Structure-Controlled Zinc Acetate Enables Depolymerization, Repolymerization, and Crystallization Enhancement of Polycaprolactone

Chemical recycling to monomer (CRM) represents a highly efficient closed-loop strategy, yet selective recovery of polymerization-grade monomers from aliphatic polyesters remains challenging. Herein, we report a multifunctional zinc salt strategy for the closed-loop recycling of poly(ε-caprolactone) (PCL), in which zinc acetate serves triple roles in the polymerization, depolymerization, and crystallization regulation of the regenerated polymer. Compared with 0.25 wt % Zn(acac)2, which afforded ε-CL with 98.0% purity at 260 °C, ball-milled Zn(OAc)2 (Zn(OAc)2-BM) reduced the depolymerization temperature to 200 °C and produced recovered ε-CL (r-ε-CL) with 98.7% purity. Density functional theory (DFT) calculations combined with localized orbital locator (LOL) analysis demonstrated that stronger Zn2+ coordination and polarization of the carbonyl oxygen, coupled with the higher conformational flexibility of acetate ligands, significantly weakened the adjacent ester C–O bond ( the relaxed force constant reduced by 26.4%) and lowered the Gibbs free energy of activation by ∼10 kcal mol–1 relative to Zn(acac)2. Unpurified r-ε-CL was directly repolymerized into regenerated PCL (re-PCL) using Zn(OAc)2-BM, which enhanced crystallization kinetics and mechanical performance through residual heterogeneous nucleation. Under the optimized conditions, Zn(OAc)2-BM forms a dehydrated, less crystalline Zn(II) acetate based material containing a minor ZnO phase, enabling selective PCL recycling and improving the properties of re-PCL.