In order to elucidate the nature of active site, the correlation between the active site of catalyst and the polymerization mechanism was investigated by using different types of catalyst system.
The effects of electron donor, i.e.; ethyl-benzoate(EB), di-n-butyl phthalate(DNBP) and phenyl-triethoxysilane(PTES), as internal and/or external donor on the catalytic activity in propylene polymerization over butylmagnesium chloride (BuMgCl)-based titanium catalysts have been investigated in terms of activity and stereospecificity. It was found that the polymerization rates were well correlated by a kinetic model where the activation and deactivation were assumed to be 1st and 2nd order, respectively. The catalyst with DNBP as internal donor showed the highest propagation rate constant($k_p$) and lowest deactivation rate constants($k_d$). When the PTES was used as an internal donor, the higher $k_d$ value was obtained. It seems that the titanium species in the catalytic system containing PTES as internal donor was easily reduced by triethyl aluminum(TEA). X-ray diffraction(XRD) pattern of the catalytic system including PTES as internal donor showed that $TiCl_4$ did not seem to diffuse into the interlayer of the $MgCl_2$-support matrix and did not show the highly disordered form compared with other internal donors. From the microstructure analysis of polypropylene, it was believed that the stereoregul- arity depended on the nature of internal donor and was controlled by the extent of reversible interconversion between the isospecific(predominantly) and the syndiospecific(predominantly) sites. In the case of PTES as an external donor, a number of syndiotactic stereoblocks were found for the polypropylene soluble in boiling heptane and showed less interchangeability between these active sites, resulting in the selective activation of both types of active sites.
Propylene was polymerized in the presence of the isospecific $rac-Et[Ind]_2ZrCl_2$(A) (Et:ethyl, Ind:indenyl) catalyst impregnated on the $SiO_2$ and $SiO_2$/MAO(MAO:methyl-aluminoxane) supports. The resulting catalysts using TMA/$Ph_3C \cdot B(C_6F_5)_4$ (TMA : trimethyl- aluminum) instead of MAO as cocatalyst showed fairly good yields and properties for isotactic polypropylene, i.e.; higher isotacticity as well as higher melting point compared with the polypropylene produced by homogeneous zirconocene catalyst. From the $^{13}C$-NMR analysis, isospecific polypropylene obtained by the catalytic system(A/MAO/$SiO_2$-TMA/ $[Ph_3C] [B(C_6F_5)_4]$) was found to have high regioregularity, and its microstructure was well correlated with enantiomorphic site model. Therefore active sites formed in the $SiO_2$ supported zirconocene catalysts seem to have more stereo-rigid single site character as compared with those in the homogeneous counterparts.
