Poly(urethane-benzoxazine)/clay hybrid nanocomposites (PU/Pa-OMMTs) were prepared from an in situ copolymerization of a polyurethane (PU) prepolymer and a monofunctional benzoxazine monomer, 3-phenyl-3,4-dihydro-2H-1,3-benzoxazine (Pa), in the presence of an organophilic montmorillonite (OMMT), by solvent method using DMAc. OMMT was made from cation-exchange of Na-montmorillonite (MMT) with dodecyl ammonium chloride. The formation of the exfoliated nanocomposite structures of PU/Pa-OMMT was confirmed by XRD from the disappearance of the peak due to the basal diffraction of the layer-structured clay found in both MMT and OMMT. DSC showed that, in the presence of OMMT, the curing temperature of PU/Pa lowered by ca. 60degreesC for the onset and ca. 20degreesC for the maximum. After curing at 190degreesC for 1 h, the exothermic peak on DSC disappeared. All the obtained films of PU/Pa-OMMT were deep yellow and transparent. As the content of OMMT increased, both the tensile modulus and strength of PU/Pa-OMMT films increased, while the elongation decreased. The characteristics of the PU/Pa-OMMT films changed from plastics to elastomers depending on OMMT content and PU/Pa ratio. PU/Pa-OMMT films also exhibited excellent resistance to the solvents such as tetrahydrofuran, N,N-dimethylformamide and N-methyl-2-pyrrolidinone. The thermal stability of PU/Pa were enhanced remarkably even with small amount of OMMT. (C) 2003 Wiley Periodicals, Inc.