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. Low density Poly(ethene)
High density Poly(ethene) How structure determines their physical properties More elastic and bends easily Softens on warming Brittle at low temperatures Strength reduced by disorganised molecules Strong Easily moulded Not easily deformed by heat * Not easily damaged by heat Melts at high temperatures 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) (? H = -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. Total word count = 999 Bibliography a) Article 1, From accident to Design : the progress of poly(ethene). b) Article 2, Shaping up : the story of poly(propene). c) Principles of Organic Chemistry, by Peter R. S. Murray. – Published by Heinemann Educational Publishers in 1977.
d) Organic Pathways, Synthesis and Analysis, by Brian Chapman. – Published by Thomas Nelson and Sons Ltd in 1998. e) Salters Advanced Chemistry, Chemical Storylines (second edition) – Published by Heinemann Educational Publishers in 2000. f) The New Penguin Dictionary of Science, by M. J. Clugston 1998. g) www. wbateman. demon. co. uk h) www. earlham. edu/~chem/pages/polymers/#chemistry Rowena Griffin Page 1 of 3 Show preview only The above preview is unformatted text This student written piece of work is one of many that can be found in our GCSE Patterns of Behaviour section.