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The Story of Poly(ethene) and Poly(propene).

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Introduction

The Story of Poly(ethene) and Poly(propene) The term "Addition Polymerisation" means when a reaction takes place where a polymer is created from its monomers without leaving other molecules behind. A monomer is a molecule that when joined up in long chains create a polymer. For example: the monomer of polyethylene is an ethylene molecule: CH2=CH2 When this molecule is joined up with other similar molecules it creates polyethylene, a polymer. CH2=CH2 + CH2=CH2 ? -CH2-CH2-CH2-CH2-CH2-CH2- ethene ethene polyethene piece When this reaction takes place nothing is lost as the monomers (i.e. ethene) join end-to-end forming no other products. The monomers double bond is opened to bond with other monomers. When Polyethene is produced there are two kinds, hdpe and ldpe. Ldpe is polymer molecules that contain branching. The carbons of an ldpe instead of having Hydrogen atoms they will have polyethene bonded to them. ...read more.

Middle

The structure of hdpe on the right of the diagram shows a much more compact structure making it a much more strong plastic. The same rule applies to polypropene. The fact that hdpe and hdpp are closely compacted together also means it can withstand higher temperatures. This can be shown simply by getting a plastic bag and a plastic chopping board. When you place the plastic bad over a source of heat you can see that is melts almost straightaway, yet the plastic chopping board doesn't. The complete control of the process of polymerisation was not something that was done overnight. It involved a lot of lucky chances in understanding how it really worked. On Friday, March 24 1933, the chemists Eric Fawcett and Reginald Gibson carried out an experiment. Ethene was heated with benzyldehyde at 1400 atmospheres in an attempt to prepare a ketone. ...read more.

Conclusion

The greater the pressure, the higher the average molecular mass. They also discovered that if the temperature within the vessel was too high, then explosions were more imminent. The solved this problem by the addition of cold ethene. Further on in the development of polyethene, the chemists Zielger and Holzkamp found that in one experiment, no polymer was produced. After thorough investigations, they found that the vessel contained small traces of Nickel from previous experiments. This made them question whether adding other metals to the reaction had the same or even opposite effects. They found that adding certain compounds (triethylaluminium catalyst) into the ethane made it polymerise more easily. The chemist Guilio Natta wanted to see whether the same methos would work for Propene. This worked well, it seemed that scientists had the knowledge to control the manufacture of polymers. However the control was not total. This was because if this catalyst were damaged in any way, the forming of chains would cease. ...read more.

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