Effect of catalyst aging on catalyst activity and stereospecificity for MgCl2/ethyl benzoate or dioctyl phthalate/TiCl4-triethylaluminium for propene polymerization

The aging of the MgCl₂/dioctyl phthalate (DOP) or ethyl benzoate (EB)/TiCl₄ catalyst was studied. Because of the strong complexation of DOP with the catalyst, only a small fraction of DOP was extracted by cocatalyst triethylaluminium (TEA) during aging, resulting in converting some highly isospecific sites into aspecific ones. No change of the overall number of sites was detected. EB, on the other hand, could be readily removed by TEA, resulting in a large increase in aspecific sites. Clustering of those sites facilitated catalyst deactivation.     

Copolymerization of propene with high-1-olefin using a MgCl2/TiCl4 catalyst

Copolymerization of propene and 1-olefins including 1-octene, 1-decene, 1-dodecene, 1-tetradecene, and 1-hexadecene were studied with the catalyst system MgCl₂/TiCl₄-Al(i-Bu)₃. It was found that the polymerization productivity and the consumption rate of propene are improved significantly in the presence of the comonomer. The total productivity of propene/1-olefin copolymerizations decreases as follows: 1-octene> 1-decene>1-dodecene>1-hexadecene>1-tetradecene. The reactivity ratios were estimated from the copolymerization results. ¹³C NMR was used to characterize the microstructures of propene/1-octene copolymer. Finally, the oxygen enrichment behavior of propene/1-octene copolymer was investigated.     

Kinetic study of gas phase olefin polymerization with a TiCl4/MgCl2 catalyst I. Effect of polymerization conditions

This study focuses on the kinetics of ethylene/propylene (homo/co) polymerization reactions using a high activity TiCl₄/MgCl₂/AlEt₃ catalyst. The reactor system is a gas phase reactor equipped with an on-line composition control scheme. As such, important kinetic data such as the instantaneous reaction rate of each monomer is readily obtained. In the investigation, experiments are performed to study the effects of comonomer composition variations, temperature variations, hydrogen concentration variations, and variations in the Al/Ti ratio. It is observed that the ethylene and propylene instantaneous reaction rates show a rather peculiar pattern with the appearance of a second peak. Our work linked the existence of this peak to the Al/Ti ratio used. A theory based on the oxidation state change is proposed. This theory is also used to explain the effects of temperature changes and hydrogen concentration changes on the system. A variety of analytical techniques are employed to study the polymer properties and evidence is provided to support the existence of polymer partial melting at relatively high reaction temperatures. The resulting diffusion limitation is believed to be partially responsible for the observed activity decrease at such elevated temperatures. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35 : 2063–2074, 1997     

Comparative characterization of MgCl2/ethyl benzoate/TiCl4 and MgCl2/2,2,6,6 tetramethylpiperidine/TiCl4 Ziegler-Natta precatalysts

In this article we present the results of the preparation and characterization of two Ziegler–Natta precatalysts: MgCl₂/Ethyl benzoate (EB)/TiCl₄ and MgCl₂/2,2,6,6 tetramethylpiperidine (TMPiP)/TiCl₄ by means of FTIR, X-ray diffraction, SEM, BET surface area measurements, and other techniques applied at different steps of their preparation procedures. The precatalysts were prepared by impregnating with TiCl₄ a given amount of MgCl₂, which was previously ball-milled with the electron donor chosen. Prior to impregnation, the ball-milled material presented different surface compounds depending on the electron donor used: [(MgCl₂)₂] · 2EB, MgCl₂ · EB, or a salt of the amine. The solid milled with EB is more homogeneous than the one milled with the TMPiP. Titanium is better retained in the solid milled with EB. This precatalyst has better morphological properties and larger BET surface area. By means of FTIR, we found evidences that an adequate surface structure for the formation of stereospecific sites in MgCl₂/TMPiP/TiCl₄ was formed. © 1994 John Wiley & Sons, Inc.     

Kinetic study of gas phase olefin polymerization with a TiCl4/MgCl2 catalyst. II. Kinetic parameter estimation and model building

Monomer transport and polymerization kinetics are two key phenomena in olefin polymerization with heterogeneous transition metal catalysts. To have a better understanding of these interrelated kinetics and diffusion phenomena, a quantitative calculation of the monomer diffusion directly from experimental study is essential. In this work, a novel temperature-perturbation technique is developed to systematically study the kinetic and diffusion limitations in catalyzed gas phase olefin polymerization. A physical model of the particle growth mechanism as well as its mathematical representation is presented and the diffusion limitations occurring in the system at high temperature are characterized and quantitatively analyzed. Finally, the practical implications of the results of this study on the operation of industrial scale polyolefin reactors are examined. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 2075-2096, 1997     

Distribution of active centers on TiCl4/MgCL2 catalyst for olefin polymerization

1-Octene was polymerized with TiCl₄/MgCl₂—AlEt₃ and the polymerization was quenched with CH₃COCl to introduce a CH₃CO— group onto each propagation chain. The polymer was fractionated by fractional precipitation, and the number of active centers in each fraction was determined by measuring the CH₃CO— content of the fraction. Distributions of the number and reactivity of active centers among the fractions were determined and discussed based on these experiments. The active center distributions were also studied by fitting the molecular-weight distribution (MWD) curve from GPC with multiple Schulz-Flory most-probable distributions. The uncontinuous reactivity distribution of active centers reveals that there exist limited types of active centers on the catalyst. Five types of active center were distinguished by the MWD fitting treatment. The correlations between the active center distributions and catalyst structures are discussed. © 1996 John Wiley & Sons, Inc.     

