| Summary: | 博士 === 國立中正大學 === 化學工程研究所 === 105 === 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, first, a series of intrinsically achiral poly(2-vinyl pyridine)s, including isotactic poly(2-vinyl pyridine) (it-P2VP), atactic poly(2-vinyl pyridine) (at-P2VP), and syndiotactic poly(2-vinyl pyridine) (st-P2VP) have been prepared to study the mechanism of induced chirality on vinyl polymers. To examine ICD phenomenon resulting, 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 stereoregular P2VPs, in particular isotactic P2VP (it-P2VP), can be driven by the formation of helical conformation with preferred helicity after associating with chiral acids or metals. Most interestingly, in contrast to the intense ICD of it-P2VP, the st-P2VP only gives rather weak ICD, suggesting that the magnitude of ICD is dependent upon isotacticity rather than syndiotacticity. Furthermore, at-P2VP gave more intense ICD than that of st-P2VP after associating with chiral guests, further demonstrating that isotacticity indeed plays important role for the ICD.
Next, for the self-assembly study by complexing with chiral dopants to exam the possibility of forming tertiary helical structures in solid state. A serious of it-P2VP-based stereoregular block copolymers, it-poly(2-vinylpyridine)-b-polystyrene (it-P2VP-b-PS) and it-poly(2-vinylpyridine)-b-poly(ε-Caprolactone) (it-P2VP-b-PCL), were prepared by atom transfer radical polymerization (ATRP) of an α -bromoester-terminated it-P2VP macroinitiator with styrene and ring-opening polymerization (ROP) of an stannous alkoxide end-capped it-P2VP macroinitiator with ε-caprolactone respectively. The α-bromoester-terminated it-P2VP was prepared from the esterification of hydroxyl-capped it-P2VP. The hydroxyl-capped it-P2VP was prepared from hydroboration of ethenyl-capped it-P2VP. The ethenyl-capped it-P2VP was generated by using the allylbromide quenched the anionic polymerization of it-P2VP. The method offers effective control over the stereoregularity of poly(2-vinylpyridine) and facilitates linking between blocks to provide stereoregular BCPs, which are capable of generating well-ordered nanostructures resulting from the self-assembly of stereoregular BCPs, as evidenced by small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM).
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