Heterogeneous Ziegler-Natta TiCl4 catalyst using MgCl2 and SiO2 as supports was prepared under controlled conditions. Mg(OEt)(2) was used as a starting material and was expected to convert to active MgCl2 during catalyst preparation. Due to the high surface area and good morphological control, SiO2 was chosen as well. Slurry copolymerization of ethylene and propylene (EPM) was carried out in dry n-heptane by using the catalyst system SiO2/MgCl2/TiCl4/EB/TiBA or TEA/MPT/H-2 at temperatures of 40-70degreesC, different molar ratios of alkyl aluminum: MPT: Ti, hydrogen concentrations, and relative and total monomers pressure. Titanium content of the catalyst was 2.96% and surface area of the catalyst was 78 m(2)/g. Triisobutyl aluminum (TiBA) and triethyl aluminum (TEA) were used as cocatalysts, while ethyl benzoate (EB) and methyl p-toluate (MPT) were used as internal and external donors, respectively. H, was used as a chain-transfer agent. Good-quality ethylene propylene rubber (EPR) of rubber was obtained at the ratio of [TiBA] : [MPT] : [Ti] = 320:16:1 and polymerization temperature was 60degreesC. When TiBA was used as a cocatalyst, a higher and more rubberlike copolymer was obtained. For both of the cocatalysts, an optimum ratio of Al/Ti was obtained relative to the catalyst productivity. Ethylene content of the copolymer obtained increased with increasing TiBA concentration, while inverse results were obtained by using TEA. Addition of H, increased the reactivity of the catalyst. The highest product was obtained when 150 ml, H-2/L solvent was used. Increasing temperature from 40 to 70degreesC decreased the productivity of the catalyst, while irregular behavior was observed on ethylene content. Relative pressure of P-P/P-E = 1.4:1 and total pressure of 1 atm was the best condition for the copolymerization. Polymers with ethylene contents of 25-84% were obtained. Increasing ethylene content of EPR decreased T-g of the polymer obtained to a limiting value. Viscosity-average molecular weight (M.) decreased with increasing temperature and TiBA and H-2 concentration. However, increasing the polymerization time increased the M-v. (C) 2004 Wiley Periodicals, Inc.