Polymeric Helical Structures à la Carte by Rational Design of Monomers

Abstract

para-positions with the two enantiomers of either -hydroxy-phenylacetic acid (1) and -chloro--phenylacetic acid (S-2) linked through an anilide bond. These monomers were further polymerized, and their secondary structure and dynamic behavior analyzed. Compiling information from these studies, and the structural data for other PPAs found in literature, we can state that anilide linkages in p-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 p-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 ortho positions, due to the steric hindrance generated by placing this group closer to the backbone.

From Sergeants and Soldiers to Chiral Conflict Effects in Helical Polymers by Acting on the Conformational Composition of the Comonomers

“…To perform these studies, the 4‐ethynylanilides of ( R )‐ or ( S )‐α‐methoxy‐α‐phenylacetic acid (MPA) were chosen as chiral soldiers (monomers ( R )‐ or ( S )‐ 1 ; Figure 3), [42–47] while the 4‐ethynylanilide of the ( S )‐mandelic acid (MA) was selected as chiral sergeant (monomer ( S )‐ 2 ) [48] . The difference between these two monomers is one substituent at the chiral carbon.…”

“…Thus, while monomer 1 bears a methoxy group, monomer 2 possess a hydroxyl group (Figure 3 a and b). However, this only apparently small difference between the two monomers produces a large difference in the dynamic behavior of the corresponding monomers and homopolymers [48] …”

“…On the other hand, monomer 2 (( R )‐ or ( S )‐Sgt.) has a major ap conformation at the (O=)C−C(−O) bond (Figure 3 b), which can be shifted to sp by adding stimuli such as Ba(ClO 4 ) 2 [48] . The homopolymer poly‐( R )‐ 2 has a cis–cisoidal structure with a preferred M helical sense in low polarity solvents ( ap pendant), while a P helicity is induced in polar solvents owing to the presence of major sp conformations at the pendants (Figure 3 d).…”

“… [56–58] CD spectra of the poly‐[( S )‐ 1 r ‐ co ‐( S )‐ 2 (1−r) ] copolymer series show in this solvent the classical sergeants and soldiers effect, where the chiral sergeant ( S )‐ 2 is able to induce a P helix in the chiral soldier ( S )‐ 1 in a very effective way: just 3 % of the chiral sergeant ( S )‐ 2 is enough to induce the maximum helical preference (CD signature at the vinylic region) in the copolymer chain, r =0.03 (Figure 5 b). [55, 56] From the mechanistic point of view, in chloroform this process is originated by the ap conformation of the sergeant ( S )‐ 2 (Figure 5 b), [48] which commands the soldier ( S )‐ 1 to adopt an ap conformation that in turn induces the same P helix orientation in the copolymer chain. The conformational composition at the pendants was determined by fixing their ap or sp conformations by addition of either monovalent or divalent metal ions (see the Supporting Information) [46–48] …”

“… [55, 56] From the mechanistic point of view, in chloroform this process is originated by the ap conformation of the sergeant ( S )‐ 2 (Figure 5 b), [48] which commands the soldier ( S )‐ 1 to adopt an ap conformation that in turn induces the same P helix orientation in the copolymer chain. The conformational composition at the pendants was determined by fixing their ap or sp conformations by addition of either monovalent or divalent metal ions (see the Supporting Information) [46–48] …”