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

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Introduction

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: Temperature About 200�C Pressure About 2000 atmospheres Initiator A small amount of oxygen as an impurity The next stage is propagation. ...read more.

Middle

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. ...read more.

Conclusion

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. Hayley Thompson 1 ...read more.

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