Catalysts for CO2/epoxide ring-opening copolymerization

Abstract

This article summarizes and reviews recent progress in the development of catalysts for the ring-opening copolymerization of carbon dioxide and epoxides. The copolymerization is an interesting method to add value to carbon dioxide, including from waste sources, and to reduce pollution associated with commodity polymer manufacture. The selection of the catalyst is of critical importance to control the composition, properties and applications of the resultant polymers. This review highlights and exemplifies some key recent findings and hypotheses, in particular using examples drawn from our own research.

Keywords: CO2; catalysis; polycarbonate; ring-opening copolymerization.

Figure 1.

The ROCOP of carbon dioxide and epoxides to produce aliphatic polycarbonates. (Online version in colour.)

Figure 2.

Catalytic cycle of CO₂/epoxide copolymerization.

Figure 3.

One of the crystal structures for zinc glutarate and a generic illustration of the structure of the repeat unit in zinc-cobalt double metal cyanide catalysts. Adapted with permission from [38]. Copyright (2004) American Chemical Society. (Online version in colour.)

Figure 4.

Bicomponent catalysts for CO₂/epoxide copolymerization. (a) General porphyrin structure [,,–65]. (b) General salen structure [55,57,66]. (c) Bifunctional salcy complex tethered to piperidinium moieties [67]. (d) Bifunctional salen complex tethered to quaternary ammonium groups [33].

Figure 5.

Possible pathways for epoxide ring opening by the bicomponent catalysts.

Figure 6.

Dinuclear catalysts for CO₂/epoxide copolymerization: (a) zinc BDI complex [56]; (b) zinc bis(anilido-aldimine) catalysts [108]; (c) tethered zinc BDI complex (TMS = trimethylsilyl) [109]; (d) tethered cobalt salen complex [91]; (e) dinuclear macrocyclic catalyst [110].

Figure 7.

The different reaction energy barriers to the formation of PCHC versus cyclic carbonate. As part of the full kinetic analysis of the polymerization, the relative barriers were determined to be E_a(PCHC) of 96.8 kJ mol⁻¹ and E_a(CHC) of 137.5 kJ mol⁻¹. Adapted with permission from [124]. Copyright (2011) American Chemical Society.

Figure 8.

The structure, as determined using X-ray crystallography, of the di-cobalt catalyst [LCo₂Cl₂(methyl imidazole)]. The ‘bowl’ shape of the ligand is notable in the structure, as are the twofaces. The convex face refers to the outside of the ‘bowl’ (i.e. where Cl and MeIm are coordinated) and the concave face refers to the inside of the ‘bowl’ (i.e. where the bridging Cl ligand is coordinated). Adapted from [122].

Figure 9.

The structures of the di-Co(II) complexes and the corresponding activities in the copolymerization of CO₂/CHO. Polymerization conditions: CHO : catalyst = 1000 : 1, 80°C, 1 bar CO₂ [122].

Figure 10.

Comparison of the experimentally determined free energy barrier (a) to polymerizationwith the value determined by DFT (b) [123]. (Online version in colour.)

Figure 11.

The structures of di-zinc (1), di-magnesium (2) and the heterodinuclear mixed catalystsystem (3) which were compared for the ROCOP of CO₂/CHO [120]. (Online version in colour.)