Ethylene copolymerizations with various penterics [1-pentene, 4-methyl-1-pentene (4M1P), 3-methyl-1-pentene (3MIP), 4.4-dimethyl-1-pentene (NHEP)l using Cp*TiCl2(O-2,6-' Pr2C6H3) (1), CpTiCl2(N = C'Bu-2) (2), [Me2Si(C5Me4)(NBu)]TiCl2, (4) - MAO catalyst systems have been explored. Both 1 and 2 exhibited high catalytic activities affording high molecular weight copolymers with unimodal Molecular weight distributions; the r(E)r(C) value,; (r(E) = k(EE)/k(EC); E = ethylene; C = comonomer) by 1 were small, suggesting that the monomer incorporations were rather alternating, whereas the copolymerization by 4 proceeded in a random manner (r(E)r(C) = ca. 1) except the copolymerization with 3M I P I exhibited remarkable both catalytic activities and 3M 111 incorporation in the ethylene/3MIP copolymerization, and the r(E) value (8.73) was much smaller than that by 4 (92), 2 (28.3). Both I and 4 showed better NHEP incorporations than 2 in the ethylene/NHEP copolymerization, and the rather large r(E): value by 2 (6.77) compared to those by 1 (2.58-2.94) was also obtained in the copolymerization with 4M I P. These results clearly indicate that the monomer reactivities (r(E) values) are influenced not only by the substituent in the olefins, but also by the nature of the catalytically active species (structure, and ligand set employed). Both I and 2 also exhibited notable catalytic activities in the copolymerization of ethylene with 1-dodecene, 1-hexadecene, affording high molecular weight copolymers with unimodal molecular weight distributions. No notable differences toward the r(E) values by 1,2,4 were seen. although the values were slightly affected by both the steric bulk of olefins (linear branching) and the catalyst structure employed.