Helical conjugated polymers: Synthesis, stability, and chiroptical properties of poly(alkyl phenylpropiolate)s bearing stereogenic pendants

Chiral poly(alkyl phenylpropiolate)s -{(C₆H ₅)C=C[CO₂(CH₂)₂OCOR*]} _n - with R* = (S)(+)-[1-(6-methoxy-2-naphthyl)ethyl (P1), (1R,2S,5A)-(-)-menthoxymethyl (P2), (S)-(+)-(α-acetoxy)benzyl (P3), and cholesteryloxy (P4) were synthesized, and their structures and properties were investigated. The monomers [C₆H₅C≡CCO ₂(CH₂)₂OCOR*; 1-4] were prepared by esterifications of stereogenic acids 8-10 or chloroformate (11) with 3-hydroxyethyl phenylpropiolate (7) in high yields. Polymerizations of 1-4 were effected by MoCl₅-Ph₄Sn at 60 or 80°C, and polymers with high molecular weights (M_w up to ∼100 × 10 ³) were obtained in moderate yields. The polymers were characterized by IR, NMR, TGA, UV, and CD analyses. All of the polymers are stable, losing little of their weights when heated to ≥300°C and undergoing no chain scissions when annealed in air at ≥150°C. The macromolecular chains take helical conformations with preferred handedness, and their helical chirality can be reversibly tuned by solvent or temperature to varying extents.