Polymerization is a chemical process in which small molecules called monomers react with each other to form long-chain polymers or three-dimensional networks. The process of polymerization is used to create polymers, which are then processed to make various kinds of plastic products. Polymerization reactions can occur via a variety of mechanisms that vary in complexity due to the functional groups present in the reactants and their inherent steric effects. There are two general types of polymerization reactions: addition polymerization and condensation polymerization.

• Addition Polymerization: In this type of polymerization, the monomers add to one another in such a way that the polymer contains all the atoms of the starting monomers. Ethylene molecules are joined together in long chains/27%3A_Reactions_of_Organic_Compounds/27.08%3A_Polymers_and_Polymerization_Reactions).

• Condensation Polymerization: In this type of polymerization, each step of the process is accompanied by the formation of a molecule of some simple compound, often water. Monomers react to form a polymer without the formation of by-products. Addition polymerizations usually are carried out in the presence of catalysts, which in certain cases exert control over structural details that have important effects on the properties of the polymer.

Polymerization reactions are used to create a wide range of materials, including natural materials such as proteins, cellulose, and starch, as well as artificial fibers, films, plastics, semisolid resins, and rubbers/27%3A_Reactions_of_Organic_Compounds/27.08%3A_Polymers_and_Polymerization_Reactions).