Ring-opening reactions of 1,3-dioxepan-2-one (1) and 1,3-dioxan-2-one (2) with several alcohols were examined. The reactions proceeded without trifluoroacetic acid (TFA) in low conversions, while they proceeded smoothly with TFA to afford the ring-opened adducts and oligomers. Ring-opening polymerizations of 1 and 2 were also carried out by alcohol-acid catalysts to afford the corresponding polycarbonates ((M) over bar(n) = 2500-6800). The molecular weights increased with increase of the conversions of 1 and 2. The observed polymerization rates of 1 and 2 were determined as 24.4 x 10(-6) and 0.8 x 10(-6) s(-1), respectively. Mechanistic aspects were studied by NMR spectroscopy. The methylene protons alpha and beta to the carbonate moieties shifted to lower fields in 0.06-0.11 ppm in the H-1-NMR spectra by the addition of TFA. Downfield shifts of the carbonyl carbon signals of 1 and 2 were observed in 3.94-4.15 ppm in the C-13-NMR spectra. These results strongly suggest that the cyclic carbonates are activated by TFA.