Living behavior of a nitroxide-mediated polymerization of 1,3-butadiene in the presence of diphenyl nitroxide (DPN) as counter radical and hydrogen peroxide (H(2)O(2)) as initiator has been studied to elucidate the bimodal molar mass distribution of the product by four approaches such as hydrolyzation of the oligomeric product, peak separation technique, which defines the oligobutadiene molar mass dependency in relation to a as a ratio of [nitroxide] to [H(2)O(2)], variation of hydroxyl value with respect to a, and para-substituted DPN. It is demonstrated that the synthesized DPN shows efficient control of the polymerization of 1,3-butadiene, but because of the formation of difunctional nitroxide during the oxidation process of aromatic diphenylamine, a bimodal distribution in the final product is observed. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 2560-2565, 2011