Theoretical calculations are carried out to study the effect of electron donating and withdrawing groups on the electronic structure and properties of a class of conjugated polymers where the basic unit consists of a thiophene and a pyrrole ring linked by a -CH=N- group. Density functional theory (DFT) and time-dependent DFT (TDDFT) were employed for this series of oligomers with different substituents and different positions of those substituents. Ground state structural parameters, HOMO-LUMO gaps, excitation energies, and the oscillator strengths for the first three allowed electronic transitions were computed. The results show that sterically demanding substituents induce twisting in the conjugated backbone of the oligomers and increase both the HOMO-LUMO gaps and the excitation energies, compared to the basic compound. Properties of the oligomers are observed to be dependent on the positions of the substituents, for both electron donating and withdrawing groups. Presence of an intramolecular S center dot center dot O interaction results in planar structure for two oligomers. Reduction in dihedral angles (leading to planarity) is associated with a reduction in excitation energies. S-0 -> S-1 transitions are found to have the largest oscillator strengths for all the compounds.