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Scientific Career

Throughout his life, Ziegler was a zealous advocate for the necessary indivisibility of all kinds of research. Because of this, his scientific achievements range from the fundamental to the most practical, and his research spans a wide range of topics within the field of chemistry. As a young professor, Ziegler posed the question: what factors contribute to the dissociation of carbon-carbon bonds in substituted ethane derivatives? This question was to lead Ziegler on to a study of free radicals, organometallics, ring compounds, and, finally, polymerization processes.

Free Radical Compounds

Example of three tri-valent carbon free radicals. 1. 1,2,4,5-tetraphenylallyl. 2. pentaphenylcyclopentadienyl. 3. triphenylmethyl.

While still a doctoral student at University of Marburg, Ziegler published his first major article which showed how halochromic (R₃C⁺Z^-) salts could be made from carbinols. Previous work had left the impression that halochromic salts or free radicals (R3C•) required R to be aromatic. He was encouraged to try to synthesize similarly substituted free radicals, and successfully prepared 1,2,4,5-tetraphenylallyl in 1923 and pentaphenylcyclopentadienyl in 1925. These two compounds were much more stable than previous tri-valent carbon free radicals, such as triphenylmethyl. His interest in the stability of tri-valent carbon free-radical compounds brought him to publish the first of many publications in which he sought to identify the steric and electronic factors responsible for the dissociation of hexa-substituted ethane derivatives.

Many-Membered Ring Compounds

Ziegler’s work with many-membered ring compounds also utilized the reactive nature of alkali metal compounds. He used strong bases such as the lithium and sodium salts of amines, to accomplish the cyclization of long-chain hydrocarbons possessing terminal cyano groups. The initially formed ring compound was then converted to the desired macrocyclic ketone product. Ziegler’s synthetic method, which included running reactions at high dilution to favor the intramolecular cyclization over competing intermolecular reactions, resulted in yields superior to those of existing procedures (Laylin): he was able to prepare large-ringed alicyclic ketones, C14 to C33, in yields of 60-80% . An outstanding instance of this synthesis was the preparation of muscone, the odiferous principle of animal musk by Lavoslav Ružička. Ziegler and co-workers published the first of their series of papers on the preparation of large ring systems in 1933. For his work in this area and in free-radical chemistry he was awarded the Liebig Memorial Medal in 1935.

Organometallic Compounds

Ziegler’s work with free radicals led him to the organo compounds of the alkali metals. He discovered that ether scission opened a new method of preparing sodium and potassium alkyls (Oesper), and found that these compounds could easily be converted to the hexa-substituted ethane derivatives. The nature of the substituent could be easily and systematically altered using this synthetic route by simply changing the identity of the ether starting material. (Laylin 450)

Lithium alkyls

Later, in 1930, He directly synthesized lithium alkyls and aryls from metallic lithium and halogenated hydrocarbons. 4Li+2RX – 2RLi This convenient synthesis spurred numerous studies of RLi reagents by others, and now organolithium reagents are one of the most versatile and valuable tools of the synthetic organic chemist. Ziegler’s own research on lithium alkyls and olefins was to lead directly to his discovery of a new polymerization technique some 20 years later.

Living polymerization

In 1927, he found that when the olefin stilbene was added to an ethyl ether solution of phenylisopropyl potassium, an abrupt color change from red to yellow took place. He had just observed the first addition of an organoalkali metal compound across a carbon-carbon double bond. Further work showed that he could successively add more and more of the olefinic hydrocarbon butadiene to a solution of phenylisopropyl potassium and obtain a long-chain hydrocarbon with the reactive organopotassium end still intact. Oligomers such as these were the forerunners of the so-called “living polymers”

Polyethylene

Since Ziegler was working at the Max Planck Institute for Coal Research, ethylene was readily available as a byproduct from coal gas. Because of this cheap feedstock of ethylene and the relevance to the coal industry, Ziegler began experimenting with ethylene, and made it a goal to synthesize polyethylene of high molecular weight. His attempts were thwarted because a competing elimination reaction kept occurring causing an anomalous result: instead of ethylene being converted into a mixture of higher aluminum alkyls, its dimer, 1-butene, was almost the only product. It was reasoned that a contaminant must have been present to cause this unexpected elimination reaction, and the cause was eventually determined to be traces of nickel salts. Ziegler realized the significance of this finding; if a nicked salt could have such a dramatic influence on the course of an ethylene-aluminum alkyl reaction, then perhaps another metal might delay the elimination reaction. Ziegler and his student H. Breil found that salts of chromium, zirconium, and especially titanium did not promote the R2AlH-elimination but, instead, enormously accelerated the “growth” reaction. Simply passing ethylene, at atmospheric pressure, into a catalytic amount of TiCl3 and Et2AlCl dissolved in a higher alkane led to the prompt deposition of polyethylene. Ziegler was able to obtain high molecular weight polyethylene (MW > 30,000) and, most importantly, to do so at low ethylene pressures. The Ziegler group suddenly had a polymerization procedure for ethylene superior to all existing processes.

Ziegler-Natta Catalyst

Ziegler-Natta Catalyst In 1952, Ziegler disclosed his catalyst to the Montecatini Company in Italy, for wwhich Giulio Natta was acting as a consultant. Natta denoted this class of catalysts as “Ziegler catalysts” and became extremely interested in their ability and potential to steroregularly polymerize α-olefins such as propene. Ziegler, meanwhile concentrated mainly on the large-scale production of polyethylene and copolymers of ethylene and propylene. Soon the scientific community was informed of his discovery. Highly crystalline and stereoregular polymers that previously could not be prepared became synthetically feasible. For their work on the controlled polymerization of hydrocarbons through the use of these novel organometallic catalysts, Karl Ziegler and Giulio Natta shared the 1963 Nobel Prize in Chemistry.