Alkali Metal Complexes for the Controlled Synthesis of Bioplastics: Tuning the Metal Environment and the Reaction Conditions

Abstract

Polylactide (PLA) is one of the most prominent bioplastics, derived from renewable feedstocks and noted for its biocompatibility. Yet, the full potential of PLA has not been fulfilled due to limitations in its production processes, especially the dependence on traditional toxic catalysts such as tin(II) octanoate. Recent studies have highlighted the advantages of alkali metal complexes as efficient, non-toxic, and versatile catalysts for the ring-opening polymerization (ROP) of lactide. Historically, alkali metals were considered too reactive or poorly controlled to be effective in ROP catalysis. However, recently it has been demonstrated that this limitation can be overcome through judicious ligand design and reaction engineering, transforming alkali metals into powerful tools for sustainable polymer chemistry. As such, the use of bulky ligands can tune the metal environment and assert a better control over the polymerization. As well, depending on the presence or not of the co-initiator, the polymerization mechanism varies significantly which influences the control of the stereoregularity of the polymers obtained, and poly-L-lactide (PLLA) with different D-lactide units can be obtained. This stereoregularity determines the thermal and mechanical properties and hence the applications of the PLLA. Furthermore, using chiral alkali compounds and controlling the aggregation, isoselective rac-lactide polymerization can be achieved. Hence, catalyst design and reaction conditions can be combined to tune polymer microstructure, molecular weight, and tacticity, advancing PLA toward a sustainable and circular material future. Furthermore, the alkali metal compounds described herein not only enable rapid lactide polymerization, but also promote PLA depolymerization under mild conditions, thereby connecting synthesis with chemical recycling.