Olefin polymerizations were carried out under high pressure and atmospheric pressure slurry reactors using three types of Ziegler-Natta catalysts synthesized by different operational methods.
The first type of catalyst to study ethylene polymerization was prepared by anchoring $TiCl_4$/THF/$MgCl_2$ solvate complex onto the inert $SiO_2$. A plausible mechanism for the reaction between the solvate complexes and hydroxyl groups in silica gel was postulated. Ethylene polymerization were then carried out in a semi-batch, constant pressure slurry reactor. The pressure and temperature employed in this study were similar to those employed in industrial practice.
Kinetic investigations were carried out on the effect of polymerization conditions and nature of aluminum alkyl cocatalyst on the morphology (size, shape, density, and texture) of nascent polyethylene synthesized with the catalyst. Depending on the nature of cocatalyst, considerably different kinetic profiles were obtained. Fibrillar, globular, cobweb, ribbon-like, worm-like, and lamellar structures were obtained depending upon the catalyst system and polymerization conditions. The effect of polymerization of nascent polyethylene was also investigated. Replication phenomenon was observed depending on the catalyst yield and polymerization time. Generally, the average particle size of polymer particles became larger as the rate of polymerization increased.
Polymerization of ethylene and copolymerization of ethylene with 1-hexene were performed by using a second type of catalyst prepared by coprecipitation method. The catalyst system is composed of $TiCl_4$/THF/$MgCl_2$ bimetallic complex. In order to look into the role of magnesium compound in the catalyst, various catalysts with different Mg content (Mg/Ti=0.42-16.5) were characterized by means of elemental analysis, FTIR, x-ray powder diffraction, and SEM techniques. The catalyst activity increased linearly with respect to the Mg/Ti ratio of catalyst throughout the experimental range. The activity of copolymerization with 1-hexene also increased with Mg/Ti ratio. The enhancement of polymerization rate by the addition of 1-hexene in the reaction medium was observed only for the catalyst of Mg/Ti ratio smaller than 2.5. The enhancement of catalyst activity by 1-hexene could be explained by the physical disintegration of the catalyst matrix particles thus exposing new potential centers.
The effect of precipitation condition during the preparation of $TiCl_4$/THF/$MgCl_2$ catalyst on the chemical composition and physical structure of catalyst was discussed. The variation caused by different precipitation condition had considerable influences on the rate profiles of homo- and co-polymerization of ethylene.
To verify the role of magnesium, titanium, and electron donor compounds in determining the activity, a series of superactive and stereospecific catalysts were prepared by chlorinating $Mg(OEt)_2$ with $TiCl_4$ both with or without benzoyl chloride (to generate ethyl benzoate (EB) in situ) in chlorobenzene. The catalysts have been characterized by means of various physico-chemical methods. Chemical compositions of the catalysts were determined by elemental analysis and gas chromatography. The infrared spectra of catalysts have been recorded to identify the complex formation between catalyst matrix and electron donor, EB. The physical states of the solids obtained during each stage of catalyst preparation were studied by BET, SEM and x-ray diffraction techniques. Final catalysts had surface are ca. 200-300 ㎡/g.
By comparing the profiles of polymerization of propylene with each catalyst and the change of catalyst isospecificity during the polymerization, it was concluded that the strength of complexation between catalyst matrix and EB generated in situ played an important role in determining the catalyst yield (g polymer/g-Ti) and isospecificity.
In order to elucidate the kinetic feature of the active sites in the catalysts, attempts were made to determine the number of active sites, $C^*$, applying CO inhibition method for propylene and ethylene polymerization. the results showed that the decrease of the $C^*$ value during the propylene polymerization was first or second order with respect to the polymerization time for the catalysts which have weak complexation with EB. However, the $C^*$ values were constant with respect to the polymerization time for the catalysts which have strong complexation with EB. The $C^*$ values for ethylene polymerization were constant for a long polymerization period for all catalysts.
The detailed kinetic behavior of the slurry polymerization of propylene with a selected catalyst characterized as above, activated by $AlEt_3$ both with or without external electron donor, was studied. The dependence of the initial polymerization rate on temperature and concentrations of $AlEt_3$ was explained on the basis of simple proposed models. The overall polymerization rate as a function of the polymerization temperature and the concentration of $AlEt_3$ showed a maximum at 42℃ and at Al/Ti = 20, respectively, and could be fitted by a Langmuir-Hinshelwood rate law. The isospecificity of catalyst was sharply increased by activating the catalyst with $AlEt_3$ containing a small amount of external donor ([PEEB]/[$AlEt_3$]~0.2) due to the fact that the external donor not only poisons the non-stereospecific sites but stabilizes the stereospecific sites. Excess amount of PEEB ($[PEEB]/[AlEt_3] ＞ 0.3$) destabilized both stereospecific and non-stereospecific sites of catalyst. Observation of the rapid decay of the polymerization rate showed that the deactivation is represented by a second order decay. The activation energy for deactivation reaction was 5.4 kcal/mol.
