Homolysis and decomposition of alkoxyamines containing PROXYL and TEMPO residues: A comparison

Quantitative ESR and H-1-NMR spectroscopies have been used to study the homolysis and decomposition, respectively of alkoxyamines derived from TEMPO and PROXYL nitroxides. It is shown that alkoxyamines substituted in the beta position with a tert-butoxy group have different activation parameters than those bearing a l-phenethyl fragment, however, there is compensation between transition state enthalpy and entropy, resulting in most cases in similar transition state free energies and homolysis kinetics. On the other hand, beta-substituted alkoxyamines show a much lower tendency to undergo decomposition, while species derived from TEMPO are more prone to decomposition than those from the substituted PROXYL derivative investigated. These findings are used to explain the observed differences in polymerisation behaviour of the various alkoxyamines.