Synthesis of star poly (L-lactic acid) and its copolymers catalyzed by lipase in supercritical carbon dioxide

Poly (L-lactic acid) (PLLA) is a bio-based biodegradable polymer with minimal immunogenicity, non-toxicity and excellent mechanical properties and has widely used in medical, pharmaceutical, food packaging and other fields. However, linear PLLA has the characteristics of high melting point, strong crystallinity, weak flowability and poor thermal stability, which cannot meet the requirements of various industrial processes and novel pharmaceutical formulations. With its low viscosity, controlled degradation cycle, and excellent melt flowability, star-shaped PLLA can be used in the fields of biocomposites, biomedical devices, drug delivery, and intelligent packaging. Presently, metal catalysts and organic solvents are frequently used in PLLA synthesis industries, and both residues continue to pose a safety risks. In existing studies, the performance of bioenzymes as optimal green catalysts in organic solvents is not satisfactory, and they are predominantly used to synthesise oligomers with low yields and lengthy reaction times. In this study, a four-armed star-shaped poly (L-lactic acid) (4s-PLLA) with a porous structure favourable for drug loading was synthesised using supercritical carbon dioxide (Sc-CO2) as a solvent, lipase as a catalyst, and erythritol as a nucleus. Using Sc-CO2 as a solvent the reaction activity and stability of the enzyme were increased so that the reaction yield could reach 83% and the reaction time was only 24 h, which provided a possibility for the practical application of the enzymes catalyze green process. Both the random copolymerization product four-armed star-shaped poly(L-lactic-glycolic acid) (4s-PLGA) and the block copolymerization product four-armed star-shaped poly (L-lactic-glycolic acid) (4s-PLLA-PGA) were subsequently synthesized by copolymerization modification, and their porous structures retained and hydrophilicity were enhanced. The response surface analyses were performed about the reaction time, reaction temperature and yield of synthetic reactions by using Design-Expert 12 software and the prediction equations were generated.