Polyethylene

The invention: An artificial polymer with strong insulating properties
and many other applications.
The people behind the invention:
Karl Ziegler (1898-1973), a German chemist
Giulio Natta (1903-1979), an Italian chemist
August Wilhelm von Hofmann (1818-1892), a German chemist
The Development of Synthetic Polymers
In 1841, August Hofmann completed his Ph.D. with Justus von
Liebig, a German chemist and founding father of organic chemistry.
One of Hofmann’s students,William Henry Perkin, discovered that
coal tars could be used to produce brilliant dyes. The German chemical
industry, under Hofmann’s leadership, soon took the lead in
this field, primarily because the discipline of organic chemistry was
much more developed in Germany than elsewhere.
The realities of the early twentieth century found the chemical
industry struggling to produce synthetic substitutes for natural
materials that were in short supply, particularly rubber. Rubber is
a natural polymer, a material composed of a long chain of small
molecules that are linked chemically. An early synthetic rubber,
neoprene, was one of many synthetic polymers (some others were
Bakelite, polyvinyl chloride, and polystyrene) developed in the
1920’s and 1930’s. Another polymer, polyethylene, was developed
in 1936 by Imperial Chemical Industries. Polyethylene was a
tough, waxy material that was produced at high temperature and
at pressures of about one thousand atmospheres. Its method of
production made the material expensive, but it was useful as an insulating
material.
WorldWar II and the material shortages associated with it brought
synthetic materials into the limelight. Many new uses for polymers
were discovered, and after the war they were in demand for the production
of a variety of consumer goods, although polyethylene was

still too expensive to be used widely.

Organometallics Provide the Key
Karl Ziegler, an organic chemist with an excellent international
reputation, spent most of his career in Germany. With his international
reputation and lack of political connections, he was a natural
candidate to take charge of the KaiserWilhelm Institute for Coal Research
(later renamed the Max Planck Institute) in 1943. Wise planners
saw him as a director who would be favored by the conquering
Allies. His appointment was a shrewd one, since he was allowed to
retain his position after World War II ended. Ziegler thus played a
key role in the resurgence of German chemical research after the war.
Before accepting the position at the Kaiser Wilhelm Institute,
Ziegler made it clear that he would take the job only if he could pursue
his own research interests in addition to conducting coal research.
The location of the institute in the Ruhr Valley meant that
abundant supplies of ethylene were available from the local coal industry,
so it is not surprising that Ziegler began experimenting with
that material.
Although Ziegler’s placement as head of the institute was an important
factor in his scientific breakthrough, his previous research
was no less significant. Ziegler devoted much time to the field of
organometallic compounds, which are compounds that contain a
metal atom that is bonded to one or more carbon atoms. Ziegler was
interested in organoaluminum compounds, which are compounds
that contain aluminum-carbon bonds.
Ziegler was also interested in polymerization reactions, which
involve the linking of thousands of smaller molecules into the single
long chain of a polymer. Several synthetic polymers were known,
but chemists could exert little control on the actual process. It was
impossible to regulate the length of the polymer chain, and the extent
of branching in the chain was unpredictable. It was as a result of
studying the effect of organoaluminum compounds on these chain
formation reactions that the key discovery was made.
Ziegler and his coworkers already knew that ethylene would react
with organoaluminum compounds to produce hydrocarbons,
which are compounds that contain only carbon and hydrogen and
that have varying chain lengths. Regulating the product chain length
continued to be a problem.
At this point, fate intervened in the form of a trace of nickel left in a
reactor from a previous experiment. The nickel caused the chain
lengthening to stop after two ethylene molecules had been linked.
Ziegler and his colleagues then tried to determine whether metals
other than nickel caused a similar effect with a longer polymeric
chain. Several metals were tested, and the most important finding
was that a trace of titanium chloride in the reactor caused the deposition
of large quantities of high-density polyethylene at low pressures.
Ziegler licensed the procedure, and within a year, Giulio Natta
had modified the catalysts to give high yields of polymers with
highly ordered side chains branching from the main chain. This
opened the door for the easy production of synthetic rubber. For
their discovery of Ziegler-Natta catalysts, Ziegler and Natta shared
the 1963 Nobel Prize in Chemistry.
Consequences
Ziegler’s process produced polyethylene that was much more
rigid than the material produced at high pressure. His product also
had a higher density and a higher softening temperature. Industrial
exploitation of the process was unusually rapid, and within ten years
more than twenty plants utilizing the process had been built throughout
Europe, producing more than 120,000 metric tons of polyethylene.
This rapid exploitation was one reason Ziegler and Natta were
awarded the Nobel Prize after such a relatively short time.
By the late 1980’s, total production stood at roughly 18 billion
pounds worldwide. Other polymeric materials, including polypropylene,
can be produced by similar means. The ready availability
and low cost of these versatile materials have radically transformed
the packaging industry. Polyethylene bottles are far lighter
than their glass counterparts; in addition, gases and liquids do not
diffuse into polyethylene very easily, and it does not break easily.
As a result, more and more products are bottled in containers
made of polyethylene or other polymers. Other novel materials
possessing properties unparalleled by any naturally occurring material
(Kevlar, for example, which is used to make bullet-resistant
vests) have also been an outgrowth of the availability of low-cost
polymeric materials.