Ligand effects on the catalytic activity [and norbornene (NBE) incorporation] for both ethylene polymerization and ethylene/NBE copolymerization using half-titanocenes (titanium half-sandwich complexes) containing ketimide ligand of type Cp′TiCl 2 [NC(R 1 )R 2 ] [Cp′=Cp (1), C 5 Me 5 (Cp ∗ , 2); R 1 ,R 2 = t Bu, t Bu (a), t Bu,Ph (b), Ph,Ph (c)]–methylaluminoxane (MAO) catalyst systems have been investigated. CpTiCl 2 [NC( t Bu)Ph] (1b) CpTiCl 2 (NCPh 2 ) (1c), and Cp ∗ TiCl 2 (NCPh 2 ) (2c) were prepared and identified; the structure of Cp ∗ TiCl 2 (NCPh 2 ) (2c) was determined by X-ray crystallography. The catalytic activity for ethylene polymerization increased in the order: 1a>1b>1c, suggesting that an electronic nature of the ketimide ligand affects the activity. However, molecular weight distributions for resultant (co)polymers prepared by 1b,c and by 2c–MAO catalyst systems were bi- or multi-modal, suggesting that the ketimide substituent plays a key role in order for these (co)polymerizations to proceed with single catalytically-active species. CpTiCl 2 (NC t Bu 2 ) (1a) exhibited both remarkable catalytic activity and efficient NBE incorporation for ethylene/NBE copolymerization.