Pyrazine is a heterocyclic ring-based material and has been spotlighted as an emerging building block for polymeric donor in organic solar cells. Pyridazine and pyrimidine are structural isomers of pyrazine which contain two nitrogen atoms in their structure, in different position. Due to their structural differences, they each show different optical, electrochemical properties. In this study, we designed and synthesized three monomers, based on pyrazine, pyridazine and pyrimidine, and three polymer donors containing each. Computational calculation showed that the conformation of two thiophene spacers adjacent to the pyrazine, pyridazine and pyrimidine core can be locked (or stabilized) due to the interaction between sulfur and nitrogen atoms. Optical, electrochemical, thermal, and photovoltaic properties of the polymers are measured and analyzed closely to confirm that the polymers are suitable candidates as active materials in high performance organic solar cells.