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 Brönsted acids with a novel bis-phosphinimine ligand (L = 4,6-(MesNPPh2)2dibenzofuran). Corresponding cationic complexes [LMgnBu]+[BR4]− (2) were produced by reaction of [MgnBu2] with [LH]+[BR4]−. Organomagnesium species 2a and 2b exhibit extremely high activity as initiators for the polymerization of ε-caprolactone, yielding near-quantitative conversion of monomer to high molecular weight (>2.0 × 105 g/mol) polymer in 4 min at ambient temperature.