One-pot three-step enzymatic ROP in situ to form polycaprolactone from cyclohexanone: Optimizing and kinetic modeling

A one-pot ring-opening polymerization (ROP) to form poly ε-caprolactone (PCL) from cyclohexanone (CHN) mediated by immobilized T. laibacchii lipase was proposed. The optimal conditions by using response surface methodology (RSM) were established as follows: the supplement of lipase , 23.55 mg, the reaction temperature, 59.4 °C and the molar ratio of urea hydrogen peroxide (UHP) to CHN, 1.31:1, under which the yield of PCL and number-average molar mass (Mn) at a high CHN concentration of 1.25 M were 96.5% and 2912, respectively. The one-pot ROP consists three steps: first, ethyl acetate (EtOAc) reacts with UHP to generate peracetic acid catalyzed by lipase; then, CHN is lactonized to ε-caprolactone (CL) with peracetic acid in situ generated in nonenzymatic, and acetic acid formed reacts with UHP to form peracetic acid; finally, CL suffers from enzymatic ROP to in situ generate PCL. There are three factors contributing to the high yield: cross-linked immobilized lipase employed, the supplement of lipase added in step 3 of ROP and optimal conditions adopted. The initial concentration of UHP was reduced from 2.5 M to 1.625 M owing to acetic acid reacting with UHP to yield peracetic acid which can help obtain a high yield of PCL. A kinetic model for this one-pot three-step enzymatic ROP reaction was developed, which fitted the observed data well. The enzymatic reaction mediated by lipase may obey the ping-pong mechanism with substrate inhibition of UHP and acetic acid, while the nonenzymatic reaction may obey a power law. Using the kinetic model , the contribution of the reaction of UHP with EtOAc and acetic acid to the yield of peracetic acid can be distinguished, contributing to better comprehend the reaction process.