Metallocenes catalyst structure

The issue begins with an article by Ittel, Johnson, and Brookhart on late metal catalysts for ethylene homo- and copolymerization. They detail the newest generation of catalysts to be commercially licensed. Alt and Koppl then introduce ethylene and propylene polymerization by metallocene catalysts. Structure-performance relationships for unbridged and bridged... 

Implications of Metallocene Catalyst Structure on Polypropylene Structure. The previous section gave a brief description of the various types of metallocenes. In this section a general relationship between metallocene structure and type of polypropylene produced will be made. It is important to note that these are generalizations. While the stereochemistry of the metallocene plays an important role in mechanism of monomer insertion and ultimately the stereo- and regiospecificity of the polymer, the substituents and location of the substituents on the carhocyclic tt-ligands also effect the microstructure of the polymer. 

Figure 14.2 Upper. Illustration of metallocene catalyst structure, which must be specific for each stereo-specific polymer structure. Lower. Microstructures of polypropylene most can be made via metallocene catalysis polymerization.

Mortazavi SMM, Arabi H, Zohuri G, et al Copolymerization of ethylene/a-olefins using bis(2-phenylindenyl)zirconium dichloride metallocene catalyst structural study of comonomer distribution, Polym Int 59(9) 1258—1265, 2010. 

Abstract. This paper presents results from quantum molecular dynamics Simula tions applied to catalytic reactions, focusing on ethylene polymerization by metallocene catalysts. The entire reaction path could be monitored, showing the full molecular dynamics of the reaction. Detailed information on, e.g., the importance of the so-called agostic interaction could be obtained. Also presented are results of static simulations of the Car-Parrinello type, applied to orthorhombic crystalline polyethylene. These simulations for the first time led to a first principles value for the ultimate Young s modulus of a synthetic polymer with demonstrated basis set convergence, taking into account the full three-dimensional structure of the crystal. 

Mention has already been made in this chapter of metallocene-catalysed polyethylene (see also Chapter 2). Such metallocene catalysts are transition metal compounds, usually zirconium or titanium. Incorporated into a cyclopentadiene-based structure. During the late 1990s several systems were developed where the new catalysts could be employed in existing polymerisation processes for producing LLDPE-type polymers. These include high pressure autoclave and... 

Since the last edition several new materials have been aimounced. Many of these are based on metallocene catalyst technology. Besides the more obvious materials such as metallocene-catalysed polyethylene and polypropylene these also include syndiotactic polystyrenes, ethylene-styrene copolymers and cycloolefin polymers. Developments also continue with condensation polymers with several new polyester-type materials of interest for bottle-blowing and/or degradable plastics. New phenolic-type resins have also been announced. As with previous editions I have tried to explain the properties of these new materials in terms of their structure and morphology involving the principles laid down in the earlier chapters. 

More recent inventions are the metallocene catalysts based on zirconium. They offer more uniform catalyst activity and can give a relatively narrow molecular weight distribution. More importantly, they offer better control over structure and copolymer composition distributions. 

Figure 21 Structure of methylaluminoxane (extreme left), and some metallocene catalyst systems. Reproduced from Kaminsky [128], with permission of The Royal Society of Chemistry.

Fig. 2 General structures of group 4 metallocene catalysts and CGCs...

Based on Chien s research results, Collins et al. modified the basic structure of the catalysts and also achieved elastic material [8,18,19]. In both cases the elastic properties of the polymers are justified in a block structure with isotactic and atactic sequences. In 1999 Rieger et al. presented a couple of asymmetric, highly active metallocene catalysts, e.g., the dual-side catalyst rac-[l-(9-r 5-fluorenyl)-2-(5,6-cyclo-penta-2-methyl-l-q5-indenyl)ethane]zirconium dichloride (Fig. 3). These catalysts allowed building of isolated stereoerrors in the polymer chain to control the tacticity and therefore the material properties of the polymers [9],... 

A special case of the chain back skip polymerization mechanism and therefore an entirely different polymerization behavior was observed for differently substituted asymmetric complexes (for example catalyst 3). Although asymmetric in structure, these catalysts follow the trend observed for C2-symmetric metallocenes [20], Chien et al. [23] reported a similar behavior for rac-[l-(9-r 5-fluorenyl)-2-(2,4,7-trimethyl-l-ri5-indenyl)ethane]zirconium dichloride and attributed this difference in the stereoerror formation to the fact that both sides of the catalyst are stereoselective thus isotactic polypropylene is obtained in the same manner as in the case of C2-symmetric metallocene catalysts. 

Deffieux A, Cramail H, Radharkrishnen K, Pedeutour JN (2001) Reactivity of metallocene catalysts for olefin polymerization influence of activator nature and structure. Macromol Rapid Commun 22 1095-1123... 

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