Polymerization or polymerisation is the process in which small molecules, called monomers, join chemically to produce a very large chain-like or network molecules, called polymers. The monomer molecules may all be alike, or they may represent two, three, or more different compounds. Usually at least 100 monomer molecules must be combined to make a product that has certain unique physical properties—such as elasticity, high tensile strength, or the ability to form fibres—that differentiate polymers from substances composed of smaller and simpler molecules. Often, many thousands of monomer units are incorporated in a single molecule of a polymer. The formation of stable covalent chemical bonds between the monomers sets polymerization apart from other processes, such as crystallization, in which large numbers of molecules aggregate under the influence of weak intermolecular forces. Alkene molecules can react with themselves, by adding polymerization to form 'plastic' or polymeric materials. When a catalyst (a substance that speeds up chemical processes) is added to the reaction, or it is heated under pressure, unsaturated alkenes link together, when the double bond partially breaks.
Polyethene is a cheap and useful plastic used for plastic bags and bottles, it is also very flexible and quite strong.