Acyclic β-Phosphonylated Nitroxides:  A New Series of Counter-Radicals for “Living”/Controlled Free Radical Polymerization

Oxidation of α-(N-alkylamino) phosphonic acid esters, carrying one or two alkyl groups as substituents on their α-carbon, by m-chloroperbenzoic acid afforded the corresponding stable β-phosphonylated nitroxides. The nitroxides derived from α-mono-tert-butyl α-alkylaminophosphonic acid esters are stable compounds despite the presence of a hydrogen atom on the α-carbon bound to the nitroxyl group. The ESR study of these nitroxides in solution showed that this β-hydrogen atom lies in the nodal plane to the nitroxyl function. These β-phosphonylated nitroxides were found to efficiently control the free radical polymerization reaction of styrene, with a much faster rate of propagation than that observed in TEMPO-mediated systems.