Valorization\nof Polycaprolactone to γ‑Caprolactone\nover Lewis Acidic Ionic Liquids

Upcycling spent polyesters into useful chemicals is of\ngreat significance\nfor achieving a circular carbon economy. Herein, we report the valorization\nof polycaprolactone (PCL) to γ-caprolactone over binary ionic\nliquid systems derived from 1-butyl-3-methylimidazolium bromide ([BMIm]Br)\nand CuBr<sub>2</sub> under mild conditions (e.g., 160 °C). It\nwas found that the in situ formed Lewis acidic [CuBr<sub>4</sub>]<sup>2–</sup> anion could activate the ester group of PCL via coordination\nwith carbonyl O, while [Br]<sup>−</sup> anion could attack\nthe C atom of alkyl C–O to achieve the decomposition of PCL,\nforming the key ionic intermediate, 6-bromohexanoate. This intermediate\nsubsequently undergoes β-elimination reaction and isomerization–lactonization\ntransformation facilitated by the synergistic effect of cation and\nanion of the ionic liquids, thus producing γ-caprolactone in\nhigh yield and selectivity. This simple, green, and efficient protocol\nto valorize PCL into γ-caprolactone holds great promise for\napplication in industry.