Polymeric Helical Structures a la Carte by Rational Design of Monomers

Preparation of helical structures a la carte by monomer design of dynamic helical polymers such as poly(phenylacetylene)s (PPAs) is a difficult task due to conformational freedom of the polyene backbone. Herein, we study the monomer/helical polymer scaffold relationship by preparation of two novel phenylacetylene monomer series substituted at the phenyl ring in ortho-, meta-, or para-positions with the two enantiomers of either alpha-hydroxy-alpha-phenylacetic acid (1) or alpha-chloro-alpha-phenylacetic acid (S-2) linked through an anilide bond. These monomers were further polymerized, and their secondary structure and dynamic behavior were analyzed. Compiling information from these studies and the structural data for other PPAs found in the literature, we can state that anilide linkages in para-substituted polymers tend to generate compressed cis-cisoidal polyene structures, which can be transformed into more elongated cis-transoidal ones by external stimuli, while benzamide linkages in para-substituted polymers form mainly cis-transoidal scaffolds. The macromolecular structure of PPAs is also largely affected by the aromatic substitution pattern, adopting more stretched scaffolds once the pendant group is placed in meta- or orthopositions, due to the steric hindrance generated by placing this group closer to the backbone.