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Addition polymerisation is the assembly of simple monomer units to form a larger molecule called a polymer.

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Addition polymerisation is the assembly of simple monomer units to form a larger molecule called a polymer. These monomers join together in a long chain under the correct conditions of temperature and pressure to become a single polymer chain. The monomers in the chain may also contain cross-links, which alter the properties of the polymer molecules. Different temperature and pressure conditions can cause monomers to join together differently, resulting in different polymers even when the same monomers are used. [1] Low density poly(ethene) (ldpe) is an extensively branched polymer, made of ethene monomers, its low density a result of the branching. The formative polymerisation reaction is carried out under high pressure and temperature with radicals, these conditions being responsible for the branching of the chains. The growth of the ldpe polymer chains is due to a radical mechanism and consists of 3 stages. At the initiation stage, the radical reduces the C=C double bond in an ethene molecule to a single C-C bond and becomes itself part of the polymer chain. Now one of the CH2� molecules in the chain is the radical. ...read more.


[6] Poly(propene) is also a branched molecule. There are 3 forms - Isotactic, syndotactic and atactic. The differences in the structure are shown below in [Figure 1.]. Isotactic and syndostactic poly(propene) chains can be arranged to lie close together and the material formed is crystalline and strong. Because of it's regular structure, isotactic poly(propene) can be easily drawn into fibres which is not possible with the other forms of poly(propene). Poly(propene) can be manipulated to give side-chains which give the material various other properties. The regular arrangement of the side-chains in syndostactic poly(propene) for example give properties such as complete transparency, better impact strength than isotactic poly(propene) and a greater resistance to gamma rays making it more suitable for use with food and in the medical industry where gamma rays are encountered. [5] Atactic poly(propene) is similar to ldpe in that the way in which it is branched does not allow the chains to lie very closely together. This gives it a low density causing it to be soft with elastic properties. [4] If it weren't for serendipity, the processes to control polymerisation may not have been revealed anywhere near as soon as they were. ...read more.


[2] Ziegler-Natta catalysts could not give complete control over the process either as althought the catalysts regulated the construction of the polymer chains, the catalyst could be damaged or easily poisoned by the polymer, giving a different outcome than was desired, for example, side-chains. The same thing may happen if a second particle of the catalyst touches the forming polymer chain. [5] Metallocene catalysts did increase the control that chemists had over the reactions, but did not give them complete control. This is because at first, although chemists knew that these catalysts helped the polymerisation reaction, they did not know how, and so could not manipulate the form of poly(propene) produced. Once they knew how the metallocenes worked, they could use the 'jaws' of a specific shape to manipulate the chain. These gave chemists the ability to control the type of poly(propene) formed but only three forms of poly(propene) could be created. [5] [1] http://www.kcpc.usyd.edu.au/discovery/9.2.1/9.2.1_Addition.html [2] Article 1 - From Accident to Design: The process of poly(ethene) [3] Chemical Storylines (Salters Publishers), Second edition, p.95 [4] Chemistry: An integrated approach (Prentice Hall/ Pearson Education), no edition number available (first published 1997), p.381 [5] Article 2 - Shaping up: the story of poly(propene) [6] http://www.newi.ac.uk/buckleyc/materials.htm#Polymers%20and%20Plastics ...read more.

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