Liquid crystalline siloxane macromolecules with steroidal mesogens

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Abstract
Several new vinyl and allyloxybenzoate steroidal‐based mesogens were synthesized and attached to cyclic tetramethylhydrosiloxane rings using standard hydrosilation chemistry. The steroids examined, all derivatives of cholesterol, 3β‐hydroxy‐24α‐ethylcholest‐5,22‐diene, 3α‐hydroxyandrostan‐17‐one, 3β‐hydroxyandrostan‐17‐one, and 3β‐hydroxypreg‐5‐en‐20‐one were chosen because structural variations in the 17‐β aliphatic chain of the tetracyclic core were present. The synthesis and phase behavior of these mesogenic compounds are discussed with respect to previous systematic investigations linking structural features to mesomorphic behavior. Macromolecular cyclic siloxanes derivatized with these steroidal mesogens were characterized for liquid crystallinity using differential scanning calorimetry (DSC), polarized optical microscopy (POM), and X‐ray diffraction. Compounds with 17‐β ketone substituents possessing vinylbenzoate spacer groups do not exhibit liquid crystallinity, whereas similar compounds with allyloxybenzoate spacer units exhibit broad smectic mesophases. The ability to from partially or fully interdigitated layered mesophases with respect to the 17‐β substituent is examined using X‐ray diffraction. The results indicate the length and flexibility of the 17‐β group must be considered along with the nature of the spacer group in determining phase behavior.