Versatile Phosphate Diester-Based Flame Retardant Vitrimers via Catalyst-Free Mixed Transesterification

We herein report a new vitrimer system integrated with UV curability, recyclability, and flame retardancy. Energy-efficiency, sustainability, and safety have been required features for next-generation polymer materials. Various attempts have been made to endow thermoset polymers with rapid prototyping capacity, recyclability, and flame retardancy. Thermoset vitrimers based on covalent adaptable networks (CANs) are recyclable and remoldable but are generally not UV curable or flame retardant. Here, we present a conceptually novel option to achieve fast exchange reactions in CANs via catalyst-free mixed transesterification of a UV curable phosphate diester-based acrylate cross-linker. In this system, the phosphate diesters serve as reversible covalent bonds, hydrogen bonding ligands, and flame-retardant structures, while acrylate groups serve as UV curable units as well as transesterification collaborators. After the facile UV curing, an intrinsic flame-retardant and mechanically strong dynamic network was achieved due to abundant hydrogen bonds between P-OH and C═O structures. Additionally, this highly cross-linked network exhibited an attractive recyclability even at temperatures lower than T_g. This phosphate diester-based mixed transesterification concept represents an efficient approach for developing multifunctional vitrimers and can also be generalized into other thermally cured polymer systems.