Steric and Polar Effects of the Cyclic Nitroxyl Fragment on the C−ON Bond Homolysis Rate Constant

Alkoxyamines and persistent nitroxyl radicals are important regulators of nitroxide mediated radical polymerization (NMP). Because polymerization times decrease with increasing rate constant of the homolysis of the C-ON bond between the polymer chain and the nitroxyl moiety, the factors influencing the cleavage rate constant are of considerable interest. Here, we present the measurements of the rate constants (k d ) of the C-ON bond cleavage in new cyclic alkoxyamine models. The homolysis rate constants of 9 new alkoxyamines and 33 others given by the literature are analyzed with regards to the contributions of the polar inductive/field (σ I ) effect, the steric (E s ) effect and the intramolecular hydrogen bonding (IHB) effect of the nitroxyl moieties, using the multiparameter equation established by Marque, i.e., log(k d /k d,0 ) = -3.07σ I - 0.88E s - 5.88. Cyclic steric constants r(R i ) for seven- and eight-membered rings are developed. Analysis of the results provides new insight on the importance of the conformation of the alkoxyamine on the values of k d .