ALKOXYAMINE-INITIATED LIVING RADICAL POLYMERIZATION - FACTORS AFFECTING ALKOXYAMINE HOMOLYSIS RATES

Previous work from these laboratories has shown that the success of al koxyamine-initiated living radical polymerization (polydispersity of p roduct, rate of polymerization) is critically dependent on the rate of homolysis of the C-O bond of the alkoxyamine initiator. Half-lives fo r a range of alkoxyamines based on initiator-derived radicals or low m olecular weight propagating species have been measured experimentally. The values show a marked dependence on the structure of both the nitr oxide and radical components. In this work, we demonstrate that semiem pirical molecular orbital calculations provide a reliable, though qual itative, prediction of the experimentally observed trends in alkoxyami ne homolysis rates. For example, for a series of alkoxyamines based on nitroxides (R(CH3)(2)C)(2)NO ., C-O bond dissociation energies are pr edicted to decrease with an increase in ring size or the C-N-C angle ( i.e., 5-membered > 6-membered > open chain > 7-membered), which is in accord with experimental results (5-membered > 6-membered > open chain ). More importantly, calculations allow an assessment of the relative importance of steric and polar factors and radical stability in determ ining the order of alkoxyamine homolysis rates. In the case of seconda ry and tertiary alkoxyamines, steric factors appear to be the dominant influence. The calculations have application in the design of new ini tiators for living radical polymerization.