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Fifty Years of Ziegler Catalysts Consequences and Development of an Invention.

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Essays
Fifty Years of Ziegler Catalysts
Fifty Years of Ziegler Catalysts: Consequences and
Development of an Invention**
Gnther Wilke*
Keywords:
biography · history of science · polymerization ·
Ziegler catalysts · Ziegler, Karl
O
n October 26, 1953 an experiment
was carried out in the Max Planck
Institut f r Kohlenforschung in M lheim an der Ruhr that was to become
the basis of a patent with the priority of
November 18, 1953—a patent that
would cause a revolution in the chemical
industry and, within a short time, provide a strong impetus to basic research
around the world. The inventors were
Karl Ziegler, Heinz Breil, Erhard Holzkamp, and Heinz Martin. The title of the
German patent was a “Process for
Preparing High-Molecular Polyethylenes” (Figure 1).[1]
Now, a half century later, it is well
worth looking back at the developments
that would to lead to the award of a
Nobel Prize[2] and bring great financial
rewards to the Institute in M lheim,
particularly because the story is a prime
example relevant to the current political
discussion on the funding of basic research and innovation for industry.
When looking at the biography of
Karl Ziegler, one cannot help noticing
that the years ending in a “3” represent
major landmarks in his life. 1923 was the
year of his habilitation under the guidance of his doctoral advisor, Karl von
Auwers. In 1933 his first publication on
“Vielgliedrige Ringsysteme” (large ring
systems)[3] appeared—this presents the
fundamentals of the Ruggli–Ziegler di-
[*] Prof. Dr. G. Wilke
Max Planck Institut f$r Kohlenforschung
Kaiser Wilhelm Platz 1, 45470 M$lheim an
der Ruhr (Germany)
Fax: (+ 49) 208-389-9761
E-mail: guenther.wilke@t-online.de
[**] Karl Ziegler's Nobel Lecture had the Title
“Consequences and Developments of an
Invention”.[2]
5000
Figure 1. Copy of the first page of the German Patent DBP 973 626.
career. On August 11, 1973, four months
before his 75th birthday, Karl Ziegler
died M lheim.
lution principle, as it was later called. In
1943 he took over as director of the
Kaiser Wilhelm Institut f r Kohlenforschung in M lheim an der Ruhr as
successor to Franz Fischer. 1953 saw
the discovery of the so-called M lheim
mixed catalysts, which were later described in a patent of Giulio Natta as
“Ziegler catalysts”, a name which is still
used today. The award of the Nobel
Prize in 1963 can rightly be regarded as
the peak of Karl Ziegler's academic
Karl Ziegler's Scientific Work from
1923 to 1949
DOI: 10.1002/anie.200330056
Angew. Chem. Int. Ed. 2003, 42, 5000 –5008
2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
When relating the story of the discovery of the organometallic mixed
catalysts—I am deliberately using the
term first used by Karl Ziegler himself—
Angewandte
Chemie
soon to lead to very fundamental discoveries. It required considerable experimental skill to handle such highly
reactive organometallic compounds successfully. Solutions of 2-phenylisopropyl
potassium in ether are exceptionally
sensitive test reagents for impurities in
gases such as ethylene, since the deep
red solutions lose their color immediately in the presence of, for example,
water or oxygen.
In a preliminary communication,
Karl Ziegler and Kurt BIhr described
the addition of 2-phenylisopropyl potassium to stilbene.[7] In the introduction to
this paper they wrote that
Figure 2. The Professors and Lecturers from the Chemical Institute in Heidelberg (January
1928): Max Trautz, Ernst M$ller, Hans Kautsky, Karl Freudenberg, Walter Hieber, Hausorth
(guest), Robert StollH, and Karl Ziegler.
then one must ask what led Karl Ziegler
into organometallic chemistry in the
first place. The answer can be found in
a paper published in 1923, the same year
as his habilitation, in which he reported
“On Alkali Metals as Reagents for
Weakened Valencies (”abgeschwchte
Valenzen“) in Organic Compounds”.[4]
In the course of investigations into
tetraarylallyl radicals, which can be
trapped by alkali metals, it was discovered that certain ethers can be cleaved
very easily by alkali metals, particularly
with sodium–potassium alloy. In addition to the alcoholates, organometallic
compounds of the alkali metals were
obtained, which were thus formed both
by cleaving the ether and by trapping
the free radicals. The tetraarylallyl radicals were the starting point of a long
series of papers that appeared under the
collective title of “Studies onCTrivalent'
Carbon”. This research is reported in
the years up to 1950 in no fewer than 24
papers which belong to Ziegler's most
fascinating publications. They earned
him the reputation of being one of the
first “physical organic chemists”, even in
the late 1920s he was already using
physico-chemical methods in his organic
chemical studies, carrying out studies of
kinetics and determining dissociation
and activation energies at a time when
most organic chemists still had little idea
of the possibilities they presented. In a
footnote to the first paper[4] he commented that “I gave a short review of
this interesting group of compounds on
Angew. Chem. Int. Ed. 2003, 42, 5000 –5008
“the polymerizing effect of the alkali metals on unsaturated hydrocarbons has
been known for a long time. … . The
mechanism of this characteristic property of the alkali metals was previously
completely unknown … . During the
course of experiments being carried out
with completely different aims, we found
by chance a reaction that is most probably of great significance for the occurrence of the polymerizations described:
alkali metal alkyls undergo addition reactions with conjugated double bonds or
double bonds adjacent to a benzene
ring. … . This reaction was discovered
while attempting to prepare dipotassium
diphenylethane by reacting stilbene with
2-phenylisopropyl potassium as follows:
April 21, 1923, in Heidelberg at the
Conference of Lecturers of the SouthWest German Universities.” Three years
later, Karl Freudenberg brought the
young lecturer to Heidelberg (Figure 2).
