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Your NCERT hydrocarbons chapter says polythene is made from ethene "at high temperature, high pressure and in the presence of a catalyst". That was the only way until late 1953. This post tells how Karl Ziegler made polyethylene at ordinary pressure, how Giulio Natta built on it, works through some numbers from their Nobel lectures, and ends with what you can learn from how they worked.
Who were Ziegler and Natta?
Karl Ziegler, according to his Nobel Prize facts page, was born on 26 November 1898 in Helsa, Germany, and died on 12 August 1973 in Mülheim. He worked at the Max-Planck-Institut für Kohlenforschung (coal research) in Mülheim. Giulio Natta, according to his facts page, was born on 26 February 1903 in Imperia, Italy, died on 2 May 1979 in Bergamo, and worked at the Polytechnic in Milan. They shared the 1963 Nobel Prize in Chemistry "for their discoveries in the field of the chemistry and technology of high polymers".
How did a trace of nickel lead to a new catalyst?
Ziegler's Nobel lecture explains that his group had been adding ethylene to aluminium triethyl in a "growth reaction". To get true polyethylene they needed about 1,000 ethylene units on each chain, but they kept getting only waxy products. Then one experiment gave mainly butene instead. They traced it to "a tiny trace of metallic nickel".
That accident suggested other metals might change the reaction too. In the middle of 1953 the group began testing substances with effects similar to nickel's. Near the end of 1953, with Holzkamp, Breil and Martin, Ziegler found that ethylene polymerised very rapidly with certain easy-to-make catalysts at 100, 20 and 5 atmospheres, and "finally, even at normal pressure". The catalyst in his demonstration was aluminium triethyl or diethyl aluminium chloride mixed with titanium tetrachloride.
The comparison in the lecture is striking. The polyethylene of Imperial Chemical Industries was made at 1,000 to 2,000 atmospheres and 200 °C. Ziegler's low-pressure polyethylene was denser, more rigid and more heat-resistant, because its chains were joined "linearly, without interruption", while the high-pressure product was strongly branched. The group published the process in Angewandte Chemie in 1955.
What did Natta add?
Natta's Nobel lecture says that early in 1954 his group polymerised propylene and other alkenes with Ziegler-type catalysts (TiCl₄ with aluminium alkyls). The crude product was a mixture: some fractions amorphous, some crystalline. The crystalline fractions had molecular weights only a little higher than the amorphous ones, so he concluded the difference was not size but the steric structure of the chains.
In propylene's polymer every second carbon carries a CH₃ group. X-ray analysis showed that in the crystalline polymer these groups all had the same configuration along the chain. Natta coined Greek-based terms for the patterns: isotactic (all the same), syndiotactic (alternating) and atactic (random). His group reported the work in the Journal of the American Chemical Society in 1955. Natta's Nobel biography notes that Montecatini began producing isotactic polypropylene on an industrial scale in 1957.
Worked example: numbers from the lectures
Chain mass. Ziegler needed about 1,000 ethylene units per chain. Ethylene, C₂H₄, has molar mass 2 × 12.01 + 4 × 1.008 = 28.05 g/mol. So a chain of 1,000 units has a molar mass of about 1,000 × 28.05 = 28,050 g/mol, roughly 28,000.
Growth of production. Ziegler gives 200 tonnes of low-pressure polyethylene in 1955 and about 120,000 tonnes in 1962. That is 120,000 / 200 = 600 times as much in seven years.
Helix repeat. Natta found a repeat distance of about 6.5 Å along the chain axis, containing three monomer units: 6.5 / 3 ≈ 2.2 Å per unit along the axis.
Is this on the JEE or NEET syllabus?
As our post on addition and condensation polymers explains, the current NCERT Class 12 book has no polymers unit, and JEE Main and NEET-UG mention polymerisation only as a reaction of alkenes. The JEE Advanced brochure lists a Polymers topic. The NCERT Class 11 hydrocarbons chapter is where polymerisation of ethene appears for everyone. Check the current year's syllabus for your exam.
If isotactic and atactic chains are hard to picture, the Fermi Simulation Lab has interactive simulations for ideas that are easier to see than read.
Common mistakes students make here
Thinking more pressure always means better polymer. The low-pressure product was the stiffer, denser one. Chain structure mattered more than conditions.
Treating isotactic as a different compound formula. Isotactic and atactic polypropylene have the same repeat unit; only the arrangement of CH₃ groups differs.
What can a student learn from how they worked?
Ziegler did not throw away the failed butene experiment; he asked what caused it, and the answer became a catalyst family. His lecture also says that being made to chase a set goal at the coal institute would have "completely dried up the springs of my creative activity". Natta noticed that two fractions of similar size behaved differently and refused to accept the easy explanation. When a result looks wrong, treat it as data first.
Today, draw four repeat units of polypropylene in isotactic and atactic forms, with every CH₃ group marked.
