Activated Ester Chemistry Enabled Thermally Triggered Self-Reinforcement of Polyurethane: Toward Elastomers with Fatigue Elimination and Mechanical Upcycling Capabilities

Upcycled elastomers based on covalent adaptable networks with diverse dynamic bonds have been designed to avoid downcycling and to tackle the problem of waste elastomers. Although thermally triggered self-reinforced materials with favorable recyclability and mechanical properties can be obtained via hydrogen bond reconstruction, a powerful strategy using multihydrogen bonding motifs like 2-ureido-4-pyrimidinone is limited to polar-functional polymer systems. Furthermore, its poor solubility necessitates large quantities of polar solvents, while their high polarity further gives rise to elastomer compatibility issues. Amides capable of formation of intermolecular hydrogen bonding serve as core functional moieties in high-performance polymers. Among the various chemistries for amide bond formation, activated ester has apparent advantages of good storage stability, mild reaction conditions, and good solubility in most organic solvents, whose application potential in polymeric materials is far from being explored. In the present work, for the first time, we report the design of a polyurethane elastomer that enables thermally triggered topological isomerization and forms new cross-linking points. The designed network features hindered urea moieties in the main chain and pendant hexafluoroisopropyl (HFI) ester groups along the side chains. Upon heating during hot pressing, the secondary amine released from the hindered urea reacts with the HFI ester to form strong amide bonds, while the released isocyanate reacts with moisture to generate urea linkages. These reactions collectively result in increased cross-linking density and microphase separation. After three thermal recyclings, the tensile strength of the material reaches 36.4 MPa, corresponding to 300% of its pristine value; this recycling efficiency and mechanical performance outperform the reported elastomers. Notably, it is intriguing that mechanical fatigue caused by repeated stretching and releasing can be eliminated via thermal treatment based on a similar mechanism. This work presents a novel chemistry tool for designing hydrogen bond reconstruction systems in materials science.