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The Development of Poly(ethene) andPoly(propene).

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The Development of Poly(ethene) and Poly(propene) Addition polymerisation involves the joining together of monomers; usually compounds containing C=C double bonds such as alkenes, to form saturated long-chain polymers1[LE1][LE2]: The reaction occurs when catalyst such as the Zeigler-Natta catalyst, used to make high-density poly(ethene), is present. Another type of poly(ethene), low-density poly(ethene) was Discovered in 1933 by Eric Fawcett and Reginald Gibson. In their experiment, the polymerisation was free radical polymerisation. This type of polymerisation has three stages. First is initiation, started off by an initiator molecule. Here, this is benzoyl peroxide[LE3]: This molecule splits into two initiator fragments, each having an unpaired electron, meaning that these molecules are free radicals2. These, combined with the oxygen that leaked into Fawcett and Gibson's experiment, provided the free radicals required to catalyse the mechanism. Electrons in The C=C double bond are easily attacked by the free radicals, forming a bond between the initiator fragment and one of the C=C atoms. The remaining electron attaches to the other carbon atom. The process now starts again because a new free radical has been formed3[LE4]. The next stage is propagation, where the new radical reacts with another ethene monomer, as with the initiator fragment. This causes the addition of more monomers and the growth of a chain[LE5]: Termination, the last stage of the mechanism occurs when all the radicals are used up. ...read more.


Serendipity has played a large part in the development of polymers, but also influenced the amount of control chemists have over the polymerisation process. At first, scientists didn't have control of the high pressure, high temperature polymerisation due to this element of lucky chance. As time has gone by, control has become greater and the reliance on serendipity has largely disappeared. Explosions plagued the production of the first polymers because scientists didn't have this control. The risk of explosions increased if the temperature became too high, so using the constant addition of cold ethene they kept the reactant cold. In one of Zeigler's experiments, he found a trace of nickel left on the apparatus from a previous experiment had prevented a polymer from forming. The subsequent investigation led to the development of the Zeigler-Natta catalysts, which allowed increased, but not total control of the polymerisation process. Zeigler-Natta catalysts allow monomers to add in a regular way, but the chain stops growing if the catalyst becomes poisoned by the polymer. Alternatively, a second particle of catalyst might cause the chain to branch. This branching and variable chain length was a problem, depending on the required properties of the plastic. Zeigler-Natta catalysts only work with small hydrocarbon monomers, limiting the range of polymers that can be made with them. ...read more.


7, http://www.psrc.usm.edu/macrog/radical.htm 3 Free Radical Vinyl Polymerisation, page 2 of 7, http://www.psrc.usm.edu/macrog/radical.htm 4 The complete A-Z of Chemistry handbook, second edition (2000), Andrew Hunt, Hodder & Stoughton 5 PR2, The Polyethene Story, SAC, Chemical Storylines, George Burton et al, Heinemann, 2000. 6 Metallocene Catalysis Polymerisation, page 2 of 11, http://www.psrc.usm.edu/macrog/radical.htm [LE1] Insert picture of polymerisation of propene from CI page 109 [LE2] insert picture of repeating until of propene from CI page 109 [LE3] insert picture of benzoyl peroxide from article page 4 [LE4] insert picture of initiation from page 2 of free radical vinyl polymerisation and the 1st stage of box 1 on page 5 of the article [LE5] insert stage 2 of box 1 on page 5 of the article [LE6] insert 4 pictures from page 5 of the article into a mini flow diagram [LE7]inset pictures from article page 7. Figure 2, the three structural forms of poly(propene) [LE8] insert picture of cyclopentadiene/cyclopentadienide ions from top of page 2 of metallocene catalysis polymerisation. Annotate: 'the cyclopentadiene acts as an acid when forming the aromatic cyclopentadienide ion'. Underneath that, inert picture from the bottom of the same page showing the sandwich formation. Loose the top annotations of both pictures. [LE9] Inset pictures of the isotactic and atactic poly(propene) [LE10] insert picture of the syndiotactic form of poly(propene) 1 ...read more.

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