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 −{HCC[(CH2)m−OCO−Biph−OC7H15]}n− [1(m), m = 2, 3, 4, 9] and orientations of ester bridge −{HCC[(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 Tm and Ti 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 Tg and Ti 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 (Δλmax >30 nm), a higher Tg and Ti (ΔT ∼40 °C), and a better packed SmAd structure.