Cationic ring-opening polymerization of cyclic carbonates with alkyl halide as initiator during which no decarboxylation takes place is described in detail, using 5,5-dimethyl-1,3-dioxan-2-one (1), 1,3-dioxan-2-one (2), 5-n-butyl-1,3-dioxan-2-one (5), and 1,3-dioxaspiro[5.5]undecan-2-one (6) as monomers. Cationic polymerizations of 2, 5, and 6 with cationic initiators such as methyl triflate and boron trifluoride etherate were carried out under various conditions. In the polymerizations decarboxylation (elimination of carbon dioxide) during the polymerization occurred to yield the corresponding polycarbonate with ether units (5-10%) in the main chain. 6 only gave a small amount of polymer. Prolonged reaction time and enhanced temperature accelerated the decarboxylation. Reactions of the polycarbonate of 2 with several cationic initiators including methyl iodide were monitored by H-1 NMR and gel permeation chromatography. Both a decrease in the polymer molecular weight and an increase in the ratio of ether unit, which strongly depended upon the kind of initiator, were observed. The occurrence of the decarboxylation in the propagation step was suggested by PM3 molecular orbital calculations using model compounds. Direct reaction of 2 with an excess of alkyl halide such as methyl iodide and benzyl bromide at 120 degrees C afforded the corresponding 1:1 adducts 7. Yield of 7a from methyl iodide was only 10%, while yield of 7h, from benzyl bromide was 78%. Cationic polymerizations of 1, 5, and 6 with a few alkyl halides such as methyl iodide, benzyl bromide, and allyl iodide were studied under various conditions, and corresponding polycarbonates without any ether unit were obtained (<(M)over bar (n)> 1000-3700), while 6 gave no polymer. A terminal structure of the polymers obtained was determined by H-1 NMR, and the polymerization with ethyl 3-iodopropyl carbonate as its model compound was examined to prove the propagation structure of the active site. The mechanism of the decarboxylation was discussed from the viewpoint of degree of interaction between HOMOs and LUMOs of cyclic carbonate and initiator and on the basis of the experimental results obtained in this work.