Four cocatalysts, referred to as ethylaluminoxanes, were synthesized by the reaction between triethyl-aluminium (AlEt3) and water under various molar ratios of H2O/Al at -78 degrees C. Aluminoxaiies were used as cocatalysts for a MgCl2-supported Ziegler-Natta catalyst for propylene polymerization at temperatures ranging from 70 to 100 degrees C. When the polymerization was activated by AlEt3, the activity as well as the molecular weight and isotacticity of the resulting polymer gradually dropped as the temperature varied from 70 to 100 degrees C. When ethylaluminoxane was employed as the cocatalyst, good activity and high molecular weight and isotacticity were obtained at 100 degrees C. Furthermore, when the cocatalyst varied from AlEt3 to ethylaluminoxane, the atactic fraction and polymer fraction with moderate isotacticity decreased and the high isotactic fraction slightly increased, which indicated that the variation of the cocatalyst significantly affects the isospecificity of active sites. It was suggested that the reactivity of the Al-Et group and the size of the cocatalyst were correlated to the performance of the Ziegler-Natta catalyst at different temperatures. (c) 2006 Wiley Periodicals, Inc.