A complex chemical reaction that takes place in drying printing inks in which several relatively simple, low-molecular-weight compounds (called monomers) combine to form a long, high-molecular-weight chainlike molecule (called a polymer). Polymerization commonly takes place in the presence of a catalyst, which can be another compound, or heat or light. Polymerization is responsible for the hardening and drying of printed ink films and commonly occurs following oxidation of components of the ink vehicle.

Polymerization is also the basis of the creation of several types of plates used in offset lithography and flexography, in which photopolymers (molecules that are polymerized by light) are applied to the surface of a plate, and exposure of these coatings, via light shined through a photographic negative or positive or by lasers, causes the image areas of the original to form hard, durable polymers in the appropriate regions of the plate. Chemical treatment after exposure removes the unexposed coating from non-image areas of the plate. (See Plate: Offset Lithography and Plate: Flexography.) Polymerization is also involved in the preparation of photostencils used in screen printing. See Photostencil.

Polymerization—brought about by exposure to either ultraviolet light or electron beams—is also the basis of the drying of many types of coatings used in finishing operations. See Binding and Finishing: Finishing: Coating.

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