Titanocene complexes based on an amido-fluorenyl ligand bridged by a dimethylsilylene group, (eta(1):eta(5)-C13H8SiMe2NCMe3)TiCl2 (3) and (eta(1):eta(5)-C13H8SiMe2NCMe3)TiMe2 (5), have been synthesized. Reaction of 5 with 1 equiv of [Ph3C][B(C6F5)(4)] (2) was almost quantitative to give the "cationic" compound [(eta(1):eta(5)-C13H8SiMe2NCMe3)TiMe](+)[B(C6F5)(4)](-) (1), as identified on the basis of elemental analyses and spectroscopic properties. The "cation" 1 was highly active and stereoselective in the copolymerization of ethylene (E) and styrene (S), depending upon the polymerization conditions, to produce predominantly a new microstructural E-S copolymer (together with some polyethylene homopolymer) with an activity of (0.65-1.58) x 10(5) g of bulk polymer/(mol of Ti mol of total monomers.h). As thoroughly characterized by solvent extraction, GPC, C-13 NMR, DSC, and DMA, the E-S copolymer obtained by catalyst 1 proved to be a perfectly alternating copolymer with well-defined isotactic polystyrene structure, together with a single glass transition (T-g = 30 degrees C) and melting temperature of 118 degrees C. The findings obtained by catalyst 1 suggest that the structure of the active species with the bulkier fluorenyl substituent and the following alternating site migratory insertion of comonomer in the chain propagation is responsible for the preferentially alternating, isotactic comonomer incorporation.