Synthesis of phosphaphenanthrene-functional monobenzoxazine for enhancing thermal properties of polybenzoxazine thermosets

A facile approach to incorporate an intramolecular 6-membered ring hydrogen bond into a benzoxazine monomer containing 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) moiety has been developed, leading to a novel monobenzoxazine, ODOPB-pa. This monobenzoxazine was successfully synthesized via a Mannich condensation from 2-(6-oxido-6H-dibenz-[c,e][1,2]oxaphosphorin-1,4-dihydroxy phenylene (ODOPB), propargylamine and paraformaldehyde. The newly obtained ODOPB-pa possesses a relatively lower polymerization temperature than most commercialized benzoxazine resins. In addition, the intramolecular 6-membered ring hydrogen bond is identified as an essential feature in its polymerization behavior. The cured product, poly(ODOPB-pa), exhibits high thermal stability with a 5% weight loss temperature of 327 °C, and good flame retardant properties with a heat release capacity value of 119 Jg−1 K−1 and a total heat release value of 41 kJg−1. Moreover, ODOPB-pa was also applied to mix in different amounts (5, 10, 15 and 20 wt%) with a tri-functional benzoxazine bearing propargyl group, TH-pa. Notably, it has been demonstrated that the TH-pa/ODOPB-pa blend derived thermoset with 10 wt% ODOPB-pa exhibits the best thermal and flame retardant properties in consideration of the high glass transition temperature (312 °C) and very low heat release capacity value (56.5 Jg−1 K−1). These combined advantages highlight the potential of intramolecular 6-membered ring hydrogen bond-containing ODOPB-based benzoxazine in the producing of high-performance thermosetting systems.