Copolymerization of ethylene with 1-octadecene was studied using [eta(5):eta(1)-C5Me4-4-R-1-6-R-C6H2O]TiCl2[R-1 = Bu-t (1), H (2, 3, 4); R = Bu-t (1, 2), Me (3), Ph (4)] as catalysts in the presence of Al(i-Bu)(3) and [Ph3C][B(C6F5)(4)]. The effect of the concentration of comonomer in the feed and Al/Ti molar ratio on the catalytic activity and molecular weight of the resultant copolymer were investigated. The substituents on the phenyl ring of the ligand affect considerably both the catalytic activity and comonomer incorporation. The 1/Al(i-Bu)(3)/[Ph3C][B(C6F5)(4)] catalyst system exhibits the highest catalytic activity and produces copolymers with the highest molecular weight, while the 2/Al(i-Bu)(3)/[Ph3C][B(C6F5)(4)] catalyst system gives copolymers with the highest comonomer incorporation under similar conditions. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 120: 1514-1519, 2011