AbstractDiethyl vinyl phosphate (1), diphenyl vinyl phosphate (2), and phenyl divinyl phosphate (3) were prepared, and their polymerization and copolymerization behaviors as well as the hydrolysis of the polymers obtained were investigated. The monomers1and2were homopolymerized to give low molecular weight polymers, probably as a result of high chain transfer reactivity to monomer. Although the monomer3gave a cross‐linked polymer, this polymerization was found to proceed mainly by a cyclopolymerization mechanism to give a polymer with a five‐membered ring unit which was readily hydrolyzed. The alkaline hydrolysis of poly(1) occurred only partly and thus pure polyvinyl alcohol was not obtained. From the copolymerizations of1and2with vinyl monomers such as vinyl chloride, the reactivities of these monomers were found to be somewhat lower than that of vinyl acet