Anomalous Stiff Backbones of Helical Poly(phenyl isocyanide) Derivatives
Anomalous Stiff Backbones of Helical Poly(phenyl isocyanide) Derivatives
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DOI：
10.1021/ma8015954
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发表时间：
2008-09
期刊：
影响因子：
5.5
通讯作者：
K. Okoshi;Kanji Nagai;Takashi Kajitani;Shin-ichiro Sakurai;E. Yashima
中科院分区：
文献类型：
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作者：
K. Okoshi;Kanji Nagai;Takashi Kajitani;Shin-ichiro Sakurai;E. Yashima
具有过量单手螺旋感觉的全合成螺旋聚合物最近引起了人们的极大关注，因为它们对生物螺旋和功能的影响以及它们作为手性材料的可能应用。1棒状合成螺旋聚合物特别有趣，因为它们经常在浓溶液或熔体中形成手性液晶(LC)，2如在生物聚合物中观察到的那样，如DNA、3多糖、4和多肽，5它们采用刚性的棒状结构，具有由分子内和/或分子间氢键网络稳定的可控螺旋感觉。这种分子内氢键已被用于构建仿生螺旋聚合物，如多异氰酸酯、1D、2c、6氨基酸结合的聚乙炔、7、8和折叠体。9最近，我们报道了一种独特的多异氰化物(图1中的Poly-1)，它是由一个带有L或D-丙氨酸对映体的苯基异氰酸酯在非手性NiCl2催化剂上聚合而成的，其螺旋度(右旋或左旋螺旋)可以通过聚合溶剂和温度来控制。由此得到的非对映异构体Poly-1在聚合物主链(约360 nm)的亚氨基发色团区域显示出强烈的First Cotton效应(ΔεFirst)-11.0至+8.14，由于其主链刚性产生于侧基之间的分子内氢键，在浓缩液中形成了具有相反扭曲感觉的溶致胆甾体LC。10为了探讨手性支链结构对多异氰酸酯手性性质的影响，在不同的溶剂中，在不同的温度下合成了一系列具有不同旋光活性官能团的光学活性多异氰酸酯，如L-乳酸(聚-2)、L-苯丙氨酸(聚-3)和L-丙氨醇(聚-4)残基与手性支链相同。与Poly-1形成鲜明对比的是，Poly2和Poly4表现出较弱的Cotton效应，而Poly3表现出较强的正Cotton效应，与聚合条件无关。结果表明，在聚合反应过程中，生长链端的侧链残基与单体之间发生的特殊相互作用(分子间氢键和空间位阻效应)决定了多异氰酸酯的螺旋结构和一个螺旋结构的过剩程度，这可能受聚合过程中溶剂的极性和温度的影响。虽然Poly-1在浓溶液中形成了胆甾型液晶，这表明了其主链的刚性棒状特征，但作为评价棒状聚合物刚性的有用指标，持续长度Q尚未被估计。我们现在展示了一系列含有不同光学活性侧基的螺旋多异氰酸酯(Poly-1-Poly-4)的持续长度，并描述了手性侧基结构对其主链刚性的影响。我们发现Poly-1具有220 nm的超长Q值，是迄今报道的所有合成螺旋聚合物中最高的。12
Fully synthetic helical polymers with an excess one-handed helical sense have recently drawn much attention because of implications for biological helices and functions as well as their possible applications as chiral materials. 1 Rodlike synthetic helical polymers are particularly interesting, since they often form chiral liquid crystals (LCs) in concentrated solutions or in a melt, 2 as observed in biological polymers, such as DNA, 3 polysaccharides, 4 and polypeptides, 5 which adopt a stiff rodlike structure with a controlled helix-sense stabilized by intra-and/or intermolecular hydrogen-bonding networks. Such intramolecular hydrogen bonds have been utilized in constructing biomimetic helical polymers, such as polyisocyanopeptides, 1d, 2c, 6 amino acid-bound polyacetylenes, 7, 8 and foldamers. 9 Recently, we reported a unique polyisocyanide (poly-1 in Chart 1) prepared by the polymerization of an enantiomerically pure phenyl isocyanide bearing an L-or D-alanine pendant with a long n-decyl chain with an achiral NiCl2 catalyst, whose helical sense (right-or left-handed helix) could be controlled by the polymerization solvent and temperature. The resulting diastereomeric poly-1s showing intense first Cotton effects (Δε1st)-11.0 to+ 8.14) at the imino chromophore regions of the polymer backbones (ca. 360 nm) formed lyotropic cholesteric LCs with opposite twist senses in concentrated solutions due to their main-chain stiffness arising from the intramolecular hydrogen-bonding between the pendant amide groups. 10 In order to explore the effect of the chiral pendant structure on the chiroptical properties of the polyisocyanide, a series of optically active polyisocyanides bearing different optically active functional groups, such as L-lactic acid (poly-2), L-phenylalanine (poly-3), and L-alaninol (poly-4) residues, with the same n-decyl chain as the pendants were prepared in various solvents at different temperatures. 11 In sharp contrast to poly-1, poly-2 and poly-4 exhibited weak Cotton effects, while poly-3 showed rather intense positive Cotton effects, independent of the polymerization conditions. We then concluded that the helical senses and the excess of one helical sense of the polyisocyanides were governed by specific interactions (intermolecular hydrogenbonding and steric effect) occurring between the pendant residues of the growing chain end and monomers during the propagation reaction, which might be influenced by the solvent polarity and temperature of the polymerization process. Although the fact that poly-1 formed a cholesteric LC in concentrated solutions, which indicated its rigid rodlike feature of the main chain, the persistence length q, a useful measure to evaluate the stiffness of rodlike polymers, has not been estimated. We now show the persistence lengths of a series of helical polyisocyanides bearing different optically active pendant groups (poly-1-poly-4) and describe the effect of the chiral pendant structures on their main-chain stiffness. We found that poly-1 possesses an extremely long q value of 220 nm, which is the highest among all synthetic helical polymers reported so far. 12