Synergistic effects of the ZnCl2-SiCl4 modified TiCl4/MgCl2/THF catalytic system on ethylene/1-hexene and ethylene/1-octene copolymerizations

The copolymerizations of ethylene with 1-hexene or 1-octene by using TiCl4/MgCl2/THF catalysts modified with different metal halide additives (ZnCl2, SiCl4, and the combined ZnCl2-SiCl4) were investigated based on catalytic activity and copolymer properties. It was found that the catalyst modified with mixed ZnCl2-SiCl4 revealed the highest activities for both ethylene/1-hexene and ethylene/1-octene copolymerization. The increase in activities was due to the formation of acidic sites by modifying the catalysts with Lewis acids. Based on the FTIR measurements, the characteristic C-O-C peaks of the catalysts modified with metal halide additives were slightly shifted to lower wavenumber when compared to the unmodified catalyst. This showed that the modified catalysts could generate more acid sites in the TiCl4/MgCl2/THF catalytic system leading to an increase in activities as well as comonomer insertion (as proven by C-13-NMR). However, Lewis acidmodifications did not affect the microstructure of the copolymers obtained. By comparison on the properties of copolymers prepared with the unmodified catalyst, it was found that polymers with ZnCl2 and/or SiCl4 modification exhibited a slight decrease in melting temperature, crystallinity and density. It is suggested that these results were obtained based on the different amount of alpha-olefins insertion, regardless of the types of Lewis acids and comonomer.