The thermotropic nematic-isotropic equilibria in an undiluted system of polydis-perse rod-like molecules obtained by melt polycondensations has been investigated both theoretically and experimentally. In this paper

effects of the polydispersity and orientation-dependent (i.e. anisotropic) interactions between rod-like molecules on the phase equilibria are included. It was shown in the experimental part of this work that the thermotropic nematic-isotropic equilibria will appear in the wide range of temperature for poly(p-hydroxybenzoates) samples with the average D.P. equal to approximately 3.