1 - Monoalkene Polymerization: Ziegler–Natta and Transition Metal Catalysts

Abstract

One of the most important achievements in the field of synthetic polymer chemistry during the past 50 years has been the discovery in 1953 by Ziegler and his co-workers1,2 that aluminum alkyl compounds used along with many transition metal halides in an inert hydrocarbon medium could polymerize ethylene under conditions of normal temperatures and pressures to yield a solid polymer of high molecular weight and of linear structure. The developments which led to the discovery of this catalyst have been described by Ziegler.2 For some time Ziegler’s group had studied the ‘aufbau’ or growth reaction in which ethylene undergoes oligomerization when heated with an aluminum alkyl compound. Although the insertion step proceeds at temperatures of 100–120°C the displacement reaction requires temperatures of greater than 120°C. By accident Holzkamp, one of Ziegler’s students, discovered that the displacement reaction could be catalyzed at lower temperatures by traces of colloidal nickel inadvertently left in a reactor. Subsequently, during wider investigations using other metals, Breil discovered that zirconium acetylacetonate, used with aluminum triethyl, was an exceedingly active catalyst with the autoclave being filled with polyethylene. Other transition metal compounds from Groups IV to VI when used with aluminum alkyl compounds gave similar results, with titanium compounds showing a high activity. Characterization studies on the polyethylenes produced by the new catalyst systems showed them to have linear unbranched structures and high molecular weights.