Systematic research on the synthesis, chemical oxidative polymerization of 3,4-ethylenedithiathiophene (EDTT) in the presence of surfactants or not, and solid-state polymerization of 2,5-dibromo-3,4-ethylenedithiathiophene (DBEDTT) and 2,5-diiodo-3,4-ethylenedithiathiophene (DIEDTT) under solventless and oxidant-free conditions has been investigated. Effects of oxidants (Fe3+ salts, persulfate salts, peroxides, and Ce4+ salts), solvents (H2O, CH3CN/H2O, and CH3CN), surfactants, and so forth on polymerization reactions and properties of poly(3,4-ethylenedithiathiophene) (PEDTT) were discussed. Characterizations indicated that FeCl3 was more suitable oxidant for oxidative polymerization of EDTT, while CH3CN was a better solvent to form PEDTT powders with higher yields and electrical conductivities. Dispersing these powders in aqueous polystyrene sulfonic acid (PSSH) solution showed better stability and film-forming property than sodium dodecylsulfate and sodium dodecyl benzene sulfonate. Oxidative polymerization of EDTT in aqueous PSSH solutions formed the solution processable PEDTT dispersions with good storing stability and film-forming performance. Solvent treatment showed indistinctive effect on electrical conductivity of free-standing PEDTT films. As-formed PEDTT synthesized from solid-state polymerization showed similar electrical conductivity, poorer stability, but better thermoelectric property than oxidative polymerization. Contrastingly, PEDTT synthesized from DIEDTT showed higher electrical conductivity (0.18 S cm-1) than DBEDTT which showed better thermoelectric property with higher power factor value (6.7 X 10-9 W m-1 K-2). (c) 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012