Molecular design of poly(ethylene dithioterephthalate) (PETS2) and poly(ethylene tetrathioterephthalate) (PETS4) has been carried out by molecular orbital calculations, NMR experiments, and single crystal X-ray structure analysis for their model compounds and refined rotational isomeric state calculations for the two polymers, and their conformational characteristics, crystal conformations, configurational properties, solubilities, crystallization behaviors, and thermal properties were predicted. For the purpose of the experimental verification, PETS2 and PETS4 were actually prepared and fully characterized. The new polymer, PETS4, was synthesized by an ionic polycondensation reaction using a complex of tetrathioterephthalic acid and piperidinium. The two polymers were characterized by elementary analysis, IR, solubility test, X-ray diffraction, thermal analysis, and solid-state NMR to yield experimental results satisfactorily consistent with the theoretical predictions. Through the actual examples, molecular design of polymers has been proved to be feasible.