A novel synthetic method for the preparation of copolyesters comprised of diols and bisphenols using tosyl chloride (TsCl)/DMF/pyridine (Py) as a condensing agent has been developed. A variety of combinations of monomers could produce relatively high molecular weight copolymers, and better results were obtained by initial oligomerization of diols followed by bisphenols. In order to demonstrate usefulness of this method, copolymers comprised of IPA/TPA (50/50), bis(2-hydroxyethyl)terephthalate (BHET),and several bisphenols were prepared and compared to the poly(ethylene terephthalate) (PET) modified by TPA and 2,2-bis(4-hydroxyphenyl)propane (BPA) diacetate in terms of their thermal properties, The length of mesogenic unit segments in the thermotropic IPA/TPA (50/50)-BHET/ 4,4'-dihydroxybenzophenone (4,4'-DHBP) (50/50) copolymer was changed by initial reaction of BHET followed by dropwise addition of 4,4'-DHBP in the two-stage polycondensation and also by varying the amounts of BHET used at the initial and final stages in the three-stage copolycondensation, and the results were studied by NMR and their thermal properties.