A polymerization process for preparing a silica-filled butyl rubber polymer. A quaternary onium ion substituted silaceous nanoclay is dispersed in the organic polymerization fluid prior to polymerization of an isoolefin monomer and a multiolefin monomer in the fluid. The resulting silica-filled polymer comprises butyl rubber intercalated with silica from the silaceous nanoclay. The rate of polymerization is substantially unaffected by addition of the nanoclay to the polymerization fluid. The resulting silica-filled polymers exhibit improved impermeability as compared with non-filled polymers.