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Open Book Paper - Polymers

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Open Book Paper A polymer is a made up of a chain of smaller molecules called monomers. Polymers melt over a range of temperature because it is a mixture of smaller molecules and not a pure compound, a different number of monomers can join onto the chain before the polymerisation stops. Addition polymerisation is when unsaturated monomers are encouraged to form a more stable, singly bonded structure. It is a chain reaction and the process occurs in a number of steps; initiation, propagation and termination. Polymerisation may be homolytic or heterolytic. INITIATION: R O O R R O� + �O R PROPAGATION: R O� C=C R O C C� R O C C� C=C R O C C C C� e.t.c... This is a chain reaction and takes place very rapidly, up to 10000 monomers per second. Termination: The reaction stops when all radicals are used up, e.g. R O (CH2)n CH2� �CH2 (CH2)m O R R O (CH2)n CH2 CH2 (CH2)m O R Poly(ethene) behaves like an alkane of high relative molecular mass, it burns well, and tends not to react with other alkalis or acids but there is a difference; poly(ethene) ...read more.


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. ...read more.


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. ...read more.

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