Mixtures comprising a low molecular weight diol, a liquid modifier compound having a boiling point above about 150.degree. C, such as a cyclic sulfone, a polyisocyanate having a functionality of at least about 2.5 and an organometallic catalyst instantly set, after a brief induction period, to a solid, dense, rigid polymeric product which can be demolded within a period of from less than 1 minute to about 5 minutes.