A very useful group of adhesives and plastics is based on condensation polymers of bisphenol A and chloromethyloxacyclopropane (epichlorohydrin). The first step in the formation of epoxy resins is to form a prepolymer by condensation polymerization of the sodium salt of bisphenol A with the epoxide: The formation of a prepolymer involves two different kinds of reactions. One is an \(S_\text{N}2\)-type displacement, and the other is oxide-ring opening of the product by attack of more bisphenol A. Usually, for practical purposes the degree of polymerization \(n\) of the prepolymer is small (5 to 12 units). The epoxy prepolymer can be cured, that is, converted to a three-dimensional network, in several different ways. A trifunctional amine, such as \(\ce{NH_2CH_2CH_2NHCH_2CH_2NH_2}\), can be mixed in and will extend the chain of the polymer and form cross-links by reacting with the oxide rings: Alternatively, a polybasic acid anhydride can be used to link the chains through combination with secondary alcohol functions and then the oxide rings.