The effect of ethylaluminium dichloride (EADC) in the heterogeneous Ziegler-Natta catalyst on the ethylene-1-butene copolymerization and the molecular structure of the resulting copolymer was investigated. SiO2-supported TiCl4 catalysts having different EADC/Ti molar ratio were prepared by changing the amount of EADC in feed during the catalyst preparation. Ti oxidation states of the prepared catalysts were changed with EADC/Ti molar ratio, and it contained the higher fraction of Ti(3+) and Ti(2+) at the higher EADC/Ti molar ratio. The change in EADC/Ti molar ratio of the catalyst was also shown to influence on the copolymerization performance. Xylene-solublility value corresponding to the fraction having low molecular weight with higher 1-butene content, decreased as EADC/Ti molar ratio decreased. The chemical compositional distribution of PE copolymer was shown not to be affected by EADC/Ti molar ratio of the catalyst significantly. (C) 2007 Elsevier B.V. All rights reserved.