The development of poly(ethene) and Poly(propene).

Hayley Thompson

The development of poly(ethene) and Poly(propene).

A polymer molecule is a long molecule made up from lots of small molecules called monomers. The properties of the polymer depend on the type of monomers and how they are connected together. Poly(ethene) and PVC are example of polymers, which contain the same monomers.

The reaction starts with the initiation stage. This involves the creation of a radical from the catalyst. In this case, it is donated from the oxygen; it is created by breaking the oxygen-oxygen bond. The catalyst in the polymerisation of ethene is benzoyl peroxide, which is organic peroxide. As an oxygen bond has been broken it leaves electrons free.

R = a group of particles

∙ = A free radical

R∙ + H2C === CH2 ➔ R – CH2 – CH2∙

There are certain conditions that need to be met for the reaction to be carried out efficiently. These are:

The next stage is propagation. This is when a free radical combines with one of the electrons from the carbon-carbon double bond in the alkane. The lack of the double bond leaves the other carbon with an unpaired electron making it a radical. An atom with an unpaired electron is very reactive. The new radical continues to react with alkenes and so the chain grows. The chain can continue to grow in this way at a great speed, which is why the reaction can be explosive.

R – CH2 – CH2∙ + H2C === CH2 ➔ R – CH2 - CH2 - CH2 – CH2∙

This chain can keep growing until the final stage termination. This is when the free radicals join together.

R – (CH2)n – CH2∙ + R∙ ➔ R – (CH2)n – CH2 + R

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R – CH2 – CH2∙ + H2C === CH2 ➔ R – CH2 - CH2 - CH2 – CH2∙

This chain can keep growing until the final stage termination. This is when the free radicals join together.

R – (CH2)n – CH2∙ + R∙ ➔ R – (CH2)n – CH2 + R

This mechanism forms poly(ethene) (LDPE) from the alkane ethene. The structure of poly(ethene) is large polymer chain, which is extensively branched. Branching occurs during propagation as the chains grow they can sometimes flail around and attack themselves by ‘backbiting’. This is when the radical end of the chain curls around and removes an H from a CH2 in the middle part of the chain, which moves the radical to the middle part, the chain then continues to grow from there. The intermolecular forces between them are smaller than those in a close-packed structure because branching makes the chains further away from each other than in unbranched chains. This results in LDPE having a low softening temperature and low tensile strength.

The structures of the different forms of poly(ethene) and poly(propene) give them different physical properties. The next major advance was when high-density poly(ethene) (HDPE). This is produced when ethene is polymerised by triethylaluminium. The ethane molecules insert themselves between the aluminium atom and the ethyl group. This results in very little branching so the chains line up side-by-side more closely. HDPE is crystalline which means it has stronger intermolecular forces and a high melting point.

In the 1990s a polymer called linear low-density poly(ethene) was produced. It has short branches, which was achieved by copolymerising ethene with small amounts of hex-1-ene. This polymer has a lower density than HDPE as the chains are not as closely packed together but the material can withstand tearing forces due to regions of it becoming sufficiently crystalline.

There are three different types of poly(propene), which are made by the polymerisation of propene using triethylaluminium as the catalyst. They are different depending on how the methyl groups are arranged. Isotactic poly(propene) is the most crystalline of the three as its methyl groups are on the same side of the carbon chain. The syndiotactic poly(propene) has its methyl groups alternating regularly from side to side. This means the carbon chains in these two can get close to each other, which gives the polymer a higher strength and rigidity. The atactic poly(propene) polymer is flexible and softer with a lower softening temperature. The chains cannot fit as closely together because the methyl groups are distributed on both sides of the chain irregularly. This polymer is used for roofing materials, sealants and waterproof coatings.

Serendipity has played an important part in improving the production of polyethene. It all started with the first experiments carried out by Fawcett and Gibson in 1933 in which they were trying to prepare a ketone from benzaldehyde and ethene. There was a leak in the vessel and oxygen was unintentionally allowed in. This reacted with the benzaldehyde to form benzyl peroxide, which initiated the polymerisation of ethene. Another example is when a student at the University of Hamburg, who was supervised by Dr Walter Kaminsky didn’t follow the correct instructions. He was working with Ziegler-natta catalysts. In earlier experiments Kaminsky had done the reactions were quite slow but when the student did it he produced a large amount of poly(ethene). The reason for this was that the Ziegler-natta catalysts are sensitive to oxygen. The student admitted that he didn’t flush the apparatus out with an inert gas so there was oxygen there for the reaction to happen. This was the accidental discovery of linear low-density poly(ethene). This meant that the reactions could be controlled a lot easier as they could be made at lower temperatures.

Chemists did not have total control over the polymerisation process when trying to polymerise ethane. This was because the polymerisation happened at such high temperatures and pressures that explosions in the reaction vessels were common through the exothermic decomposition of ethene. The use of zieglar-natta catalysts did not provide total control of the polymerisation reactions because if the catalyst becomes poisoned or damaged then the polymer chain stops growing altogether. Another catalyst metallocene causes a problem, as it is difficult to prepare. But at least this catalyst allows chemists to control the polymers molecular mass as well as it structure. ‘As time has gone by, control has become greater and the reliance on serendipity has now largely disappeared.’

Article 1 Chemical ideas

Article 2 Chemical storylines

Polymers like poly(ethene) are formed by addition polymerisation. This process involves three stages, initiation, propagation and termination. The ways in which the polymer chains are arranged determine the strength and melting point of the polymer, catalysts give more control to polymerisation. Serendipity has played an important part in the production of polymers.