Polymers are formed through chemical reactions known as polymerizations. A majority of polymers are produced through two basic reaction types. The first type of polymerization reaction is known as a condensation polymerization or step-growth polymerizations. The second type of reaction is known as chain-growth or Addition polymerization. In Condensation polymerizations, when two monomers react to yield a repeat unit and a smaller molecule such as water. An example: Polymerization of nylon from monomers with carboxylic acids and basic amines. This reaction shows a chain of linkage between each monomer and produces H2O as a by-product. It is also used in clothing to produce nylon fibres for clothing. Addition polymerization occurs when a monomer forms a highly reactive free radical, or molecule with an unpaired electron. The free radical reacts quickly with another monomer and causes a repeat unit with another free radical. A rapid chain reaction continues the polymer chain and the polymerization to grow longer. One example of a polymer made through a chain-growth polymerization is polystyrene and it is used in disposable drinking cups. Chain growth polymerization divided into cationic addition polymerization and anionic addition polymerization. A special case of chain-growth polymerization leads to living polymerization. Most of the photopolymerization reactions and Ring open polymerization are chain-growth polymerizations reactions. Other polymerization reactions include emulsion polymerization, Dispersion, suspension and Plasma polymerization etc. Copolymerization is a process in which a mixture of more than one or different monomeric species are allowed to polymerise and form a copolymer. The copolymer is defined as a polymer obtained by linking two or more different types of monomers in the same chain. Copolymers are classified into Alternating copolymers, Random copolymers, Graft copolymers and Block copolymers. Nylon 66 is a copolymer of hexamethylenediamine and adipic acid.