Abstract: The present invention discloses a method to prepare the polymer-clay nanocomposite latex. At first, a clay and a water-soluble initiator are mixed in water to form an intermediate solution, in which the initiator is absorbed or intercalated into the clay. Then, at least one monomer from vinyl ester, acrylic and acrylamide derivatives is added into the intermediate solution for soap-free emulsion polymerization, which may also be added with some other monomers such as acrylic acid, maleic anhydride, vinyl chloride, acrylonitrile ethylene, et al. for copolymerization. During soap-free emulsion polymerization, the radicals from the dissociated initiators will react with the monomers to form the monomer radicals, which tend to diffuse into the interlayer region of clays for further polymerization and eventually exfoliate the clays. The prepared exfoliated polymer-clay nanocomposite latex can be cast into a vapor-impermeable film so that it can be used as a sealing material or coating.