Design of covalent adaptable networks with intrinsic flame retardancy

Thermosetting polymers crosslinked through covalent bonds are widely used in various fields due to their excellent mechanical properties and chemical stability. However, the non-recyclable nature of their fossil-based constituents hinders their post-service recycling and reshaping. This not only results in resource waste but also leads to environmental pollution, contradicting the principles of green and sustainable development. Dynamic covalent crosslinks are incorporated to turn conventional thermoset into covalent adaptable networks (CANs), enabling them to be processible and recyclable via bond exchange under specific stimuli. Normal covalent adaptable networks are highly flammable, which limits their applications and poses safety hazards. This review aims to provide insights to address this challenge. The state of the art of the molecular design of flame retarded CANs is summarized and classified through types of dynamic covalent bonds including classical ester exchange, imine exchange, disulfide exchange, Diels Alder reaction and transformation based on recently developed diselenide bonds, borate esters bonds, phenol carbamate bonds. Mechanical strength, dynamic behavior and flame retardancy of each type of CANs are described and compared quantitatively. Furthermore, the future prospects and challenges for development and application of these novel high-performance polymeric materials will be discussed.