Factors influencing C-ON bond homolysis in alkoxyamines:: Unexpected behavior of SG1 (N-(2-methyl-2-propyl)-N-(1-diethylphosphono-2,2-dimethylpropyl)-N-oxyl)-based alkoxyamines

Alkoxyamines and persistent nitroxides are important regulators of nitroxide-mediated radical polymerization (NMP). Since the polymerization time decreases with the increasing equilibrium constant K (k(d)/k(c)), i.e., the increasing rate constant k(d) of the homolysis of the C-ON bond between the polymer chain and the nitroxide moiety, the factors influencing the cleavage rate constants are of considerable interest. SG1-based alkoxyamines have turned out to be the most potent alkoxyamine family to use for NMP of various monomers. Therefore, it is of high interest to determine the factors which make SG1 derivatives better regulators than TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl) derivatives. Contrary to what we had observed with TEMPO derivatives, we observed two relationships for the plot E-a vs BDE(C-H), one for the nonpolar released alkyl radicals (E-a (kJ/mol) = -133.0 + 0.72BDE) and the other one for the polar released alkyl radicals (E-a (kJ/mol) = -137.0 + 0.69BDE). However, for both families (SG1 and TEMPO derivatives), the rate constants kd of the C-ON bond homolysis were correlated to the cleavage temperature T-c (log(k(d)(S-1)) = 1.51 - 0.058T(c)). Such correlations should help to design new alkoxyamines to use as regulators and to improve the tuning of NMP experiments.