Indeed, TAD-indole covalent bond exchanges at elevated temperatures have been well-studied in the context of covalent adaptable networks 24, 25, 26. It is known that the TAD-indole chemistry platform offers a range of selective and predictable covalent links that are quantitatively formed under equimolar conditions at ambient temperature without the need for a catalyst, while the corresponding adducts are known to be bench-stable, yet are reversible upon heating. Our strategy to introduce force-reversible C-N bond exchanges at ambient temperature is inspired by reversible click chemistry between triazolinedione (TAD) and indole-based building blocks (Fig. As a result, developing an ambient-stable dynamic covalent bond with reversible stress-responsiveness in real time would afford a useful route toward designing multiple force-responsive functions for covalently crosslinked polymers, such as simultaneously enhancing mechanical strength and ductility, adaptability to dynamic environments and network autonomy. However, the delayed recovery of the DFSN linkers did not allow for their self-reassociation in real time. 21 developed a notable example based on a difluorenylsuccinonitrile (DFSN) linker whose central C-C bond is readily cleaved under mechanical stress to generate a relatively stable radical species. In particular when exposed to ambient conditions for a long time, radical species are expected to lose their activity to de-bond and re-bond. However, radical recombination reactions are often susceptible to undergo irreversible reactions, for example with molecular oxygen, moisture, and other surrounding molecules, which is a concern in the context of dynamic covalent chemistry 20. In addition, homolytic cleavage into relatively stable organic radicals showed promising reversible dissociation/association 19, 20. Despite the fact that weak force-activated covalent bonds could be re-formed by external stimulation 15, 16, 17, 18, such as UV or visible light irradiation, heating or a catalyst, the broken bonds cannot be reformed in real time under ambient conditions, thereby leading to irreversible damage within the polymer network in the long-term application process.
Currently, a series of weak force-activated covalent bonds have been introduced as crosslink points into covalent polymer networks to increase simultaneously the mechanical strength and ductility of a polymeric material 9, 13, 14. Generally, a weak force-activated covalent bond can break preferentially in order to dissipate energy from external force perturbation 10, 11, 12. This phenomenon significantly weakens the mechanical and functional properties of covalently crosslinked polymer materials, as well as shorten their service life, and even bring safety risks to their application. However, long-term force perturbation is inevitable in crosslinked polymers and typically induces irreversible covalent bond scission, resulting in a chemically damaged crosslinked network 9. Most of the covalently crosslinked polymer materials are employed under ambient conditions, where the covalent polymer network integrity directly affects polymer materials’ properties and lifetime 5, 6, 7, 8. Overall, our strategy represents a general method to create toughened covalently crosslinked polymer materials with simultaneous enhancement of mechanical strength and ductility, which is quite challenging to achieve by conventional chemical methods.ĭue to their excellent mechanical strength and thermal stability, polymer materials fabricated with covalent crosslinks have been widely used in numerous fields of daily life 1, 2, 3, 4. Moreover, the nascent TAD moiety can spontaneously and immediately be recombined after dissociation with an indole reaction partners at ambient temperature, thus allowing for the adjustment of the polymer segment conformation and the maintenance of the network integrity by force-reversible behaviors. Whereas the exergonic TAD-indole reaction results in the formation of bench-stable adducts, they were shown to dissociate at ambient temperature when embedded in a polymer network and subjected to a stretching force to recover the original products. Here, we displayed that TAD-indole adducts, acting as crosslink points in dry-state covalently crosslinked polymers, enable materials to display reversible stress-responsiveness in real time already at ambient temperature. Force-reversible C-N bonds, resulting from the click chemistry reaction between triazolinedione (TAD) and indole derivatives, offer exciting opportunities for molecular-level engineering to design materials that respond to mechanical loads.