In the context of the studies on
radicals, the organometallic compounds
of the alkali metals at first appeared to
be mere by-products, but Karl Ziegler
soon recognized that the high reactivity
of these compounds opened up completely new synthetic possibilities. His
work in this field led to a further series
of 18 publications titled “Investigations
into
Organo–Alkali-Metal
Compounds”, which likewise appeared up
to the year 1950.
For the time being, however, the
investigations concentrated on new
ways of synthesizing the ethane derivatives required to study their dissociation
into tetraarylallyl radicals. It was disThese substances did not react with
covered that when the organometallic
compounds of the alkali metals formed each other in the way that was intended.
during the course of the ether cleavage, Instead it was observed that 2-phenylparticularly the potassium compounds, isopropyl potassium undergoes an addiwere treated with tetramethylethylene tion reaction with stibene.
bromide, the corresponding ethanes were
formed very smooth- “It was the first example of the addition of an organomely.[5] It was during this tallic compound to a C=C double bond, a reaction that
project[6] that 2-phenyl- was hitherto completely unknown. Just a single series of
isopropyl
potassium experiments sufficed to show that this reaction was clearwas prepared by ether ly a general one with a wide range of applicability.”
cleavage: the study of
this deep red compound, which is readily
soluble in ether, was
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Essays
Based on this fundamental discovery, Karl Ziegler formulated at once a
method for polymerizing butadiene to
1,2 polybutadiene.[7] The “further investigation of this addition reaction” were
“planned”.
The above quotations reveal the
characteristic way in which Karl Ziegler
thought and worked—to recognize immediately the general significance of a
new and unexpected result and then to
follow this up systematically. “In the
course of this project we have made a
comprehensive study of the reactions
between unsaturated hydrocarbons and
alkali metals and report on these in the
following.” Just 11=2 years after the
preliminary communication he submitted an extended three-part publication
on the “Reactions between Unsaturated
Hydrocarbons and Alkali-Metal Alkyl
Species.” in which Ziegler observed that
“the reactivity of the alkali-metal compounds have something capricious
about them, so we are not yet able to
formulate general rules that are valid for
all our experimental data.”[8]
The extension of these studies to
include lithium alkyl compounds was to
influence their development in a big
way. Lithium alkyl compounds still had
to be prepared by treating lithium metal
with mercury alkyls—a not particularly
satisfactory method! “This remarkable
procedure is not exactly convenient.”
All the same, very significant results
were obtained. Just a year later, Karl
Ziegler and Herbert Colonius published
“A Convenient Synthesis of Simple
Lithium Alkyl Species”.[9] They were
able to show that by working in benzene
and keeping the temperature to 35 8C,
butyllithium could be prepared from
butyl chloride and lithium metal in
yields of between 80 % and 100 %. This
process was then patented, since it could
clearly be of industrial value. However,
attempts to distil butyllithium were unsuccessful because it decomposed at
higher temperature. This observation
was re-examined years later and would
then lead to unexpected consequences.
Karl Ziegler's postulate on the polymerization of butadiene[7] was followed
by detailed experimental studies on the
“Mechanism of the Polymerization of
Unsaturated Hydrocarbons by Alkali
Metals and Alkyls.”[10] “As is well
known, these reactions are the basis of
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this particularly in view of the statements made by Midgley and Henne[13]
and Wagner-Jauregg.[14] On the contrary,
all the evidence obtained to date speaks
for these processes being entirely due to
organometallic syntheses.”
Without doubt, Karl Ziegler's habilitation thesis, which he submitted to the
Faculty in Marburg and which was
published in the Annalen der Chemie,
provided the basis for the work that he
then carried out during a short stay at
the Johann-Wolfgang-Goethe-University in Frankfurt and particularly in Heidelberg after that. He soon achieved
international recognition, but because
of his clearly discernible rejection of
ðC6 H5 Þ3 CNa þ Bu !
National Socialism he had to wait before
ðC6 H5 Þ3 CCH2 CH¼CHCH2 Na þ ðC6 H11 Þ2 NH
he received a call to a university chair.
! ðC6 H5 Þ3 CCH2 CH¼CHCH3 þ ðC6 H11 Þ2 NNa
Finally, in 1936, he could be overlooked
no longer. The call to the Chair at the
At first, triphenylmethane had been University of Halle an der Saale opened
used to try to trap the primary addition up new ways of widening his research
product. Ziegler wrote that “the princi- and led to further contacts with industry.
ple of working at high dilution to detect
His investigations on the polymerless favored reaction paths was first used ization of butadiene continued to gain in
for this reaction at about the same time importance with the construction of the
as for the recently published experi- Buna Works nearby at Schkopau. Thus
ments to prepare large ring systems.”[3] it is not surprising that two major review
However, the trapping reactions proved articles on the polymerization of butato be too slow, so dicyclohexylamine was diene[15, 16] were published during his
used instead.
time in Halle together with an original
The proof was finally found by a publication[17] in which he reported on
method analogous to that used to inves- experiments to steer the polymerization
tigate the polymerization of butadiene towards either 1,2- polybutadiene or 1,4
induced by sodium and lithium met- polybutadiene. His paper concluded:
als:[12]
the formation of the so-called ”sodiumrubbers“, of which sodium-butadiene
rubber has gained a certain amount of
attention in the industrial field.”
