The cationic polymerization of 3-methyl-3-(phthalimidomethyl)oxetane (1) afforded two kinds of polymers. One was a polyacetal, i.e., poly{oxy(8,9-benzo-4-methyl-7-oxo-2,6-oxazabicyclo[4.3]nona-8-ene-1,4-diyl)methylene} (3), produced at 50 degrees C and below, and the other was a polyether, i.e., poly[oxy(2-methyl-2-phthalimidomethyltrimethylene)](4), produced at 80 degrees C and above. This new polymerization with the two ring-opening courses was accompanied by the monomer-isomerization process of 1 to give 5,6-benzo-1-methyl-8,11,3-dioxazatricyclo [5.2.2.0(3,7)] undeca-5-en-4-one (2). The cyclic acetal 2, therefore, functioned as the real monomer, and either single or double ring-opening polymerization took place to give 3 or 4, respectively, depending on the reaction temperature. The strain of 2 for ring-opening is discussed on the basis of the result of X-ray structure analysis. The single ring-opening process of 2 was an equilibrium polymerization having the standard enthalpy and entropy changes in dichloromethane equal to -15.8 kJ mol(-1) and -46.0 J mol(-1) K-1, respectively, and the ceiling temperature (T-c) equal to 70.1 degrees C for 1 mol L-1 solution. The critical temperature, where the structure of the product polymer changed, was related to a T-c calculated for the conditions employed (59.1 degrees C for polymerization in 0.83 mol L-1 dichloromethane solution). Therefore, 3 was successfully transformed to 4 with a thermal latent catalyst at a higher temperature than the T-c through the ring-closure depolymerization of 3 to 2 followed by the repolymerization of 2 in the double ring-opening manner. When 1 was polymerized in a temperature step-down mode, a block copolymer with the 3-4 sequence was prepared from the single monomer. Moreover, 1 was copolymerized with a nonisomerizable oxetane, accompanying the monomer isomerization of 1.