Architecture of Chiral Poly(phenylacetylene)s: From Compressed/Highly Dynamic to Stretched/Quasi-Static Helices

The remarkable consequences in elongation, dynamic character, response to external stimuli (e.g., solvent effects, metal cations), and aggregation observed in helical poly(phenylacetylene)s (PPAs) when either the type of linkage with the pendant groups (i.e., anilide, benzamide) or the aromatic substitution pattern (i.e., ortho, meta, para) of the parent phenylacetylene monomer undergo modification are analyzed in depth. Two series of PPAs substituted at the phenyl ring in ortho, meta, and para with either (S)-α-methoxy-α-phenylacetic acid (MPA) or (S)-phenylglycine methyl ester (PGME) linked through anilide or benzamide bonds were prepared (i.e., o-, m-, p-poly-1 and poly-2 series) and characterized both in solution and in the solid state (CD, UV-vis, Raman, NMR, DSC, TGA, X-ray, AFM, SEM). Para-substituted polymers (p-poly-1 and p-poly-2) present the most compressed and dynamic helices, which respond easily to external stimuli. Meta-substituted PPAs (m-poly-1 and m-poly-2) exist as a mixture in equilibrium of two different helices (compressed and stretched), both less dynamic than the para counterparts and with a weak response to external stimuli. Moreover, in the solid state, m-poly-1 and m-poly-2 show separate fields for the compressed and for the stretched helices. For its part, the ortho-substituted PPA (o-poly-1) presents a highly stretched, almost planar and practically rigid helical structure, inert to external stimuli and prone to aggregate. These structural changes (elongation/dynamic behavior) are rationalized on the basis of the increasing difficulties imposed by the meta- and ortho-substitution on the accommodation of the pendants within the helical structure.