In Situ Versus Isolated Zinc Catalysts in the Selective Synthesis of Homo and Multi-block Polyesters

The in situ generation of metal-alkoxide complexes is a common initiation method in cyclic ester ring-opening polymerization. However, this method is often a “black box”, where the species 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 ring-opening polymerization, 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 homopolymerization of multiple cyclic esters (rac-lactide, ε-caprolactone, and rac-β-butyrolactone). The stability of this isolated catalyst also controls the chain exchange and backbiting, allowing for one-pot synthesis of multiblock 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.