Two novel electron-donating quinonoid compounds, 2,5-bis(4',5'-dimetliyl-1',3'-dithiolan2'-ylidene)-2,5-dihydrothiophene (1b) and 2,5-bis(1',3'-dithian-2'-ylidene)-2,5-dihydrothiophene (1c), were synthesized successfully. The homopolymerizations of 1b and 1c took place only in the presence of oxygen to give the corresponding polymers composed of three different structural units. The ratios of the structural units in the polymers were significantly dependent on the polarity of the solvents used for the polymerizations. On the basis of NMR analysis of the resulting polymers and of the reaction products of lb and Ic with water, a possible polymerization mechanism via a radical-cation intermediate was proposed.