A convenient synthetic method for vinylketene cyclic acetals, 2-propenylidene- (Ia) and 4-phenyl-2-propenylidene-1,3-dioxolane (Ib), was developed by the reaction-of 2-bromoethyl or 2-bromo-2-phenylethyl 3-butenoate with potassium tert-butoxide. The radical and cationic polymerizations of vinylketene cyclic acetals were investigated. In the radical polymerization, Ia gave copolymers consisting. of the 1,2-, 1,4-, and 1,7-polymerization structures whereas Ib afforded homopolymer with the complete 1,7-polymerization structure. In the cationic polymerization, the structure of the polymers was dependent on the employed initiator. Although Ia gave copolymers composed of the 3,4- and 3,7-polymerization structures when boron trifluoride, trifluoromethanesulfonic acid, or tin(IV) chloride was used, Copolymers with the 1,4- and 1,7-polymerization structures were obtained with iodine or aluminum chloride. On the other hand, homopolymers of Ib with the 3,4-polymerization structure and the 1,7-polymerization structure were obtained with boron trifluoride, trifluormethanesulfonic acid, or tin(IV) chloride, and iodine or aluminum chloride, respectively.