A particularly valuable class of prepolymers which can be blocked by use of the blocking agents of this invention can be prepared by reacting a polyisocyanate with a glycol formed by reacting from about 1.67 to 2.0 moles of phenol or cresol having from 5 to 9 carbon atoms in the para position such as p-octyl or p-nonyl phenol or p-octyl or nonyl-o-cresol and the like, with one mole of formaldehyde