Abstract: A process is provided for the preparation of rigid forms which comprises the step of reacting an organic polyisocyanate with an isocyanate-reactive material in the presence of a blowing promotor being an isocyanate-reactive cyclic compound of formula: ##STR1## wherein Y is O or NR.sup.1 wherein each R.sup.1 independently is a lower alkyl radical of C.sub.1 -C.sub.6 or a lower alkyl radical substituted with an isocyanate-reactive group;each R independently is hydrogen, a lower alkyl radical of C.sub.1 -C.sub.6 or (CH.sub.2).sub.m --X wherein X is an isocyanate-reactive group which is OH or NH.sub.2 and m is 0, 1 or 2; andn is 1 or 2;with the proviso that at least one of R.sup.