Intramolecular hydrogen bonding:: The case of β-phosphorylated nitroxide (= aminoxyl) radical

Alkoxyamines and persistent nitroxide (=aminoxyl) radicals are important regulators of nitroxide-mediated radical polymerization. Since polymerization times decrease with the increasing homolysis rate constant of the C-ON bond homolysis between the polymer chain and the aminooxy moiety, the factors influencing the cleavage rate constant are of considerable interest. It has already been shown that the value of the homolysis rate constant k(d) is very sensitive to the stabilization of both released radical species. X-Ray, EPR, and kinetic data showed that the intramolecular H-bonding radical in the 1-(diethoxyphosphoryl)-2,2-dimethylpropyl 2-hydroxy-1,1-dimethylethyl nitroxide (3a) (homologue of 2-hydroxy1,1-dimethylethyl 1-phenyl-2-methylpropyl nitroxide (2a)) did not occur with the nitroxide moiety as expected but with the phosphoryl group. However, the polymerization rate of styrene (=ethenylbenzene) was significantly enhanced.