Alkoxyamine Re-Formation Reaction. Effects of the Nitroxide Fragment: A Multiparameter Analysis
Résumé
A few years ago, Studer and co-workers (Macromolecules 2006, 39, 1347-1352) reported the dramatic effect of the reaction of re-formation of alkoxyamines on the fate of the nitroxide-mediated polymerization (NMP) of styrene. This prompted us to investigate more carefully the effects of the nitroxide structure on the re-formation rate constant k(c). Ten new values of k(c) were obtained for the reaction of imidozalidine nitroxide and the phenethyl radical. These values were combined with the 21 values of k(c) reported in the literature for a multiparameter analysis (log(k(c)/M-1 s(-1)) = (10.22 +/- 0.10) + (0.46 +/- 0.02)E-s + (0.41 +/- 0.17)sigma(1)) using the electrical Hammett constant sigma(1) to describe both the stabilization and polar effects as well as the modified Taft steric constant E-s of the nitroxide. The same analysis was performed for the k(c) values of the cross-coupling reaction of nitroxides with tert-butoxylcarbonyl-2-prop-2-yl radical (log(k(c)/M-1 s(-1)) = (11.10 +/- 0.25) + (0.57 +/- 0.05)E-s + (1.42 +/- 0.18)sigma(1)) and tert-butoxycarbonylethyl radical (log(k(c)/M-1 s(-1)) = (10.23 +/- 0.16) + (0.35 +/- 0.03)E-s + (0.93 +/- 0.25)sigma(1)). These correlations were applied for the analysis of the NMP of styrene controlled by 6 pi(.), 6 theta(.), and 6p(.) using a Fischer phase diagram.