Eugenol-Derived Thiol–Ene Photopolymerization Networks with Exceptional Flame Retardancy, Mechanical Robustness, and Chemical Degradability

Preparing polymer materials from renewable resources with intrinsic flame retardancy, mechanical robustness, and chemical degradability will significantly enhance their application potential in fire safety, practical requirements, and sustainable development. In this study, a phosphorus-containing tetrafunctional allyl compound (TEDP) was first synthesized using renewable eugenol. Subsequently, a thiol–ene polymer network (named TEPN1) was constructed by cross-linking TEDP with pentaerythritol tetra(3-mercaptopropionate) (PETMP) through a thiol–ene photopolymerization reaction. TEPN1 presents exceptional mechanical robustness, storage modulus, and charring capability due to its high cross-linking density. Moreover, TEPN1 exhibits superior flame retardancy, achieving Underwriters Laboratories-94 (UL-94) V-0 classification. The cone calorimeter test results show that TEPN1 exhibits a peak heat release rate (PHRR) of 425 kW/m2 and a total smoke production (TSP) of 6.9 m2. Both values are lower than most reported for eugenol-based flame-retardant thermosetting resins. The flame-retardant mechanism indicates that the flame-retardant effect of TEPN1 primarily acts in the condensed phase. Furthermore, TEPN1 also exhibits high transparency and satisfactory chemical degradability. Considering the inherent flame retardancy, superior transparency, and high mechanical robustness, TEPN1 is suitable for use as a UV-curable adhesive and flame-retardant coating.