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Ziegler, Karl Waldemar

ZIEGLER, KARL WALDEMAR

(b. Helsa, near Kassel, Germany, 26 November 1898; d. Mülheim (Ruhr), Federal Republic of Germany, 11 August 1973)

organic chemistry, organometallic chemistry.

Karl was the youngest son of Carl August Ziegler, a Protestant minister, and Caroline Helene Louise, née Rall. In 1910 the family moved to Marburg, where he attended the Realgymnasium. In 1916 he began studying chemistry at the University of Marburg. Due to previous private chemical experimentation, Ziegler soon proved to be so well advanced that he was allowed to forgo the first two semesters.

In 1920 he received his Ph.D. under the direction of Karl von Auwers, an organic chemist noted for his use of physical methods in determining molecular constitution. As Auwers’s assistant from 1919 through 1926, Ziegler first studied Grignard reactions, but soon turned to another and more promising field, namely, the theory of valency and the study of organic radicals. His starting points were a paper of 1919 by Hans Meerwein on the pinacol-pinacolone rearrangement and Johannes Thiele’s theory of partial valencies, which aimed at explaining the reactivity of unsaturated bonds by assuming localized affinity residues. At this time the most controversially discussed example of partial valency was Moses Gomberg’s triphenylmethyl radical, discovered in 1900 as a dissociation product of hexaphenylethane. The existence of a stable, trisubstituted carbon compound was a serious threat to the firmly established doctrine of the tetravalent carbon atom. In 1923 Ziegler began to study the steric and electronic factors that influence the dissociation into, and the stability of such free radicals. In a series of papers (“Zur Kenntnis des ‘dreiwertigen’ Kohlenstoffs”) published over a period of twenty-seven years, the first of which qualified him as Privatdozent at Marburg in December 1923, he determined activation energies and dissociation kinetics of various hexasubstituted ethanes that he had synthesized. Among them, 1, 2, 4, 5-tetraphenylallyl and pentaphenylcyclopentadienyl, prepared in 1923 and 1925, proved to be almost entirely dissociated into free radicals.

In 1922 Ziegler married Maria Kurtz of Marburg, with whom he had two children. In 1925 the young family had to move, since Ziegler was given leave from his assistantship to fill a vacancy at the University of Frankfurt. The following year Karl Freudenberg offered him an assistantship at Heidelberg’s Chemical Institute; Ziegler became a professor there in 1927. In 1924 Ziegler observed that phenyliso-propylpotassium, prepared by his “ether method,” easily added to substituted olefines such as stilbene. This first example of an organometallic synthesis opened an entirely new field of research and provided the key to subsequent studies on the mechanism of sodium-induced butadiene polymerization. Ziegler’s inquiry into the chemistry of alkali organic compounds was extended to include the organolithium compounds in 1930, when the metal became easily available. In 1933, while studying butadiene polymerization, he investigated the relationship between intermolecular polymerization and intramolecular cyclization. Expanding on observations made by Paul Ruggli twenty-one years earlier, he introduced his “dilution principle” as a means of obtaining large-ringed alicyclic compounds.

In 1936 he left Heidelberg to become, for a short time, visiting professor at the University of Chicago, and, later in that year, full professor and director of the Chemical Institute at the University of Halle, a position awarded to him despite his reserved attitude toward the Nazi regime. In Halle, Ziegler continued studying alkali organic compounds, free radicals, polymerization mechanisms, and ring syntheses, which he successfully applied to some naturally occurring polycyclic substances.

The political and economic situation made fundamental research of this type increasingly difficult. Eventually, in 1943, the Kaiser Wilhelm Institute offered Ziegler the opportunity to succeed Franz Fischer as director of the society’s Institute for Coal Research at Mülheim, in Germany’s coal mining region. Ziegler accepted under the condition that his research would not be bound to coal chemistry and its industrial use but would be allowed to deal with the chemistry of carbon compounds in general. After a two-year period of divided responsibility for Halle and Mülheim, he finally moved permanently from Halle to Mülheim, to the Kaiser Wilhelm Institute, which after the war became the Max Planck Institute.

Ziegler’s initial research in Mülheim continued his earlier studies of organometallic syntheses with alkyllithium and opened up the way to organoaluminium compounds, now easily accessible through his direct synthesis of aluminum trialkyls and lithium aluminum hydride in 1949. One year later Ziegler’s stepwise addition of aluminium alkyls to olefines resulted in a new method of preparing hydrocarbons of any desired molecular size, which immediately was adopted for the industrial synthesis of straight-chain primary alcohols; the corresponding displacement reaction yielded a technically important intermediate for isoprene, the basis of synthetic rubber.

The breakthrough for industrial application came in 1953, when Ziegler unexpectedly observed that traces of nickel influence the relationship of growth-to-displacement reactions. He soon realized that combinations of trialkyl aluminum with transition elements worked as extremely active catalysts that polymerize ethylene at atmospheric pressure, as opposed to the widely accepted belief that high pressures and high temperatures were required for producing polyethylene. The impact on industrial chemistry was rapid and revolutionary. Giulio Natta, a consultant for an Italian chemical company, extended the use of “Ziegler catalysts” to stereospecific polymerization in order to obtain polymers with defined molecular structures and properties. Within a few years after the public announcement of Ziegler’s method in 1955, various industrial polymerization processes using his invention were in operation all over the world. In 1963 Ziegler and Natta shared the Nobel Prize for this discovery.

A great many honors and awards followed, including foreign membership in the Royal Society (London). Meanwhile the Mülheim institute had developed into one of Germany’s leading research institutions for applied chemistry. There Ziegler continued his research on organometallic, especially alkyl aluminum, compounds and explored a new approach to metal alkyls by electrolysis of complex organometal compounds. He retired in July 1969. Almost two hundred publications, with about the same number of coauthors, in journals and hand-books numerous patents, and about 150 doctoral theses completed under his supervision document the breadth of his research interests.

BIBLIOGRAPHY

I. Original Works. Full bibliographies are included in the obituaries by Günther Wilke in Justus Liebigs Annalen der Chemie (1975), 804–833; and C. E. H. Bawn in Biographical Memoirs of Fellows of the Royal Society, 21 (1975), 569–584, Ziegler’s personal recollections include his Noble lecture, “Folgen und Werdegang einer Erfindung,” in Angewandte Chemie, 76 (1964), 545–553, translated in Rubber Chemistry and Technology, 38 (1965), xxiii–xxxvi; and “A Forty Years’ Stroll Through the Realms of Organometallic Chemistry,” in Advances in Organometallic Chemistry, 6 (1968), 1–17. For a report on the Mülheim institute, see his “Max-Planck-Institut für Kohlenforschung,” in Jahrbuch der Max-Planck-Gesellschaft, 1961 (Göttingen, 1962), pt. 2, 492–510. This institute holds Ziegler’s scientific papers and correspondence.

II. Secondary Literature. John J. Eisch, “Karl Ziegler, Master Advocate for the Unity of Pure and Applied Research,” in Journal of Chemical Education60 (1983), 1009–1014; Christoph Meinel, Die Chemie an der Universität Marburg seit Beginn des 19, Jahrhunders (Marburg, 1978), 304–311.

Christoph Meinel

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