The most important results to
emerge from this work are that butadiene in ether is polymerized rapidly by
butyllithium, that when the reaction is
stopped abruptly by adding water, unsaturated hydrocarbons with the general
formula C4H9(C4H6)nH (n = 1–6) are
obtained, and that the first step in this
organometallic synthesis can be trapped
by the reaction of triphenylmethyl sodium and butadiene in the presence of
dicyclohexylamine:[11]
“However, clear conclusions on the nature of the initial reaction products can
be reached based upon the results of
two groups of experiments. It was the
study of the reaction of butadiene with
alkali metals in the presence of trapping
reagents and the surprisingly clear reaction between certain butadienes and lithium metal that finally gave us the missing pieces of evidence required to clear
up all remaining hypothetical assumptions.”
The investigations also produced the
important realization that[12]
“our results produced no evidence at
all for the formation of radical intermediates in the course of the alkali
metal polymerization. We emphasize
2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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“In summary, of all the factors, namely
(1) temperature, (2) rate of addition of
butadiene, (3) the solvent chosen, and
(4) the type of alkali metal, the temperature alone determines the constitution of
the polymer formed.”
At 70 8C a nearly pure 1,2-polybutadiene is formed, while at 150 8C the
1,4-polymer is obtained. This steered
polymerization was the result of pure
basic research, but its technical importance is clear, since the industrial properties of the rubber are determined by
the structure of the polymer constituents.
In 1943 came the call to the Kaiser
Wilhelm Institut f r Kohlenforschung in
M lheim an der Ruhr as successor of
Franz Fischer. The Institute derived its
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Angewandte
Chemie
support from the Kaiser-Wilhelm-Gesellschaft, the coal mining industry, and
the town of M lheim.
Karl Ziegler recounted his first reaction to this offer as follows:[18]
the Institute in M lheim for a very wide
range of processes.
In 1950, the last original publication
in the series about butadiene and its
polymerization appeared,[19] and the
more about this and they were
able to show that at elevated temperature lithium alkyls react with ethylene under pressure to form longer
chains:[21]
C4 H9 Li þ n CH2 ¼CH2 ! C4 H9 ðCH2 CH2 Þn Li
“In early 1943 I received a letter in my office at the Chemistry Institute of the University of Haale an der Saale from the unforgotten President of the Kaiser Wilhelm Gesellschaft, A. VLgler, asking me if I would be prepared to take over as director of the
Kaiser-Wilhelm-Institut f$r Kohlenforschung in M$lheim an der Ruhr. I must freely
admit that my immediate reaction was completely negative. There were good reasons
for this: in my scientific career I had experienced happy years in which my work was
guided purely by the pleasure of working on the problems that I had chosen myself,
even though this was within the framework of the modest resources of a university
professor of chemistry. Moreover, the period of directed research immediately preceding had made it clear to me that I was less suited for projects with predetermined
aims. With just a few exceptions, my research had developed from observations that
had been made during the immediately preceding work. I was very convinced of the
fruitfulness of the policy of allowing my work to develop from the interaction of observation, theoretical interpretation and new experiments, without regard to the fields
into which this would lead me. It is therefore quite natural that I was disturbed that
the name of the institute being offered to me expressed its purpose. However, in the
course of the negotiations that now took place I found that both the President of the
Kaiser-Wilhelm-Gesellschaft and the Chairman of the Institute's Board of Governors,
Generaldirektor Bergassessor H. Kellermann, showed great understanding for the
condition I placed on going to M$lheim an der Ruhr: I insisted that I must be given
complete freedom to pursue the entire field of compounds of carbon (“organic
chemistry”) irrespective of whether a clear relationship could be recognized between
my work and coal, or not.”
Even at that time as the German
Reich headed for defeat, Ziegler's conditions were accepted. This decision
should also prove instructive for the
present day, whenever decisions have to
be made about granting support for
research.
Karl Ziegler accepted the call to
M lheim and between 1943 and 1945 he
shuttled back and forth between Halle
and M lheim as he had to supervise
both institutes. His family did not finally
move to M lheim until 1945. Although
most of M lheim had been destroyed
by aerial bombing, the Institute remained undamaged, so that the research
work there could carry on at a modest
rate.
M lheim also offered contacts with
industry, since his work on the polymerization of butadiene was as interesting
for the Chemische Werke H ls in Marl
as for the Bunawerke in Schkopau. In
the following decades the Chemische
Werke H ls would take licenses from
Angew. Chem. Int. Ed. 2003, 42, 5000 –5008
lithium alkyls once again became the
focus of the organometallic investigations. Attention turned afresh to the
behavior that was observed when butyllithium is distilled. Decomposition took
place at the temperature required to
distil it and the liquid became cloudy as
lithium hydride precipitated out. The
distillation was finally achieved in special equipment under high vacuum.[20]
Ethyllithium proved to be more stable
than butyllithium, but it, too, decomposed at higher temperature:
“It is evident that this addition of ethylene to lithium alkyls is probably the simplest and at the same time the most effective way of lengthening the chain using organometallic reagents. When starting with any lithium alkyl having an even
number of carbon atoms, then alkyl
chains with an even number of carbon
atoms are obtained, while starting with
lithium alkyl compounds having an odd
number produces alkyl chains with an
odd number of carbon atoms.”
As interesting as these observations
were from a scientific point of view, Karl
Ziegler asked himself[22]
“whether, in the very difficult period after
the war, one could really justify continuing investigations in a coal research institute into the properties of exotic materials with no industrial significance whatsoever—something that appeared to be
the pursuit of private pleasure?”
