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Addition polymerisation is the reaction that forms many of the polymers.

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Introduction

Open Book Paper Addition polymerisation is the reaction that forms many of the polymers (a long molecule made up from lots of small molecules called monomers). Usually there is a C=C double bond as there is in alkenes. Addition polymers were found by accident and are know as A-A polymers because all the monomer molecules are the same. Polymerisation is a three-stage process. Below shows the polymerisation of ethane to low density poly(ethene) (ldpe) Stage 1 : INITIATION This is when there is a creation of a radical from a catalyst Stage 2 : PROPAGATION A radical now combines with one of the electrons from the double bond in the alkene: New radical then reacts with more alkene: The chain grows longer and eventually ends up attacking itself, back-biting. ...read more.

Middle

These different properties are due to the fact that the structures of the poly(ethene)s are different. During one experiment (conducted by Karl Ziegler) to observe polymerisation there was nickel left in the reaction vessel that was not know about. The product was not a polymer but when the cause was discovered it lead to investigations about what would happen if different metals were added. They found that when ethene was passed in to a solution of triethylaluminium and a tiny amount of titanium(IV) the ethene readily polymerised. This polymer had different properties to the one first discovered. Its relative molecular mass, density, softening temperature and tensile strength were higher. Its wear properties were also more superior. ...read more.

Conclusion

The two forms that he discovered differ in the ways that the methyl groups are arranged. One crystalline, named isotactic as the methyl groups are all on one side of the carbon chain and the other non-crystalline named atactic as the methyl group is irregularly distributed on both sides of the chain. Another form is also now been discovered. This is called syndiotactic, which is also crystalline and has the methyl groups in a regular alternate pattern of each side of the carbon chain. Isotactic Crystalline Atactic Non- Crystalline Syndiotactic Crystalline I both the crystalline forms the carbon chains can fit close together which results in it having stronger intermolecular forces so therefore is a more stronger and rigid material. In the atactic form the methyl groups are randomly distributed so the chains cannot fit that closely together. This polymer is softer then the other two varieties and more flexible. ...read more.

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