Nucleophilic Aromatic Substitution Tempering the Closed-Loop Recycling of Shape Memory Polymers

The dynamic covalent bond (DCB) strategy endows the polymer with recyclability, enabling resourceful recycling, energy conservation, and addressing environmental pollution issues, thereby garnering significant attention currently in the polymer field. Here, we achieved closed-loop recycling of a series of shape memory polymers (SMPs) through a nucleophilic aromatic substitution (S_NAr) reaction. First, the S_NAr reaction facilitated the recycling of high-strength, high-transition-temperature shape memory cyanate esters (SMCEs) and their composite (SMCEC) into triethoxytriazine (TETA) and diols. The recycled monomers can then be used to synthesize SMPs with tailored mechanical properties and a broad range of transition temperatures, including shape memory epoxy resins, cyanate ester elastomers, and 4D printing elastomers. TETA can be recycled again from 4D printing elastomers and cyanate elastomers. Furthermore, we devised two innovative strategies for the 4D printing of recycled materials to produce actuators with high resolution. Notably, the polyacrylate elastomer, which undergoes in situ cross-linking after postheating through S_NAr reactions, presents a groundbreaking method for the 4D printing of high-performance SMPs utilizing DCBs. This work not only provides an innovative strategy for achieving closed-loop recycling of high-performance polymers but also introduces an interesting approach for 4D printing SMPs.