Ethylene polymerization reactions with multicenter Ziegler-Natta catalysts-Manipulation of active center distribution (original) (raw)
Related papers
Ethylene polymerization with heterogeneous Ziegler-Natta catalysts
Polymer Bulletin, 1995
TiCl4/SiO2, Ti(OC4H9)4/SiO2, MgCl2/TiCl4/SiO2 and MgCl2/Ti(OC4H9)4/SiO2 catalysts were prepared by treating silica gel with TiCl4, Ti(OC4H9)4, MgCl2/TiCl4 or MgCl2/Ti(OC4H9)4 in tetrahydrofuran (THF) solution. Ethylene polymerization was performed with these catalysts activated by common alkylaluminum compounds. The influence of magnesium dichloride on catalyst performance was investigated. MgCl2 has enhanced the catalyst activity for both titanium compounds. In addition, all catalyst systems were only active when they were washed with AlCl(C2H5)2 (DEAC).
It was found that the observed order of the polymerization rate with respect to ethylene concentration at ethylene polymerization over two titanium-magnesium catalysts of different compositions is significantly higher than 1 (1.6–2.1). The data on the effect of ethylene concentration on the number of active centers (C P) and the propagation rate constant (k P) at ethylene polymerization over these catalysts were obtained by method of polymerization quenching with 14 CO. An increase in ethylene concentration was found to increase the number of active centers. In some cases the increase of ethylene concentration proceeds to the narrowing of the molecular weight distribution of the resulting polyethylene and an increase in the calculated value of propagation rate constant. These effects were shown to be most pronounced at low ethylene pressure and increased concentration of an activator (AlEt 3). Based on the experimental data, we proposed a scheme of reactions to explain the effects of ethylene and AlR 3 concentrations on the number of active centers, the average values of propagation rate constant and molecular weight distribution of polymers produced over these multi-site catalysts.
Polymer Bulletin, 1995
This study investigated the copolymerization of ethylene with l-hexene using the homogeneous Et[Ind]2ZrC12 and [lnd]2ZrC! 2 catalysts. The Et[Ind]2ZrC12 catalyst gave a higher catalytic activity than the [Ind]2ZrCl 2 and also showed a better incorporation of 1-hexene for the same comonomer concentration in the feed. Thermal analysis (DSC) and viscosity measurements showed that an increase of the l-hexene incorporated in the copolymer results in a decrease of the melting point, crystallinity and molecular weight of the polymer formed. The reactivity ratios for ethylene and 1hexene confirmed the more successful incorporation of the comonomer for the polymerization catalyzed by Et[Ind]2ZrCI 2.
Journal of Polymer Science Part a Polymer Chemistry, 1999
The previously developed kinetic scheme of ethylene polymerization reactions with heterogeneous Ziegler-Natta catalysts (refs 1-3) states that the catalysts have several types of active centers which have different activities, different stabilities, produce different types of polymers, and respond differently to reaction conditions. Each type of center produces a single polymer component (Flory component), a material with the same structure (copolymer composition, isotacticity, etc.) and a narrow molecular weight distribution with Mw/Mn=2.0. This paper examines several features of ethylene polymerization reactions in the view of this mechanism. They include temperature and cocatalyst effects on molecular weight distribution, as well as the effect of reaction parameters (temperature, ethylene and hydrogen partial pressure, -olefin and cocatalyst concentration) on molecular weights of Flory components.
Journal of Polymer Science Part A: Polymer Chemistry, 2008
Homogeneous and silica-supported Cp 2 ZrCl 2 /methylaluminoxane (MAO) catalyst systems have been used for the copolymerization of ethylene with 1-butene, 1-hexene, 4-methylpentene-1 (4-MP-1), and 1-octene in order to compare the ''comonomer effect'' obtained with a homogeneous metallocene-based catalyst system with that obtained using a heterogenized form of the same metallocene-based catalyst system. The results obtained indicated that at 70 8C there was general rate depression with the homogeneous catalyst system whereas rate enhancement occurred in all copolymerizations carried out with the silica-supported catalyst system. Rate enhancement was observed for both the homogeneous and the silica-supported catalyst systems when ethylene/4-MP-1 copolymerization was carried out at 50 8C. Active center studies during ethylene/4-MP-1 copolymerization indicated that the rate depression during copolymerization using the homogeneous catalyst system at 70 8C was due to a reduction in the active center concentration. However, the increase in polymerization rate when the silica-supported catalyst system was used at the same temperature resulted from an increase in the propagation rate coefficient. V
Polymer Engineering & Science, 1999
A Ziegler-Natta catalyst was modified with a metallocene catalyst and its polymerization behavior was examined. In the modification of the TiCl, catalyst supported on MgCb (MgCb-Ti) with a rac-ethylenebis(indeny1)zirconium dichloride (rac-Et(Ind)&Cb. EIZ) catalyst, the obtained catalyst showed relatively low activity but produced high isotactic polypropylene. These results suggest that the EIZ catalyst might block a non-isospecific site and mod@ a Ti-active site to form highly isospecific sites. To combine two catalysts in olefin polymerization by catalyst transitioning methods, the sequential addition of catalysts and a co-catalyst was tried. It was found that an alkylaluminum like triethylaluminum (TEA) can act as a deactivation agent for a metallocene catalyst. In ethylene polymerization. catalyst transitioning was accomplished with the sequential addition of bis(cyclopentadieny1)zirconium dichloride (Cp&rClJ/methylaluminoxane (MAO), TEA, and a titanium tetrachloride/vanadium oxytrichloride (TiC14/VOCl,, Ti-V) catalyst. Using this method, it was possible to control the molecular weight distribution (MWD) of polyethylene in a bimodal pattern. In the presence of hydrogen, polyethylene with a very broad MWD was obtained due to a different hydrogen effect on the Cp,ZrCl, and Ti-V catalyst. The obtained polyethylene with a broader MWD exhibited more apparent shear thinning.
Preparation of Highly Active Heterogeneous Ziegler-Natta Catalyst for Polymerization of Ethylene
2007
H ighly active supported catalyst of Ziegler-Natta type was prepared by reaction of a Grignard reagent. The starting chemicals for preparing MgCl 2 , the support, were butyl magnesium chloride which mixed with AlCl 3 and CH 3 Si(OC 2 H 5) 3 , following to two steps treatment of the chemicals with TiCl 4 in toluene. Ti Concentration in the solid catalyst was 3.19%. Slurry polymerization of ethylene was carried out using the catalyst in dry heptane, while; triethylaluminium was used as cocatalyst. The cocatalyst effects, such as cocatalyst/catalyst molar ratio, polymerization time, temperature, H 2 concentration and the monomer pressure on activity of the catalyst were studied. There was an optimum [Al]/[Ti] molar ratio and temperature to obtain the highest activity of the catalyst. The maximum activity was obtained at 60ºC, and [Al]/[Ti]=714:1. Productivity of 14700 g PE/mmol Ti.h was obtained at monomer pressure of 8 bar. Addition of hydrogen decreased the activity of catalyst and the viscosity average molecular weight (M v) of the polymer obtained, while; increasing monomer pressure increased its activity. Density of the polymer obtained was 0.93-0.95 g/cm 3 which is in the range of high density polyethylene. Melting point of the polymer was in the range of 140-144ºC.