Polypropylene metallocene polymerization catalysts

Unlike the conversion of ethylene to linear polyethylene (PE), propylene polymerization to polypropylene (PP) introduces stereochemical complexity because we can obtain 12.8,12.9 or a random atactic product. Surprisingly, selective formation of syndiotactic propylene (12.9) is seen for many metallocene polymerization catalysts. To see why, we need to know that (f [Cp2ZrR]+ is pyramidal (12.12 in Fig. 

Metallocene catalysts have allowed greater control and advances in polymerization. The polymerization of isostatic propylene up to about 1995 resulted in a structure with approximately 5% atactic polypropylene. Metallocene catalysts made it possible to produce 100%... 

Early metal-metallocene-alkene polymerization catalysts permit the synthesis of highly isotactic polypropylene . They rely on controlling the stereochemistry of alkene insertion by the use of chiral C2 symmetric metallocenes . Late metal systems for alkene polymerization , and copolymerization of alkenes and CO , have also been developed. 

Work in the application of metallocene-based catalysis to olefin polymers has become a research topic of growing interest in recent years. A great number of symmetrie and chiral zirconocenes have been synthesized to give totally different structures of isotactic, syndiotactic, atactic or block polymers. The isotactic sequence length of polypropylene is influenced by the nature of the ligands of the metallocene. New ring or bridge substituted metallocene/methylalumoxane catalysts for the olefin polymerization are described. 

Materials that promote polymerization may be devided into true catalysts such as metal complexes, metal oxides, and anionic and cationic catalysts and initiators, which appear as end groups in the final polymer. It seems to be clear that the most expensive polymerization catalysts are the single-site metallocene catalysts. Sales has grown from 1 million US in 1994 to 100 million US in 2000. Ziegler catalysts used for polypropylene and polyethylene, and the dibutyl tin and special basic catalysts for polyurethanes are also expensive. 

The growth of polypropylene has largely depended on the development of its polymerization catalysts. Following the invention of Ziegler catalyst, the refinement by Natta and the subsequent introduction of metallocene catalysts brought about significant improvement in... 

Sun, T. and Garces, J. M. 2002. High-performance polypropylene-clay nanocomposites by in-situ polymerization with metallocene/clay catalysts. Advanced Materials 14 128-130. 

Cyclopentadiene itself has been used as a feedstock for carbon fiber manufacture (75). It is also a component of supported metallocene-alumoxane polymerization catalysts in the synthesis of polyethylene and polypropylene polymers (76), as a nickel or iron complex in the production of methanol and ethanol from synthesis gas (77), and as Group VIII metal complexes for the production of acetaldehyde from methanol and synthesis gas (78). 

The cover background photograph shows a reactor in the laboratory of Professor Stephen A. Miller, containing syndiotac-tic polypropylene prepared by graduate student Levi). Irwin. The Miller-Bercaw metallocene catalyst employed (catalyst s-90 in Chapter 2) is one of the most syndioselective propylene polymerization catalysts currently known. Unlike isotactic polypropylene, which can also be prepared with Ziegler-Natta initiators, syndiotactic polypropylene is only obtained in appreciable quantities using single-site catalysts. 

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