Received March 18, 2004; Revised Manuscript Received July 22, 2004 Norbornene polymerization via "vinyl addition" proceeded with (t-BuNSiMe(2)Flu)TiMe2 (1) in the presence of various cocatalysts [methylaluminoxane (MAO), modified methylaluminoxane (MMAO), and Ph3CB(C6F5)(4)/Oct(3)Al] at wide range of temperatures (20-80degreesC) with high activity. The 1-Ph3CB-(C6F5)(4)/Oct(3)Al system produced 4.8 x 10(3) kg of polymer per mole of Ti per hour at 20degreesC. All the catalyst systems produced high molecular weight (M-n) polymers. In the 1-MMAO system the yield and M values increased linearly keeping narrow molecular weight distribution against the Al/Ti ratio, suggesting that MMAO should take part not only in the initiation but also in the propagation reaction. The rate enhancement with higher Al/Ti ratio was also observed in the Ph3CB(C6F5)(4)/Oct(3)Al system. The activity of the 1-dried MAO and 1-MMAO systems increased with raising reaction temperature up to 60degreesC, whereas that of 1-Ph3CB(C6F5)(4)/Oct(3)Al decreased. The polynorbornenes obtained with 1 were amorphous, soluble in halogenated aromatic solvents, and stable up to 420degreesC. The film of polynorbornene prepared by solvent casting was highly transparent in the UV-vis region.