Abstract: The gaseous phase homopolymerization of ethylene has been studied in the presence of TiCl₄, Ti(OBu)₄/MgCl₂, SiO₂, ZnCl₂/SiCl₄/alcohol/AlR₃ highly active catalysts (named Cat# A and Cat# B) which were prepared by different precipitating-reaction procedures. It was found that the catalytic efficiency increased and there was an optimum value respectively in bulk density and mass distribution of polymer with the increase of the amount of ZnCl₂ AlEt₃ used to treat SiO₂. The kinetics curves of gaseous phase etheylene polymerization with two catalysts showed a decline and the decay rate with Cat# A was larger than that with Cat# B. The analysis of the components of the catalysts showed that zinc compound is one of the important compositions of supports, which can notably control the molecular weight of the products. This should be ascribed to the fact that diethyl zinc compound formed in site of the ethylene polymerization system due to the interaction between ZnCl₂ and trialkyl aluminnium is liable to induce transfer reaction of PE propagation chains. The particle structures and the properties of the catalysts and the obtained products of PE were characterized by the determination of surface area and SEM, FTIR and DSC. The morphology of the catalysts and products of polymerization by SEM showed that all the particle shapes were nearly spherical. Compared with the matrix PEprepared by MgCl₂-supported titanium catalyst, the products prepared with Cat# A and Cat# B have a higher branching degree, lower crystallinity and wider melting peak. The products prepared with Cat# B with melting dual peak(peak temperature 133.0 ℃ and 140.0 ℃) have newly been found.

Key words: Gas-phase polymerization of ethylene, Highly active catalyst, ZnCl₂, Molecular weight regulator