polyaddition

Polyaddition is a class of polymerization in which monomer units join by successive addition reactions with little or no elimination of small molecules. It is distinguished from polycondensation, in which the stepwise reaction of monomers typically releases a byproduct such as water or methanol.

Most polyadditions involve multifunctional monomers, allowing chain or network growth. Common examples include the formation of polyurethanes and polyureas from diisocyanates reacting with diols or diamines, and epoxy resins formed by the polymerization of epoxides with amines or acids. Ring-opening of cyclic monomers with nucleophiles and certain click-chemistry routes are also described as polyadditions.

The mechanism can be step-growth, with progressive linkage formation between all reactive functional groups, or more chain-growth-like in systems where reactive end groups rapidly add monomers. The key feature is the absence of a detectable small-molecule byproduct in the main polymerization step.

Industrial relevance includes polyurethane foams and coatings, polyureas, and epoxy polymers. The term polyaddition is sometimes used to emphasize the absence of condensation byproducts and to distinguish these processes from classical condensation polymerizations.