Renewable Magnolol-Derived Benzoxazine Resins with Ultrahigh Glass Transition Temperature, Superior Flame Retardancy, and Valorization into High-Performance CO ₂ Adsorption

The design and development of high-performance resins, along with the resourceful utilization of their waste, are of great significance to modern industry. This study employed magnolol, a renewable phenolic compound, combined with meta-/para-aminobenzonitrile through solvent-free synthesis to develop two benzoxazine monomers (M-mBN and M-pBN). The incorporation of cyano groups enabled the in situ formation of triazine rings during thermal curing. The research results demonstrated that the cured benzoxazine resins (poly(M-mBN) and poly(M-pBN)) exhibited outstanding comprehensive properties: ultrahigh glass transition temperature (Tg) exceeding 400 °C, initial thermal decomposition temperatures (Tdi) of 434 and 416 °C, respectively, and char residues at 800 °C reaching 59% and 57%. In terms of flame retardancy, poly(M-mBN) showed a remarkably low heat release capacity of 98.5 J·g–1·K–1, meeting the criteria for noncombustible materials. These cyano-functionalized resins also exhibited low dielectric constants and inherent hydrophobicity, demonstrating a breakthrough in balancing renewable feedstock utilization with excellent performance. Moreover, poly(M-mBN) could be efficiently converted into a microporous carbon material (poly(M-mBN)-800) through carbonization. The resulting carbon exhibited a high specific surface area (1858 m2/g) and nitrogen-containing functionalities, enabling superior CO2 adsorption capacity (up to 5.52 mmol·g–1). The integrated strategy of molecular engineering, solvent-free processing, and functional group optimization addresses the longstanding performance gap between petroleum-based and biobased thermosetting resins and also offers a pathway for the green synthesis and environmentally friendly recycling of high-performance resins.