The sulfur analog of spiroorthocarbonate (SOC) capable of undergoing tandem double ring-opening polymerization (dibenzo[3,4;10,11]-1,6,8,13-tetrathiaspiro[6.6]tridecane, 4) was synthesized, and its polymerization behavior was studied. Some properties of the polymers obtained and the volume change on polymerization of 4 were also examined. The synthesis of 4 was performed by the acid-catalyzed ester-exchange reaction of tetrakis(methylthio)methane with o-xylenedithiol and was obtained in 98% yield. The structure of 4 was determined by the spectral and elemental analysis data. Cationic polymerization of 4 with latent thermal initiator benzyl tetrahydrothiophenium hexafluoroantimonate in nitrobenzene proceeded efficiently at more than 150 degrees C to give the corresponding poly(thioether-trithiocarbonate) 5. The polymer structure was determined by the spectral data and further confirmed by the structures of the decomposition products formed by the reductive cleavage of the trithiocarbonate bond of 5 with LiAlH4. The mechanism of the polymerization was postulated from the results obtained. Thermal properties of 5 such as T-g and T-m were examined and compared with those of its oxygen analog (13). The volume change was studied in the polymerizations of 4 and its oxygen analog (2), during which 5.2 and 6.7% volume expansions were observed, respectively.