Conformational Study of Helical Poly(propiolic esters) in Solution

Optically active poly(phenylacetylene) copolymers composed of non-racemic phenylacetylenes bearing L-and D-alanine decyl esters as the side groups (poly(1L m -co-1D n ), m4n) were prepared by the copolymerization of the corresponding L-and D-phenylacetylenes with different enantiomeric excesses using a rhodium catalyst; their chiral amplification of the helical conformation as an excess of one-handedness in a dilute solution, in a lyotropic liquid-crystalline (LC) state and in a twodimensional (2D) crystal on substrate was investigated by measuring the circular dichroism spectra of the copolymers at different temperatures, mesoscopic cholesteric twist in the LC state (cholesteric helical pitch) and high-resolution atomic force microscopy images of the self-assembled 2D helix-bundle structures of the copolymer chains, respectively.

Section: Resultsmentioning

confidence: 99%

Amplification of macromolecular helicity of dynamic helical poly(phenylacetylene)s bearing non-racemic alanine pendants in dilute solution, liquid crystal and two-dimensional crystal

Ohsawa

Sakurai

Nagai

et al. 2011

“…Either static or dynamic helical polymers with an excess one-handedness, such as poly(quinoxaline-2,3-diyl)s (4), 25,26 polyguanidines (5), 27,28 poly(phenyl isocyanide)s (6) 29,30 and polyacetylenes (7), [8][9][10][11][12][31][32][33][34] have also been prepared by the polymerization of analogous monomers bearing different chiral or achiral substituents, that is, the boundary between static and dynamic helical conformations is totally dependent on the helix inversion barrier. In addition, the helical conformations of dynamic helical polymers can be locked, resulting in static helical polymers as demonstrated by the memory effect of induced helical poly(phenylacetylene)s (7) 35 and poly(4-carboxyphenyl isocyanide) (6: R¼COOH).…”

mentioning

confidence: 99%

Synthesis and structure determination of helical polymers

Yashima

2010

During the last decade, remarkable progress in developing synthetic helical polymers with a controlled helical sense has been achieved. However, the exact helical structures of most of the already prepared synthetic helical polymers remain unsolved. In this review, the recent progress in the synthesis of helical polymers and their structural determination, including helical pitch and handedness based on X-ray diffraction and spectroscopic measurements, together with high-resolution atomic force microscopy, is described. Polymer Journal (2010) 42, 3-16; doi:10.1038/pj.2009 Keywords: atomic force microscopy; circular dichroism; helical polymer; helical structure; X-ray diffractionThe helix is a unique structure as observed in nature from microscopic to macroscopic levels and is inherently chiral. Inspired by biological helices, such as DNA and proteins, chemists have been challenged to synthesize helical polymers with a controlled helical sense that aims not only to mimic biological helical structures but also to develop chiral materials for the separation of enantiomers and asymmetric catalysis. [1][2][3][4][5][6][7][8][9][10][11][12] The history of synthetic helical polymers with optical activity extends back to the 1960s when Pino and Lorenzi 13 investigated the structural and chiroptical properties of isotactic vinyl polymers prepared by the polymerization of a-olefins bearing optically active substituents. Although helical polyolefins are totally dynamic in nature and consist of short helical segments separated by frequently occurring helical reversals among disordered, random coil conformations, 14 this study was significant in the field of synthetic helical polymers, from which a number of helical polymers have been synthesized.On the basis of the pioneering studies by Nolte et al., 15 Okamoto et al. 16 and Green et al., 17 the existing synthetic helical polymers that exhibit an optical activity solely because of their macromolecular helicity can be classified into two categories with respect to their helix inversion barriers, that is, static and dynamic helical polymers (Figure 1). 9,12 Optically active helical polymers, such as poly(triphenylmethyl methacrylate) (1), 16 poly(t-butyl isocyanide) (2) 15 and polychloral (3), 18 have a sufficiently high helix inversion barrier and belong to the family of static helical polymers. Therefore, these helical polymers with either a right-or left-handed helix can be prepared by the helix-sense-selective polymerization of the corresponding achiral monomers using chiral catalysts or initiators under kinetic control. On the other hand, dynamic helical polymers, such as polyisocyanates (8) 17 and polysilanes (9), 19 possess a very low helix inversion barrier.As a consequence, they consist of interconvertible right-and lefthanded helical segments separated by rarely occurring helical reversals, as demonstrated by Green et al.,6,20,21 so that a preferred-handed helical conformation can be induced in the presence of a small amount of chiral residues at the pendant 6 or ter...

