Synthesis and Novel Mesomorphic Properties of the Side-Chain Liquid Crystalline Polyacetylenes Containing Phenyl Benzoate Mesogens with Cyano and Methoxy Tails

The synthesis of side-chain liquid crystalline polyacetylenes containing phenyl benzoate mesogens with octyl spacers and cyano (2a) or methoxy tails (2b) is accomplished, and the tuning of the molecular alignments of 2b is realized by simple mechanical perturbations. The acetylene monomers, 10-[4-(4′-R-phenoxycarbonyl)phenoxycarbonyl]-1-decynes [R = cyano (1a), methoxy (1b)], are prepared by two consecutive esterification reactions of 10-undecynoic acid. None of the two monomers are liquid crystalline, but 1a exhibits an unusual crystal-crystal transition at 70.5°C, probably due to its strong intermolecular ≡C-H⋯ N≡C- hydrogen bonding. The polymerizations of 1 initiated by the WCl₆-Ph₄Sn/dioxane complex yield yellow powdery polymers (2) with M_w of ca. 30 000, whose molecular structures are characterized by NMR, IR, and UV spectroscopy. The mesomorphic behavior of 2 is investigated by DSC, POM, and XRD: 2a exhibits smecticity in a temperature range as wide as over 100°C (k 80.4 s_A 194.8 i), while 2b displays smectic and nematic mesophases in the temperature region of 95-140°C (k 95.6 s_A 113.9 n 140.8 i). Rotational agitation of the nematic 2b generates the disclinations of high strengths (s up to 2), and translational shear creates the inversion walls, solidification of which produces the well-ordered bands aligned parallel to the shear direction. Such phenomena have not previously been observed in the "conventional" side-chain liquid crystalline polymers with flexible backbones, suggesting that the rigid polyacetylene backbone of 2b plays an important role in inducing the novel molecular alignments aided by the external mechanical force.