End-capping enables phase-switchable cyclization to high-purity glycolide

Degradable polyesters are attracting sustained interest as practical materials to mitigate plastic-waste accumulation, yet their large-scale deployment critically depends on access to high-purity cyclic ester monomers for ring-opening polymerization. Here we report a phase-switchable end-capping strategy for glycolide (GL) synthesis that couples cyclization and purification through spontaneous liquid-liquid phase separation (LLPS). In this route, a weakly polar and sufficiently non-volatile alcohol (ROH) end-caps glycolic acid oligomers to regulate chain length and suppress unproductive interchain reactions, while its low miscibility with GL enables built-in purification by direct collection of a GL-rich phase after depolymerization. Combining polarity descriptors and thermodynamic analysis with experimental validation establishes polarity–volatility design rules for ROH selection. Guided by these criteria, behenyl alcohol affords a GL-rich phase with consistently high purity (≥99%) and high recovery, and the ROH-rich phase can be recycled over multiple cycles without loss of product quality. Conceptual process simulations further indicate a reduced separation burden and improved process economics relative to conventional glycolic acid oligomer depolymerization. Beyond GL, the approach is extendable to other sufficiently polar cyclic esters, as demonstrated by LLPS-enabled isolation of l -lactide with 97.6% purity.