In an updated version of the polymerization-filling technique, the surface of particulate fillers has been activate by metallocenes. Homogeneously filled polyolefins have accordingly been produced by the in situ copolymerization of ethylene and alpha-olefins e.g., 1-octene and 1,9-decadiene. Different fillers have been successfully used, including inorganic particles with either an acidic surface (kaolin, glass beads, silica) or a basic surface (magnesium hydroxide, wollastonite), graphite and nickel particles. Hydrogen has proved to be efficient in reducing the molecular weight of the produced polymers.