Different structures of poly(ethene) and poly(propene) give the respective polymers different physical properties. The physical characteristics of ldpe are a result of the extensively branched structure of the polymer chains. Ldpe has a low density, low melting point and low tensile strength because the chains on the polymer mean that the polymer chains cannot lie close together. As a result, the intermolecular forces between the chains are weak giving properties as those described above. [2]
In hdpe (high density poly(ethene)) the chains lie much closer together as the chain is not branched but is said to be crystalline. The intermolecular forces are stronger and it is therefore tougher and more durable. This gives the polymer a higher density and greater strength. It has a higher melting point and can therefore be used to make containers for hot water such as water tanks and piping. [3] Its strength and rigidity are the reason hdpe has become a major structural material, as well as being used to make replacement knee and hip joints. [5] Because of its density, it is optically dense, unlike ldpe, which is transparent and appears a whitish colour. Another property of this polymer is its behaviour when heated. It may have a high melting point but its pattern of transition from a solid to liquid is not a sharp change – the solid melts to become the consistency of treacle rather than a runny liquid. [6]
Poly(propene) is also a branched molecule. There are 3 forms – Isotactic, syndotactic and atactic. The differences in the structure are shown below in [Figure 1.].
Isotactic and syndostactic poly(propene) chains can be arranged to lie close together and the material formed is crystalline and strong. Because of it’s regular structure, isotactic poly(propene) can be easily drawn into fibres which is not possible with the other forms of poly(propene).
Poly(propene) can be manipulated to give side-chains which give the material various other properties. The regular arrangement of the side-chains in syndostactic poly(propene) for example give properties such as complete transparency, better impact strength than isotactic poly(propene) and a greater resistance to gamma rays making it more suitable for use with food and in the medical industry where gamma rays are encountered. [5]
Atactic poly(propene) is similar to ldpe in that the way in which it is branched does not allow the chains to lie very closely together. This gives it a low density causing it to be soft with elastic properties. [4]
If it weren’t for serendipity, the processes to control polymerisation may not have been revealed anywhere near as soon as they were. ICI first produced a poly(ethene) polymer by chance – they were originally trying to produce a ketone. In an experiment to try and replicate this polymer, modifications to the method of production had to be made, as the required pressure was not obtained. This again produced an unexpected result, which lead scientists to experiment more with polymerisation reactions. This experiment also showed, by chance, the use of radicals in these polymerisation reactions. Another discovery at the Max Planck Institute made whilst investigating organomettalic reagents showed that pressure could be altered to control the branching of the polymer chains, this discovery being made by chance and again, not whilst investigating polymers. [2]
A mistake in a research experiment to produce poly(ethene) gave rise to the discovery of the influence of a modified Ziegler-Natta catalyst when, in this case erroneously, used alongside water. From this it was found that the rate of polymerisation could be sped up by around a million times. This showed chemists the way forward in controlling polymerisation reactions and the investigation of the effect of various metallocenes on polymerisation reactions involving ethene and then propene commenced. [5]
During the high-pressure, high-temperature polymerisation of ethene and propene, chemists did not have a lot of control over the reactions taking place. This was because at the high temperatures involved, ethene tended to explode, ruining the experiments. Also, as this polymerisation reaction was only in its investigative stages, little was known about the specific temperature and pressure conditions required for the reaction to occur successfully so it could not therefore be controlled very well. Different conditions gave different resulkts with regards to branching and structure, but this was again only in the preliminary stages of discovery so could not be manipulated. [2]
Ziegler-Natta catalysts could not give complete control over the process either as althought the catalysts regulated the construction of the polymer chains, the catalyst could be damaged or easily poisoned by the polymer, giving a different outcome than was desired, for example, side-chains. The same thing may happen if a second particle of the catalyst touches the forming polymer chain. [5]
Metallocene catalysts did increase the control that chemists had over the reactions, but did not give them complete control. This is because at first, although chemists knew that these catalysts helped the polymerisation reaction, they did not know how, and so could not manipulate the form of poly(propene) produced. Once they knew how the metallocenes worked, they could use the 'jaws' of a specific shape to manipulate the chain. These gave chemists the ability to control the type of poly(propene) formed but only three forms of poly(propene) could be created. [5]
[1]
[2] Article 1 – From Accident to Design: The process of poly(ethene)
[3] Chemical Storylines (Salters Publishers), Second edition, p.95
[4] Chemistry: An integrated approach (Prentice Hall/ Pearson Education), no edition number available (first published 1997), p.381
[5] Article 2 – Shaping up: the story of poly(propene)
[6]