A new type of half-metallocene catalyst for the polymerization of ethylene and styrene, Cp*M((O)(3)O9Si7(c-C5H9)(7)) (M = Ti (1); Zr (2)), is prepared by the reaction of (HO)(3)O9Si7(c-C5H9)(7) with Cp*MCl3 in the presence of triethylamine. Complex 1 is stable to hear and air, but its heavier congener 2 is slightly air-sensitive. The catalytic systems 1/MMAO and 2/MMAO, in which MMAO is modified methylaluminoxane, show good activities in ethylene polymerization, with that of 2/MMAO greater than that of 1/MMAO. Polyethylenes with bimodal molecular weight distributions were obtained. In addition, the catalytic system 1/MMAO shows fairly good activities for the syndiospecific polymerization of styrene. The molecular weight distribution of PE obtained by the 2/MMAO system as a function of the polymerization temperature.