 The final type of reaction of alkenes we're going to look at is polymerization, just very briefly. From an industrial point of view this is a massively important reaction because we use so many plastics these days. The basic idea is this, a polymer is a very very long molecule that's made by joining smaller molecules together in a chain, much like a chain of paperclips. The smaller molecules are known as monomers. Mono means one and myrrh means part, one part. So when monomers are joined together they make a polymer, many parts. Your body contains many biopolymers, polymers made by biological processes like DNA and RNA and proteins. But the simplest example of polymerization is the formation of polyethene, also known as polyethylene, from the alkene called ethene. Here it's just drawn as symbols, each E represents an ethene molecule and they string together to make a great long chain. However if we draw things out in more detail here are the ethene monomers. The polymerization reaction occurs when a double bond, when the second bond in the double bond breaks in an ethene monomer and that allows each of the end carbons to form a new bond with an adjacent molecule and this continues on in a kind of chain reaction linking more and more molecules together. Note that the resulting long molecule has all single bonds, it is in fact a giant alkane. To save writing the whole thing out a polymer is often written like this with the repeating unit placed in brackets and a subscript telling you how many of these repeating units there are.