The synthesis and characterization of rac-[ethylenebis(1-(tert-butyldimethylsilyl)-3-indenyl)] zirconium dichloride (3) is reported. The silyl substituted 3/MAO was compared to its siloxy substituted analogue (4) in ethylene homo- and in ethylene-1-hexene copolymerizations to elucidate the effect of the heteroatom on polymerization performance. The influence of monomer and cocatalyst concentration and the polymerization temperature was investigated. The oxygen between the indenyl ligand and the bulky tert-butyldimethylsilyl group in the siloxy substituted 4/MAO was found to have a positive influence on polymerization activity and copolymerization performance. (C) 2000 John Wiley & Sons, Inc.