Catalysis metallocenes

During the 1990s there was enormous activity in the development of a further type of polyethylene based on metallocene catalysis methods. One patent search... 

The application of metallocene catalysis to the preparation of polypropylenes reached a commercial stage with the production by Exxon of their Achieve range in 1996 and in 1997 by Targor, the BASF-Hoechst joint venture with the introduction of Metocene. Such metallocene polypropylenes are, however, only a small proportion of the total polypropylene market, predicted at only about 3% of the total in 2005. 

Polypropylenes produced by metallocene catalysis became available in the late 1990s. One such process adopts a standard gas phase process using a metallocene catalyst such as rac.-dimethylsilyleneto (2-methyl-l-benz(e)indenyl)zirconium dichloride in conjunction with methylaluminoxane (MAO) as cocatalyst. The exact choice of catalyst determines the direction by which the monomer approaches and attaches itself to the growing chain. Thus whereas the isotactic material is normally preferred, it is also possible to select catalysts which yield syndiotactic material. Yet another form is the so-called hemi-isotactic polypropylene in which an isotactic unit alternates with a random configuration. 

Metallocene catalysis can also make possible the production of copolymers of propylenes with monomers such as long-chain olefins, cyclic olefins and styrene which is not possible with more conventional Ziegler-Natta catalysts. 

Isotactic polypropylenes produced by metallocene catalysis are now being produced by a number of different manufacturers and because different systems are used there is some variation in properties. Typically however such materials have similar density, hardness and tensile strength to conventional homopolymers but differ in having... 

Because of the chain-stiffening effect of the benzene ring the TgS of commercial materials are in the range 90-100°C and isotactic polymers have similar values (approx. 100°C). A consequence of this Tg value plus the amorphous nature of the polymer is that we have a material that is hard and transparent at room temperature. Isotactic polystyrenes have been known since 1955 but have not been of commercial importance. Syndiotactic polystyrene using metallocene catalysis has recently become of commercial interest. Both stereoregular polymers are crystalline with values of 230°C and 270°C for the isotactic and syndiotactic materials respectively. They are also somewhat brittle (see Section 16.3). 

Stable transition-metal complexes may act as homogenous catalysts in alkene polymerization. The mechanism of so-called Ziegler-Natta catalysis involves a cationic metallocene (typically zirconocene) alkyl complex. An alkene coordinates to the complex and then inserts into the metal alkyl bond. This leads to a new metallocei e in which the polymer is extended by two carbons, i.e. 

CPE Produced by ADMET poly condensation.23 JSee ref 38 for reviews on metallocene PE catalysis. eFrom ref. 39. 

Metallocenes Mg(II)-catalyzed Microwave irradiation MO calculation Montmorillonite Moraceous plants Morphinadienes Mulberry tree Multifunctional catalysis Multi-step... 

Molander G, Dowdy EC (1999) Lanthanide- and Group 3 Metallocene Catalysis in Small Molecule Synthesis. 2 119-154 Monnier F, see Dixneuf (2004) 11 1-44 MoriM (1998) Enyne Metathesis. 1 133-154... 

For recent symposiums on zirconocene chemistry, see E. Negishi, Recent Advances in the Chemistry of Zirconocene and Related Compounds, Tetrahedron Symposia-in-print No. 57, Tetrahedron 1995, 51 (special issue). R. F. Jordan Metallocene and Single Site Olefin Catalysis, f. Md. Catal. 1998, 128 (special issue) and references cited therein. R. F. Jordan, A. S. Guram, in Comprehensive Organometallic Chemistry II, E. W. Abel, F. G. A. Stone, G. Wilkinson, M. F. Lappeet (eds.), Pergamon Press, Oxford, 1995, Vol. 4, p. 589. 

It is the purpose of this section to review ways in which processes involving electrons are either explicitly accounted for in calculations on polymeric systems or in which a more or less rigorous abstraction from the electronic degrees of freedom into effective models of a coarser-grained nature is performed. The next level up from electrons is obviously atoms. Hence, this section deals mainly with the connection between quantum chemistry and atomistic (force field) simulations. Calculations which exclusively use quantum chemistry are not covered. This excludes, for example, all of the recent work on metallocene catalysis. 

A final example of homogeneous catalysis is the use of metallocene catalyst systems in chain growth polymerization processes. The metallocene, which consists of a metal ion sandtviched between two unsaturated ring systems, is activated by a cocatalyst. The activated catalyst complexes with the monomer thereby reducing the reaction s energy of activation. This increases the rate of the reaction by up to three orders of magnitude. 

One such reported example is the synthesis of polypropylene-6-polymethyl-methacrylate (PP-6-PMMA) copolymers utilizing metallocene catalysis and the borane chemistry. In the initial step, PP with chain-end olefinic unsaturations was prepared using metallocene catalysts such as Et(Ind)2ZrCl2/MAO. The unsaturation sites were then hydroborated by 9-borabicyclo[3.3.1]nonane (9-BBN) to produce borane-terminated PP (43) (Fig. 30), which was selectively oxidized and interconverted to a... 

Metallocene catalysis has been combined with ATRP for the synthesis of PE-fr-PMMA block copolymers [123]. PE end-functionalized with a primary hydroxyl group was prepared through the polymerization of ethylene in the presence of allyl alcohol and triethylaluminum using a zirconocene/MAO catalytic system. It has been proven that with this procedure the hydroxyl group can be selectively introduced into the PE chain end, due to the chain transfer by AlEt3, which occurs predominantly at the dormant end-... 

Ldfgren, B., Kokko, E. and Seppala, ]. Specific Structures Enabled by Metallocene Catalysis in Polyethenes. Vol. 169, pp. 1-12. 

Kaminsky W (1996) New polymers by metallocene catalysis. Macromol Chem Phys 197 3907... 

Kaminsky W, Arndt M (1997) Metallocenes for polymer catalysis. Adv Polym Sci 127 143-187... 

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