A Ziegler-Natta catalyst is a catalyst used in the synthesis of polymers of 1-alkenes (α-olefins). Three types of Ziegler-Natta catalysts are currently employed:
- Solid and supported catalysts based on titanium compounds. They are used in polymerization reactions in combination with cocatalysts, organoaluminum compounds such as triethylaluminium, Al(C2H5)3.
- Metallocene catalysts, combination of various mono- and bis-metallocene complexes of Ti. Zr or Hf. They are usually used in polymerization reactions in combination with a different organoaluminum cocatalyst, methylaluminoxane (or methylalumoxane. MAO).
- Post-metallocene catalysts based on complexes of various transition metals with multidentate oxygen- and nitrogen-based ligands. These complexes are also activated with MAO.
Ziegler-Natta catalysts are used to polymerize terminal 1-alkenes (ethylene and alkenes with the vinyl double bond):
n CH2=CHR → -[CH2-CHR]n-
German Karl Ziegler, for his discovery of first titanium-based catalysts, and Italian Giulio Natta, for using them to prepare stereoregular polymers, were awarded the Nobel Prize in Chemistry in 1963. Ziegler-Natta catalysts have been used in the commercial manufacture of various polymeric materials since 1956. In 2010, the total volume of plastics, elastomers, and rubbers produced from alkenes with these catalysts worldwide exceeds 100 million metric tons. Together, these polymers represent the largest-volume commodity plastics as well as the largest-volume commodity chemicals in the world.
Stereochemistry of poly-1-alkenes
Giulio Natta used first polymerization catalysts based on titanium chlorides to polymerize propylene and other 1-alkenes. He discovered that these polymers are crystalline materials and ascribed their crystallinity to a special feature of the polymer structure called stereoregularity.
The concept of stereoregularity in polymer chains is illustrated in the picture above with polypropylene. Stereoregular poly(1-alkene) can be isotactic or syndiotactic depending on the relative orientation of the alkyl groups in polymer chains consisting of units -[CH2-CHR]-, like the CH3 groups in the figure. In the isotactic polymers, all stereogenic centers CHR share the same configuration. The stereogenic centers in syndiotactic polymers alternate their relative configuration. A polymer that lacks any regular arrangement in the position of its alkyl substituents (R) is called atactic. Both isotactic and syndiotactic polypropylene are crystalline, whereas atactic polypropylene, which can also be prepared with special Ziegler-Natta catalysts, is amorphous. The stereoregularity of the polymer is determined by the catalyst used to prepare it. Once the polymer chain is formed, its stereochemistry does not change.
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