Dynamically Cross-Linked Semicrystalline Thiol-Ene Photopolymers for Additive Manufacturing of Recyclable and Adaptive Materials

Current photopolymers are mainly produced by (meth)acrylate-based photoresins, possessing permanently cross-linked structures that prohibit their chemical recycling. This study reports a one-step, scalable synthesis of dynamically cross-linked semicrystalline thiol-ene photopolymers comprising exchangeable disulfide bonds with excellent material performance, shape memory/adaptive characteristics, and chemical recyclability. These photopolymers are synthesized via stoichiometric thiol-ene click reactions using commercial reagents, including difunctional ene, alkyl dithiol giving rise to crystallinity, thiol-terminated oligomer comprising dynamic disulfide bonds, and trithiol cross-linker. Cross-link density is systematically controlled by varying trithiol cross-linker content to tune degrees of crystallinity and mechanical properties over a wide range, e.g., Young's modulus from 4 to 138 MPa, elongation-at-break from 40 to 370%, and toughness from 0.3 to 32 MJ m^-3. Importantly, these dynamically cross-linked photopolymers undergo complete decross-linking via thiol-disulfide exchange reactions with small-molecule thiols at 120 °C in a solvent- and catalyst-free manner, yielding liquid thiol-terminated oligomers. These recycled thiol-terminated oligomers are used to synthesize next-generation dynamically cross-linked thiol-ene photopolymers with identical material composition and full property retention, demonstrating closed-loop recycling. Promisingly, these new thiol-ene photoresins demonstrate excellent feasibility for Digital Light Processing (DLP)-based 3D printing of high-resolution objects with complex geometries, preserved crystallinity and recyclability, and unique shape memory/adaptive characteristics.