Amino Acid-Containing Polyacetylenes: Synthesis, Hydrogen Bonding, Chirality Transcription, and Chain Helicity of Amphiphilic Poly(phenylacetylene)s Carryingl-Leucine Pendants

Abstract

l-Leucine−phenylacetylene adducts 6 and 14 undergo self-association in concentrated solutions and heterocomplexation with solvent molecules through hydrogen bond formation. The adducts are polymerized by rhodium catalysts into poly(4-ethynylbenzoyl-l-leucine methyl ester) (1e) and poly[11-(4-ethynylphenoxy)undecanoyl-l-leucine methyl ester] (2e), with the polymerizations of 6 producing polymers with high molecular weights (Mw up to 1.47 × 106) and high stereoregularity (Z content up to 100%) in high yields (up to 92%). The ester groups of “polyesters” 1e and 2e are selectively hydrolyzed under basic conditions to afford “polyacids” 1a and 2a carrying free leucine pendants. The polymers are thermally stable (up to ∼260 °C) and undergo Z−E isomerization at ∼200 °C. The chirality transcription from the l-leucine pendants induces the chain segments of polymers 1 to helically spiral predominantly in one preferred direction, while the helicity induction processes in 2 are interrupted by the relaxations of the long flexible spacers between the chiral pendants and the polyene backbones. The chiroptical properties of the amphiphilic polymers change with solvent. In nonpolar solvents, 1 shows high optical activities and strong Cotton effects, whose helical structures are stabilized by intra- and interchain hydrogen bonding. The chain helicity of the polymers decreases in polar solvents due to the partial breakage of the hydrogen bonds within and between the polymer strands.