Dual Cross-Linked, Polymer Thermosets: Modular Design, Reversible Transformation, and Triggered Debonding

Abstract

We demonstrated the modular synthesis of polymer thermosets exhibiting programmed responses that arise from two predesigned functional units: a reversible cross-linker and a self-immolative linker. The former unit contains a 1,2,3-triazolium moiety that provides the cross-linked network but undergoes dynamic covalent bond exchange via transalkylation, while the latter phenyl-based unit generates a signal-specific elimination reaction that promotes selective de-cross-linking of the entire network. The two units form a functional, dual cross-linked structure after thiol–ene click polymerization followed by a thermal curing reaction, allowing large-scale synthesis. By changing the ratio of functional units, the physicochemical properties of the materials during polymerization can be finely tuned, and adequate combinations result in tailored thermosets that are reversible yet removable. Thus, we were able to reuse the thermosets in bulk, solid state, or restructure without overall structural collapse. Furthermore, by exploiting the designed nature, the materials can be used as a stimuli-responsive adhesive. We observed a high adhesive strength when gluing glass pieces and rejoined them upon heating after they were detached by force. However, exposure to a molecular signal significantly diminished the adhesion under benign conditions, and the joint was easily and irreversibly separated. The design concept and materials presented herein elucidate or advance the chemical concepts for the recycling and disposal of commercial thermosetting plastics when they are no longer needed.