The effect of the cyclopentadienyl fragment in the copolymerization of ethylene with norbornene (NBE) by various non-bridged (aryloxo)(cyclopentadienyl)titanium(IV) complexes of the type, Cp′TiCl2(OAr) [Cp′=indenyl (1), C5Me5 (Cp*, 2), t-BuC5H4 (3), 1,2,4-Me3C5H2 (4); OAr=O-2,6-i-Pr2C6H3], MAO catalyst systems has been explored. The catalytic activity and the NBE incorporation were highly dependent upon substituent on Cp′ employed, and 1 exhibited both a remarkable catalytic activity and an efficient NBE incorporation, affording high molecular weight polymers with unimodal molecular weight distributions. NBE repeat units were observed in the resultant poly(ethylene-co-NBE)s prepared by 1, 3, and 4, and the degree of NBE dyads as well as alternating sequences was strongly influenced by the Cp′ employed. These complexes (1,3,4) also exhibited high catalytic activities for ethylene/cyclopentene (CPE) copolymerization, but the CPE incorporation by 1 was less efficient than the NBE incorporation under the same conditions. The ab initio density functional (DF) molecular orbital (MO) methods were utilized to explore the effect of substituents on Cp′ toward the catalytic activity, the NBE incorporation and the monomer sequence distribution. Good correlations between the ΔEcoordEt (coordination energy of ethylene) after NBE insertion and the activity, and between ΔEcoordNBE (coordination energy of NBE) after ethylene (or NBE insertion) and NBE content were observed, strongly suggesting that the capability producing NBE repeat units was dependent upon ΔEcoordNBE after NBE insertion.