Co(III)/Alkali-Metal(I) Heterodinuclear Catalysts for the Ring-Opening Copolymerization of CO₂ and Propylene Oxide

The ring-opening copolymerization of carbon dioxide and propene oxide is a useful means to valorize waste into commercially attractive poly­(propylene carbonate) (PPC) polyols. The reaction is limited by low catalytic activities, poor tolerance to a large excess of chain transfer agent, and tendency to form byproducts. Here, a series of new catalysts are reported that comprise heterodinuclear Co­(III)/M­(I) macrocyclic complexes (where M­(I) = Group 1 metal). These catalysts show highly efficient production of PPC polyols, outstanding yields (turnover numbers), quantitative carbon dioxide uptake (>99%), and high selectivity for polyol formation (>95%). The most active, a Co­(III)/K­(I) complex, shows a turnover frequency of 800 h^–1 at low catalyst loading (0.025 mol %, 70 °C, 30 bar CO₂). The copolymerizations are well controlled and produce hydroxyl telechelic PPC with predictable molar masses and narrow dispersity (Đ < 1.15). The polymerization kinetics show a second order rate law, first order in both propylene oxide and catalyst concentrations, and zeroth order in CO₂ pressure. An Eyring analysis, examining the effect of temperature on the propagation rate coefficient (k_p), reveals the transition state barrier for polycarbonate formation: ΔG^‡ = +92.6 ± 2.5 kJ mol^–1. The Co­(III)/K­(I) catalyst is also highly active and selective in copolymerizations of other epoxides with carbon dioxide.