6,6′-(Propane-2,2-diyl)bis(3-(3-(diethoxy(methyl)-silyl)propyl)-3,4-dihydro-2H-benzo[e][1,3]oxazine) (BA-mdes) monomers were thermally polymerized to form a polybenzoxazine network containing Ph-OH and Si-OEt groups, and then these groups further reacted to obtain Si–O–Ph bond additionally cross-linked organic–inorganic hybrid polybenzoxazine. The glass transition temperature (T_g, tan δ) of the resulting polybenzoxazine reaches ≈400 °C.