Studies on Helicity Induction of Polymers and its Memory

近年来，具有可控螺旋意义的螺旋聚合物的合成因其有趣的结构及其在材料科学中的潜在应用而引起了聚合物化学的极大兴趣。最近，我们发现在光学不活跃的聚苯乙炔(如聚(4-羧基苯基)乙炔)(PCPA)与DMSO中的手性胺络合时，可以诱导出以单手螺旋为主的大分子螺旋，从而在聚合物主链上表现出诱导的圆二色(ICD)。此外，手性胺被非手性胺取代后，诱导的螺旋结构被记忆。在本研究中，我们通过红外光谱、紫外光谱和圆二色谱研究了螺旋性诱导和螺旋性记忆的机理。我们发现，当PCPA的羧基与手性胺形成离子对时，在PCPA上协同地诱导出单手螺旋。另一方面，手性胺被无手性胺取代后，自由电子的形成对于PCPA的大分子螺旋性记忆是必不可少的，因为相邻的羧酸根离子之间的分子内静电斥力有助于减少PCPA的反常异构化过程。根据所解决的机理，我们发现过量的大体积无手性胺与少量的手性胺共存导致ICD显著增加，与过量手性胺引起的完全ICD相当。诱导的单手螺旋PCPA进一步被非手性胺成功地记忆，对于光学非活性多异氰酸酯来说，类似的大分子螺旋度的诱导和螺旋手性的记忆是可能的，因此这一概念可能是通用的，可以作为一种有前景的方法来在单手螺旋阵列中构建具有所需支链的新型功能螺旋聚合物。

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.