Catalyst-free β-hydroxy phosphate ester exchange for robust fire-proof vitrimers

Abstract Herein, we introduce a new catalyst-free β-hydroxy phosphate ester exchange chemistry in fabricating mechanically robust vitrimer with excellent recyclability and intrinsic fire safety. The desire to use polymers sustainably and safely has driven rapid development of recyclable thermosets and flame-retardant thermosets, respectively. However, few efforts have been devoted to address the desire simultaneously in an individual polymer. In this study, we integrate robust mechanical, recyclable, shape memorable, and fire-proof properties into a single β-hydroxy phosphate esters based covalent adaptable network (PE-CAN). These properties rely on the unique behaviors of β-hydroxy phosphate esters at low, medium, and high temperatures, respectively. At room temperature, the abundant hydrogen bonds in the PE-CANs contribute to outstanding toughness (5.44 MJ/m3). Around 100 °C, the rapid exchange reaction between phosphate esters and neighboring β-hydroxyls endows the vitrimer with almost 100% recycling efficiency. Above 250 °C, a cellular layer of charred phosphoric acid generated from β-hydroxy phosphate esters could separate/insulate heat effectively, providing fire protection. Additionally, a flame-triggered shape memory effect is demonstrated. The strategy of integrating fire retardancy in CANs by dynamic β-hydroxy phosphate ester exchange can be scaled up for mass production and for a wide range of applications in chemical engineering, and can be generalized to other monomers.