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Understanding metal synergy in heterodinuclear catalysts for the copolymerization of CO2 and epoxides

Abstract

Carbon dioxide and epoxide copolymerization is an industrially relevant means to valorize waste and improve sustainability in polymer manufacturing. Given the value of the polymer products—polycarbonates or polyether carbonates—it could provide an economic stimulus to capture and storage technologies. The process efficiency depends upon the catalyst, and previously Zn(ii)Mg(ii) heterodinuclear catalysts showed good performances at low carbon dioxide pressures, attributed to synergic interactions between the metals. Now, a Mg(ii)Co(ii) catalyst is reported that exhibits significantly better activity (turnover frequency > 12,000 h−1) and high selectivity (>99% CO2 utilization and polycarbonate selectivity) for carbon dioxide and cyclohexene oxide copolymerization. Detailed kinetic investigations show a second-order rate law, independent of CO2 pressure from 1–40 bar, to produce polyols. Kinetic data also reveal that synergy arises from differentiated roles for the metals in the mechanism: epoxide coordination occurs at Mg(ii), with reduced transition state entropy, while the Co(ii) centre accelerates carbonate attack by lowering the transition state enthalpy. This rare insight into intermetallic synergy rationalizes the outstanding catalytic performance and provides a new feature to exploit in other homogeneous catalyses.

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Fig. 1: Ring opening copolymerization (ROCOP) of cyclohexene oxide (CHO) and carbon dioxide, catalysed by a heterodinuclear catalyst.
Fig. 2: Preparation, characterization and polymerization kinetics of the MgCo catalyst.
Fig. 3: The chain shuttling mechanism for the copolymerization (CHO–CO2 ROCOP) using MgCo.
Fig. 4: Insight into the factors governing heterodinuclear synergy in polymerization catalysis using kinetic data to compare the MgCo heterodinuclear catalyst with homodinuclear analogues MgMg and CoCo.

Data availability

All the data supporting the findings of this study are available within the paper and its Supplementary Information files or from the corresponding author on request.

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Acknowledgements

We acknowledge the EPSRC (EP/L017393/1, EP/S018603/1, EP/K014668/1) and EIT Climate KIC (EnCO2re) for research funding. A.C.D. acknowledges an EPSRC CASE award, with Econic Technologies, for financial support. A.F.R.K. acknowledges the support of Wadham College, Oxford for an RJP Williams Junior Research Fellowship. A.R. acknowledges Imperial College London for a Junior Research Fellowship (JRF).

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A.C.D. and C.K.W. conceived the project. A.C.D. designed and performed the synthetic experiments. A.F.R.K. designed and performed the cyclic voltammetry and SQUID experiments. A.R. designed and performed the XPS study. A.C.D. and C.K.W. prepared the manuscript.

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Correspondence to Charlotte K. Williams.

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C.K.W. is a director of Econic Technologies.

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Experimental characterization; analysis of the experimental data; proposed mechanisms.

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Deacy, A.C., Kilpatrick, A.F.R., Regoutz, A. et al. Understanding metal synergy in heterodinuclear catalysts for the copolymerization of CO2 and epoxides. Nat. Chem. 12, 372–380 (2020). https://doi.org/10.1038/s41557-020-0450-3

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