Synthesis of new side-chain liquid-crystalline polyoxetanes with two mesogenic groups connected by a flexible spacer in the side chain

Full Paper: The synthesis of a different geometry of side-chain liquid-crytalline polymers is reported where two mesogenic units connected by a flexible spacer are attached as pendent groups of a polymeric chain. The resulting structure is the following, where R-1, R-2 and R-3 are methylenic or oxymethylenic segments: [GRAPHICS] The monomers were obtained by linking the dimer structure to an oxetane ring. These substituted oxetanes were polymerised by a boron trifluoride-initiated cationic ring-opening reaction to obtain the corresponding polymers. The polymerisation proceeds with high yields, in spite of the steric hindrance derived from the bulkiness of the side chain, and the presence of some functional groups in the molecule that compete against the cyclic either for the nucleophilic attack at the propagating centre. A fraction of cyclic oligomer is obtained together with high molecular-weight polyoxetane, as a result of back-biting reactions during the polymerisation. Several systems with spacers of different length and parity were synthesized in order to analyse the influence of the nature of the spacer on the phase behaviour. The formation of mesophases was proved by means of differential scanning calorimetry, X-ray diffraction and microscopic analysis. Most of the monomers form smectic structures at subambient temperatures. The polymers display a similar phase behaviour, although shifted to higher temperatures.