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 CO 2 ). 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 CO 2 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.

The controlled synthesis of polyesters via epoxide/anhydride ring-opening copolymerization is a versatile and generally applicable method to make many sustainable polymers, but catalyst activities are limited and the required catalyst loadings are typically high. Here, novel heterodinuclear complexes, featuring Al­(III)/M­(I) (M = Na, K, Rb, Cs), show exceptional activities for phthalic anhydride and cyclohexene oxide copolymerization (catalyst = Al­(III)/K­(I), turnover frequency = 1072 h–1, 0.25 mol % catalyst loading vs anhydride, 100 °C). The Al­(III)/K­(I) catalyst is also tolerant to low loadings, maintaining a good performance at 0.025 mol % catalyst vs anhydride loading and 0.005 mol % vs epoxide. It rapidly polymerizes other epoxide/anhydride combinations yielding various semi-aromatic, rigid, and/or functionalizable polyesters and also shows activity in carbon dioxide/epoxide copolymerizations. The results of structure–activity, X-ray crystallography, polymerization kinetics, and density functional theory investigations support a mechanism with chain growth alternation between the metals. The rate-limiting step is proposed to involve epoxide coordination at Al­(III) with K­(I) carboxylate attack. Future exploitation of abundant and inexpensive Group 1 metals to deliver synergic polymerization catalysts is recommended.

Gas chromatography traces of representative catalytic runs Figure S1. Gas chromatogram of final reaction mixture after hydrogenation of levulinic acid using Triphos Ph /[Ru(acac) 3 ] to generate the catalyst in situ. The stated areas were corrected using K F values determined by calibration experiments with pure substance mixtures.

A combined computational and experimental investigation into the catalytic cycle of carbon dioxide and propylene oxide ring-opening copolymerization is presented using a Co(III)K(I) heterodinuclear complex (Deacy, A. C.et al. Co(III)/ Alkali-Metal(I) Heterodinuclear Catalysts for the Ring-Opening Copolymerization of CO 2 and Propylene Oxide. J. Am. Chem. Soc. 2020, 142(45), 19150−19160). The complex is a rare example of a dinuclear catalyst, which is active for the copolymerization of CO 2 and propylene oxide, a large-scale commercial product. Understanding the mechanisms for both product and byproduct formation is essential for rational catalyst improvements, but there are very few other mechanistic studies using these monomers. The investigation suggests that cobalt serves both to activate propylene oxide and to stabilize the catalytic intermediates, while potassium provides a transient carbonate nucleophile that ringopens the activated propylene oxide. Density functional theory (DFT) calculations indicate that reverse roles for the metals have inaccessibly high energy barriers and are unlikely to occur under experimental conditions. The rate-determining step is calculated as the ring opening of the propylene oxide (ΔG calc † = +22.2 kcal mol −1 ); consistent with experimental measurements (ΔG exp † = +22.1 kcal mol −1 , 50 °C). The calculated barrier to the selectivity limiting step, i.e., backbiting from the alkoxide intermediate to form propylene carbonate (ΔG calc † = +21.4 kcal mol −1 ), is competitive with the barrier to epoxide ring opening (ΔG calc † = +22.2 kcal mol −1 ) implicating an equilibrium between alkoxide and carbonate intermediates. This idea is tested experimentally and is controlled by carbon dioxide pressure or temperature to moderate selectivity. The catalytic mechanism, supported by theoretical and experimental investigations, should help to guide future catalyst design and optimization.