Reversibly Cross-Linked Polymers: A New Method for High-Performance and Sustainable Polymer Materials

ConspectusConventional polymeric materials have profoundly shaped modern society by enabling the large-scale production of lightweight and mechanically robust products. However, their massive consumption and rapid proliferation have led to extensive environmental pollution and severe resource depletion. These escalating concerns underscore the urgent need for sustainable alternatives that exhibit inherent healing, reprocessing, and closed-loop recycling capabilities. Although noncovalent interactions and dynamic covalent bonds endow polymer materials with reversibility that is essential for sustainability, their intrinsically weaker and more labile nature relative to permanent covalent cross-links presents a critical challenge: how to retain dynamic functionality while simultaneously enhancing structural stability and achieving mechanical performances comparable to, or even surpassing, those of conventional polymers. To address this challenge, we recently developed the concept of reversibly cross-linked polymers (RCPs), a class of three-dimensional polymer networks fabricated by reversibly cross-linking polymer chains via noncovalent interactions and/or dynamic covalent bonds and feature intrinsic healing, reprocessing, or chemical recycling capabilities. Using polymers rather than small-molecule monomers as the primary building blocks maximizes the fraction of stable covalent bonds relative to reversible cross-links, ensuring sufficient mechanical strength and structural integrity. Furthermore, employing polymers with self-assembling or immiscible segments enables the in situ formation of reversibly cross-linked phase-separated nanostructures that act as nanofillers, significantly enhancing both the mechanical performance and structural stability of RCPs.This Account provides a comprehensive overview of our recent advances in the fabrication of high-performance RCPs, including plastics, elastomers, and ionogels/hydrogels. We begin by outlining the general design principles and versatile synthetic strategies for the development of RCPs. Central to our approach is the deliberate engineering of in situ formed, reversibly cross-linked phase-separated nanostructures with tunable rigidity, deformability, and dissociability. Rigid nanostructures endow RCPs with mechanical strengths comparable to or even exceeding those of conventional plastics and elastomers, whereas tough and deformable nanostructures dissipate energy efficiently under external loading, imparting both high strength and exceptional toughness to RCPs. This design enables the fabrication of RCPs with mechanical properties that are rarely attainable in conventional counterparts. For instance, reversibly cross-linked elastomers and ionogels/hydrogels can be endowed with extraordinary damage tolerance, ultrahigh tensile strength and modulus, and high-strength, low-hysteresis elasticity. The confinement of dynamic reversible cross-links within densely packed, hydrophobic phase-separated nanostructures or microenvironments markedly improves the thermal stability and solvent resistance of RCPs, thereby broadening their applications in demanding engineering and environmental conditions. Moreover, the dynamic nature of reversible cross-links enables efficient depolymerization of RCPs into (macro)monomers under mild, catalyst-free conditions, facilitating chemical recycling of both neat RCPs and carbon fiber/RCP composites. These advances establish RCPs as a promising materials platform capable of overcoming the long-standing trade-off between mechanical robustness and dynamic recyclability, opening new avenues for the development of sustainable, high-performance polymeric materials.