Coordination polymerization and Ziegler-Natta Catalyst

The first efficient and stereospecific catalytic polymerization method was developed by Karl Ziegler and Giulio Natta in the 1950's. Their method permitted the synthesis of unbranched, stereospecific, high molecular weight polyolefins for the first time. Important examples of pure isotactic (and in some cases syndiotactic) polymers are cis-polyisoprene, cis-polybutadiene, and highly crystalline, isotactic polyethylene and polypropylene. In the case of ethylene, rapid polymerization occurrs at atmospheric pressure and moderate to low temperature, giving a stronger and more crystalline polymer (HDPE) than that produced from radical polymerization (LDPE). For this important discovery Ziegler and Natta received the Nobel Prize in chemistry in 1963.¹

Ziegler-Natta catalysts can be prepared by reacting transition metal halides from groups IV - VIII, such as titanium tetrachloride (TiCl₄), with organometallic compounds from groups I - III of the periodic table, such as triethylaluminum (Al(C₂H₅)₃). The titanium catalysts have been widely studied, but other metals like vanadium and zinc have also been proven to be very effective.

Commercial catalysts are usually supported, that is, they are bound to a solid with a high surface area. In the case of TiCl₄ or TiCl₃ as the active catalyst, the support is usually MgCl₂. These catalysts are incorporated in the so-called carrier, which is a material that determines the size and the shape of catalyst particles. One common carrier are spheres of microporous, amorphous silica . The titanium compounds on MgCl₂ are packed into the pores and are activated with organoaluminum compounds.

In some cases, especially for catalyzing the polymerization of propylene, an organic modifier is added to control stereoregularity. It can either be incorporated into the catalyst during synthesis (internal donor) or it can be added to the polymerization reactor with the catalyst during polymerization (external donor). The modifier is usually an ester of an aromatic diacid (internal donor) or an alkoxysilane or a hindered amine (external donor). The modifier reacts both with inorganic ingredients of the solid catalysts as well as with organoaluminum co-catalysts.

The Ziegler-Natta catalyst reactions are coordination polymerizations. They involve complexes formed between a transition metal and the π electrons of the monomer. These reactions are similar to anionic polymerizations and lead to linear and stereo-regular polymers.

The metal organic compound acts as a weak anionic initiator forming a complex whose nature is still open to debate. Polymerization then proceeds by insertion of monomers; the transition metal ion is connected to the end of the growing chain and simultaneously coordinates the incoming monomer at a vacant orbital site.

References & Notes

1. Karl Ziegler was awarded the Nobel price in chemistry in 1963 for the discovery of the first titanium-based catalysts, and Guilo Natta for using them to prepare stereoregular polymers from polypropylene.

2. Giulio Natta, J. of Poly. Sci., Vol. 34, 531 - 549 (1959)
3. K. Ziegler, Angew. Chem., Vol. 76, Iss. 17, pp. 545 - 553 (1964)
4. K. Ziegler, E. Holzkamp, H. Breil and H. Martin, Angew. Chem., 67, 426 (1955)
5. K. Ziegler, Angew. Chem., Vol. 76, Iss. 17, pp. 545-553 (1956)