A novel polymer-nanoclay hybrid nanocomposite based on polyvinyl acetate (PVAc)-organophilic montmorillonite (OMMT) has been reported via an in situ intercalated polymerization technique. The hybrid material was synthesized by one-step emulsion polymerization of vinyl acetate in the presence of OMMT using polyvinyl alcohol as the stabilizing agent. The intercalated polymerization was characterized by X-ray diffraction (XRD). The XRD patterns show that the interlayer spacing of OMMT after polymerization increased from 2.64 to 3.78 nm, indicating that the large macromolecular chain of PVAc was formed in the OMMT interlayer space. The Fourier transform infrared spectrum showed the characteristic absorption of PVAc in the OMMT particles separated from the nanocomposite, and the position of peaks shifted to high wave numbers. This showed that there was an interaction between PVAc and OMMT nanoparticles. A two-fold blend composed of PVAc-nano-OMMT/PP was prepared by the melt-blending technique. XRD and transmission electron microscopy images of the PVAc-nano-OMMT/PP composite further confirmed the formation of a partially delaminated nanocomposite structure. Thermogravimetry results showed that the thermal stability of PVAc-nano-OMMT/PP was greater than that of either polypropylene (PP) or Nano-OMMT/PP blend. PVAc-nano-OMMT/PP had better toughness, as the mass fraction of OMMT was 5 wt %. The flame retardancy of PP, Nano-OMMT/PP, and PVAc-nano-OMMT/PP composites was also studied. According to the limiting oxygen index (LOI) data and Cone calorimeter test, the addition of PVAc-OMMT resulted in higher LOI and lower heat release rate, effective heat of combustion, smoke release course, and better flame retardancy and barrier properties. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012