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Propylene homopolymerizations were carried out with rac-dimethylsilylenebis(indenyl)zirconium dichloride, methylaluminoxane-modified silica, and common alkylaluminum cocatalysts. Supported catalysts were prepared by the in situ immobilization technique. The effects of the type and concentration (Al/Zr ϭ 40 -1000) of alkylaluminum on the propylene polymerization were evaluated with triethylaluminum (TEA), isoprenylaluminum (IPRA), and triisobutylaluminum (TIBA) as cocatalysts. The polymers were analyzed by gel permeation chromatography, differential scanning calorimetry, and 13 C-NMR. The polypropylene molar mass varied according to the nature of the alkylaluminum in the following order: TIBA Ͼ IPRA Ͼ TEA Ͼ no alkylaluminum. The polymers made with an in situ supported catalyst had lower crystallinities and melting points than the ones produced by homogeneous polymerization. The isotacticity was not affected by the polymerization conditions examined in this investigation.