Tuning the Chain Helicity and Organizational Morphology of an l-Valine-Containing Polyacetylene by pH Change

The helical chirality and self-assembling structures of an unnatural polymer, poly(p-ethynylbenzoyl-l-valine) (1), are readily manipulated by a simple environmental perturbation of pH change. The amino acid appendages of l-valine create an asymmetric force field, inducing the polyacetylene backbones to helically rotate, and form intra- and interchain hydrogen bonds, stabilizing the screw-sense conformation of the polymer chains. The polymer exhibits a large Cotton effect in methanol, which decreases with an increase in pH upon addition of KOH into the polymer solution. The change in the chain helicity is reversible: the unfolded polymer chains refold back to their original helical conformations when the solutions are neutralized. Natural evaporation of the methanol solutions of 1 on mica gives long, bundled nanofibers of macromolecular assemblies; in contrast, evaporation of the methanol/KOH solutions yields short, unraveled nanofibers with sizes of roughly single macromolecular chains. The ionization of the carboxy groups of the valine moieties by KOH breaks the hydrogen bonds, and the entropy-driven randomization leads to the observed chain helicity attenuation. The electrical repulsion between the polyelectrolyte chains carrying the negatively charged carboxylate ions disassembles the macromolecular association, resulting in the formation of the nanofibers of single chain dimension.