Thermoset polymers assume a permanent shape or set once cured. Once set,

they cannot be reshaped. They are formed by a large amount of cross-linking

of linear prepolymers (a small amount of cross-linking will produce

elastomers) or by direct formation of networks by the reaction of two

monomers. The latter is the more prominent of the two methods. It is a

stepwise or condensation method that has been defined as “the reaction of

two monomers to produce a third plus a by-product, usually water or

alcohol.” Because in some cases a by-product is not produced, this definition

is no longer exactly correct. The reaction is now referred to as a “stepwise”

polymerization. When the reaction results in a by-product, it is called a

“condensation reaction.” Table 3.1 lists the principal thermoset polymers.