Tunable
chirality of helical polymers through external achiral
stimuli is highly valuable for fabrication of intelligent chiral materials.
Recently, we reported that through dendronization of phenylacetylene
(PA) with threefold dendritic oligo(ethylene glycols)
(OEG) via alanine linkage, the corresponding polymers feature water
solubility, thermoresponsiveness with cloud points (T
cps) around 31.5 °C, and helical structures. In the
present study, effects of various anions on the chiral structures
and properties of dendronized PA homopolymer (PG1) and copolymers (PG1
m
EB
n
) from dendronized macromonomer (G1) and the hydrophobic
comonomer 4-ethynylbenzaldehyde (EB) were examined. The T
cp of PG1 largely increased in
the presence of so-called salt-in anions such as PF6
– (47.0 °C) and SCN– (37.7 °C),
whereas it slightly decreased with salt-out anions like SO4
2– (29.2 °C) and Cl– (30.8
°C). These results can be correlated with Hofmeister series (HS),
and essentially explained in terms of the competitive interactions
of the three components, i.e., OEG moiety, water molecule, and the
anions. PG1 assumed a right-handed helical conformation
at room temperature in the absence and presence of salt-in anions
including PF6
– and underwent helix inversion
above T
cp according to circular dichroism
(CD) spectroscopy. On the other hand, in the presence of salt-out
anions like SO4
2–, the CD spectral pattern
changed above T
cp with a red shift, suggesting
formation of a different type of helix. Phase transition processes
were further clarified by IR spectroscopy. Copolymers PG1
m
EB
n
with different OEG coverages were
utilized to confirm the crowding effects of dendritic pendants. When
carrying a lower coverage of OEG dendrons, chirality of the copolymer PG1
14
EB
1
became much dependent on anions. We assume that the crowded
OEG moieties along the poly(phenylacetylene) (PPA) backbone provide
a molecular envelope, which plays a key role for the differential
interactions between polymers and anions below and above the T
cps.