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What are polymers and how do they form?

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Chemistry Open Book Exam Firstly a polymer is a long chained molecule which is made up of many hundreds or thousands on monomer units. Addition polymerisation is the joining of these monomers to produce the polymer molecule. In most cases there are C=C double bonds, which are found in alkenes. These double bonds are then broken in order to form a long chain of monomer units. The mechanism of polymerisation of ethene to form low density polyethene requires a catalyst, which can be dioxygen or an organic peroxide. The reaction involves radicals and can be shown in three steps. The first step is the initiation which involves the creation of the radical: R* + H2C=CH2 R-CH2-CH2* Where R is the alkyl group From the Peroxide and * Indicates the Radical The radical formed now reacts with another ethene molecule, during the propagation reaction, which then happens again and again to form a long chain polymer R-CH2-CH2* + H2C=CH2 R-CH2-CH2-CH2-CH2* R-CH2-CH2-CH2-CH2* + H2C=CH2 R-CH2-CH2-CH2-CH2-CH2-CH2* etc During this reaction the radical combines with one of the electrons from the double bond of the ethene. ...read more.


LDPE has branched chains which arises from back-biting. As a result of the branched chains, the chains can not fit very tightly and compactly together, which causes it to take up more space and therefore to decrease in density. For this reason it is called ldpe. There are, however, other properties which arise from the branched chains being spaced apart. Any IMF that form are very weak as the chains are spaced apart from one another. The dipole formed (instantaneous dipole- induced dipole) is also very weak. This results in the polymer being quite weak and is the reason polyethene can be remoulded and the reason for it being weak. This also applies to poly(propene). HDPE on the other hand, when manufactured, does not have branches. This means the chains can fit very tightly together and therefore it takes up a smaller area. This means it is higher in density. Due to the chains being closer the IMF are stronger and so give the polymer a high tensile strength and melting and boiling point. ...read more.


The chains are said to be in an amorphous arrangement, which is also shown in the diagram below. Diagram taken from Salters Advanced Chemistry: Chemical Ideas 5.5 Page 113 Luck has played a large role in giving chemists more control over the polymerisation process. The development of hdpe, by Karl Ziegler, resulted from accidental impurity of nickel compounds in his apparatus. This in turn led him to investigate the effects of adding other metal ions in the reaction. It is therefore by chance that he worked out how to control the process of producing unbranched polymers. As well as this it was by chance that the rate of reaction of polymerisation was increased. A student who was working with a Zeigler-Natta catalyst was not expecting much to happen in the polymerisation process, and to his surprise found a reasonably large yield of poly(ethene). It was later discovered that this was due to the student's laziness of not carrying out the reaction properly. It was discovered that water in the air sped up the reaction by about one million times. It is therefore clear that luck had a large role in allowing chemists more control over the reaction. ...read more.

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