized and the polymerization of styrene (St) initiated with 10 was investigated. The NO-C bond of 9 is very weak, cleaving at room temperature. On the other hand, alkoxyamine 10 is stable at room temperature and the A(act) and E-act for the NO-C bond homolysis were determined to be 1.4 x 10(15) s(-1) and 124.5 kJ mol(-1), respectively. When the polymerization of St was carried out at 70 degrees C, the resultant poly(St) showed narrow polydispersities below 1.25. In the polymerization at 90 degrees C, the resulting poly(St) showed narrow polydispersity until 60% conversion, but Mw/Mn was rapidly increased above 60% conversion. On the other hand, the polymerization at 120 degrees C gave poly(St) with broad polydispersities. The unusual polymerization behavior was discussed on the basis of the SEC and ESR results. (c) 2006 Elsevier Ltd. All rights reserved.