A series of novel phosphorus-containing polyesterimides were prepared from diols-a mixture of a new aromatic phosphorus-containing bisphenol, namely 1,4-bis[N-(4-hydroxyphenyl)phthalimidyl-5-carboxylate]-2-(6-oxido-6H-dib enz < c,e >< 1,2 > oxaphosphorin-6-yl)-naphtalene, with aliphatic diols such as 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, and 1,12-dodecanediol-and an aromatic diacid chloride containing two preformed ester groups, namely terephthaloyl-bis-(4-oxibenzoyl-chloride), via high-temperature polycondensation in o-dichlorobenzene. The structures of monomers and polymers were verified by means of Fourier transform infrared (FTIR) spectroscopy and H-1 NMR spectroscopy. The molar ratio of aromatic bisphenol to aliphatic diol was varied to generate a series of copolyesterimides with tailored physicochemical properties, structure-properties relationships being established. The effect of the phosphorus content on the thermal properties and the flame retardancy was evaluated by means of thermogravimetric analysis (TGA), TGA-FTIR, and scanning electron microscopy. The polymers were stable up to 340 degrees C showing a 5% weight loss in the range of 340-395 degrees C and a 10% weight loss in the range of 370-415 degrees C. The char yields at 700 degrees C were in the range of 13.6-38% increasing with the content of phosphorus-containing bisphenol. The effect of the aliphatic content on the liquid crystalline behavior was investigated by polarized light microscopy, differential scanning calorimetry, and X-ray diffraction. The transition temperatures from crystal to liquid crystalline melt were in the range of 209-308 degrees C. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 5391-5403, 2010