Phthalonitrile functional polybenzoxazines show high thermal stability with temperature at 5% weight loss (T-5%) UP to 450 degrees C for monofunctional benzoxazine-based polymers, and up to 550 degrees C for polymers from the bifunctional precursors. High char yield up to 80% by weight is obtained by thermogravimetric analysis in an inert atmosphere at 800 degrees C. Char yield up to 70% by weight at 600 degrees C is achieved in air with T-5% in the 380-420 degrees C range. Differential scanning calorimetry (DSC) is used to determine curing conditions. These resins do not require ultrahigh curing temperature as it is typical for the other phthalonitrile functional prepolymers and 250 degrees C is used as a maximum post-cure temperature to achieve desired properties. Dynamic mechanical analysis (DMA) reveals unusually broad dependence of loss modulus vs. temperature with the maximum at 300 degrees C for polybenzoxazines from bifunctional precursors and 278 degrees C for polymer from monofunctional precursors. Additionally, these materials exhibit low flammability.