A mechanistic analysis of propylene polymerization was performed, in which the catalyst system was Me2Si(R(1)Ind)(2)ZrCl2/SMAO/AIR(3)(2) (in situ supported catalyst onto MAO-modified silica) or Me2Si(R(2)Ind)(2)ZrCl2/MAO (homogeneous), where R-1 = H or CH3, cocatalyzed by AIR 2 3 = TEA (triethylaluminum), IPRA (isoprenylaluminum), or TIBA (triisobutylaluminum). The catalyst activity of the homogeneous system Me2Si(2-Me-Ind)(2)ZrCl2/MAO was almost 8 times higher than that observed for Me2Si(Ind)(2)ZrCl2/MAO (38 vs 4.6 kg PP/g cat h), while the polypropylene molar mass was 3 times higher (M-w: 93 vs 34 kg/mol). Conversely, the in situ supported systems Me2Si(Ind)(2)ZrCl2/SMAO/ AlR3 and Me2Si(2-Me-Ind)(2)ZrCl2/SMAO/AIR(3) showed similar activities, ranging from 0.2 to 1.5 kg PP/g cat h. The molar mass of the resulting polymers prepared using the in situ procedure was dependent on the AIR3 nature and on the Al/Zr ratio. Generally, the heterogeneous catalysts produced PP with higher molecular weights than that obtained with homogeneous ones. The influence of the alkylaluminum, used as the cocatalyst, on the chain-transfer termination reaction to the alkyl compound was evident from the activity and the molecular weight of the produced polymers. (c) 2007 Elsevier Ltd. All rights reserved.