Conjugated polymers containing phenyl and bithiophene units are synthesized by conventional polycondensation and crossing-coupling method of carboxylic acids with halides. The structure of both polymers formed by adding the ester group to the first polymer was confirmed by means of Fourier transform infrared (FTIR), nuclear magnetic resonance techniques, differential scanning calorimetry, and thermogravimetry and elemental analysis. The molecular weight distribution values of polymers were determined with a gel permeation chromatography (GPC) system. The optical properties of these different polymers with synthesis methods and structures were investigated for different molarities. The molar extinction coefficient values of the polymers decrease with increasing molarity. Transmittance (T) values of the polymers are increased with ester group addition and decreasing molarity. We obtained two types (direct and indirect allowed band gaps) of band gaps. The direct allowed optical band gap (E (gd)) values of poly(2,2' bithiophene terephthalate) (polyBTTP) are lower than the indirect allowed optical band gap (E (gid)) values of poly(5-phenyl-2,2' bithiophene) (polyPBT). The direct and indirect allowed band gap values of the polymers can be decreased with ester group addition and increasing molarity. The refractive index of the polymers can be decreased with decreasing molarity, increasing wavelength, and ester group addition.