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

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.

Cooperative Heterometallic Catalysts for Lactide Ring-Opening Polymerization: Combining Aluminum with Divalent Metals.

While homometallic (salen)Al catalysts display excellent performance in lactide ring-opening polymerization (ROP), heterometallic (salen)Al complexes have yet to be reported. Herein, we describe four heterobimetallic (salen)Al catalysts and show that the choice of the heterometal is key. Cooperative Al/Mg and Al/Zn combinations improved the catalyst activity by a factor of up to 11 compared to the mono-Al analogue, whereas the mono-Mg and mono-Zn analogues were completely inactive.