Heterometallic Li/Zn, Li/Al and Li/In catalysts for rac-lactide ring-opening polymerisation: “ate” or “non-ate” pathways?

Abstract

The ring-opening polymerisation (ROP) of lactide (LA) is an attractive route to produce aliphatic polyesters, with bimetallic catalysts displaying some of the highest catalyst activities to date. While a range of heterometallic catalysts have been reported to outperform their homometallic analogues, the origins of cooperativity are not always well understood. Previous studies indicate that the reaction pathways may differ for different metal heterocombinations, especially when an alkali metal is combined with zinc or aluminium. Here, a series of homo- and hetero-metallic complexes combining Li with Al, Zn or In, supported by an asymmetric methyl-ester substituted salen ligand (H₂L), have been synthesised and characterised by single-crystal X-ray diffraction, to probe potential differences. The heterobimetallic LLiZnCl, LLiAlCl₂, and LLiInCl₂ complexes were all active for rac-LA ROP in the presence of an epoxide initiator, with LLiInCl₂ offering the most efficient polymerisation while homobimetallic LLi₂ was inactive. Investigations into the roles of the different metals through X-ray diffraction and DFT structural studies suggest that oxophilicity, Lewis acidity, and electronegativity difference between the two metals all play a role, with the high oxophilicity and Lewis acidity of Al overriding the “ate” pathway.