The copolymerization of epoxides and a six-membered cyclic carbonate, 5,5-dimethyl-1,3-dioxan-2-one (DM6CC), was carried out with 1,8-diazabicyclo[5.4.0]undec-7-ene as an initiator. In the copolymerization of glycidyl phenyl ether (GPE) and DM6CC, DM6CC remarkably accelerated the polymerization rate of GPE and also effectively suppressed chain-transfer reactions, which occur in the homopolymerization of the epoxide. This suppression resulted in the formation of the corresponding copolymer with a higher molecular weight. Similar effects of DM6CC were also observed in a curing system with a Novolac-type multifunctional epoxide (Novolac glycidyl ether). The curing reaction of the epoxide in the presence of DM6CC smoothly proceeded and yielded the corresponding networked polymer, showing a decrease in the volume shrinkage as the DM6CC content increased. (C) 2005 Wiley Periodicals, Inc.