1,3-alkylene-diazetidinediones-2,4, wherein alkylene contains at least 8 carbon atoms, are described. These compounds, which can be regarded as intramolecular dimers of alkylene diisocyanates, give rise to the latter on heating. Accordingly, these compounds are useful in a variety of situations in which it is desirable to have a compound which is stable on storage in admixture with other compounds such as polyols, polyamines and the like (which latter would react with a compound containing free isocyanate groups) but which can, at any desired moment, be converted by heating to the free diisocyanate. The latter then reacts with the other component(s) of the mixture to form a polymer. The above compounds are derived by cyclization of the appropriate cyclic allophanoyl halide.