Reversibly Cross-Linking Polyimide and Cyclophosphazene Toward Closed-Loop Recyclable Plastics with High Mechanical Strength, Excellent Flame Retardancy, and Chemical Resistance

Traditional flame-retardant plastics are technically diffi-cult to chemically recycle. The development of new types of flame-retardant plastics that are intrinsically capable of being closed-loop recycled and are sufficiently robust and stable to satisfy their practical application is urgently needed. In this study, closed-loop recyclable flame-retar-dant plastics with high mechanical strength and excellent chemical resistance are fabricated by cross-linking ami-no-terminated polyimide (PI-NH2) and aldehyde-termi-nated cyclophosphazene (CP-CHO) with imine bonds. The resultant flame-retardant plastic, which is denoted as PI-CP, exhibits a tensile strength of-115.6 MPa, Young's modulus of-2.5 GPa, and glass transition temperature of 316 & DEG;C. In the PI-CP plastic, the imine bonds are isolated within hydrophobic microenvironments generated by the rigid and hydrophobic polyimide chains and the benzene ring of cyclophosphazenes. As a result, the PI-CP plastics are highly stable in highly acidic and basic aqueous solutions and other commonly used organic solvents. The PI-CP plastic shows outstanding flame retardancy with a limiting oxygen index value of 48.8%. More impor-tantly, the PI-CP plastic can be depolymerized to generate the original PI-NH2 and CP-CHO monomers in high yields (-97%) and purity. The recovered monomers can be used to refabricate the original plastics, establish-ing highly efficient polymer-monomer-polymer circula-tion and a sustainable plastics economy.