The answer came in the successes
that soon followed.
“Various observations led us to conclude
that these elimination reactions are basically reversible, so that one should actually write:
CH3CH2Li Q CH2=CH2+LiH
C2 H5 Li ! CH2 ¼CH2 þ LiH
Remarkably, small amounts of butene were detected in the gas produced,
which indicated that some of the ethylene formed in the primary process
had dimerized. Apparently ethyllithium
was able to undergo an addition reaction with ethylene. Karl Ziegler and
Hans-Georg Gellert immediately undertook systematic experiments to discover
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An extensive series of experiments
aimed at observing this reverse reaction,
for example, the addition of lithium
hydride to ethylene, was unsuccessful.
Lithium hydride is completely insoluble
and is therefore unable to react with the
ethylene. “It appeared at first that this
part of our work on organolithium
compounds was heading for a dead
end.”[21]
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Essays
1949—The Beginning of Organoaluminum Chemistry
The turn-around came about in
1949. More or less as a last resort,
Hans-Georg Gellert, who had worked
with Karl Ziegler for many years, treated ethylene under pressure with a solution of lithium aluminum hydride, a
material that had just recently been
discovered, in ether at 180–2008C. The
pressure fell rapidly and the product
isolated was identified as a mixture of
pure a-olefins. It appeared that the
lithium hydride in the form of soluble
LiAlH4 had not only added to ethylene
in the manner being sought, but that in
addition an “Aufbau” (chain growth)
reaction had occurred: the aluminum
component had apparently rendered the
reagent soluble. However, it soon became clear that aluminum hydride itself
could undergo an addition reaction with
ethylene, indeed that the triethylaluminum formed as the primary product of
this reaction was itself undergoing the
Aufbau reaction.[22, 23]
This discovery was fundamental,
because it was now possible to employ
metal alkyl species of a common metal,
aluminum, and this finding appeared to
open up the way even to industrial
applications. Pure a-olefins could now
be prepared by a homogeneous catalytic
process. When the reaction was carried
out at high temperature the olefins were
formed by chain formation then cleavage. The cleavage, later called elimination, could be suppressed by lowering
the temperature at which the reaction
was carried out from 180–200 8C to
100 8C. The chain-forming process produced higher aluminum alkyls, which on
hydrolysis gave straight-chain (unbranched) paraffins. Thus, a method
had been found of polymerizing ethylene under very mild conditions. At the
same time, however, even under the
most favorable circumstances it produced waxy products with molecular
weights of up to 5000: polyethylenes
with true plastic properties could
apparently not yet be prepared in
this way because, after about a hundred
addition steps, the chain formation
was terminated by an elimination reaction.
Thus, when triethylaluminum was
used as the initial catalyst, ethylene,
5004
under pressure at high temperature,
yielded mixtures of a-olefins, a class of
substances of industrial interest since
they could not prepared in a pure form
before. At normal pressure and temperatures of around 170 8C, a smooth dimerization of the ethylene to butene-1 was
achieved. We will come back to this
process later. With typical thoroughness,
Karl Ziegler had the reactions of triethylaluminum with propylene or butene-1 tested to see what reactions
occurred. The result was a smooth
dimerization to 2-methyl-1-pentene
and 2-ethyl-1-hexene, respectively, since
a further extension of the chain did not
occur with either of these olefins. The 2methyl-1-pentene later found use as the
starting material for the large-scale
industrial synthesis of isoprene, while
the 2-ethyl-1-hexene opened up the way
to terephthalic acid, since the aromatization of the 2-ethylhexene leads to the
formation of para-xylene.
Initially, these reactions of aluminum alkyl species with potential industrial applications all suffered a grave
disadvantage—aluminum alkyl compounds were difficult to prepare. Although the syntheses known at that time
could be used to obtain these compounds on a small laboratory scale, they
were unsuitable for production on an
industrial scale. I remember how valuable the small amounts of triethylaluminum were that had been prepared in the
Institute. It was vitally important to find
a way of preparing it on the kilogram
scale. The reaction of aluminum with
ethyl bromide to give so-called ethylaluminum sesquibromide, an equimolar
mixture of diethylaluminum bromide
and ethylaluminum dibromide, was
known but the ethyl bromide starting
material was expensive. Whereas ethyl
chloride is cheap, either it does not react
under similar conditions or decomposition occurs. I finally discovered that the
reaction of aluminum with ethyl chloride can be catalyzed by the sesquibromide prepared in situ. The reaction then
proceeds so smoothly that in the end we
were able to use 100-litre vessels, since
we also had to show that such compounds that ignite in the air and explode
with water can be handled safely on this
a scale:
cat:
2 Al þ 3 C2 H5 Cl ƒ! ðC2 H5 Þ2 AlCl þ C2 H5 AlCl2
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The sesquichloride, which was later
to play a role in the development of the
Ziegler catalysts, had to be dehalogenated to obtain triethylaluminum. Metallic sodium was found to be the best
material for doing this: when pieces of
sodium metal are added to ethylaluminum sesquichloride at around 170 8C the
sodium takes up the chlorine, aluminum
is precipitated, and the triethylaluminum is formed:
ðC2 H5 Þ3 Al2 Cl3 þ 3 Na ! ðC2 H5 Þ3 Al
þ 3 NaCl þ Al
In 1952 we synthesized in the pilot
plant of our Institute our first 20 kg
batch of triethylaluminum—a material
that could hitherto be prepared only in
small quantities.
