# 15.11: Epoxy Resins - The Advent of Modern Glues

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.