Truly sustainable plastics require renewable feedstocks coupled with efficient production and end-of-life degradation/recycling processes. Some of the most useful degradable materials are aliphatic polyesters, polycarbonates and polyamides, which are often prepared via ring-opening (co)polymerisation (RO(CO)P) using an organometallic catalyst. While there has been extensive research into ligand development, heterometallic cooperativity offers an equally promising yet underexplored strategy to improve catalyst performance, as heterometallic catalysts often exhibit significant activity and selectivity enhancements compared to their homometallic counterparts. This review describes advances in heterometallic RO(CO)P catalyst design, highlighting the overarching structure-activity trends and reactivity patterns to inform future catalyst design.

The in situ generation of metal-alkoxide complexes is a common initiation method in cyclic ester ring-opening polymerisation. Yet this method is often a "black box", where the species so generated are assumed to be the same as the isolated complex. We now demonstrate that an isolated Zn-benzoxide catalyst gives a remarkable reactivity enhancement in lactide ROP, with kobs values 10 times higher than the in situ generated analogue. The dinuclear zinc catalyst, built using the Trost ProPhenol ligand, offers these excellent activities and good control over homopolymerisation of multiple cyclic esters (rac-lactide, ε-caprolactone and rac-βbutyrolactone). The stability of this isolated catalyst also controls chain exchange and back-biting, allowing for one-pot synthesis of multi-block polyesters without loss of activity, selectivity and control. To the best of our knowledge, this is the first catalyst reported for the selective preparation of block terpolymers of ε-CL, rac-LA and rac-β-BL.

The synthesis of cyclic carbonates from epoxides and CO2 is a well-established reaction, whereas the synthesis of cyclic carbonates from diols and CO2 is considerably more challenging, and few efficient catalysts are available. Here, we describe heterocyclic carbene catalysts, including one derived from a cheap and efficient thiazolium salt, for this latter reaction. The reaction proceeds at atmospheric pressure in the presence of an alkyl halide and Cs2CO3. Reaction mechanisms for the transformations involved are also proposed.