The application of high-temperature bismaleimide resins in structural composites is inherently limited by the trade-off between thermal performance and processability. Here, we report a targeted molecular design strategy to overcome this operational bottleneck. By introducing bulky ortho-ethyl substituents into a monomeric architecture, premature homopolymerization was selectively suppressed up to 300 °C while maintaining the rigidity of the aromatic backbone. The resulting resin system provides an optimized processing profile for vacuum-assisted resin transfer molding (VARTM), characterized by low melt viscosity (<400 mPa·s) across a 120–140 °C range and isothermal stability exceeding 8 h. The fabricated carbon fiber reinforced polymer (CFRP) exhibits a high glass transition temperature (up to 277 °C) and retains 92.5% of its room-temperature flexural modulus at 250 °C (49 GPa). This strategy serves as a technological basis for the autoclave-free manufacturing of heat-resistant structural materials.

To overcome the hydrolytic instability inherent in contemporary phosphorus-containing phthalonitrile resins, this work reports the synthesis and characterization of novel trifunctional phthalonitrile monomers: m-(3-phthalonitriloxyphenyl) phosphoramide (m-PHPA) and p-(3-phthalonitriloxyphenyl) phosphoramide (p-PHPA). By incorporating P‒N bonds in place of P−O linkages, these innovative monomers exhibit exceptional resistance to hydrolysis, remaining stable under acidic (pH ≈ 3), neutral (pH ≈ 7), and alkaline (pH ≈ 10) conditions. Furthermore, the monomers were discovered to possess self-curing capabilities. Upon heating, they release an amino-containing fragment that initiates polymerization without the need for additional catalysts. A comparative analysis revealed p-PHPA as a superior candidate for high-performance materials. While the high reactivity of the m-isomer led to microcrack formation, the p-PHPA-based system produced carbon fiber reinforced polymers (CFRPs) with a defect-free microstructure. Consequently, the p-PHPA composites exhibit a high glass transition temperature (449°C), significantly better thermo-oxidative stability, and superior retention of mechanical properties at 300°C.