Examining the Impact of Side-Chain Chirality on Conformation of a Helical Poly(3‑(S‑1-ethylhexyl)esterfuran)

A key
challenge with synthetic chiral helical polymers is the precise
determination of their structure, particularly their pitch and handedness.
In past work, we demonstrated that poly­(3-hexylesterfuran) (P3HEF)
adopts a compact helical conformation (pitch ∼3.4 Å),
driven by the syn conformational preference for regioregular,
α-linked furan-3-carboxylates. Chiral side chains (either R or S) were attached to the furan monomer
to synthesize poly­(3-(1-ethylhexyl)­esterfurans) (R- or S-P3­(1EH)­EF) with excess helix sense, but the
branched alkyl group clearly impacted the folding behavior of these
polymers. Here, through combined experimental and computational analyses,
we assigned helix sense in these ester-functionalized polyfurans:
where the S configuration for the side chain results
in a left-handed helix bias, while the opposite R enantiomer results in a right-handed helix bias. While helix handedness
can be biased by the attachment of the chiral side chain, the ethyl
branch disrupts the formation of well-ordered helices when compared
to the P3HEF analog. Solid-state characterization of S-P3­(1EH)­EF revealed isotropic grazing-incidence wide-angle X-ray
scattering (GIWAXS) patterns, in contrast to the anisotropic edge-on
orientation observed for P3HEF films. Thermal analysis with powder
X-ray diffraction showed that while P3HEF remains stable up to 350
°C, the S-P3­(1EH)­EF melts near 150 °C
in the solid state. Additionally, the chiral conformation of the S-P3­(1EH)­EF polymer is lost upon heating in THF solution
above 30 °C, as evidenced by temperature-dependent circular
dichroism (CD) studies. The results demonstrate that while the 1-ethylhexyl
chiral side chains can bias helix sense, they also partially disrupt
the formation and stability of a compact helical structure for ester-functionalized
polyfurans.