Summary of the Ziegler-Natta Catalysis

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. 

Polymers prepared by metallocene catalysts have special properties. Metallocene-polyethylene is an excellent film wrap and has excellent barrier properties to oxygen and moisture. Special polymers such as linear low-density polyethylene (LLDPE) which is a copolymer of ethylene and 1-butene (or 4-methyl-l-pentene) can be manufactured readily by the use of metallocene catalysts. LLDPE is a high strength film-forming polymer. 

A new generation of Ziegler-Natta catalysts does away with titanium or zirconium complexes altogether. Instead, conventional transition-metal complexes involving Fe(II) or Ni(II) are being used. 

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