The frequent occurrence of abnormal reactions leading to gelation or high viscosities in prepolymers prepared from diisocyanates and polyols has prompted an investigation of the side reactions which take place, and methods for their control. It has been observed in this laboratory and elsewhere (l)
that undesirable gelation reactions can be tempered or avoided entirely by judicious employment of acidic additives, e.g., acid chlorides. One important source of acidity is the hydrolyzable chlorine in the diisocyanate itself. It is known that abnormal reactions are sometimes prevented merely through use of a different lot of the same diisocyanate and for this reason the source of difficulties often has been ascribed to the diisocyanate. The purpose of this paper is to elucidate the interrelationship of the hydrolyzable chlorine content of the diisocyanate with the purity of the polyol. The observed phenomena are illustrated in model compound studies as well as in typical commercial formulations. Methods for avoiding undesirable side reactions are presented.