Main chain liquid crystalline polyphosphonates containing semi-flexible phenylester mesogen with even number of methylene spacers (2 - 10) have been synthesized. The monomers and polymers were characterized by IR, H-1, C-13 and P-31 NMR spectroscopy. The spectral details are in accordance with the structures. All the polymers were exhibited liquid crystalline property in the Hot stage optical polarized microscope (HOPM). DSC thermal analysis confirms the mesophase formation for all the polymers. The grain size of the liquid crystalline mesophase is increasing with increasing methylene chain. T-g, T-m and T-i of the polymers decreased with increase in spacer length. The T-g of these phosphorus-containing polymers is much lower than that of non-phosphorus polymers containing triad ester mesogens. Energy minimized structures for the molecules which mimic the polymer chain suggests that the reduction in T-g may be due to entanglement raised by incorporation of phosphorus heterogeneity. (C) 2004 Elsevier Ltd. All rights reserved.