Mechanistic Aspects of the Copolymerization of CO₂ with Epoxides Using a Thermally Stable Single-Site Cobalt(III) Catalyst

The mechanism of the copolymerization of CO2 and epoxides to afford the corresponding polycarbonates catalyzed by a highly active and thermally stable cobalt(III) complex with 1,5,7-triabicyclo[4,4,0] dec-5-ene (designated as TBD, a sterically hindered organic base) anchored on the ligand framework has been studied by means of electrospray ionization mass spectrometry (ESI-MS) and Fourier transform infrared spectroscopy (FTIR). The single-site, cobalt-based catalyst exhibited excellent activity and selectivity for polymer formation during CO2/propylene oxide (PO) copolymerization even at temperatures up to 100 °C and high [epoxide]/[catalyst] ratios, and/or low CO2 pressures. The anchored TBD on the ligand framework plays an important role in maintaining thermal stability and high activity of the catalyst. ESI-MS and FTIR studies, in combination with some control experiments, confirmed the formation of the carboxylate intermediate with regard to the anchored TBD on the catalyst ligand framework. This analysis demonstrated that the formed carboxylate intermediate helped to stabilize the active Co(III) species against decomposition to inactive Co(II) by reversibly intramolecular Co−O bond formation and dissociation. Previous studies of binary catalyst systems based on Co(III)−Salen complexes did not address the role of these nucleophilic cocatalysts in stabilizing active Co(III) species during the copolymerization. The present study provides a new mechanistic understanding of these binary catalyst systems in which alternating chain-growth and dissociation of propagating carboxylate species derived from the nucleophilic axial anion and the nucleophilic cocatalyst take turns at both sides of the Co(III)−Salen center. This significantly increases the reaction rate and also helps to stabilize the active SalenCo(III) against decomposition to inactive SalenCo(II) even at low CO2 pressures and/or relatively high temperatures.