CIEC Catalysis -- Polyethene

discovery and development of Ziegler catalyst

Discovery and Development of Ziegler Catalyst

One catalyst by accident leads to another

The discovery of a catalyst came after years of research into organometallic chemistry. By 1930 low molecular mass poly(ethene) could be made on lithium alkyls.

In 1946 Ziegler restarted work on poly(ethene). He found that adding ether gave better results and so tried the ether-soluble LiAlH₄. Unexpectedly the Al atom was better at polymerisation than Li and so he switched to triethyl aluminium.

As a reducing agent in organic chemistry

Question

You may have come across LiAlH₄ before now. What was it being used for?

Question

What is the angle between the growing chains on the Al atom, and why might this favour the process?

120^o The chains would spread out as widely separated as possible

Sadly growth stops when chains reach masses of 3000 to 30000, giving soft waxes rather than useful solids.

One day a strange thing happened. Instead of wax the reaction vessel contained butene. Chains had terminated very quickly. But why?

Eventually, an Al-H bond forms, displacing the growing chain.

But why? Ziegler reasoned that something had got into the vessel which catalysed the termination. He found traces of nickel from previous hydrogenations. So transition metal salts could affect the polymerisation.

He tested a whole series of transition metal salts with triethylaluminium. Some behaved like nickel, but zirconium and titanium did not. They gave a solid poly(ethene) with high molecular mass. At last a catalyst had been found.

Answer

Either of the atoms coloured red

Question

Identify a hydrogen atom, which could have moved to cause the chain to terminate?

Today's commercial plants using Ziegler catalysts operate at between 1 and 50 atmospheres and 40-90^oC, much milder conditions than the original ICI process.

Answer

Cheap, easy to produce, make the reaction occur at lower pressure and lower temperature

Question

If you had been told that someone had discovered a good catalyst for polymerising ethene what would you expect or hope that it would be like?

Ziegler

Ziegler and Natta shared
the Nobel Prize for
chemistry in 1963

The most active catalyst was developed for large-scale production. Ziegler concentrated on poly(ethene), but was in contact with Natta who worked on poly(propene).

Over time a very large number of Ziegler- Natta catalysts have been developed for specific polymerisations.