End-group fidelity in nitroxide-mediated living free-radical polymerizations

In this study, new nitroxides based on the 2,2,5-trimethyl-4-phenyl-3-azahexane-3-oxy skeleton were used to examine chain-end control during the preparation of polystyrene and poly(t-loutyl acrylate) under living free-radical conditions. Alkoxyamine-based initiators with a chromophore attached to either the initiating fragment or the mediating nitroxide fragment were prepared, and the extent of the incorporation of the chromophores at either the initiating end or the propagating chain end was determined. In contrast to 2,2,6,6-tetramethyl piperidinoxy (TEMPO), the incorporation of the initiating and terminating fragment into the polymer chain was extremely high. For both poly(t-butyl acrylate) and polystyrene with molecular weights less than or equal to 70,000, incorporations at the initiating end of greater than 97% were observed. At the terminating chain end, incorporations of greater than 95% were obtained for molecular weights less than or equal to 50,000. The level of incorporation tended to decrease slightly at higher molecular weights because of the loss of the alkoxyamine propagating unit, which had important consequences for block copolymer formation. These results clearly show that these new alpha -H nitroxides could control the polymerization of vinyl monomers such as styrene and t-butyl acrylate to an extremely high degree, comparable to anionic and atom transfer radical polymerization procedures. (C) 2000 John Wiley & Sons, Inc.