Among self-assembled architectures, helical morphology is probably the most fascinating texture in nature. By introducing the chirality into synthetic molecules, helical textures in different length scales can be obtained by self-assembling through interplay of secondary interactions (i.e., non-covalent bonding forces). For synthetic polymers, helical polymers could be classified into two types: static and dynamic helical polymers, depending on the nature of helical conformation. Interestingly, through non-covalent interaction, dynamic helical polymers with functional groups capable of interacting with optically active small molecules may adopt one-handed helical conformation through induced chirality. Most dynamic helical polymers have conjugated or peptide-based backbones so as to give rigidity of the main chain for the formation of helical conformation and also the absorption of chromophore on the backbone for examination of induced circular dichroism (ICD). However, the examples of polyolefins and vinyl polymers with induced chirality are rare.
In this study, a series of intrinsically achiral poly(vinyl pyridine)s, including isotactic poly(2-vinyl pyridine) (iP2VP), atactic poly(2-vinyl pyridine) (aP2VP), and syndiotactic poly(2-vinyl pyridine) (sP2VP) have been synthesized to study the mechanism of induced chirality on vinyl polymers. Carbon-13 nuclear magnetic resonance spectroscopy (13C-NMR) was used to determine the tacticity of synthesized P2VPs by the signal of quaternary aromatic carbon which has been claimed to be the most stereosensitive one among the aromatic carbons. In contrast to stereoirregular aP2VP, the 13C-NMR spectrum exhibited high stereoregularity for the iP2VP and sP2VP synthesized. To examine ICD phenomenon resulting from induced chirality or induced helicity on stereoregular polymers, Circular dichroism (CD) spectroscopy was used to measure the optical activity of the polymers in solution. All P2VPs are optical inactive polymers adopting random-coil conformation in dilute solution. However, through non-covalent bonding of chiral dopant, ICD of the tactic P2VPs, in particular isotactic P2VP (iP2VP), can be driven by the formation of helical conformation with preferred helicity after associating with chiral acids. The characteristic split-type ICD suggests the handedness of the helical conformation on the basis of exciton chirality method. Because of the nature of acid-base equilibrium, the magnitude of ICD would increase with the increase of acidity of chiral acid, solvent polarity, and the ratio of chiral acid to P2VP. In addition, the bulkiness of chiral acid was also found to affect the magnitude of ICD significantly. Most interestingly, in contrast to the intense ICD of iP2VP, the sP2VP only gives rather weak ICD, suggesting that the magnitude of ICD is dependent upon isotacticity rather than syndiotacticity. Furthermore, stereoirragular aP2VP gave more intense ICD than that of sP2VP after associating with chiral acid, further demonstrating that isotacticity indeed plays important role for the ICD. As a result, unlike most dynamic helical polymers having conjugated or peptide-based backbones, the formation of helical poly(vinyl pyridine)s with C-C backbones, in particular for iP2VP, could be successfully achieved by using chiral acids because of the isotacticity of P2VPs.

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