9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) is an organophosphorus compound that is used to produce fire retardants. From the chemistry perspective, it is a derivative of hypophosphorous acid as well as a derivative of phenanthrene. The reactivity of the P-H center allows for the conjugation of DOPO to many monomers. Such monomers are incorporated into polymers, conferring fire retardant properties. It is mainly used for nylon and polyesters, as well as epoxides.
The compound is prepared from 2-phenylphenol, which condenses with one equivalent of phosphorus trichloride. Heating that species in the presence of aluminium trichloride results in cyclization. The procedure is completed by hydrolysis.