Specifically, in the process of this invention, the polyisocyanate is reacted with the polyol in the presence of from about 0.1 to about 10 parts, and preferably, from about 0.1 to about 5.0 parts (by weight, per hundred parts of the polyol) of a nitrogen compound selected from the group consisting of ##STR3## wherein R is a divalent alkylene radical containing from about 1 to about 4 carbon atoms