Quantifying CO 2 Insertion Equilibria for Low-Pressure Propene Oxide and Carbon Dioxide Ring Opening Copolymerization Catalysts.

While outstanding catalysts are known for the ring-opening copolymerization (ROCOP) of CO 2 and propene oxide (PO), few are reported at low CO 2 pressure. Here, a new series of Co(III)M(I) heterodinuclear catalysts are compared. The Co(III)K(I) complex shows the best activity (TOF = 1728 h -1 ) and selectivity (>90% polymer, >99% CO 2 ) and is highly effective at low pressures (<10 bar). CO 2 insertion is a prerate determining chemical equilibrium step. At low pressures, the concentration of the active catalyst depends on CO 2 pressure; above 12 bar, its concentration is saturated, and rates are independent of pressure, allowing the equilibrium constant to be quantified for the first time ( K eq = 1.27 M -1 ). A unified rate law, applicable under all operating conditions, is presented. As proof of potential, published data for leading literature catalysts are reinterpreted and the CO 2 equilibrium constants estimated, showing that this unified rate law applies to other systems.