Valorization of Polycaprolactone to γ-Caprolactone over Lewis Acidic Ionic Liquids

Abstract

Upcycling spent polyesters into useful chemicals is of great significance for achieving a circular carbon economy. Herein, we report the valorization of polycaprolactone (PCL) to γ-caprolactone over binary ionic liquid systems derived from 1-butyl-3-methylimidazolium bromide ([BMIm]Br) and CuBr₂ under mild conditions (e.g., 160 °C). It was found that the in situ formed Lewis acidic [CuBr₄]^2– anion could activate the ester group of PCL via coordination with carbonyl O, while [Br]⁻ anion could attack the C atom of alkyl C–O to achieve the decomposition of PCL, forming the key ionic intermediate, 6-bromohexanoate. This intermediate subsequently undergoes β-elimination reaction and isomerization–lactonization transformation facilitated by the synergistic effect of cation and anion of the ionic liquids, thus producing γ-caprolactone in high yield and selectivity. This simple, green, and efficient protocol to valorize PCL into γ-caprolactone holds great promise for application in industry.