Alkenes also link together or polymerize into long chains -a reaction that is of great importance in plastics and rubbers. When ethylene polymerizes, the double bond opens up to join monomer units into a continuous, saturated hydrocarbon chain, as shown to the right.

Most plastics must have at least 1000 repeating monomer units in each chain before they begin to show familiar "plastic" properties.

Chain lengths usually are not uniform, but the range of lengths can be controlled by the conditions of polymerization. Polyethylene for use in laboratory ware has 5000 to 50,000 carbons per chain.

Changing the substituents on the polyethylene chains gives polymers with a variety of properties.

Polyvinyl chloride chains, which have as many as 25,000 units, are used for phonograph records and plastic pipe, and with the addition of a plasticiser, as artificial leather.