Ketenes contain the functional group -C=C=O. They have been classified as aldoketenes and ketoketenes. The ketenes are prepared by modifications of the general methods for the synthesis of olefins. The ketenes are much more reactive than simple olefins, however, and are more likely to enter into combination with the reagents from wich they are prepared or with solvents used or into self-condensation to yield dimers or polymers, Many of the aldoketenes, which are generally more reactive than ketoketenes, have not been isolated as the pure monomers. The dimers of aldoketenes have some of the properties of ketenes and some are useful.