Ethylene polymerization using Tp*TiMe2(OAr) [Tp* = HB(3,5-Me-2-pyrazolyl)(3); Ar = O-2, 6-Me2C6H3 (2a), O-2,6-(Pr2C6H3)-Pr-i (2b)]-cocatalyst systems [cocatalyst: MAO, [Ph3C][B(C6F5)(4)], or B(C6F5)(3) and (AlBu3)-Bu-i or Al(n-octyl)(3)] or[Tp*TiMe(O-2,6-Me2C6H3)](+)[MeB(C6F5)(3)](-) center dot 2THF (3a) in the presence of Al alkyls have been explored. The catalytic activity by 2a,b-cocatalyst systems was highly affected by the Al/Ti molar ratios. Ethylene polymerizations by 2a,b-MAO catalyst systems proceeded in a quasi living manner between -30 and 25 degrees C, whereas certain degree of chain transfer was observed when the polymerizations by 2a,b were conducted in the presence of borate cocatalysts. The activity by the cationic Ti(IV)-methyl complex (3a) in the presence of Al(n-octyl)(3) was close to those by 2a - B(C6F5)(3)-Al(n-octyl)(3), (AlBu3)-Bu-i catalyst systems and the M-n values in the resultant polymers were close with uniform distributions; the facts clearly indicate that the same catalytically active species play a role in this catalysis.