Ziegler-Natta catalysts for olefin polymerization: Mechanistic insights from metallocene systems (original) (raw)
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Ziegler-Natta catalysts for olefin polymerizations
Progress in Polymer Science, 1997
The current development of the metallocene-based Ziegler-Natta catalysts has been reviewed. The discovery of these catalysts has offered the opportunity to obtain a deeper insight into the mechanism of Ziegler-Natta polymerizations. In this review, some mechanistic models for polymerization and stereoregulation, as well as the factors which affect the activity and stereospecificity of the catalysts, have been discussed. The technology of olefin polymerization with the metallocene-based catalysts is in the early stage of commercialization. Using these catalysts, a large number of novel polymers with special properties have been obtained.
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.
Materials, 2014
50 years ago, Karl Ziegler and Giulio Natta were awarded the Nobel Prize for their discovery of the catalytic polymerization of ethylene and propylene using titanium compounds and aluminum-alkyls as co-catalysts. Polyolefins have grown to become one of the biggest of all produced polymers. New metallocene/methylaluminoxane (MAO) catalysts open the possibility to synthesize polymers with highly defined microstructure, tacticity, and steroregularity, as well as long-chain branched, or blocky copolymers with excellent properties. This improvement in polymerization is possible due to the single active sites available on the metallocene catalysts in contrast to their traditional counterparts. Moreover, these catalysts, half titanocenes/MAO, zirconocenes, and other single site catalysts can control various important parameters, such as co-monomer distribution, molecular weight, molecular weight distribution, molecular architecture, stereo-specificity, degree of linearity, and branching of the polymer. However, in most cases research in this area has reduced OPEN ACCESS Materials 2014, 7 5070 academia as olefin polymerization has seen significant advancements in the industries. Therefore, this paper aims to further motivate interest in polyolefin research in academia by highlighting promising and open areas for the future.
From Unstable to Stable: Half-Metallocene Catalysts for Olefin Polymerization †
Inorganic Chemistry, 2008
The reaction of LAlMeOH [L) CH(N(Ar)(CMe)) 2 , Ar) 2,6-i-Pr 2 C 6 H 3 ] with CpTiMe 3 , Cp*TiMe 3 , and Cp*ZrMe 3 was investigated to yield LAlMe(µ-O)TiMe 2 Cp (2), LAlMe(µ-O)TiMe 2 Cp* (3), and LAlMe(µ-O)ZrMe 2 Cp* (4), respectively. The resulting compounds 2-4 are stable at elevated temperatures, in contrast to their precursors such as CpTiMe 3 and Cp*ZrMe 3 , which already decompose below room temperature. Compounds 2-4 were characterized by singlecrystal X-ray structural analysis. Compounds 2 and 3 were tested for ethylene polymerization in the presence of methylaluminoxane. The half-metallocene complex 3 has higher activity compared to 2. The polydispersities are in the range from 2.8 to 4.2. A copolymerization with styrene was not observed.
2004
New group IV compounds containing a 9-dialkylaminofluorenide ligand have been prepared. Their properties and reactivity have been investigated. For example, single crystal X-ray analysis of the metallocene (9-(N,N-dimethylamino)fluorenyl)2ZrCl2 (4) reveals a bonding mode that includes a zirconium-nitrogen bond. In combination with methylaluminoxane (MAO), several aminofluorenide complexes afford atactic polypropylene. A statistical model has been developed that quantifies the unidirectional site epimerization probability, e, for singly- and doubly-bridged C1- symmetric metallocene polymerization catalysts. The unidirectional site epimerization model allows deconvolution of the site sequence probability and the stereochemical probability for producing a given pentad. Highly stereoregular syndiotactic polypropylene is obtained with the catalyst systems Ph2C(Oct)(C5H4)ZrCl2/MAO (8/MAO) (Oct = octamethyloctahydrodibenzofluorenyl, C-29H36) and Me2C(Oct)(C5H4)ZrCl2/MAO (12/MAO). Melting t...
Journal of Catalysis, 1964
A mechanism for the polymerization of olefins is proposed. An essentially octahedrally coordinated ion of a transition element with empty or nearly empty tzg orbitals carrying in its coordination sphere one alkyl group and having one vacant octahedral position is supposed to be the active center. In this model the monomeric olefin is coordin&ed in the vacant position through a 'Gr bond."
A few considerations on some catalysts for olefin polymerization
Makromolekulare Chemie. Macromolecular Symposia, 1993
Among many precursors and catalysts for alpha‐olefins polymerization, one seems to be particularly interesting, because it has not been completely clarified yet.We refer especially to precursors obtained via reaction between Mg‐alkyls and SiCl4.The products of this reaction are not well known; in fact, under some operating conditions, a special form of MgCl2 is obtained, showing x‐ray diffraction peaks in the angular region lower than 15° (2 theta), which corresponds to the 5.9 A interplanar spacing, characteristic of alpha‐MgCl2.Under other conditions, MgCl2 is obtained in the well known and strongly disordered delta structure.By employing these precursors, some catalytic systems for alpha‐olefins polymerization have been prepared.In this paper, the peculiar aspects of these precursors and catalysts are described, particularly focusing on the correlation between structure and performances in ethylene and propylene polymerization.
Catalysts for olefins polymerization
Catalysis Today, 1998
Polyole®ns are still protagonist of an exciting innovation, due to a continuous development of new catalysts, processes and products. The positive solutions given by polyole®ns to the environmental and energetic issues are among the factors responsible for their success. The most relevant breakthrough occurred in the last years is the discovery of metallocenes, and more in general, of single centre catalysts. They are, in most cases, highly active catalysts and are already employed on the industrial scale for the preparation of both``drop-in'' products with improved properties, and of totally new materials. # 1998 Elsevier Science B.V. All rights reserved.