Poly(ethene) a simple polymer with no double bonds and only contains carbon and hydrogen atoms. It is a linear polymer. A represents a monomer molecule.
∼A-A-A-A-A-A-A-A-A-A-A-A-A-A∼
Low density poly(ethene) (Ldpe) was discovered by Fawcett and Gibson in 1933, they reacted ethene with benzaldehyde at around 1500 atm hoping to make ketone. There was a leak in their apparatus which allowed a tiny quantity of oxygen to catalyst the radical polymerisation of ethene.
1400 atm
+ C=C
Benzaldehyde ethene Ketone
The oxygen was found to have entered the experiment with the benzaldehyde, which also contains benzoyl peroxide
Benzaldehyde benzoyl peroxide
O2 initiator
nCH2 =CH2 ~ CH2 - CH2 - CH2 - CH2 ~
1500 atm, 200°c Poly(ethene)
The chains that they produced, had a comparatively large number of branched methyl groups and wrapped around itself in a disorganised way taking up lots of room therefore having a low density.
The result was varied and often ended up in an explosion due to the exothermic decomposition of ethene, this resulted in Fawcett and Gibson being banned for doing this specific reaction for two years. The amount of oxygen in the reaction is important as if there is too little oxygen no polymerisation takes place and if there is too much present an explosion takes place. When they restarted their research they were able to control the reaction by adding cold ethene at the correct rate, this prevented the explosions.
C2H4 (g) 2C (s) + 2H2 (g)
(ΔH = -60 kJ mol -1 )
Poly(ethene) is tough, durable and a good electrical insulator. It has two main forms, low or high density and both of these have different characteristics. High-density Poly(ethene) (hdpe), has fewer branched chains and so can line up closer together and therefore take up less room, so has a higher density.
Low density poly(ethene) High density poly(ethene)
Plastics are substances that are not as springy as elastomers, which are soft and spring and easily reformed after deformation, but undergo permanent or plastic deformation. Fibres are strong polymers that don't deform easily and can be made into strong thin threads such as nylon and used for weaving.
Poly(ethene) when deforms it tends to stay out of shape and is therefore a plastic.
Linear low density poly(ethene) is a polymer where the molecules aren't as tightly packed together as hdpe making its density lower. There are short branches which allows sufficient crystalline regions for the material to withstand tearing force.
Poly(propene) is on the edge of the plastic-fibre boundary. It has two main forms which are described by the arrangement of the molecules in the polymer, these are isotactic and atactic
Later syndiotactic poly(propene) was discovered which is a polymer that has the methyl groups on alternate sides. This structure allows the chains to be relatively close. Compared with isotactic poly(propene) it has a better impact of strength and is more transparent and therefore more suitable for the insulation in optoelectronic devices.
Fawcett and Gibson were lucky in that there was a leak and that it was just the right size in their apparatus. Because of this the right amount of air could enter because otherwise the formation of poly(ethene) would never have been discovered. Karl Ziegler's finding of high density poly(ethene) (hdpe) was also by chance as he was investigating the reactions of organometallic reagents. One reaction was with ethene and (C2H5)3Al and this sometimes produced poly(ethene) but was more crystalline, this turned out to be hdpe. One experiment didn't produce any poly(propene) and so
Ziegler researched the reason and found that there were traces of nickel compounds in the reaction vessel. This lead to more investigations with other metal ions, some were found to inhibit the reaction but others didn't:
Ethene (aq) of TiCl4 + (C2H5 )3 Al
In liquid alkene
When ethene was passed into a solution of triethylaluminium and a small amount of titanium(IV) chloride or zirconium(IV) chloride in a liquid alkane at room temperature and pressure, the ethene readily polymerised.
This discovery also lead to Giulio Natta to experimenting in polymerising propene and formed two forms of poly(propene) Natta then worked on individual catalysts to try and be able to control which form of poly(propene) was formed.
Titanium chloride catalyst isotactic poly(propene)
Vanadium chloride catalyst syndiotactic poly(propene)
Metallocene catalysts are more specific catalysts than the Ziegler-Natta catalyst. Poly(ethene) and poly(propene) can be produced using a metallocene catalyst, this can be used as thin films with interesting properties such as; even more impermeable to air and moisture than other polymers and is very strong and tear-resistant.
Ferrocenes, the first large family of metallocene 'sandwich' compounds.
A zirconocene
Ferrocene A Zirconocene
Chemists did not have total control of the processes of polymerisation in the high pressure, high temperature polymerisation of ethene and propene because the conditions to produce these had to be very specific and exact and it was hard to repeat the results. The high temperature was easy to achieve but the pressure was harder, as in Article 1; "Perrin (who was supervising the experiment) had difficulty in reaching the specified pressure, suggesting there might be a leak in the vessel."
C2H4 (g) 2C (s) + 2H2 (g)
(ΔH = -60 kJ mol -1 )
The Ziegler-Natta- catalyst only works with small hydrocarbon monomers so only a limited range of polymers can be produced. The first metallocene catalyst was only able to produce one stereo form of poly(propene) and three different metallocenes had to be found so that the three different forms of poly(propene) could be produced.
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Bibliography
- Article 1, From accident to Design : the progress of poly(ethene).
- Article 2, Shaping up : the story of poly(propene).
- Principles of Organic Chemistry, by Peter R.S. Murray. - Published by Heinemann Educational Publishers in 1977.
- Organic Pathways, Synthesis and Analysis, by Brian Chapman. - Published by Thomas Nelson and Sons Ltd in 1998.
- Salters Advanced Chemistry, Chemical Storylines (second edition) - Published by Heinemann Educational Publishers in 2000.
- The New Penguin Dictionary of Science, by M.J. Clugston 1998.
- www.wbateman.demon.co.uk
- www.earlham.edu/~chem/pages/polymers/#chemistry
