Ziegler-Natta catalysts metallocene-based systems

New Ziegler-Natta catalysts are homogeneous systems. These are based on a combination of metallocenes and MAO. By appropriate use of these catalysts linear polyethylene, and all the three types of polypropylene (atactic, isotactic and syndiotactic) can be produced in a specific manner. A cationic alkyl-metallocene complex is the active species in the polymerization. MAO helps in alkylating the metallocene (if the metallocene is a metallocene dihalide like Cp2ZrCl2) and also in assisting the removal of a CHa. Further it stabilizes the cationic center and also possibly scavenges impurities. 

Supported metallocene catalyst systems are preferred to soluble versions in conventional polyolefin plants, which were designed to use supported Ziegler-Natta or Cr203-based catalysts. Metallocenes can be supported on a number of substrates, such as Si02, MgC or AI2O3. Supported catalysts also provide polypropylene with fewer stereochemical defects. 

Many other catalysts capable of polymerizing olefins have become available since the original Ziegler-Natta catalyst based on crystalline titanium chloride was introduced. More recently, the discovery of soluble metallocene/aluminoxane catalysts opened the doors to a new revolution in the production of polyolefins. These catalyst systems are able to make polyolefins in very high yields and with a degree of microstructural control not possible to achieve using conventional Ziegler-Natta catalysts. 

Brookhart and coworkers [1] have recently developed Ni(II) and Pd(II) bis-imine based catalysts of the type (ArN=C(R)-C(R)=NAr)M-CH3+ (la of Figure 1) that are promising alternatives to both Ziegler-Natta systems and metallocene catalysts for olefin polymerization. Traditionally, such late metal catalysts are found to produce dimers or extremely low molecular weight oligomers due to the favorability of the P-elimination chain termination process [2],... 

The annual production of various polymers can be measured only in billion tons of which polyolefins alone figure around 100 million tons per year. In addition to radical and ionic polymerization, a large part of this huge amount is manufactured by coordination polymerization technology. The most important Ziegler-Natta, chromium- and metallocene-based catalysts, however, contain early transition metals which are too oxophiUc to be used in aqueous media. Nevertheless, with the late transition metals there is some room for coordination polymerization in aqueous systems [1,2] and the number of studies published on this topic is steadily growing. 

Haselwander, T., Beck, S. and Brintzinger H. H., Binuclear Titanocene and Zirconocene Cations with //-Cl and fi — CH3 Bridges in Metallocene-based Zieg-ler-Natta Catalyst Systems - solution-NMR studies , in Ziegler Catalysts, Springer-Verlag, Berlin, 1995, pp. 181-197. 

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