Syntheses, hydrogen-bonding interactions, tunable chain helicities, and cooperative supramolecular associations and dissociations of poly(phenylacetylene)s bearing L-valine pendants: Toward the development of proteomimetic polyenes

4-Ethylnylbenzoyl-L-valine methyl ester (1e), an acetylene-valine adduct, is polymerized by organorhodium catalysts to the corresponding "polyester" (P1e) of high molecular weights (M_w up to 371 000) and high stereoregularities (Z content up to 100%) in high yields (up to ∼95%). The amino acid residues form intrastrand and interstrand hydrogen bonds within and between the polymer chains. The ester groups of P1e are selectively deprotected by base-catalyzed hydrolysis, giving "polyacid" P1a with "free" valine pendants. While 1e is CD-inactive at λ ≤ 300 nm, both P1e and P1a exhibit intense Cotton effects in the long wavelength region where the polyacetylene backbone absorbs, confirming that the chiral valine pendants have induced the polymer chain to take a helical conformation with an excess in one handedness. The helicity of the chain segments is sensitive to the variations in their environmental surroundings. Utilizing this environmental susceptibility, the chain helicity of the polymers is tuned continuously by such external stimuli as solvent, temperature, pH, and additive, with cooperativity being observed in most systems. The manipulation of the chain helicity by solvent and pH is fully reversible.