Influence of the nitroxide structure on the homolysis rate constant of alkoxyamines: a Taft-Ingold analysis.
Alkoxyamines and persistent nitroxyl radicals are important regulators of living radical polymerizations. Because polymerization times decrease with the increasing rate of the homolytic C-O bond cleavage between the polymer chain and the nitroxide moiety, the factors influencing the homolysis rate are of considerable interest. Here, we present an analysis of the cleavage rate constants for 28 alkoxyamines carrying the styryl (PhEt) group as leaving alkyl radical in terms of polar inductive/field (sigmaL) and steric (Es) effects of the nitroxide substituents, using the Taft-Ingold equation, i.e., log(k/k0) = rhoLsigmaL + deltaEs. The rate constants are shown to increase with the increasing electron-donating capacities, the steric demand, and the intramolecular (hydrogen) bonding capabilities of the substituents. A good correlation, (R2 = 0.95, 23 data) log kd = -3.07sigmaL - 0.88Es - 5.88, is obtained, which should facilitate the design of new nitroxyl radicals and alkoxyamine regulators.