Copolymerization of ethylene and styrene with the catalytic system Cp*TiMe3-B(C6F5)(3) under suitable conditions affords a new polymer having a polyethylenic backbone with 4-phenyl-1-butyl branches as the main solid product. Analysis of the polymerization mixtures evidences the formation of significant amounts of unsaturated ethylene-styrene co-oligomers; A detailed characterization of the oligomeric fraction by CC-MS and NMR analysis suggests some selectivity of the co-oligomerization, i.e., the prevailing formation of phenylhexenes and the absence of phenylbutenes. These unexpected results have been ascribed to the multisite nature of the catalytic system, containing a species able to co-oligomerize ethylene and styrene to phenylhexenes and another species able to copolymerize the formed g-phenyl-l-hexene with ethylene. A mechanism involving Ti(II) active species and metallacycle intermediates has been proposed to explain the observed selectivity in the co-oligomerization, by analogy with the mechanism proposed for the selective trimerization of ethylene to l-hexene by the same catalyst. Extension of this study to catalysts based on other half-titanocenes activated by methylalumoxane results in similar findings; optimization of the reaction conditions for the CpTiCl4-MAO catalyst results in styrene conversions to unsaturated co-oligomers >90%. Dual-catalyst systems,with one component co-oligomerizing ethylene and styrene and the other copolymerizing the formed g-phenyl-l-hexene with ethylene, have been used to selectively produce polyethylenes with 4-phenyl-1-butyl branches. Use of substituted styrenes, i.e., p-methylstyrene, p-tert-butylstyrene, p-chlorostyrene, and m-methylstyrene, as the comonomers results in the formation of polyethylenes having the corresponding 4-aryl-1-butyl branches as well as oligomeric fractions analogous to those obtained from styrene.