I have mentioned quite deliberately
that we synthesized the triethylaluminum on the kilogram scale, since this
leads me to comment on present day
conditions. Nowadays, if one were to
work in a research institute like ours
under conditions that were normal in
1952, the institute's director would be in
court faster than at the patent office.
The overregulation as regards safety
would without doubt hinder or presumably prevent progress in such work
altogether. I am not making a case here
for carelessness but against the exaggerated ordinances of bureaucrats.
On May 14, 1952, Karl Ziegler gave
his first comprehensive report on the
new work under the title “Novel Catalytic Conversion of Olefins”.[22] Just a
few days later, on May 19, he lectured
on “Organoaluminum Synthesis in the
Field of Olefinic Hydrocarbons” at the
Annual Meeting of the Gesellschaft
Deutscher Chemiker (GDCh, German
Chemical Society) at the Achema in
Frankfurt.[24]
The First Licenses are Granted
This second lecture in particular was
to have far reaching consequences, since
Giulio Natta and Sir Robert Robinson
were in the audience. They had close
connections with industry, Natta with
Montecatini in Italy and Sir Robert with
Petrochemicals in Manchester. Both
were convinced by Ziegler's presentaAngew. Chem. Int. Ed. 2003, 42, 5000 –5008
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Chemie
tion that what they had heard could be
of importance for the future development of these companies. The lecture
created general excitement, and an
invitation to give a lecture in Milan,
the contents of which had already been
published in Italian in La Chimica &
Industria in September 1952,[25] resulted
in negotiations soon being opened with
Montecatini for a license for the processes that had been presented. An initial
comprehensive license was granted in
January 1953 covering the new organoaluminum reactions and further developments in this field. The contract was a
major financial success for the Institute,
since the so-called “down payment” was
DM 600 000, an enormous sum in those
days.
In 1952, Karl Ziegler, who was an
enthusiastic climber, reached another
summit in the truest sense of the word—
the ascent and traverse of the Matterhorn starting from HNrnligrat in Switzerland and descent via the ridge on the
Italian side to Cervinia (Figure 3). He
was accompanied by his son Erhard and
me. Unfortunately, we were caught in a
sudden change of weather which forced
us to stay overnight in the Cabanne
Luigi Amadeo at an altitude of 3900 m.
On the summit there was no glorious
view, just a driving snow.
On the way back home through the
Aosta valley we made a short detour to
Champoluc, where the Natta family had
a large holiday house. We were given a
friendly welcome as their guests and
with a large circle of their friends
enjoyed a multilingual evening—Signora Natta was fluent in no fewer than four
languages.
On the basis of the license contract,
two young chemists, Paulo Chini and
Roberto Magri, and an engineer, Giovanni Crespi, came from Montecatini to
M lheim in early 1953 to gain experience in the field of organoaluminum
chemistry. Paulo Chini and Roberto
Magri came to me in my laboratory,
since I had the syntheses of aluminum
alkyl compounds and the dimerization
of ethylene at normal pressure running
continuously in small glass apparatus.
Giovanni Crespi went to the pilot plant,
where Dr. Zosel had a larger apparatus
for the continuous dimerization of propylene under pressure. The guests were
also highly interested in the experiments
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material for bio-degradable detergents.
This did not remain just an idea—a few
years later large-scale production plants
produced hundreds of thousands of tons
of primary alcohols on the basis of the
M lheim process. However, this process
really only got going after the elegant
direct syntheses of aluminum alkyl compounds, in particular that of triethylaluminum, had been developed.[27]
In the case of triethylaluminum the
synthesis has to be carried out in two
steps because otherwise the Aufbau
reaction takes place:
Figure 3. The mountaineer Karl Ziegler.
on the Aufbau reaction being carried
out by Dr. Gellert and Dipl.-Chem.
Erhard Holzkamp.
About the same time, contracts were
made with Farbwerke HNechst, and the
companies Petrochemicals in England
and Hercules Powder Co., USA. Dr. A.
Glasebrook from Hercules Powder
came as a guest to the laboratory of
Erhard Holzkamp. The purely organoaluminum syntheses reported in the
Frankfurt lecture were now the focus
of attention of industry.
The reaction of triethylaluminum
with ethylene under pressure, the Aufbau reaction, had at this time already
been studied systematically in long series of experiments. Depending upon the
conditions, higher aluminum alkyl species could be prepared. Since it was clear
that no high molecular weight polyethylene could be prepared this way, Karl
Ziegler had the splendid idea of using
this process industrially in a completely
different way. The chain growth must be
steered to give an average length of 6–8
ethylene molecules. When the aluminum alkyls were subsequently oxidized
in the air, the corresponding aluminum
alcoholates were obtained, which on
hydrolysis yielded primary alcohols.[26]
These compounds are an ideal starting
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The synthesis of triisobutylaluminum was achieved in a one-step reaction
because with this compound, olefin
chain-growth cannot occur.[27] Today,
many thousand tons of aluminum trialkyls are produced annually by direct
synthesis in large-scale industrial plants.
The Nickel Effect and Its
Consequences
As already described, a long series of
experiments had shown that the Aufbau
reaction could always be reproduced
exactly. However, early in 1953 something odd happened. Instead of obtaining the higher aluminum alkyls expected, Erhard Holzkamp observed in the
course of experiments for his doctoral
work that when ethylene reacted with
triethylaluminum under the usual conditions, a smooth dimerization of the
ethylene to butene occurred instead. He
was able to repeat the experiment time
and again using triethylaluminum from
the same charge. Understandably, what
was at first was regarded as an annoying
mishap caused considerable excitement.
