A chemical reaction (also known as curing) in which the molecules of a simple substance (monomer) are linked together to form large molecules (polymers) whose molecular weight is a multiple of that of the monomer. Addition polymerization is a polymer formed by the reaction between two molecules to produce a larger molecule without the elimination of a smaller molecule e.g. water. If the two molecules are the same, a homopolymer is formed; if they are different, a heteropolymer is formed. Condensation polymerization is the reaction between two usually dissimilar molecules to form a larger molecule with the elimination of a smaller molecule (often water). Polymerization shrinkage is a potential problem, particularly with restorative polymers, which can result in cuspal fracture and is minimized by the addition of fillers.