Effect of ligand in copolymerization of ethylene with 1-hexene by (cyclopentadienyl)(aryloxy)titanium(IV) complexes of the type, Cp TiCl 2 (OAr) [OAr = O-2,6-i Pr 2 C 6 H 3 and Cp = C 5 Me 5 (1), 1,3-t Bu 2 C 5 H 3 (2), t Bu 2 C 5 H 4 (3), 1,3-Me 2 C 5 H 3 (5), Cp (6); OAr = O-2,6-Me 2 C 6 H 3 and Cp = C 5 Me 5 (7), 1,3-t Bu 2 C 5 H 3 (8)], has been explored in the presence of MAO, and [Me 2 Si(C 5 Me 4)(N t Bu)]TiCl 2 (4) was chosen as the reference. It was revealed that 1 exhibited the highest catalytic activity, and the resultant poly(ethylene-co-1-hexene)s possessed relatively high molecular weights with narrow molecular weight distributions (M w = 29.5 × 10 4 to 34.5 × 10 4 , M w /M n = 1.74–1.88), and contained 1-hexene in relatively high extents (36.6–43.5 mol%). Effect of both cyclopentadienyl and aryloxy groups plays an essential key role in the copolymerization not only to exhibit high catalytic activity but also to afford relatively high molecular weight copolymer with narrow polydispersity. It also turned out that -olefin incorporation into the copolymer is highly affected by the substituents on the cyclopentadienyl group. The monomer reactivity ratios were not affected by both the polymerization temperature and Al/Ti molar ratios, but the monomer sequence distributions and the monomer reactivity ratios depended upon cyclopentadienyl fragment used. In spite of the rather wide bond angle of Cp–Ti–O (ca. 120.5 •) in 1–3 than that of Cp–Ti–N in 4 (107.6 •), a high level of 1-hexene incorporation and smaller r E r H value by 1–3 than those by 4 were attained, which would be due to the flexible internal rotation of both cyclopentadienyl and aryloxy groups. © 2001 Elsevier Science B.V. All rights reserved.