Convergent Molecular and Process Engineering of π-Conjugated Flame Retardant for High-Performance Epoxy Resins

The development of flame-retardant epoxy resins (EPs) through molecular incorporation is persistently challenged by a fundamental trade-off among high additive loading, retention of mechanical and optical properties, and synthetic environmental friendliness. Herein, we transcend this compromise via a convergent molecular and process engineering strategy. A benzothiazole phosphine oxide (BTPO) flame retardant, featuring a tailored π-conjugated structure, is synthesized under mild conditions using a visible-light photocatalytic continuous-flow platform. This green method achieves direct C(sp2)–P coupling with excellent sustainability metrics: 90% atom economy, an E-factor of 4.5, and a process mass intensity (PMI) of 5.5. The molecular design enables exceptional flame suppression at an ultralow loading of 2 wt %, granting EPs a UL-94 V-0 rating and a limiting oxygen index (LOI) of 34.0%. Pyrolysis analysis reveals a biphasic flame-retardant mechanism: (1) rapid release of phosphorus- and nitrogen-centered radicals effectively quenches H•/OH• radicals in the gas phase, while (2) sulfur and phosphorus species catalyze char graphitization in the condensed phase. This synergy significantly enhances fire safety, increasing the char yield from 13.1 to 20.8%, reducing total smoke production (TSP) by 10.2%, and lowering total heat release (THR) by 16.0%. Crucially, the extended π-conjugation ensures superior matrix compatibility, mitigating phase separation to simultaneously enhance tensile strength by 7.7% and optical transmittance by 8.0% (vs neat EP). This work establishes a sustainable paradigm for multifunctional additive design, demonstrating that integrative molecular and process engineering can resolve long-standing trade-offs in high-performance polymer composites.