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Macromolecular Chirality Induction on Optically Inactive Poly(4-carboxyphenyl isocyanide) with Chiral Amines:  A Dynamic Conformational Transition of Poly(phenyl isocyanide) Derivatives

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journal contribution
posted on 29.05.2002, 00:00 authored by Masayoshi Ishikawa, Katsuhiro Maeda, Eiji Yashima
Optically active polyisocyanides (poly(iminomethylenes)) have been prepared with much interest in developing new functional materials. Polyisocyanides have been considered to have a stable 41 helical conformation even in solution when they have a bulky side group. However, the conformational characteristics of poly(phenyl isocyanide) (PPI) derivatives are still under debate. We now report that an optically inactive PPI derivative, poly(4-carboxyphenyl isocyanide) (poly-1), shows optical activity in the polymer backbone induced by external, chiral stimuli through acid−base interactions under thermodynamic control and exhibits induced circular dichroism (ICD) in the UV−visible region in DMSO. The ICD intensities of the poly-1-chiral amine complexes in DMSO gradually increased with time, and, in one case, the value reached 3 times that of the original value after 2 months at 30 °C. The conformational changes also occurred very slowly for poly-1 alone and its ethyl ester with time on the basis of 1H NMR spectroscopic analysis. These results indicate that PPIs bearing a less bulky substituent may not have a 41 helical conformation but have a different type of prochiral conformation, for instance, an s-trans (zigzag) structure which may transform to a dynamic, one-handed helical conformation when the PPIs have a functional group capable of interacting with chiral compounds. The mechanism of helicity induction on poly-1 through a dynamic conformational transition is discussed on the basis of the above results together with molecular dynamic simulation results for PPI.