Synthesis and properties of azobenzene-containing poly(1-alkyne)s with different functional pendant groups

1-Alkynes containing azobenzene mesogenic moieties [HC=C(CH ₂)₉-O-ph-N=N-ph-O-R; R = ethyl (1), octyl (2), decyl (3), (S)-2-methylbutyl (4), or (S)-1-ethoxy-1-oxopropan-2-yl (5); ph = 1,4-phenyl] were synthesized and polymerized in the presence of a Rh catalyst {(nbd)Rh ⁺[B(C₆H5)₄]^-; nbd = 2,5-norbornadiene} to yield a series of liquid-crystalline polymers in high yields (e.g., > 75%). These polymers had moderate molecular weights (number-average molecular weight ≥ 12,000), high cis contents in the main chain (up to 83%), good thermal stability, and good solubility in common organic solvents, such as tetrahydrofuran, chloroform, and dichloromethane. These polymers were thoroughly characterized by a combination of infrared, nuclear magnetic resonance, thermogravimetric analysis, differential scanning calorimetry, polarized optical microscopy, and two-dimensional wide-angle X-ray diffraction techniques. The liquid-crystalline behavior of these polymers was dependent on the tail group attached to the azobenzene structure. Poly-1, which had the shortest tail group, that is, an ethyl group, showed a smectic A mesophase, whereas poly-2, poly-3, and poly-5, which had longer or chiral tail groups, formed smectic C mesophases, and poly-4, which had another chiral group attached to the azobenzene structure, showed a chiral smectic C mesophase in both the heating and cooling processes.