“…29 In this study, we synthesized a series of m-terphenyl-based random copolymers containing chiral and achiral amidines (poly-A x ) and their complementary homopolymers containing achiral carboxylic acids (poly-C), and investigated the effect of the chiral/achiral amidine contents on the amplification of the helical chirality 30,31 during the complementary double-helix formation ('the sergeants and soldiers effect') 3,4,32,33 (Figure 1) using absorption and circular dichroism (CD) spectroscopies. Such a unique amplification of the helical chirality along the polymer backbones assisted by a small chiral unit has been proven to be applicable to some stiff single-stranded helical polymers 3,7,9,[31][32][33][34][35][36][37][38][39] and supramolecular helical systems. 30,[40][41][42][43][44] In some artificial double-stranded helical oligomers, an excess of the one-handed helical structure was produced by a small number of chiral units that were introduced into an achiral strand.…”

Section: Introductionmentioning

confidence: 99%

Chiral amplification in double-stranded helical polymers through chiral and achiral amidinium–carboxylate salt bridges

Makiguchi

Kobayashi

Furusho

et al. 2012

A series of m-terphenyl-based random copolymers of chiral and achiral amidines, and their complementary homopolymers of achiral carboxylic acids were prepared by the copolymerization of a p-diiodobenzene derivative, with the diethynyl monomers containing a chiral or achiral amidine group and a carboxyl group using the Sonogashira coupling reaction. The obtained chiral/achiral amidine copolymers assembled into a double-stranded helical structure upon complexation with the complementary achiral homopolymer of carboxylic acids through interstrand amidiniumÀcarboxylate salt bridges. The complexes exhibited characteristic induced cotton effects in the p-conjugated main-chain chromophore regions, indicating that the interstrand duplexes possess a preferred-handed double-helical structure. The effect of the chiral and achiral amidine contents on the amplification of the helical chirality ('the sergeants and soldiers effect') during the interstrand double-helix formation was investigated by comparing the cotton effect patterns and intensities of the duplexes with those of the corresponding all-chiral amidine-based double-helical polymer. Keywords: amidine; carboxylic acid; chirality; chiral amplification; circular dichroism; double helix INTRODUCTIONThe complementary double-helical structure of DNA is a key structural motif for its vital functions in biological systems, such as replication and the storage of genetic information, which has prompted chemists to develop synthetic double-helical polymers and oligomers (foldamers). [1][2][3] Although a large number of singlestranded helical polymers and oligomers have been reported, 2-12 examples of double-stranded helical polymers and oligomers remain relatively scarce. 2,3,8,9,[13][14][15][16][17][18][19][20][21][22] We have recently reported on the rational design and synthesis of a series of complementary double-stranded helical oligomers [23][24][25][26][27][28] with an optical activity that consists of a crescent-shaped m-terphenyl-based backbone containing amidine and carboxylic acid groups. In our design, the formation of the intertwined duplex is driven by the formation of an amidiniumcarboxylate salt bridge, and the helicity of the duplexes can be readily controlled by the introduction of chiral substituents on the nitrogen atoms of the amidine residues. 9,15,18,19 We also reported on mterphenyl-based conjugated polymers, which contain optically active amidine groups (poly-A 1 ) and achiral carboxylic groups (poly-C), that folded into an intertwined double-stranded helical structure through the chiral amidinium-carboxylate salt bridges. 29 In this study, we synthesized a series of m-terphenyl-based random copolymers containing chiral and achiral amidines (poly-A x ) and