Ethene, C2H4, is a small molecule with a C=C double bond. Small molecules that can join together into a long chain are called monomers. Ethene is the monomer used to make poly(ethene).
In the reaction, thousands of ethene molecules join one after another. In each molecule one of the two bonds in the C=C double bond opens up, and each carbon atom uses the freed bond to join to the next molecule. The double bond becomes a single bond in the chain. The result is one very long molecule called a polymer.
Only one product forms and nothing else is given off, so this is an addition polymerisation. The polymer is named poly followed by the name of the monomer in brackets, so the polymer made from ethene is poly(ethene). It can be written as n C2H4 becomes (-CH2-CH2-)n, where n is a very large number. For example, 100 ethene molecules make a chain with 200 carbon atoms and 400 hydrogen atoms.
