LIQUID-CRYSTALLINE SILOXANE MACROMOLECULES WITH STEROIDAL MESOGENS

Several new vinyl and allyloxybenzoate steroidal-based mesogens were s ynthesized and attached to cyclic tetramethylhydrosiloxane rings using standard hydrosilation chemistry. The steroids examined, all derivati ves of cholesterol, 3 beta-hydroxy-24 alpha-ethyl-cholest-5,22-diene, 3 alpha-hydroxyandrostan-17-one, 3 beta-hydroxyandrostan-17-one, and 3 beta-hydroxypreg- 5-en-20-one were chosen because structural variatio ns in the 17-beta aliphatic chain of the tetracyclic core were present . The synthesis and phase behavior of these mesogenic compounds are di scussed with respect to previous systematic investigations linking str uctural features to mesomorphic behavior. Macromolecular cyclic siloxa nes derivatized with these steroidal mesogens were characterized for l iquid crystallinity using differential scanning calorimetry (DSC), pol arized optical microscopy (POM), and X-ray diffraction. Compounds with 17-beta ketone substituents possessing vinylbenzoate spacer groups do not exhibit liquid crystallinity, whereas similar compounds with ally loxybenzoate spacer units exhibit broad smectic mesophases. The abilit y to form partially or fully interdigitated layered mesophases with re spect to the 17-beta substituent is examined using X-ray diffraction. The results indicate the length and flexibility of the 17-beta group m ust be considered along with the nature of the spacer group in determi ning phase behavior.