立體規則高分子之掌性誘導與自組裝行為研究

Induced Circular Dichroism and Self-Assembly of Stereoregular Vinyl Polymers

余敍理

指導教授 ： 何榮銘

國立清華大學/工學院/化學工程學系/碩士(2013年)

摘要

Herein, we aim to systematically examine the ICD behavior of stereoregular vinyl polymers, poly(2-vinyl pyridine)s (P2VPs) with C-C single bond as the soft backbone through the association of a variety of chiral guest molecules. With increasing the molar ratio of [acid]/[2VP], the intensity of the ICD approximately increases linearly in the region of low molar ratio of [acid]/[2VP], and the increase in the intensity of ICD experiences a level off by further increasing the molar ratio of [acid]/[2VP], suggesting a saturation state for the association of the acid with the P2VP. As the induced helical conformation of iP2VP is highly dependent upon the concentration of the acid, the ICD will be affected by the chain length of P2VP. P2VPs with different molecular weight and controlled tacticity are synthesized to clarify the molecular weight dependence of optical activity. Our results indicate that the intensity of ICD is independent upon molecular weight while molecular weight is over 4500 g/mol due to a very low helical inversion barrier resulting in the reversal of handedness.
To systematically investigative the isotacticity effect on the ICD of iP2VP associated with chiral acid, a series of iP2VP with different isotacticity ranging from 40mm% to 97mm% are synthesized. A nearly linear relationship between the ICD and isotacticity can be identified, indicating that the isotacticity of P2VP is the primary factor to determine the ICD of its complex. Also, an interesting phenomenon can be found at which, instead of splitting-type Cotton effect, the CD spectra of the iP2VP complex only show negative Cotton effect while the isotacticity of the P2VP used is low. According to the simulated results by using ChemDraw, the tacticity of stereoregular P2VP will significantly affect the intramolecular distance of 2-pyridine moieties in P2VP so that the formation of splitting-type Cotton effect will be justified by the stereoregularity of the P2VP.
To quantitatively investigative the effect of bulkiness of chiral acid on the ICD of vinyl polymers, chiral acids with different molecular size are used to associate with the iP2VP. As found, while the bulkiness of chiral acid decreases, the steric hindrance will be too low to maintain helical conformation for the iP2VP associated with chiral acid. As iP2VP associated with chiral acids via hydrogen bonding, which is a secondary force (i.e., non-covalent bond interaction), the association will be highly dependent upon temperature. Moreover, the intensity in the ICD can be recovered after cooling, and the intensity variation in the ICD is almost identical with the results from heating, suggesting that the association/dissociation between stereoregular P2VP and chiral acid is a thermally reversible process. Because iP2VP serves as organic base, the ICDs of iP2VP associated with chiral acid can be determined by the equilibrium between the free acid (AH) and base (B) that can be determined by pKa. As found, adding organic base (n-Butylamine, nBA) in polymer solution will reduce the intensity of iP2VP and finally to cause the CD silence. These results suggest that the intensity in ICDs is reversible by changing pKa environment in solution, and one-handed helical conformation of the P2VPs induced by chiral acids might not be memorized after the chiral acids is neutralize by n-BA.

關鍵字

誘導掌性 ； 螺旋 ； 圓二色光性

並列摘要

Herein, we aim to systematically examine the ICD behavior of stereoregular vinyl polymers, poly(2-vinyl pyridine)s (P2VPs) with C-C single bond as the soft backbone through the association of a variety of chiral guest molecules. With increasing the molar ratio of [acid]/[2VP], the intensity of the ICD approximately increases linearly in the region of low molar ratio of [acid]/[2VP], and the increase in the intensity of ICD experiences a level off by further increasing the molar ratio of [acid]/[2VP], suggesting a saturation state for the association of the acid with the P2VP. As the induced helical conformation of iP2VP is highly dependent upon the concentration of the acid, the ICD will be affected by the chain length of P2VP. P2VPs with different molecular weight and controlled tacticity are synthesized to clarify the molecular weight dependence of optical activity. Our results indicate that the intensity of ICD is independent upon molecular weight while molecular weight is over 4500 g/mol due to a very low helical inversion barrier resulting in the reversal of handedness.
To systematically investigative the isotacticity effect on the ICD of iP2VP associated with chiral acid, a series of iP2VP with different isotacticity ranging from 40mm% to 97mm% are synthesized. A nearly linear relationship between the ICD and isotacticity can be identified, indicating that the isotacticity of P2VP is the primary factor to determine the ICD of its complex. Also, an interesting phenomenon can be found at which, instead of splitting-type Cotton effect, the CD spectra of the iP2VP complex only show negative Cotton effect while the isotacticity of the P2VP used is low. According to the simulated results by using ChemDraw, the tacticity of stereoregular P2VP will significantly affect the intramolecular distance of 2-pyridine moieties in P2VP so that the formation of splitting-type Cotton effect will be justified by the stereoregularity of the P2VP.
To quantitatively investigative the effect of bulkiness of chiral acid on the ICD of vinyl polymers, chiral acids with different molecular size are used to associate with the iP2VP. As found, while the bulkiness of chiral acid decreases, the steric hindrance will be too low to maintain helical conformation for the iP2VP associated with chiral acid. As iP2VP associated with chiral acids via hydrogen bonding, which is a secondary force (i.e., non-covalent bond interaction), the association will be highly dependent upon temperature. Moreover, the intensity in the ICD can be recovered after cooling, and the intensity variation in the ICD is almost identical with the results from heating, suggesting that the association/dissociation between stereoregular P2VP and chiral acid is a thermally reversible process. Because iP2VP serves as organic base, the ICDs of iP2VP associated with chiral acid can be determined by the equilibrium between the free acid (AH) and base (B) that can be determined by pKa. As found, adding organic base (n-Butylamine, nBA) in polymer solution will reduce the intensity of iP2VP and finally to cause the CD silence. These results suggest that the intensity in ICDs is reversible by changing pKa environment in solution, and one-handed helical conformation of the P2VPs induced by chiral acids might not be memorized after the chiral acids is neutralize by n-BA.

參考文獻

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