chemistry open book:formation of natural and synthetic rubber

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Formation of natural and synthetic rubber

Natural rubber is a polymer of the monomer isoprene, formed by addition polymerisation, one double bond in isoprene open to form a bond with the next isoprene monomer, so they create a large chain.

Isoprene polymers form cis isomers, this means that all the groups orientate in the same direction, producing crystalline alignment, which makes the polymer strong.

Synthetic rubber is similar to natural rubber, in that polymerisation is used to produce long chains. Initially when double bonded alkenes (dienes) were used the polymers were found to contain both cis and trans isomers (shown in the diagram).

Now we use butadiene, which forms poly (cis-1, 3-butadiene), which contains cis bonds.#

The best synthetic rubbers are produces from copolymers, which have been made by join together a diene and an additional alkene.

The polymer is made using emulsion polymerisation, the diene and alkene are polymerised in water. A detergent is added so the polymer spreads out to form a colloidal.

Another man made process to produce rubber is to polymerise an alkene, 2-methylpropene, which only contains one double bond. A saturated polymer is formed unlike the other processes where an unsaturated double bond is present, this leads to unique properties.

Figure 6 The monomer is different as it only contains one double bond, it means has good heat resistance, and air retention.

Structures and properties of natural and vulcanised rubber

The properties of a polymer are determined by

  • chain length-the longer the chain the stronger the polymer        #                        
  • branching- unbranched chains fit closer together so that there are more intermolecular forces holding chains together
  • flexibility of the chain- hydrocarbon chains are very flexible so can move around a lot, but more rigid chains are stronger
  • Cross linking- having atoms which cross links chains increases the bonds and therefore the strength of a polymer increases.
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Natural rubber, made from butadiene, contains a double bond, so the length of the polymer chain, continues to extend until the polymerising process is stopped. The long chains become entangled and the high amount of intermolecular forces makes the polymer stronger. #

Natural rubber does not contain any additional side groups, because it is made up solely of alkenes, this means the chains can hold together closer, so there is stronger intermolecular force between chains.

 Natural rubber is flexible, because it’s made of hydrocarbons this weakens the rubber but it can be stretch to improve strength.

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