Re‐formation Reaction of Cyclic Nitroxide‐Based Alkoxyamines: Steric and Polar/Stabilization Effects

In nitroxide-mediated radical polymerization, the polymerization times decrease with the increasing re-formation rate constant of the CON bond ( alkoxyamine) between the growing polymer chain and the nitroxide radical. The factors influencing the re-formation rate constant are of considerable interest, but up to now, the polar/stabilization effects have not been addressed thoroughly. The combination of new data with previously reported data now showed that the re-formation rate constant kc increases with the increasing polar character of the substituents attached to the nitroxide moiety. The polar/stabilization effects are weaker for the re-formation than for the homolysis of the CON bond, and may be mainly attributed to the relocation of the odd electron onto the O-atom of the NO moiety, i.e., the stabilization of the nitroxide moiety. Hence, it is possible to predict the values of kc by combining both the polar/stabilization (σI) and steric effects (E), i.e., log(kc/M−1 s−1) = 9.86 + 0.57 ⋅ σI + 0.40 ⋅ Es.