When ldpe forms, it is branched. This occurs when the growing chain attacks itself and is known as ‘back-biting.’
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CH2 CH2
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-CH2-CH2-CH2* + CH2 -CH2-CH2-CH3 + *CH
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CH2 CH2
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Attack by another radical can then take place at that site which causes the chain to grow out of that radical. This makes the polymer branched. This is shown in the diagram below.
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CH2 CH2
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-CH2-CH2-CH2* + *CH -CH2-CH2-CH2-CH
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CH2 CH2
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Finally the radical will react with another radical which causes the reaction to be terminated.
Therefore it is clear that the polymers can be branched or not branched, which leads to poly(ethene) and poly(propene) having different physical properties.
We have seen that if back-biting occurs during the polymerisation, the polymers formed will be branched, which leads to other properties.
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. Again this applies to poly(propene) also. The crystalline regions in hdpe- i.e. when the chains are very ordered and close together- also mean it is very strong.
Poly(propene) is however stronger than poly(ethene) as it is a longer chain, giving more surface area contact between the chains. Therefore more instantaneous- induced dipoles can form giving poly(propene) more strength.
As well as this the arrangement of the atoms can also lead to different physical properties- i.e. isotactic, atactic and syndiotactic polymers. This is shown in the diagram below.
Diagram taken from Salters Advanced Chemistry: Chemical Storylines PR3 page 98
The isotactic structure, which results from the methyl groups having the same orientation, means that the polymer chains can fit together in a regular order/ pattern. This means the chains are closer together which is why this type of polymer is strong. This same thing applies to syndiotactic polymers. The chains are said to be in a crystalline arrangement, which is shown in the diagram below.
The atactic structure, which results from the methyl groups being randomly arranged, means the chains are further spaced apart and not as well ordered/ regular pattern. As a result IMF between chains are weaker which causes this type of polymer to be soft and flexible. 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.