Benzoxazine was synthesized through the Mannich condensation of phenol, formaldehyde, and primary amines through ring-opening polymerization. Polybenzoxazines are phenolic-like materials that possess dimensional and thermal stability, and they release no toxic byproducts during their polymerization. To further improve the thermal stability of polybenzoxazines, a hydrosilane-functionalized polyhedral oligomeric silsesquioxane (H-POSS) was incorporated into the vinyl-terminated benzoxazine monomer (VB-a) which we then subjected to ring-opening polymerization. In addition, we also prepared hybrids from a non-reactive POSS (IB-POSS) and VB-a. The glass transition temperature (T-g) of a regular polymerized VB-a (i.e. PVB-a) is 307 degrees C, while the hybrid containing 5 wt% of H-POSS is 333 degrees C. The IB-POSS modified PVB-a hybrids, in general, results in lower T-g than the pure PVB-a due to poor missibility. (c) 2005 Elsevier Ltd. All rights reserved.