Polymerization with Metallocene Catalysts

For example, it is possible to synthesize isotactic as well as syndiotactic polypropylene in high configurational purity and high yields. The same holds for syndiotactic polystyrene. Furthermore, metallocene catalysts open the possibility to absolutely new homopolymers and copolymers like, e.g., cycloolefin copolymers (COG) and even (co)polymers of polar monomers.The simplest metallocene catalyst consists of two components. The first one is a n-complex (the actual metallocene) that can be bridged via a group X and therefore can become chiral  

The second component is a special alumina-organic compound, methylalumoxane (MAO), that is prepared by partial hydrolysis of trimethylaluminum and that contains linear as well as cyclic structures in the molecules. 

The mixture of metallocene and co-catalyst is soluble. Its active center, which is chiral, induces with a very low rate of defects only one type of monomer linkage ( single site catalysts )- That is why high activities (some 1,000 kg polymer/g 

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Resconi L, Cavallo L, Fait A, Pietmontesi F (2000) Selectivity in propene polymerization with metallocene catalysts. Chem Rev 100 1253-1345... 

High-pressure polymerization with metallocene catalysts G. Luft... 

Figure 9.5-5. Mechanism of ethylene polymerization with metallocene catalyst.

The advantages of high pressure known from the radical polymerization process for low density polyethylene are listed in Table 1. They can also be observed in the polymerization with metallocene catalysts. 

NEW DEVELOPMENTS IN OLEFIN POLYMERIZATION WITH METALLOCENE CATALYSTS... 

Selectivity in Propene Polymerization with Metallocene Catalysts... 

As a lagniappe, the ratio of alumoxane to metal can be reduced considerably. It has been suggested that large excesses of MAO (aluminum-to-metal ratios of 1000—10 000) are needed in homogeneous polymerizations with metallocene catalysts in order to prevent bimolecular deactivation processes (Scheme 1). 

Zapata, P, Quijada, R., Covarrubias, C., Moncada, E., and Retuert, J. 2009. Catalytic activity during the preparation of PE/clay nanocomposites by in situ polymerization with metallocene catalysts. Journal of Applied Polymer Science 113 2368-2377. 

Olefin polymerization with metallocene catalysts includes a coordination step between the monomer molecule and the catalyst active site, followed by insertion into the metal-alkyl bond between the catalyst center and the growing polymer chain. These steps are repeated many times to form a polymer chain. 

Langhauser, E, Kerth, J., Kersting, M., Kolle, R, Lilge, D. and Muller, R (1994) Propylene polymerization with metallocene catalysts in industrial processes. Angew. Makromol. Chem., 223,155-164. 

Sacchi, M. C. Barsties, E. Tritto, L Locatelli, R Brintzinger, H.-H. Stehling, U. Stereochemistry of first monomer insertion into metal-methyl bond A tool for evaluating Hgand-monomer interactions in propene polymerization with metallocene catalysts. Macromolecules 1997, 30, 3955-3957. 

Yoder, J. C. Bercaw, J. E. Chain epimerization during propylene polymerization with metallocene catalysts Mechanistic studies using a doubly labeled propylene. J. Am. Chem. Soc. 2002, 124, 2548-2555. 

For some leading reviews see (a) Resconi, L. Cavallo, L. Fait, A. Piemontesi, F. Selectivity in propene polymerization with metallocene catalysts. Chem. Rev. 2000,100, 1253-1345. (b) Alt, H. G Koppl, A. Effect of the nature of metallocene complexes of Group IV metals on their performance in catalytic ethylene and propylene polymerization. Chem. Rev. 2000, 100, 1205-1221. (c) Halterman, R. L. Synthesis and applications of chiral cyclopentadienyhnetal complexes. Chem. Rev. 1992, 92, 965-994. (d) Halterman, R. L. Synthesis of chiral titanocene and zirconocene dichlorides. In Metallocenes ... 

Arndt, M. Kaminsky, W. Polymerization with metallocene catalysts. Hydroohgomerization and NMR investigations concerning the microstructure of poly(cyclopentenes). Macromol. Symp. 1995, 95, 167-183. 

Results of 1,5-Hexadiene Polymerization with Metallocene Catalysts 2b, 4b, and 7... 

Olefin polymerization with metallocene catalysts involves initiation, chain propagation, formation of a dead chain with a saturated chain end through the chain transfer agent, p-hydride elimination to form a dead chain with a vinyl terminal double bond, insertion of a macromer with a vinyl end group, and catalyst deactivation. Assuming that all the reactions associated with each step are first order, the reaction processes can be expressed as... 

Pawlak A, Galeski A (2005) Plastic deformation of crystalline polymers the role of cavitation and crystal plasticity. Macromolecules 38 9688-9697 Peterlin A (1971) Molecular model of drawing polyethylene and polypropylene. J Mater Sci 6 490 Popli R, Mandelkem L (1987) Influence of structural and morphological factors on the mechanical properties of the polyethylenes. J Polym Sci B Polym Phys 25 441 Read D, Duckett R, Sweeny J, Mcleish T (1999) The chevron folding instability in thermoplastic elastomers and other layered material. J Phys D Appl Phys 32 2087-2099 Resconi L, Cavallo L, Fait A, Piemontesi F (2000) Selectivity in propene polymerization with metallocene catalysts. Chem Rev 100 1253... 

Sun T, Garces JM (2003) Acidic lamellar aerogel nanoplate activated olefin polymerization with metallocene catalysts. Catal Commun 4(3) 97-100... 

Metallocene-based catalysts have broadened the scope of synthesizing polyolefins with different functional groups. The polar monomers could now be polymerized with metallocene catalysts of cationic nature. 

Figure 12 Chain migratory mechanism for olefin polymerization with metallocene catalysts.

For Ziegler-Natta polymerization of alkenes, second-order deactivation decay explains much better experimental data than the first-order decay. Such behavior in supported systems was attributed to simultaneous deactivation of adjacent catalytic species on the surface. In homogeneous polymerization with metallocene catalysts (Fig. 9.53), a metallocene complex Cp2ZrCl2 reacts first (kj) with a cocatalyst MAO ([—O—Al—CH (CH3-) ). Subsequently, alkenes molecules are inserted (kpi) into the Zr-C bond of the formed metaUocenium ion Cp2Zr -CH3. A polymer molecule grows in length by numerous insertion reactions (kpi). 

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