Factors Influencing the C−O−Bond Homolysis of Trialkylhydroxylamines

Trialkylhydroxylamines and persistent nitroxide radicals are important regulators of living radical polymerizations. Since the polymerization times decrease with increasing rate of the C−O-bond dissociation between the polymer chain and the nitroxide moiety, the factors influencing the homolysis rate constants are of considerable interest. Here, data are presented for 27 model trialkylhydroxylamines in solution containing TMIO, TEMPO, TEDIO, TIPNO, DBNO, SG1, and similar nitroxide groups and primary, secondary and tertiary carbon-centered leaving radicals R. On the average, the frequency factor is A = 2.6 × 10 14 s -1, and the activation energies reveal steric effects of both the radical and the nitroxide substituents. Moreover, they increase strongly with increasing C−H-bond energy of the corresponding R−H compounds, and for the same R, decrease in the order TMIO > TEMPO > TIPNO > TEDIO > SG1 ≈ DBNO. The rate constants agree well with data reported for polymerizing systems, and increments are given for radical and nitroxide substitutions which should facilitate predictions for other alkoxyamines.