Theoretical work was performed to investigate the geometrical and electronic structures of four new hypothetical thiophene copolymers : poly(thienylene cyclopentadienylene) (PThPD), poly(thienylene silolylene) (PThS), poly(thienylene oxocyclopentadienylene) (PThOPD) and poly(thienylene thiocyclopentadienylene) (PThTPD). AM1 band calculations showed that the ground-state geometries of PThPD and PThS copolymers are of the aromatic forms whereas those of PThOPD and PThTPD are quinoid. Each intraring structure of the copolymers is predicted to be nearly identical to that of their corresponding parent homopolymers and the bond-length alternation (Sr) of each copolymer is equal to the average of Sr values for the two corresponding homopolymers. Modified extended Huckel band calculations produced that the band gaps (which correspond to the absorption peaks of pi-pi* band transition) of the copolymers in their ground states are 1.7 for PThPD, 1.55 for PThS, 1.9 for PThOPD, and 2.09 eV for PThTPD. These values, except for PThPD, are quite smaller than the band gaps calculated for the corresponding homopolymers in the ground states. Decomposition of the band gaps reveals that the gaps of the aromatic forms of PThPD and PThS are dominated by the delta r contribution and the electronic effect of the bridging groups and that the gaps of the quinoid types of PThOPD and PThTPD correspond primarily to the Sr contribution.