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

Abstract

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.