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

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:

When this molecule is joined up with other similar molecules it creates polyethylene, a polymer.

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. Under certain pressures (1500-3000 atmospheric pressures), temperatures (100-300°c) and using peroxide catalysts. These are used as initiators. An Initiator is a substance that is added to the reaction in small quantities. They are decomposed by light or heat and produce free radicals (R’). A radical is formed when a covalent bond is broken and a bonding electron is left on each part of the broken molecule. Since the O-O covalent bond is weak, free radicals are easily formed from oxygen or peroxides. This radical then reacts easily with the products in the reaction and “initiates” the reaction. The radical reacts with an ethane molecule creating an ethane radical. This ethene radical then reacts with a normal ethene molecule to form a polyethene molecule. When two of these polyethene molecules react, a polyethene molecule is created.

Because ldpe has branches it makes it a lot less strong and therefore only used for plastic bags etc.

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 ...

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Because ldpe has branches it makes it a lot less strong and therefore only used for plastic bags etc.

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. But this experiment was left unsupervised over the weekend and when they came back to look at what had happened the following Monday, the found a white waxy solid covering its walls. Once analysed they found out that it was a polymer of ethene and found that the pressure in the vessel was much lower than expected. So this showed that they weren’t exactly looking to create a polymer of ethene. There was a risk though; this was the risk of explosions. Even so that the research of these polymers was abandoned due to explosion hazards in 1933, it was resumed though, in 1935. This time the vessel was reinforced and modified. They used temperatures of 300°c and 2000 atmospheres pressure. They continuously added ethane into the vessel until they ran out and it resulted in 8 grams of white powdery polymer. The experiment was plagued with explosions, but they learnt more about the control of the reaction as time went on. They figured out that the factor that affects the molecular mass of the polymer is the pressure in the vessel. 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.