Intrinsically Flame‐Retardant Liquid Crystal Elastomers With Flame‐Triggered Actuation Enabled by Main‐Chain P─C Bond Incorporation

Abstract

ABSTRACT Liquid crystal elastomers (LCEs) are promising soft actuators, but their practical use is constrained by the incompatibility between fire safety and reversible actuation. Extrinsic flame‐retardant strategies can disrupt the liquid‐crystal organization and network integrity required for deformation, whereas existing phosphorus‐containing liquid crystal polymers (LCPs) are mainly rigid, aromatic thermoplastics designed for structural heat resistance rather than actuation. Here, we report a radical‐mediated P─H/ene step‐growth polymerization strategy for intrinsically flame‐retardant organophosphorus LCPs and LCEs using hypophosphorous acid and diene‐terminated mesogenic monomers. This main‐chain P─C bond‐forming strategy avoids rigid aromatic phosphorus units and affords phosphorus‐containing liquid‐crystalline systems with reduced phase‐transition temperatures together with a high phosphorus content of up to 6.8 wt.%. The resulting LCEs retain thermotropic liquid‐crystalline behavior, exhibit reversible thermoactuation, and show pronounced flame retardancy characterized by reduced heat release, self‐extinguishing behavior, and phosphorus‐promoted char formation. Notably, these organophosphorus LCEs also enable flame‐triggered actuation within only 0.4 s while preserving structural integrity and reversible function after flame exposure. This work establishes a practical molecular design route toward intrinsically flame‐retardant LCE actuators for thermally harsh environments.