Polymerisation and Cracking

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Polymerisation

Polymerisation is when you join together lots of little monomers to form a long chain, which is called a polymer. Polymerisation can only be done with alkenes. This is because alkenes are unsaturated, meaning they have at least one c=c. When they are 'added' together, the double bond opens up and joins together the many monomers. However, polymerisation doesn't just happen at room temperature and pressure, and it the conditions depend on what alkene you are polymerising. For the polymerisation of ethane, you require a temperature of 200oC, a pressure of 2000 atmospheres. Each polymerisation reaction also requires something to start the process, called an initiator. It is like a catalyst, except it gets used up in the reaction. The chain length of the polymer varies from about 2000 to 40,000 carbon atoms.

Example process of polymerisation:

The top layer shows the monomers of ethane. This consists of two carbon atoms and four hydrogen atoms (C2H4). The carbon atoms and the hydrogen atoms in a hydrocarbon are bonded covalently, so each bond (a line on the diagram) consists of 2 electrons.

You can see how the second line shows the two electrons separated, i.e. a