Investigation of the structural features and polymerization mechanism of a novel bio-based benzoxazine using different lewis acids

Benzoxazines are heterocyclic compounds known for their high thermal stability, flame retardancy, and excellent mechanical performance, making them promising candidates for high-performance polymer applications. This study aimed to synthesize a benzoxazine monomer (Ca-c) derived from cardanol (Ca), the main component of cashew nut shell liquid (CNSL), using hexamethylenetetramine (HMTA) as a source of both methylene and nitrogen units, and to evaluate the influence of Lewis acids (AlCl3, FeCl3, MgCl2, and ZnCl2) on the polymerization process. The monomer was synthesized from cardanol and HMTA, producing a resin that was employed without further purification. Formulations were prepared by incorporating metal salts at 7.5% (mol/mol), followed by curing under a controlled heating protocol. This approach enabled evaluation of the influence of the salts on the curing behavior and properties of the benzoxazine system. Characterization by FTIR and NMR (1H and 13C) confirmed the formation of the oxazine ring. DSC analyses revealed a significant reduction in the onset polymerization temperature, especially with MgCl2 (up to 29 °C). TGA showed good thermal stability, and diffuse reflectance spectroscopy indicated metal–ligand interactions, particularly in the Zn2+-containing systems. The results demonstrate that metal salts act as effective catalysts, enabling efficient polymerization under milder conditions.