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 CON 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 CON bond, and may be mainly attributed to the relocation of the odd electron onto the O‐atom of the NO 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$E\rm{_{{s}}^1 }$), i.e., log(kc/M−1 s−1) = 9.86 + 0.57 ⋅ σI + 0.40 ⋅ Es.