Somewhat bad tempered, Karl Ziegler
came to me and said “Now we have
these large quantities of triethylaluminum that are useless. I warned right
from the beginning about using sodium
for the dehalogenation.” The real reason
for these problems was not found until
some weeks later. It was not the trie-
2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Essays
thylaluminum that was to blame but
traces of nickel compounds that had
been left behind after the V2A autoclaves had been cleaned. The mishap
soon proved to be a piece of luck,
because it marked the beginning of a
completely new chapter in catalysis.[28]
The method of dimerizing ethylene
that Holzkamp had discovered was
much more effective than the one at
normal pressures that I had developed.
The development of the new process,
particularly as regards its industrial
application, was much too large a topic
for a doctoral study, so Karl Ziegler put
me in charge of the further development
up to the stage of a continuously running
pilot plant. At that time the catalytic
dimerization of ethylene opened up a
new route to butene, which was not yet
available from refinery production.
However, first of all Holzkamp's experiments had to be reproduced and here
again unexpected difficulties arose: the
very first attempts to repeat the nickel
cocatalysis went wrong and instead of
continuously producing butene I was
obtaining longer chain products again!
After several further experiments it was
discovered that Holzkamp had used
technical-grade ethylene whereas I had
used ethylene purified with sodium
aluminum tetraethyl. Gas analyses revealed that the technical grade ethylene
contained about 0.5 % acetylene and we
discovered that these traces of acetylene
stabilized the nickel cocatalyst.[28] Thus
began our “organonickel chemistry”.[29]
The discovery of the “nickel effect”,
as it has become known in the chemical
literature,[28] appeared to be only the
beginning of a new wide-ranging line of
research that would far exceed the
bounds of a doctoral study. Nevertheless, as early as May 1953 Karl Ziegler
told Erhard Holzkamp to carry out
initial experiments with chromium compounds to see whether other metals had
a similar cocatalytic effect to nickel. At
first these experiments proceeded without a clear direction, since in addition to
olefins, small amounts of solid products
(which were possibly polyethylene)
were obtained. The systematic investigation did not begin until after the
summer holidays. Another diploma student, Heinz Breil, was brought in, as it
was clear that they would have to work
through the whole Periodic Table to see
5006
if there were any other metals that were
equally effective. Experiments using the
acetylacetonates of chromium, manganese, vanadium, and platinum did not
produce any spectacular results.
The revolution in the chemistry of
polymers began quite unexpectedly on
October 26, 1953 when Heinz Breil
carried out the experiment using zirconium acetylacetonate. In comparison
with the conditions required to produce
the high-pressure polyethylene known
at that time (300 8C, 1500–2000 bar), he
obtained under sensationally mild conditions (ca. 100 8C, 100 bar) a polyethylene that could be compressed into a
foil. One needs little fantasy to imagine
the excitement that went round the
Institute! The further development
could not rest on the shoulders of one
diploma student, Heinz Breil, alone.
For a long time Dr. Heinz Martin
had been trying to find conditions under
which the Aufbau reaction would, after
all, produce chains of 1000 ethylene
molecules or more, which could ultimately have led to the formation of a
plastic polyethylene. The latest results
had shown that influences, such as the
nickel effect, had to be excluded by all
means possible. Therefore, experiments
were in progress at that time using
aluminum-lined autoclaves. Now, however, it was important develop systematically the sensational results of Heinz
Breil as rapidly as possible so that they
could be patented. Understandably, this
assignment was given to Heinz Martin,
who had already been working on the
preparation of a polyethylene, although
from different direction. The situation
was similar to mine in the case of the
ethylene dimerization.
It should be mentioned at this point
that almost 40 years later Heinz Martin
was indeed able to prepare high molecular-weight polyethylene using triethylaluminum alone. At room temperature
triethylaluminum really does react with
ethylene under pressure, albeit extremely slowly, without the chain formation
being interrupted by an elimination
reaction to form an olefin. Thus, in
principle, the original aim had been
achieved. This observation is of scientific interest but, of course, of no value
industrially.[30]
Heinz Breil's first polymerization
experiments with titanium tetrachloride
2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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as the transition metal component in
combination with triethylaluminum
showed that titanium catalysts that are
considerably more efficient than zirconium. He observed a highly exothermic
reaction, a rapid fall in pressure in the
autoclave, and the only product was
high-molecular-weight
polyethylene.
With the help of further variation in
the catalyst, Heinz Martin finally achieved the so-called normal-pressure polymerization, which he was able to carry
out in a large glass for home preserves
modified for the purpose. This experiment was later demonstrated to numerous license holders and guests and it
never ceased to amaze them. It has even
been recorded on film. In a further
important development, it was discovered how the molecular weight of the
polyethylene being produced could be
varied at will.
As early as November 17, 1953 Karl
Ziegler submitted a patent application
to the German Patent Office that he had
prepared himself[31]—the Institute's patent lawyer was away on business at the
time. The main claim of the application
is:
“1. A method of preparing highmolecular polyethylene using aluminum
trialkyls as catalysts, characterized by
bringing together ethylene at pressures
greater than 10 atmospheres and temperatures above 50 8C with mixtures of
aluminum trialkyls and compounds of
the metals of groups IVa to VIa of the
Periodic Table with the atomic numbers
22 to 74 (namely titanium, zirconium,
hafnium, niobium, tantalum, chromium,
molybdenum, and tungsten).”
