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

Arron C Deacy,Charlotte K Williams,Andreas Phanopoulos,Emma Moreby

doi:10.1021/jacs.0c07980

Arron C Deacy, Charlotte K Williams + Show 2 more

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https://doi.org/10.1021/jacs.0c07980

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Abstract

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 (kp), 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.

Highlights

Carbon dioxide utilization is a grand challenge for contemporary chemistry, and several efficient reactions are known, few are viable or true utilizations when considered more broadly.[1,2] Many require esoteric, greenhouse gas emitting, and expensive stoichiometric reagents, while others yield products with no current large-scale application, or need unacceptably high catalyst loadings or operate under conditions, including carbon dioxide purity, incompatible with scale-up.[3]
Epoxide and carbon dioxide ringopening copolymerization (ROCOP) is well suited for largescale deployment since it applies already commercial monomers, enables significant carbon dioxide sequestration, and produces valuable polymers whose properties allow for the replacement of existing petrochemicals.[3−5] This process can sequester up to 50 wt % carbon dioxide into the polymer and it is truly catalytic allowing for multiple turnovers and high converson of epoxide.[3−5] At the cutting-edge are carbon dioxide and propylene oxide derived polyols which are low molar mass, hydroxyl end-capped polypropylene carbonate (PPC) or polyethercarbonates
These polyols are applied to make polyurethanes to construct mattresses and furniture foams, insulation sheet foam, coatings, sealants, and elastomers.[4,6−8] Life-cycle assessment shows that polyols with just 20 wt % CO2 content display a ∼20% reduction in green-house gas emissions and fossil fuel consumption compared to currently used petrochemicals.[9]

Summary

■ INTRODUCTION

Carbon dioxide utilization is a grand challenge for contemporary chemistry, and several efficient reactions are known, few are viable or true utilizations when considered more broadly.[1,2] Many require esoteric, greenhouse gas emitting, and expensive stoichiometric reagents, while others yield products with no current large-scale application, or need unacceptably high catalyst loadings or operate under conditions, including carbon dioxide purity, incompatible with scale-up.[3] In contrast, epoxide and carbon dioxide ringopening copolymerization (ROCOP) is well suited for largescale deployment since it applies already commercial monomers, enables significant carbon dioxide sequestration, and produces valuable polymers whose properties allow for the replacement of existing petrochemicals.[3−5] This process can sequester up to 50 wt % carbon dioxide into the polymer and it is truly catalytic allowing for multiple turnovers and high converson of epoxide.[3−5] At the cutting-edge are carbon dioxide and propylene oxide derived polyols which are low molar mass, hydroxyl end-capped polypropylene carbonate (PPC) or polyethercarbonates.

■ RESULTS AND DISCUSSION

■ CONCLUSION

■ REFERENCES