Synthesis and properties of liquid crystalline polyacetylenes with different spacer lengths and bridge orientations

A group of mesomorphic polyacetylenes with different lengths of alkyl spacer -{HC=C[(CH₂)_m- OCO-Biph-OC₇H₁₅]}_n-[1(m), m = 2, 3, 4, 9] and orientations of ester bridge -{HC=C[(CH₂)₃-CO₂- Biph-OC₇H₁₅]}_n- [2(3)] are synthesized, and the effects of the structural variables on the properties of the polymers are investigated. The liquid crystalline acetylene monomers n-{[(4′-heptoxy-4-biphenylyl)oxy]carbonyl }-1-alkynes 3(m) and 5-{[(4′-heptoxy-4- biphenylyl)carbonyl]oxy}-1-pentyne 4(3) are prepared by consecutive etherification and esterification reactions. The T_m and T_i values of 3(m) decrease with an increase in the spacer length (m). While 3(3) shows a monotropic SmA phase, its counterpart with a different ester orientation 4(3) exhibits an enantiotropic SmB phase. The monomers are polymerized by transition-metal halides and carbonyls, and the polymerizations catalyzed by WCl₆-Ph₄Sn under optimal conditions produce polymers with high molecular weights (up to 1.2 × 10⁵) in high yields (up to 92%). The structures and properties of the polymers are characterized and evaluated by IR, UV, TGA, DSC, POM, and XRD analyses. With the increase in the spacer length, the thermal stability of the polymers increases, their T_g and T_i decrease, and the packing arrangements in their mesophases change from mixed mono- and bilayer structures to a homogeneous monolayer structure. Compared to 1(3), 2(3) shows a blue-shifted absorption spectrum (Δλ_max >30 nm), a higher T_g and T_i (ΔT approx. 40°C), and a better packed SmA_d structure.