A novel dinitroxide mediating agent that was suitable for stable free-radical polymerization was synthesized and used in the block copolymerization of styrene and t-butyl styrene. Quantitative yields of a novel dinitroxide based on 1,6-hexamethylene diisocyanate and 4-hydroxy-2,2,6,6-tetramethyl-1-piperidinyloxy were obtained. Various experimental parameters, including the nitroxide-to-initiator molar ratio, were examined, and it was determined that the polymerization was most controlled under conditions similar to those of conventional 2,2,6,6-tetramethyl-1-piperidinyloxy-mediated stable free-radical polymerization. Moreover, the dinitroxide mediator proved to be a viable route for the facile two-step synthesis of triblock copolymers of styrene and t-butyl styrene. However, the dinitroxide mediation process resulted in a higher than expected level of nitroxide decomposition, which resulted in polymers possessing a terminal alkoxyamine and an adjacent hydroxylamine rather than a preferred internal bisalkoxyamine. This decomposition resulted in the formation of diblock copolymer species during the triblock copolymer synthesis. Gel permeation chromatography was used to monitor the chain-end decomposition kinetics, and the determined observed rate constant (5.89 X 10(-5) s(-1)) for decomposition agreed well with previous studies for other dinitroxide mediating agents. (C) 2004 Wiley Periodicals, Inc.