Rate enhancement and retardation strategies in living free radical polymerizations mediated by nitroxides and other persistent species: A theoretical assessment

Three strategies to improve the performance of living and controlled polymerizations mediated by persistent radicals are analyzed kinetically to explore their advantages and limits. First, rate enhancements by an additional conventional initiation or by the monomer self-initiation are found only compatible with high degrees of polymer livingness and control if the additional initiation rate is much smaller than the activation rate of the dormant polymer. Second, rate-enhancing reductions of the persistent radical concentration by reactions with additives or by a natural decay lead to living and controlled polymers even if these processes are surprisingly fast. The enhanced monomer conversion rates generally diminish the detrimental effects of a direct precursor decay or of the radical disproportionation to unreactive products. Third, a retarding initial excess of the persistent species can force unfavorably fast systems to livingness and control. The limits of these procedures are specified. Analytic equations are given which facilitate the extraction of kinetic data from conversions, degrees of polymerization and polydispersities. They agree with experimental findings.