Cationic cyclopolymerization of 2-methyl-5,5-bis(vinyloxymethyl)-1,3-dioxane (1), a divinyl ether with a cyclic acetal group, was investigated with the HCl/ZnCl2 initiating system in toluene and methylene chloride at -30 degrees C. The reaction proceeded quantitatively to give gel-free, soluble polymers in organic solvents. The number-average molecular weight (M-n) of the polymers increased in direct proportion to monomer conversion, and further increased on addition of a fresh monomer feed to the almost completely polymerized reaction mixture, indicating that the polymerization proceeded in living/controlled manner. The contents of the unreacted vinyl groups in the produced soluble polymers were less than similar to 3 mol %, and therefore, the degree of cyclization was determined to be similar to 97%. In contrast, the pendant cyclic acetal groups remained intact in the polymers under the present cationic polymerization conditions. These facts show that cyclopolymerization of 1 almost exclusively occurred and the poly(vinyl ether)s with the cyclized repeating units and cyclic pendant acetal rings were obtained. Glass transition temperature (T-g) and thermal decomposition temperature (T-d) of poly(1) (M-n = 7870, M-w/M-n = 1.57) were found to be 166 and 338 degrees C, respectively, indicating that poly(1) had high T-g and high thermal stability. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 952-958, 2010