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Addition polymerisation is the process in which the same small molecules called monomers react together to form one long molecule, known as a polymer.

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Addition polymerisation is the process in which the same small molecules called monomers react together to form one long molecule, known as a polymer. This can be summarised by a general equation: ....A+A+A+A....�....-A-A-A-A-... An addition polymerisation reaction usually takes place at a high temperature and pressure and under the presence of a catalyst. These variables control the properties of the polymer. An example of an addition polymerisation reaction is that of ethene to form poly(ethene). The temperature and pressure control the level of side branching in the polymer, which greatly affect its properties. Ldpe is made at a high temperature and pressure, which increases side branching. The polymerisation of ethene can be separated into three stages: initiation, propagation and termination. Stage 1: Initiation R� + H2C=CH2 � R-CH2-CH2� This is the start of the reaction. R� represents a radical; it reacts with an ethene molecule to begin the chain. Stage 2: Propagation R-CH2-CH2� + H2C=CH2 � R-CH2-CH2-CH2-CH2� In this stage the chain grows by the addition of more monomer units of ethene to the chain. The chain always remains a radical during propagation so that it can grow. This stage determines the properties of the poly(ethene). For ldpe, the temperature and pressure of polymerisation are increased to create branching of the chain. ...read more.


The chains do not pack as regularly as in hdpe, so it has a lower density, but the presence of short chains allows for sufficient crystalline regions for the polymer to withstand tearing forces. Lldpe has properties somewhere between ldpe and hdpe. Poly(propene) can exist in three different forms, isotactic, syndiotactic and atactic, each with different physical and molecular properties. In isotactic poly(propene) the orientation of the methyl groups is always on the same side of the chain. This results in a more crystalline structure. In syndiotactic poly(propene) the methyl groups alternate regularly along the chain. This is less crystalline than isotactic, but more crystalline than atactic. In atactic poly(propene) the methyl groups are randomly distributed on both sides of the chain. This has the most amorphous structure. Due to the regular arrangement of methyl groups in isotactic and syndiotactic the chains can pack closer together than the atactic form. This results in them having stronger intermolecular forces and these materials are stronger and more rigid. As the chains can pack closest in isotactic, then syndiotactic, then atactic, isotactic has the highest density, followed by syndiotactic, then atactic. The same is true for strength and boiling point. Today chemists have a great degree of control over polymerisation reactions. However, this was not always the case. ...read more.


This fortunate break through paved the way for more investigation and a new catalyst was discovered methyl alumoxane, MAO. This reacts with the metallocene to produce a catalyst suitable for large-scale polymerisation. This new catalyst was then tried in the polymerisation of propene. It was found that by changing the molecular structure of this catalyst it could affect the structure of the poly(propene) formed. So from a "lazy" research student came isotactic, atactic and syndiotactic poly(propene). Serendipity played an important role into the level of control we have over polymerisation today. The first polymerisation reaction of ethene could not be totally controlled by chemist due to the fact work was at a purely experimental stage. The chemists were not sure what temperature and pressure they reaction should take place at so explosions occurred and the poly(ethene) produced would vary in quantity and physical and molecular properties. Explosions would occur due to the exothermic deposites of ethene. At this stage chemists could not control the amount of branching that the poly(ethene) had because the reaction had to be carried out at high temperatures and pressures. This led to the poly(ethene) all being of ldpe. When polymerising propene the chemists were not in complete control as the catalyst could become poisoned or damaged so that the chain would stop growing. Also, a secondary catalyst particle could result in a side-branch growing from the chain. ...read more.

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