Cationic ring-opening copolymerization behavior of trioxane (TOX) and a seven-membered cyclic carbonate, 1,3-dioxepan-2-one (7CC) is described. When TOX and 7CC were cationically copolymerized under various feed ratios using trifluoromethane sulfonic acid (TfOH) as an initiator in nitrobenzene at 30 degrees C, 7CC was consumed faster than TOX and the decarboxylation was accompanied to afford the corresponding polyacetal-polycarbonate type copolymers containing poly(oxytetramethylene) units. The copolymer composition could be controlled by the feed ratio of 7CC, whose increase resulted in the high copolymer composition of the 7CC unit. The solubility of the copolymers increased as the increase of the 7CC content. Thermogravimetric, size-exclusion chromatographic, and X-ray analyses of the copolymers suggest that the sequences of the copolymer chains consist of the segments containing the units originated from 7CC and those with TOX unit-rich compositions. (c) 2007 Wiley Periodicals, Inc.