Enzymatic ring-opening polymerization (ROP) of lactides and lactone in ionic liquids and organic solvents: Digging the controlling factors

Polylactides (PLAs) and polylactones are sustainable and biodegradable polymers with important applications in packaging, horticultural materials, and biomedical fields. The enzymatic ring-opening polymerization (ROP) method represents a 'green' approach to the synthesis of these polyesters. However, there are lots of discrepancies in the literature on the lipase specificity towards a particular monomer (such as l-lactide), and many disagreements on the molecular weights produced by similar methods. To provide a better understanding of the key factors regulating the ROP reaction, we carried out a systematic investigation of the polymerization process under different temperatures catalyzed by different types and batches of lipases with various water contents, in different solvents including ionic liquids with known water contents, and by using different initiators. Our data clearly indicate that the key reaction parameters include the types of lipases and solvents, the amount of solvents, water contents in both enzymes and solvents, and the reaction temperature. N,N-Dimethylacetamide (DMA) and 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF₆]) at low concentrations were found suitable for the ROPs of lactides and ϵ-caprolactone leading to M_w of about 20 000 (polydispersity indexes mostly below 1.8) and moderately high yields (up to 60%). Our study suggests that all of these reaction parameters need to be fully controlled during the enzymatic ROP reaction although some earlier studies often neglected some conditions (such as the exact water content in enzymes and solvents).