Cationic Organomagnesium Complexes as Highly Active Initiators for the Ring-Opening Polymerization of ε-Caprolactone

Species of the form [LH](+)[BR4](-) (R = pentafluorophenyl, phenyl, 1) were synthesized by reaction of Bronsted acids with a novel bis-phosphinimine ligand (L = 4,6-(MesN-PPh2)(2)dibenzofuran). Corresponding cationic complexes [LMg '' Bu](+)[BR4](-) (2) were produced by reaction of [Mg '' Bu-2] with [LH](+)[BR4](-). Organomagnesium species 2a and 2b exhibit extremely high activity as initiators for the polymerization of epsilon-caprolactone, yielding near-quantitative conversion of monomer to high molecular weight (>2.0 x 10(5) g/mol) polymer in 4 min at ambient temperature.