Functional polyacetylenes: synthesis, thermal stability, liquid crystallinity, and light emission of polypropiolates

Polypropiolates with different kinds and numbers of substituents (-{(R)C=C[CO₂(CH₂)₆-OCO-Biph-OC₇H ₁₅]}_n-; R = H (1), CH₃ (2), C₆H₅ (3), Biph = 4,4′-biphenylyl) were synthesized, and the effects of the structural variations on the mesomorphic and luminescent properties of the polymers were investigated. The propiolate monomers RC≡CCO₂(CH₂)₆OCO-Biph-OC₇H ₁₅ [R = H (8), CH₃ (9), C₆H₅ (10)] were prepared by esterification of (substituted) propiolic acids (RC≡CCO₂H) in high yields. While 8 and 9 formed enantiotropic SmA phases, 10 was nonmesomorphic. Polymerizations of the monomers were effected by Mo- and Rh-based catalysts and polypropiolates with high molecular weights (M_W up to 3.5 × 10⁵ Da) were obtained in moderate yields. The polymers were characterized by IR, NMR, TGA, DSC, POM, XRD, UV, and PL analyses. All the polymers were thermally stable. Polymer 1 obtained from the Mo catalyst possessed a lower stereoregularity but exhibited a better-packed monolayer SmA mesostructure, in comparison to the polymer from the Rh catalyst. With an increase in the backbone rigidity from 1 to 3, the glass transition temperature of the chain segments increased, but packing order of the mesogen pendants decreased. Upon photoexcitation, all the polymers emitted UV light of 369 nm. The emission efficiency was dramatically affected by the chain stereoregularity and backbone structure, with 2 exhibiting a quantum yield as high as 0.7.