Poly(lactic acid) (PLA) is a degradable polyester which shows promise as an alternative to some petroleum-derived polyolefins, due to relatively low production costs and useful physical properties. Lactide ring-opening polymerisation (ROP) catalysed by homogenous salen complexes is a useful means to prepare PLA, combining efficient polymerisation with good control over the polymer microstructure. Incorporating a secondary heterometal into homogenous organometallic architectures often gives cooperative catalysts that can significantly outperform their homometallic counterparts, yet this concept remains underexplored in cyclic ester ROP.<br/>This presentation describes the synthesis, characterisation and catalytic activity of a series of heterobimetallic complexes for <i>rac</i>-lactide ROP. Derived from a functionalised salen ligand, these complexes combine aluminium with a mono- or divalent metal (lithium, magnesium, zinc or calcium). Through the addition of secondary metals the catalytic efficiencies are enhanced by up to 10 times compared to the monometallic analogue. Mechanistic studies and DFT calculations suggest that activity enhancements occur through mixed-metal cooperativity, incorporation of an additional monomer coordination site and optimised geometry around the metal centres. While magnesium and zinc enhance the catalyst activity, lithium and calcium both reduce the polymerisation rate, highlighting the importance of the heterometal nature in achieving heterometallic cooperativity.

Cooperative heterometallic catalysts for lactide ring-opening polymerisation: combining aluminium with mono- and divalent metals

ACS Fall 2020 Virtual Meeting

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