The bisupported Ziegler-Natta catalyst system SiO2/MgCl2 (ethoxide type) /TiCl4/di-n-butyl phthalate/ triethylaluminum (TEA)/dimethoxy methyl cyclohexyl silane (DMMCHS) was prepared. TEA and di-n-butyl phthalate were used as a cocatalyst and an internal donor, respectively. DMMCHS was used as an external donor. The slurry polymerization of propylene was studied with the catalyst system in n-heptane from 45 to 70degreesC. The effects of the TEA and H, concentrations, temperature, and monomer pressure on the polymerization were investigated. The optimum productivity was obtained at [Al] / [DMMCHS] / [Ti] = 61.7:6.2:1 (mol/mol/mol). The highest activity of the catalyst was obtained at 60degreesC. Increasing the H-2 concentration to 100 mL/L increased the productivity of the catalyst, but a further increase in H-2 reduced the activity of the catalyst. Increasing the propylene pressure from I to 7 bar significantly increased the polymer yield. The isotacticity index (II) decreased with increasing TEA, but the H-2 concentration, temperature, and monomer pressure did not have a significant effect on the II value. The viscosity-average molecular weight decreased with increasing temperature and with the addition of H-2. Three catalysts with different Mg/Si molar ratios were studied under the optimum conditions. The catalyst with a Mg/Si molar ratio of approximately 0.93 showed the highest activity. (C) 2003 Wiley Periodicals, Inc.