The clear limitation of the claim to
ethylene was to cause the Institute
considerable problems in later years,
but at that time Karl Ziegler claimed
only what had been demonstrated experimentally. Although Heinz Breil had
already carried out an experiment with
propylene, he had not been able to
isolate any polypropylene, a material
that was not yet known. Thus, Heinz
Martin was the first to show that the new
catalysts were also able to polymerize
propylene.[33]
The sensational news spread like
wildfire around the world when Karl
Ziegler first reported the low-pressure
polymerization of ethylene at the IUPAC Congress in Zurich on July 22,
Angew. Chem. Int. Ed. 2003, 42, 5000 –5008
Angewandte
Chemie
1955. Giulio Natta also announced at
the same conference the stereospecific
polymerization of olefins to isotactic
polymers with the aid of Ziegler catalysts without, however, naming them in
detail. Natta had had access to all the
relevant information through the license
contract between Ziegler and Monticatini, so that he was one of the first
outside the Institute in M lheim to be
able to work in the new field. On
September 14, 1955 Karl Ziegler held
the same lecture as in Z rich at the
Main Meeting of the Gesellschaft
Deutscher Chemiker in Munich.[32]
In 1954 Karl Ziegler was able to
grant further license contracts. It became clear what an advantage it was that
he was able to act as both director of the
Institute and at, the same time, as
managing director of the Studiengesellschaft Kohle m.b.H., which was the
trustee for the Institute. The licensees
got to know him as a versed business
man. Based on these first licenses, the
whole field also developed outside the
Institute with enormous speed, resulting
in numerous patent applications. From
the Institute came the homopolymerization of propylene and of butene,[33] a
clear proof that propylene too could be
polymerized. Without going into details
about where further processes were
developed, I will just list the milestones
of the further development. In the USA
it was found how isoprene could be
polymerized with Ziegler catalysts to
form either a natural rubber-like material (cis-1,4-polyisoprene) or guttapercha (trans-1,4-polyisoprene). With the
new catalyst butadiene formed either
cis- or trans-1,4-polybutadiene. I myself
found at the same time as Natta how to
produce 1,2-polybutadiene[34] and then,
completely unexpectedly, the cyclotrimerization of butadiene to give trans,trans,cis-1,5,9-cyclododecatriene,[35]
which opened up the way novel polyamides.
The business of licensing flourished.
Leaving the chronological report for a
moment, it should be mentioned that
between 1953 and 1990 no less than 80–
90 contracts were agreed, bringing the
Institute income that guaranteed its
complete financial independence for
more than 40 years and freeing the
Max Planck Society from having to fund
it. Thus the financial benefits continued
Angew. Chem. Int. Ed. 2003, 42, 5000 –5008
to flow for more than 20 years after Karl
Ziegler retired as director of the Institute. The licenses and income were one
side of the coin, the other was, however,
the numerous infringements of the Institute's patents, which led to numerous
lawsuits, particularly in the USA. One of
these cases ran for no less than 18 years
until finally a judgment was made in
favor of the Institute. As managing
director of the Studiengesellschaft
Kohle m. b. H., to which he was appointed in 1970, along with his role as a
scientist, Heinz Martin was engaged full
time in looking after the interests of the
Institute both in the granting of licenses
and in the defense of the patent rights.
As the major co-inventor of the polypropylene process together with Karl
Ziegler, he was ideally equipped both as
a scientist and through the experience
that he had gained as an insider in the
world of patents. In his book “Polymere
und Patente” published in 2002[36] he
gives a fascinating detailed account of
both sides of the coin. Particularly worth
reading is his account of the unique
situation regarding the classification of
the pioneering M lheim patents in the
USA, since this judgment provided
income to the Institute on the basis of
the 1954 basic patent until 1995. It is
estimated that today about 25–30 million tons of polypropylene, 10–12 million tons of low-pressure polyethylene,
and 15 000 tons of aluminum alkyl compounds are produced every year using
the M lheim processes.
After 30 years of basic research
came the great breakthrough in industrial chemistry, but the scientific success
was in no way less important. The
honors bestowed upon Karl Ziegler are
legion.[37] After 1953 excitement grew in
the Institute every year in the period
leading up to the annual announcement
of the new Nobel Prize winners, since
everyone was convinced that one day
Karl Ziegler would be awarded the
Nobel Prize. Outside the Institute others
were of the same opinion. Surprisingly,
he had to wait for ten years until the
Prize Committee honored him—surprising in as far as, according to the constitution of the Nobel Foundation, the
prize should be awarded to the person
that has done the most for humanity in
the previous year. There was great
elation and it was all the greater for
Hans-Georg Gellert, Heinz Martin, and
myself, and for our wives, as Karl
Ziegler invited us to accompany him
and his family to Stockholm (Figure 4).
What a difference it made to be present
at the festivities rather than just watching them on television!
This pinnacle of success cannot be
better documented than by the following sentences taken from the Nobel
Prize Committee's document giving
the grounds for awarding the Nobel
Prize to Karl Ziegler and Giulio Natta,
since they were made after a careful
analysis of the contributions of the
laureates.[38]
Figure 4. Picture from the Nobel Prize ceremony in Stockholm (1963) left King Gustav VI. Adolf,
right Karl Ziegler.
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5007
Essays
“However, Professor Ziegler has found
entirely new methods of polymerization.
Ziegler catalysts, now widely used, have
simplified and rationalized polymerization processes, and given us new and
better synthetic materials.
However, Professor Natta has found that
certain types of Ziegler catalysts lead to
stereoregular macromolecules, i.e., macromolecules with spatially uniform structure. In such chains, all the side chains
point to right or to left.”
