| Research Abstract |
The synthesis of helical polymers with controlled helix-sense has aroused considerable interest in polymer chemistry in recent years because of their interesting structures and their possible applications in materials science. Recently, we found that macromolecular helicity with a predominantly one-handed helix can be induced in optically inactive poly(phenylacetylenes), for instance, poly((4-carboxyphenyl)acetylene) (PCPA) upon complexation with chiral amines in DMSO, thus exhibiting an induced circular dichroism (ICD) in the polymer main chain. Moreover, the induced helical structure was found to be memorized after replacement of the chiral amines by achiral amines. In this study, we investigated the mechanism of the helicity induction and the memory of helical chirality by means of IR, UV, and CD spectroscopies.
We found that a one-handed helix is cooperatively induced on PCPA upon ion pair formation of the carboxy groups of PCPA with chiral amines. On the other hand, the free tons formation is essential for the macromolecular helicity memory of PCPA after the replacement of the chiral amines with achiral amines, since the intramolecular electrostatic repulsion between the neighboring carboxylate ions contributes to reduce the atropisomerization process of PCPA. On the basis of the solved mechanism, we found that the coexistence of the excess bulky, achiral amine with a small amount of chiral amine caused a dramatic increase in the ICD being comparable to the full ICD induced by excess chiral amine. The induced one-handed helical PCPA was further successfully memorized by achiral amines.
A similar macromolecular helicity induction and the memory of the helical chirality was possible for an optically inactive polyisocyanide, so that this concept may be general and can be used as a promising method to construct novel, functional helical polymers with the desired pendant in a one-handed helical array.
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