Polyurethanes are very commonly prepared via the prepolymer route. This method offers a means to reduce the volatility of toxic diisocyanates while producing materials that are easily handled. An optimum prepolymer species would be a single molecule of the diol capped with two molecules of the diisocyanate. Formation of the prepolymer is a random process, of course, and the ideal cannot be achieved. In practice, the initial concentration of the diisocyanate in the reaction mixture is always maintained at more than two times that of the diol to suppress the formation of higher oligomeric species. This excess diisocyanate in the initial mixture will result in unreacted diisocyanate in the final prepolymer. If this unreacted diisocyanate is not subsequently removed, the prepolymer will be more hazardous to handle.