Integrating DOPO and Disulfide Bonds into a Reactive Epoxy Monomer for Flame-Retardant, Toughened, and Transparent Thermosets

Epoxy resins (EP) are widely employed in advanced engineering fields, yet their inherent flammability, cross-linking-induced brittleness, and the optical opacity caused by conventional flame retardants pose long-standing challenges for multifunctional applications. In this work, we report a molecular design strategy to simultaneously overcome these limitations by synthesizing a reactive epoxy monomer, DPVACO, which integrates a 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO)-derived flame-retardant moiety and disulfide dynamic covalent bonds within a single molecular backbone. Through covalent incorporation into the epoxy network, DPVACO ensures molecular-level uniformity and excellent matrix compatibility, circumventing the phase separation and property deterioration observed with its nonepoxidized counterpart. The resulting thermosets exhibit a rare combination of high mechanical performance, impact toughness, flame retardancy, and optical transparency. With only a 15 wt % DPVACO loading, the composite achieves a UL-94 Vertical Burning (UL-94) V-0 rating, a limiting oxygen index of 35.0%, and significant reductions of 21.5 and 17.2% in peak heat release rate and total heat release, respectively. The enhanced flame retardancy is attributed to a compact graphitic char layer that serves as an efficient physical barrier in the condensed phase. Meanwhile, the embedded disulfide bonds function as intrinsic energy-dissipating units, substantially improving impact strength to 32.36 kJ/m2 and tensile strength to 94.20 MPa, while the fracture surface morphology reveals pronounced plastic deformation indicative of a brittle-to-ductile transition. Notably, despite the incorporation of rigid aromatic and phosphorus-containing structures, all DPVACO-modified resins maintain high optical transparency comparable to that of neat epoxy, with transmittance even surpassing that of the pristine resin beyond 460 nm─a direct consequence of the homogeneous covalent integration that preserves optical homogeneity. This work demonstrates that the rational integration of flame-retardant and dynamic covalent functionalities into a single reactive epoxy monomer offers an effective platform to resolve the traditional conflicts among stiffness, toughness, transparency, and flame retardancy. It not only delivers a high-performance multifunctional epoxy thermoset but also provides insights into the synergistic role of disulfide dynamic bonds in the molecular design of advanced polymer networks for optoelectronic, aerospace, and other demanding applications.