Cationic copolymerizations of 4-methyl-2-methylene-1,3-dioxane, 2 (M-1), with 2-methylene-1,3-dioxane, 1 (M-2); of 4,4,6-trimethyl-2-methylene-1,3-dioxane, 3 (M-1), with 2-methylene-1,3-dioxane, 1 (M-2); of 4-methyl-2-methylene-1,3-dioxolane, 5 (M-1), with 2-methylene-1,3-dioxolane, 4 (M-2); and of 4,5-dimethyl-2-methylene-1,3-dioxolane, a (M-1), with 2-methylene-1,3-dioxolane, 4(M-2) were conducted. The reactivity ratios for these four types of copolymerizations were r(1) = 1.73 and r(2) = 0.846; r(1) = 2.26 and r(2) = 0.310; r(1) = 1.28 and r(2) = 0.825; r(1) = 2.23 and r(2) = 0.515, respectively. The relative reactivities of these monomers towards cationic polymerization are : 3 > 2 > 1; and 6 > 5 > 4. With both five-and six-membered ring cyclic ketene acetals, the reactivity increased with increasing methyl substitution on the ring.