Ti-based Ziegler-Natta catalysts supported on MgCl2 doped with AlCl3 were prepared by the reaction Of MgCl2/AlCl3-ethanol adduct with TiCl4. No AlCl3 crystallites were found in the AlCl3-doped catalysts by WAXD analysis, suggesting that AlCl3/MgCl2 solid solution was formed. The effect of doping on the catalyst performance in ethylene polymerization was investigated. The results showed that the catalysts based on AlCl3-doped MgCl2 support exhibited a slightly higher activity than did the MgCl2-supported catalyst and the molecular weight distribution (MWD) of polyethylene (PE) markedly increased (from 10.8 to 47.9) with the increase of AlCl3 content in catalysts. The changes in catalyst's active center distribution were studied based on nonlinear fitting of the polymer GPC curves by multiple Flory functions. It was found that increase of types of active centers by introducing AlCl3 into the support should be responsible for the broadening of MWD of PE. (c) 2006 Wiley Periodicals, Inc.