Adsorption of TiCl4 and electron donor on defective MgCl2 surfaces and propylene polymerization over Ziegler-Natta catalyst: A DFT study

The formations of defective MgCl₂ surfaces, and subsequent adsorption of Ti species and electron donor, as well as propylene polymerization over the Ziegler-Natta catalyst have been investigated using density functional theory (DFT) method. Twelve possible support models of regular and defective MgCl₂ (110) and (100) surfaces were built. The individual adsorptions of titanium chlorides as mononuclear or dinuclear, and ethyl benzoate (EB) as electron donor, on these models were evaluated. The analysis of energies presented the cases of EB adsorption were generally more stable than titanium chlorides on both surfaces. Thus, EB as internal electron donor mainly prevented TiCl₄ from coordinating on the MgCl₂ surfaces where mostly non-stereospecific active sites could be formed. Exceptionally, A5 the site model with terminal Cl-vacancy on the MgCl₂ support, presented stronger adsorption of TiCl₄ than that of EB on (110) surface. Since the TiCl₄ and ethyl benzoate (EB) would compete to adsorb on the support surface, it seems reasonable to assume that TiCl₄ might predominately occupy this site, which can act as the most plausible active site for propylene polymerization. The first insertion of propylene monomer into the A5 active site model showed that it exhibited good regioselectivity but poor stereospecificity in the absence of electron donor.     

Effect of EtOH/MgCl(2) molar ratios on the catalytic properties of MgCl(2)-SiO(2)/TiCl(4) Ziegler-Natta catalyst for ethylene polymerization

MgCl(2)-SiO(2)/TiCl(4) Ziegler-Natta catalysts for ethylene polymerization were prepared by impregnation of MgCl(2) on SiO(2) in heptane and further treatment with TiCl(4). MgCl(2)·nEtOH adduct solutions were prepared with various EtOH/MgCl(2) molar ratios for preparation of the MgCl(2)-supported and MgCl(2)-SiO(2)-supported catalysts in order to investigate the effect on polymerization performance of both catalyst systems. The catalytic activities for ethylene polymerization decreased markedly with increased molar ratios of [EtOH]/[MgCl(2)] for the MgCl(2)-supported catalysts, while for the bi-supported catalysts, the activities only decreased slightly. The MgCl(2)-SiO(2)-supported catalyst had relatively constant activity, independent of the [EtOH]/[MgCl(2)] ratio. The lower [EtOH]/[MgCl(2)] in MgCl(2)-supported catalyst exhibited better catalytic activity. However, for the MgCl(2)-SiO(2)-supported catalyst, MgCl(2) can agglomerate on the SiO(2) surface at low [EtOH]/[MgCl(2)] thus not being not suitable for TiCl(4) loading. It was found that the optimized [EtOH]/[MgCl(2)] value for preparation of bi-supported catalysts having high activity and good spherical morphology with little agglomerated MgCl(2) was 7. Morphological studies indicated that MgCl(2)-SiO(2)-supported catalysts have good morphology with spherical shapes that retain the morphology of SiO(2). The BET measurement revealed that pore size is the key parameter dictating polymerization activity. The TGA profiles of the bi-supported catalyst also confirmed that it was more stable than the mono-supported catalyst, especially in the ethanol removal region.     

Development of novel MgCl2 supported catalyst with trivalent titanocene complex of CP2TiCl2AlCl2 for propylene polymerization

Three kinds of MgCl₂‐supported trivalent titanocene catalyst (Cat. 1: Cp₂TiCl₂AlCl₂/MgCl₂, Cat. 2: CpCp*TiCl/MgCl₂, Cat. 3: Cp₂TiCl/MgCl₂) were prepared and tested for propylene polymerization. It was found that Cat. 1, combined with ordinary alkylaluminum as cocatalyst, produced PP containing 31.8 wt % of isotactic PP in fairly good yield. On the other hand, Cats. 2 and 3 hardly showed any activity. The effects of diisopropyldimethoxysilane (DIPDMS) on isospecificity of the Cat. 1 also were investigated. The isotactic index (I.I.) of PP was improved drastically by the addition of DIPDMS as external donor and reached the value as high as 98.4%, even in the absence of any internal donors. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3355–3359, 2000     

Copolymerization of ethylene–1-Hexene over a thermally pretreated MgCl2/THF/TiCl4 bimetallic catalyst

MgCl₂/THF/TiCl₄ (TT-0) were thermally pretreated at 80°C for 5 min (TT-1) and 60 min (TT-2), and at 108°C for 5 min (TT-3) and 60 min (TT-4). These thermally pretreated catalysts showed comonomer enhancement effects in the ethylene-1-hexene copolymerization, while TT-0 catalyst did not. Comonomer enhancement effect of thermally pretreated catalysts could come from the generation of new active sites and change of its nature after heat treatment. 1-Hexene content in copolymer obtained with TT-1 was higher than those of TT-4 and TT-0. The morphology of homopolyethylene (PE) obtained with TT-1, 2, 3, and 4 was more regular and homogeneous than that of TT-0. This result could be due to the generation of active sites and change of its nature after thermal treatment of bimetallic catalyst. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35 : 2769–2776, 1997