This Essay is intended to keep alive
the memory of a development in
chemistry that is almost unique—a development that arose from basic research but the industrial success of
which around the world was a result of
recognizing its potential and systematically following it up. The quotations
from Karl Ziegler's publications are
intended to give our young chemists an
insight into the especially successful way
of thinking of this great research scientist. But above all, I hope that this
Highlight will remind everyone who has
to make political decisions on research
that truly novel developments can only
emerge from unfettered basic research.
[1] K. Ziegler, H. Breil, E. Holzkamp, H.
Martin, DBP No. 973 626, 1960.
[2] K. Ziegler, Angew. Chem. 1964, 76, 545 –
553.
[3] K. Ziegler, H. Eberle, H. Ohlinger,
Justus Liebigs Ann. Chem. 1933, 504,
94 – 130.
5008
[4] K. Ziegler, F. Thielmann, Ber. Dtsch.
Chem. Ges. 1923, 56, 1740 – 1745.
[5] K. Ziegler, Justus Liebigs Ann. Chem.
1923, 434, 34 – 78 (Habilitation thesis).
[6] K. Ziegler, B. Schnell, Justus Liebigs
Ann. Chem. 1924, 437, 227 – 255.
[7] K. Ziegler, K. BIhr, Ber. Dtsch. Chem.
Ges. 1928, 61, 253 – 263.
[8] K. Ziegler, F. CrNssmann, H. Kleiner, O.
SchIfer, Justus Liebigs Ann. Chem.
1929, 473, 1 – 35.
[9] K. Ziegler, H. Colonius, Justus Liebigs
Ann. Chem. 1930, 479, 135 – 149.
[10] K. Ziegler, F. Dersch, H. Wollthan,
Justus Liebigs Ann. Chem. 1934, 511,
13 – 44.
[11] K. Ziegler, L. Jakob, Justus Liebigs Ann.
Chem. 1934, 511, 45 – 63.
[12] K. Ziegler, L. Jakob, H. Wollthan, A.
Wenz, Justus Liebigs Ann. Chem. 1934,
511, 64 – 88.
[13] T. Midgley, Jr., A. L. Henne, J. Am.
Chem. Soc. 1929, 51, 1294.
[14] T. Wagner-Jauregg, Justus Liebigs Ann.
Chem. 1932, 496, 52 – 77.
[15] K. Ziegler, Chem.-Ztg. 1938, 62, 125 –
127.
[16] K. Ziegler, Handbuch der Katalyse, Vol.
VII, Springer, Heidelberg, 1943, pp.
106 – 135.
[17] K. Ziegler, H. Grimm, R. Willer, Justus
Liebigs Ann. Chem. 1939, 542, 90 – 122.
[18] “Das Kohlenforschungsinstitut in M lheim a. d. Ruhr zehn Jahre lang unter
neuer Leitung”: K. Ziegler, Aus der
Deutschen Forschung, Georg Thieme,
Stuttgart, 1956. See also G. Wilke, Justus
Liebigs Ann. Chem. 1975, 804 – 833.
[19] K. Ziegler, E. Eimers, W. Hechelhammer, H. Wilms, Justus Liebigs Ann.
Chem. 1950, 567, 43 – 96.
[20] K. Ziegler, H.-G. Gellert, Justus Liebigs
Ann. Chem. 1950, 567, 179 – 184.
[21] K. Ziegler, H.-G. Gellert, Justus Liebigs
Ann. Chem. 1950, 567, 195 – 203.
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[22] K. Ziegler, Brennst.-Chem. 1952, 33,
193 – 200.
[23] K. Ziegler, H.-G. Gellert, H. Martin, K.
Nagel, J. Schneider, Justus Liebigs Ann.
Chem. 1954, 589, 91 – 162. First paper in
the series “Metallorganische Verbindungen” (Organometallic Compounds)
XIX–XLII, which continued the series
“Alkaliorganische Verbindungen” (Organometallic Compounds of the Alkali
Metals) I–XVII.
[24] K. Ziegler, Angew. Chem. 1952, 64, 323 –
329.
[25] K. Ziegler, Chim. Ind. 1952, 34, 520 –
527.
[26] K. Ziegler, F. Krupp, K. Zosel, Angew.
Chem. 1955, 67, 425 – 426.
[27] K. Ziegler, H.-G. Gellert, H. Lehmkuhl,
W. Pfohl, K. Zosel, Justus Liebigs Ann.
Chem. 1960, 629, 1 – 13.
[28] K. Ziegler, H.-G. Gellert, E. Holzkamp,
G. Wilke, Brennst.-Chem. 1954, 35, 321 –
325.
[29] G. Wilke, Angew. Chem. 1988, 100, 189 –
211; Angew. Chem. Int. Ed. Engl. 1988,
27, 189 – 211.
[30] H. Martin, Makromol. Chem. 1992, 193,
1283 – 1288.
[31] Ref. [1].
[32] K. Ziegler, Angew. Chem. 1955, 67, 541 –
547.
[33] K. Ziegler, H. Breil, H. Martin, E.
Holzkamp, Deutsche Patentschrift
1 257 430.
[34] G. Wilke, Angew. Chem. 1956, 68, 306 –
307.
[35] G. Wilke, Angew. Chem. 1957, 69, 397 –
398.
[36] H. Martin, Polymere und Patente, WielyVCH, Weinheim, 2002.
[37] G. Wilke, Justus Liebigs Ann. Chem.
1975, 804 – 833.
[38] Nobel-Lectures, Chemistry, Elsevier
Publishing Company, Dordrecht, 1972.
Angew. Chem. Int. Ed. 2003, 42, 5000 –5008
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