Ziegler–Natta catalyst
Two broad classes of Ziegler–Natta catalysts are employed, distinguished by their solubility: Ziegler–Natta catalysts are used to polymerize terminal alkenes (ethylene and alkenes with the vinyl double bond): The 1963 Nobel Prize in Chemistry was awarded to German Karl Ziegler, for his discovery of first titanium-based catalysts, and Italian Giulio Natta, for using them to prepare stereoregular polymers from propylene.
As of 2010, the total volume of plastics, elastomers, and rubbers produced from alkenes with these and related (especially Phillips) catalysts worldwide exceeds 100 million tonnes.
A few years later, Ziegler discovered that a combination of titanium tetrachloride (TiCl4) and diethylaluminium chloride (Al(C2H5)2Cl) gave comparable activities for the production of polyethylene.
In the 1970s, magnesium chloride (MgCl2) was discovered to greatly enhance the activity of the titanium-based catalysts.
These catalysts were so active that the removal of unwanted amorphous polymer and residual titanium from the product (so-called deashing) was no longer necessary, enabling the commercialization of linear low-density polyethylene (LLDPE) resins and allowed the development of fully amorphous copolymers.
A polymer that lacks any regular arrangement in the position of its alkyl substituents (R) is called atactic.
Commercial catalysts are supported by being bound to a solid with a high surface area.
The modifiers react both with inorganic ingredients of the solid catalysts as well as with organoaluminum cocatalysts.
The idealized metallocene catalysts have the composition Cp2MCl2 (M = Ti, Zr, Hf) such as titanocene dichloride.
Depending on the type of their cyclopentadienyl ligands, for example by using an ansa-bridge, metallocene catalysts can produce either isotactic or syndiotactic polymers of propylene and other 1-alkenes.
[6][7][9][10] Ziegler–Natta catalysts of the third class, non-metallocene catalysts, use a variety of complexes of various metals, ranging from scandium to lanthanoid and actinoid metals, and a large variety of ligands containing oxygen (O2), nitrogen (N2), phosphorus (P), and sulfur (S).
A polymer molecule grows by numerous insertion reactions of C=C bonds of 1-alkene molecules into the Zr–C bond in the ion: Many thousands of alkene insertion reactions occur at each active center resulting in the formation of long polymer chains attached to the center.
