Upcycling Waste Cyclic Poly(phenylene sulfide) as Valuable Movable Crosslinkers for Toughening and Ring-Size-Controlled Enzymatic Degradation of Polycaprolactone

Reconciling mechanical robustness with controlled end-of-life degradability represents a central challenge in the design of sustainable polymeric materials. Here, we develop a solvent-free in situ ring-opening polymerization strategy for incorporating size-specific cyclic poly(phenylene sulfide) macrocycles (c[n]PS, n = 5, 7, and 9), industrial byproducts of poly(phenylene sulfide) (PPS) synthesis, into polycaprolactone (PCL) to construct pseudorotaxane-based movable crosslinks. This approach simultaneously addresses mechanical reinforcement and controlled enzymatic degradation without chemical modification of the PCL backbone. The resulting PCL⊂c[7]PS achieves a two-fold increase in toughness while fully preserving the Young's modulus and retaining thermoplastic reprocessability. Strikingly, the movable crosslinks impart a pronounced ring-size-dependent modulation of enzymatic degradation kinetics: PCL⊂c[7]PS exhibits a markedly improved stability, while PCL⊂c[9]PS shows accelerated enzymatic degradation within 48 h. Complementary Fourier transform infrared analysis and surface morphology observations are consistent with a mechanism in which the ring size modulates the accessibility of the amorphous domains to enzymatic attack. By upcycling an industrial polymer waste stream into a functional macrocyclic platform, this work offers a green and scalable route to biodegradable polymers with programmable properties, contributing to the development of sustainable polymeric materials.