Remarkable Effects of Anions on the Chirality of Thermoresponsive Helical Dendronized Poly(phenylacetylene)s

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(cp)s) around 31.5 degrees 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 degrees C) and SCN- (37.7 degrees C), whereas it slightly decreased with salt-out anions like SO42- (29.2 degrees C) and Cl- (30.8 degrees 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 SO42-, 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(cp)s.