Malleable and Recyclable Thermosets: The Next Generation of Plastics

Malleable thermosets are crosslinked polymers containing dynamic covalent bonds, which can be reversibly cleaved and reformed. They have attracted considerable attention in recent years due to their combined advantages of thermosets and thermoplastics. They have excellent mechanical properties and thermal and chemical stabilities like traditional thermosets yet are reprocessable and recyclable like thermoplastics. Although the chemical composition plays an important role in determining the mechanical and thermal properties of materials, the application of dynamic covalent chemistry is the key to achieving the unique properties of malleable thermosets. The mechanism of reversible bond cleavage and reformation, bond activation energies and kinetics, and the conditions triggering such reversibility define the malleable properties of the materials, how and why they can be reprocessed, and when the materials fail. In this review, we introduce fundamental concepts and principles of malleable thermosets, dynamic covalent chemistry, and the characteristic materials properties, including reprocessability, rehealability, and possible recyclability. We categorize the recent literature examples based on the underlying chemistry to demonstrate how dynamic covalent chemistry is exploited in malleable thermosets and how their malleable properties can be achieved and altered; we also discuss intriguing future opportunities based on such exploitation.