Polymerization of 1‐butene with thiobis(phenoxy)titanium dichloride (TBPTiCl2) with modified methylaluminoxane (MMAO) yielded atactic and regioirregular poly(1‐butene) with low molecular weight. However, productivity was increased and the yielded polymer structure was changed when water‐modified MMAO (WM‐MMAO) was used as a cocatalyst. The molecular weight of obtained polymer with TBPTiCl2/WM‐MMAO was dramatically increased and reached over 9 million g/mol. Poly(1‐butene) produced by this catalytic system was almost regioregular and slightly isotactic. 1‐Hexene and 1‐octene were also polymerized by this catalytic system. In all cases, the ultra‐high‐molecular‐weight polymers were obtained. These results showed that the modified structure of the MMAO cluster by water affects the productivity, molecular weight, regioregularity, and stereoregularity of yielded polyolefins. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1107–1111, 2004