A prepolymer is created when a cure is held at an intermediate stage by
attention to the reaction stoichiometry. An excess of one or other reagent
is used to limit the extent of cure. When an excess of isocyanate is used
in a polyurethane formulation the intermediate product is called an isocyanate
prepolymer. When an excess of phenol is used in a phenol-formaldehyde
resin formulation, the intermediate product is called a novolak.