The chemical interaction of the catalyst MgCl2/ethylbenzoate/TiCl4 with the cocatalysts triethylaluminum and trisobutylaluminum was investigated to establish a relationship between the titanium oxidation states and the catalytic activity, polymer isotacticity, and polymer molecular weight in propylene polymerizations. This interaction was studied using different Al:Ti molar ratios by measuring the changes of the titanium oxidation states at different polymerization times. Both hydrogen and alkyl aluminum caused a reduction of Ti4+ species to lower oxidation states species Ti3+ and Ti2+. However, the Ti4+ species reduction appeared to be incomplete. It was found that the Ti4+ species undergoes a severe reduction as the Al: Ti molar ratio increases from 50 to 230 as overreduction takes place. This change of the Ti3+ species percentage with time was found to correlate with the rate-time profiles of propylene polymerization. From this observation, it would be fair to conclude that the trivalent titanium species is more likely to be the active titanium species for propylene polymerization than the aforementioned catalyst system. On the other hand, hydrogen addition was found to cause an increase in Ti3+ species. The increases in both hydrogen amount and/or Al: Ti molar ratio were found to cause a decrease in both molecular weight and polypropylene isotactic index. (C) 2004 Wiley Periodicals, Inc.