The molecular variables which control the structure-property relationships in thermotropic liquid crystalline polyesters are under investigation in this laboratory. A wide variety of polymers based on rigid, linear aromatic ester mesogenic units, with and without flexible or rigid spacers, have been prepared and characterized for their ability to form a liquid crystalline melt, the type of phase formed, their transition temperatures, and the morphology of the mesophase. Flexible spacers reduce both the melting and clearing temperatures, and the type and length of the spacer can determine whether either a nematic, cholesteric or smectic phase is formed. Variations in the structure of the rigid mesogenic group, both in the specific type and arrangement of the aromatic ester groups and the presence of pendant substituent, also cause profound changes in the properties of the mesophase melt formed.