Enzymatic synthesis of poly(ɛ-caprolactone) in supercritical carbon dioxide medium by means of a variable-volume view reactor

Poly (ɛ-caprolactone) (PCL) is a biodegradable polyester approved for applications in the human body such as drug delivery devices and sutures. Conventional synthesis of PCL involves metal catalysts and organic solvents that may leave toxic residues in the products and contribute to environmental pollution. Polymerization processes catalyzed by enzymes are becoming more attractive due to the importance of clean processes, which produces substances free of residues, ideal for pharmaceutical and food applications. The aim of this work was to investigate the enzymatic ring-opening polymerization (e-ROP) of PCL in supercritical carbon dioxide (scCO2) solvent medium through a set of experiments assessing the influence of pressure (120–280 bar), solvent/monomer ratio (2:1–1:2 mass ratio) and enzyme percentage related to monomer (5–15 wt%) on the reaction yield, number-average molecular weight (Mn), weight-average molecular weight (Mw) and polydispersity index (P.I.). The results of these first experiments were used in the selection of the conditions for the kinetic experiments evaluating the influence of catalyst content and temperature on reaction yield, Mn, Mw, P.I. and on the characteristics of the polymer produced. This study also evaluates the enzyme reuse in order to reduce the impact of the enzyme cost on the process. Results for ANOVA statistical analysis for the first set of experiments show that the pressure or the solvent density has no significant influence over the parameters evaluated, while the solvent/monomer mass ratio presented significant effect on Mn and on Mw, with the best results obtained for the solvent/monomer mass ratio of 1:2. As expected, the enzyme content affects significantly all parameters evaluated. Polymerization results for the kinetic experiments indicate reaction yields up to 90 wt%, Mn up to 13,700 Da and Mw up to 22,200 Da, with P.I. ranging from 1.2 to 1.7. Taking into account the reaction productivity, the conditions for the reuse assays were chosen: 120 bar, 1:2 solvent/monomer ratio, 3 wt% of enzyme, 65 °C and 12 h of reaction. The enzyme recycling experiments suggested viability up to the second cycle as an alternative to improve the enzyme use. The variable-volume view reactor was adequate to provide simultaneous control of the process variables.

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► Reaction performed in a variable-volume view reactor.
► Independent control of pressure, temperature and solvent/monomer ratio.
► Polymerization kinetics dependant on the enzyme content.
► Enzyme reuse is suitable for cost reduction.
► Feasibility to employ an enzymatic catalyzed reaction in supercritical medium.