N,N-Dimethylanilinium (Ph(Me)(2)NH+) salt of tungsten disulfide (WS2) was developed as a novel cocatalyst for metallocene catalysts. The cocatalyst is composed of N,N-dimethylanilinium ion as a cationic part and "topotactic" reduction product of WS2, obtained by acquisition of an clectron by the neutral host lattice of WS2 without structural alteration, as an anionic part. Notable improvement of the catalytic activity for ethylene polymerization using the bis(indenyl)zirconium dichloride (Ind(2)ZrCl(2))/triethylaluminum (Et3Al) catalyst was observed upon the addition of the Ph(Me)(2)NH+ salt of WS2. The addition of the corresponding molybdenum disulfide (MoS2) one that had smaller crystallite size than the Ph(Me)(2)NH+ salt of WS2 showed the lower catalytic activity. The resultant poly(ethylene) prepared by the Ind(2)ZrCl(2)/Et3Al/Ph(Me)(2)NH+ salt of WS2 possessed similar properties like narrow polydispersity to that prepared by conventional metallocene type catalysts. The Zr loadings on the precipitate of the Ind(2)ZrCl(2)/Et3Al catalyst activated by the Ph(Me)(2)NH+ salt of WS2 increased with a decrease in the crystallite size of the Ph(Me)(2)NH+ salt of WS2. However, the catalytic activities in ethylene polymerization decreased drastically, indicating that the decrease of the crystallite size led to the significant increase of inactive species for ethylene polymerization. (C) 2003 Elsevier B.V. All rights reserved.