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.