![]() ![]() Catalytic cycle of CO 2/epoxide copolymerization. The polymerization is terminated by exposure to conditions/reagents that lead to hydrolysis of the growing polymer chain and formation of a polymer chain end-capped with a hydroxyl group.įigure 2. Propagation therefore involves the ‘cycling’ between metal alkoxide and carbonate intermediates. The metal coordinates another molecule of epoxide, and nucleophilic attack by the carbonate group leads to the ring opening of the epoxide and formation of a new metal alkoxide species. During chain propagation, carbon dioxide inserts into the metal alkoxide intermediate to form a metal carbonate species. The polymerization is initiated by coordination of an epoxide molecule and the subsequent ring opening by the nucleophilic attack of a carbonate group or ligand (X), so as to form a metal alkoxide intermediate. Prior to an examination of these catalysts, it is worth considering the series of reactions that are proposed to occur at the metal active site during polymerization, the major ones of which are illustrated in figure 2. Polymerization pathways and mechanismsĪ range of different catalysts are known but all catalysts contain metals, with Zn(II), Co(III) and Cr(III) being particularly common. Furthermore, the catalyst showed a high tolerance to various impurities present in captured CO 2, including water, N 2, CO, thiols and amines. The catalysts showed near-equivalent performances using such captured gases compared to using ‘pure’ carbon dioxide. Studies have shown that some homogeneous magnesium catalysts can be used in polymerizations where the carbon dioxide is captured at a CCS demonstrator plant attached to a UK power station. It has also recently been demonstrated that ROCOP catalysts are compatible with carbon capture and storage (CCS) processes. įurthermore, the replacement of fossil-derived epoxides is economically attractive and is stimulating a number of commercialization studies ( ). 20%) in both fossil resource depletion and greenhouse gas emissions for the CO 2-derived polymers. A recent detailed life-cycle analysis study compared these two types of polyols, showing significant reductions (approx. Given that polyether polyols are prepared by epoxide homopolymerization, the ROCOP of CO 2 and epoxides can also be viewed as a means to ‘replace’ a substantial portion of petrochemically derived resource (epoxide) with a renewable one (CO 2). It has been shown that the properties of CO 2-derived polyols are suitable to replace polyether polyols in some applications. Polyurethanes themselves are applied as flexible/rigid foams, adhesives, coatings and elastomers, as well as in many other areas. The primary application for the polymer products is as low-molecular-weight ( M n), (poly)hydroxyl-terminated ‘polyols’, which are widely used in the manufacture of polyurethanes. Depending on the epoxide and the selectivity of the catalyst, up to 31% (polycyclohexene carbonate) or 43% (polypropylene carbonate) of the polymer mass derives from CO 2. The most commonly studied epoxides are cyclohexene oxide (CHO) and propylene oxide (PO). The ROCOP reaction is a rare example of a truly catalytic process with the potential to deliver large-scale quantities of product, which genuinely consumes carbon dioxide. The ROCOP of carbon dioxide and epoxides to produce aliphatic polycarbonates. The intention is not to provide a comprehensive review of all known catalysts indeed, such reviews are already available. ![]() The focus for this review article will be to highlight and exemplify some of the key findings in this area of catalysis, in particular using examples drawn from our own research. The ROCOP process is strongly dependent on the selection of the catalyst, with various homogeneous and heterogeneous catalysts having been reported. However, it remains important to pursue CO 2 utilization as a means to reduce emissions, particularly those associated with existing, large-scale industrial processes and as an economic driver to support carbon capture. It should be made clear that the use of CO 2 in any chemical manufacturing process is unable to make a large impact on overall CO 2 levels in the atmosphere. The reaction was discovered more than 40 years ago and has since continued to attract attention as a means to reduce pollution associated with polymer manufacture and to ‘add value’ to carbon dioxide. The ring-opening copolymerization (ROCOP) of carbon dioxide and epoxides is an interesting method to synthesize a range of aliphatic polycarbonates ( figure 1). ![]()
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