Anionic equilibrium polymerization behavior of several six-membered cyclic carbonates was examined. The conversions of the monomers reached a constant below 100%, and the final conversion decreased in the order of 1,3-dioxan-2-one (1) > 5,5-dimethyl-1,3-dioxan-2-one (2) > 5,5-diethyl-1,3-dioxan- 2-one (3) greater than or equal to 5-methyl-5-phenyl-1,3-dioxan-2-one (4) > 5-ethyl-5-phenyl-1,3-dioxan-2-one (5). The reactions of 2,2-disubstituted-1,3-propanediols were carried out with phosgene dimer to find that the cyclic carbonate (5) was formed quantitatively in the reaction of 2-ethyl-2-phenyl-1,3-propanediol, while the corresponding polycarbonate was formed in the reaction of 2,2-diethyl-1,3-propanediol in 24% yield besides 3. Thermodynamic parameters were estimated in the anionic ring-opening polymerizations of cyclic carbonates (1-5) by Dainton＇s equation. The obtained Delta H-p value in the ring-opening polymerization of each cyclic carbonate reflected the polymerizability. Molecular orbital calculations of the model compounds of the polymers were carried out to fmd that the polymerizabilities of the cyclic carbonates correlated with the stabilities of the corresponding polymer structures. The concentrations of the monomers formed in the depolymerizations well agreed with the equilibrium monomer concentrations in the anionic ring-opening polymerizations.