Polymer engineering based on reversible covalent chemistry: A promising innovative pathway towards new materials and new functionalities

Reversible covalent polymers are able to change their bond arrangement and structure via reversible reaction triggered by external stimuli including heating, light and pH, while retaining the stability of irreversible covalent polymers in the absence of the stimuli. In recent years, more and more research has been devoted to utilization of reversible covalent bonds in synthesizing new materials, which not only overcomes disadvantages of permanent covalent polymers, but also brings in new functionalities. More importantly, a series of novel techniques dedicated to polymerized products with features such as properties regulation, self-healing, reprocessing, solid state recycling, and controllable degradation are developed, heralding the opportunity of upgrading of traditional polymer engineering. Although the exploration of this emerging topic is still in its infancy, the advances so far are encouraging and clearly directed to large scale applications. This review systematically outlines this promising trend, following a bottom-up strategy, taking into account both theoretical and experimental achievements. It mainly consists of four parts, involving design and preparation: (i) the basis of reversible covalent chemistry, (ii) rheology of reversible covalent polymers, (iii) methods of construction of reversible covalent polymers, and (iv) smart, adaptive properties offered by reversible covalent chemistry. The key elements for realizing reorganization of polymers containing reversible covalent bonds are covered. The advantages and weaknesses of representative reaction systems are analyzed, while the challenges and opportunities to engineering application of the equilibrium control based on reversible covalent chemistry for producing end-use polymers are summarized. In this way, the readers may grasp both the overall situation as well as insight into future work.