(A) study on the Characterizations of active sites in olefin polymerization over metallocene catalysts systems

Abstract

For the characterization of active site, the correlation between the active site of catalyst and the catalytic behavior in olefin polymerization(polymerization of styrene and ethylene) was investigated over the various types of metallocene catalysts systems. In order to understand the characteristics of homogeneous catalyst, the polymerization of ethylene and styrene were carried out over different types of organometallic catalysts; i.e., Type Ⅰ catalyst without cyclopentadienyl ligand($TiCl_4$) and Type Ⅱ catalyst, with one Cp ligand($CpTiCl_3$). The activity of Type Ⅱ catalyst was much higher than that of Type Ⅰ catalyst. Molecular weight distribution(MWD) of syndiotactic polystyrene prepared with Type Ⅰ catalyst was broad and the shape of MWD was bimodal. Whereas MWD of syndiotactic polystyrene prepared with Type Ⅱ catalyst was narrow and the shape of MWD was unimodal. As the results of the catalytic behavior and the polymer analysis, Type Ⅰ catalyst seems to be unstable at the activation stage with MAO. From the results of XRD, the crystalline structure of polystyrene with different types of catalyst was investigated. The form fraction of melt-crystallized polystyrene increased according to the syndiotacticity of polystyrene. Therefore, the form is more compact structure in the crystalline packing than form. In order to examine the copolymerization behaviors with different types of catalyst, the styrene-ethylene copolymerization was carried out. The styrene response of Type Ⅱ catalyst was higher than that of Type Ⅰ catalyst in styrene-ethylene copolymerization. It might be due to the different electrophilicity of cationic titanium site in the catalyst systems. The crystalline structures of melt-crystallized styrene-ethylene copolymers were depended on the types of catalysts. Homogeneous catalysts with half-metallocene($CpTiCl_3$, $Cp^*TiCl_3$, $Cp^*Ti(CH_3)_3$, and $IndTiCl_3$) and several kinds of silica supported metallocene catalysts($CpTiCl_3/SiO_2$, ...