This article describes ethylene/1-hexene copolymerization reactions with a supported titanium-based, multicenter Ziegler-Natta catalyst. The catalyst was modified by pretreating its solid precursor with AlEt2Cl and with similar organoaluminum chlorides, Al2Et3Cl3, AlEtCl2, and AlMe2Cl. Testing of the untreated and the pretreated catalysts in copolymerization reactions under standard reaction conditions demonstrated that the modifying agents produce two changes in the catalyst. First, the pretreatment significantly reduces the reactivity of active centers that produce high molecular weight, highly crystalline copolymer components with a low 1-hexene content. Second, the pretreatment noticeably increases the reactivity of active centers that produce low molecular weight copolymer components with a high 1-hexene content. The first effect is caused by Lewis acid-base interactions of the modifiers with the active centers, whereas the second (activating) effect is due to the removal of catalyst poisons (organosilicon compounds generated in the process of the catalyst synthesis) by AlEt2Cl. (c) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 4219-4229, 2010