Radical crossover in nitroxide mediated 'living' free radical polymerizations

The efficiency of exchange between the mediating nitroxide moieties at the termini of growing polymer chains during 'living' free radical polymerizations has been probed by a series of crossover experiments using functionalized unimolecular initiators. The design of appropriately substituted initiators permitted the synthesis of specifically functionalized model polymers which could be readily distinguished using high-performance liquid chromatography (HPLC). Using these models, the mixture of macromolecules obtained from a 1:1 combination of disparate initiators was separated and identified. The results reveal that exchange of the mediating nitroxide free radicals is a facile process and at essentially all stages of the polymerization a nearly statistical mixture of crossover products is obtained. The HPLC techniques developed are also useful in evaluating the extent of chain termination in nitroxide mediated 'living' free radical polymerizations.