The ethene-styrene copolymerization has been investigated using the dimethylsilylene-bridged (amidocyclopentadienyl)dichloro-titanium(IV) complexes [TiCl2 {eta(5)-1-(SiMe(2)Nt-Bu-N-K)-2,3,4-Me-3-5-R-C-5}], where R =Me (1), H (2), Bu (3), Ph (4),4-fluorophenyl (5),and but-2-en-2-yl (6) in combination with methylalumoxane (MAO) as catalysts. The nature of the substituent R strongly influenced the catalyst activity and selectivity and the copolymer microstructure and molecular weight. The catalysts derived from 1 to 3 were by about one order more active than those derived from 4 to 6. At the optimum Al/Ti molar ratio of 900, the highly active catalysts produced a pseudo-random copolymer (95-97 wt.%) containing up to 47.8 mol% of incorporated styrene. The low-active catalysts gave mixtures of a pseudo-random copolymer (76-85 wt.%) with polyethene (10 wt.%) and polystyrene sequences (3-7 wt.%). The X-ray diffraction crystal structures of 2 and 4 were determined. Comparison of crystal structures of 1 and 2 versus 4 and 5 revealed a slightly shorter distances Ti-Cg (Cg-centroid of the cyclopentadienyl ring) and slightly larger Cl-Ti-Cl angles in 1 and 2, indicating a higher electron density at the titanium atom. An electron attracting effect of phenyl or alkenyl substituents as well as their steric hindrance can account for a low catalytic performance of 4-6/MAO catalysts. (C) 2004 Elsevier B.V. All rights reserved.