Boron carbonitride polymers have been arising abiding interest due to their excellent thermal and mechanical properties. In this study, a novel kind of hyperbranched polymer based on boron carbonitride skeleton, polyborazine, has been synthesized by a convenient A2+B3 approach. The chemical structure of polyborazine was confirmed by Fourier-transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR), X-ray photoelectron spectroscopy (XPS), and size exclusion chromatography (SEC). FT-IR and X-ray diffraction (XRD) pattern exhibited that the boron carbonitride compound formed during the pyrolysis process contributed to the improved thermal property of polyborazine. Nevertheless, the polyborazine can catalyze the cure process of benzoxazine thermosets, and enhance the thermal stability of benzoxazine thermosets owning to the presence of the preceramic compound. The approach for fabricating reactive ceramic precursor polymer with prepolymer blend system reveals promising results for thermosets that demanding remarkable thermal stability and oxidation resistance. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers