Synthesis and structure determination of helical polymers

Abstract

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...

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

“…21,22 We note that when poly(1L m -co-1D n ) has a left-handed helical array of the pendant groups, the main chain has an opposite, right-handed helical structure. 21,49,50 Although the poly(1L 0.85 -co-1D 0.15 ) and poly(1L 0.75 -co-1D 0.25 ) consist of a onehanded (left-handed) helical block (Figures 5a and b), the opposite right-handed helical segments were visualized in the AFM images of poly(1L 0.65 -co-1D 0.35 ) and poly(1L 0.55 -co-1D 0.45 ) (blue colors in Figures 5c and d (right)). On the basis of an evaluation of B1500 helical blocks, the helical-sense excesses (% ee h ) of poly(1L 0.85 -co-1D 0.…”

Copolymerization of chiral propargylethers and N-propargylamide and conformational control of the copolymers

“…Helically chiral polyacetylenes involve particular structures and morphology, which can be used in the several different fields on optical and stimuli-responsive materials, chiral cognition, biosensor [14][15][16][17]. Poly(phenylacetylene) derivatives bearing a chiral pendant have been designed to form the consecutive one-handed twist of the main chain induced by chiral molecules [18][19][20].…”

Chiral anion-triggered helical poly(ionic liquids)

“…In recent years, the design and synthesis of helical polymers with a controlled helixsense have attracted significant attention for its possible applications like sensing, separation of enantiomers, enantioselective catalysis etc. [9][10][11][12][13][14][15][16][17][18][19][20][21][22] The helical polymers are mainly divided into two classes, namely static and dynamic helical polymers. In static helical polymers, the helical sense is fixed during the observation period and the interconvertibility is irrelevance.…”

“…On the other hand, in dynamic helical polymers, the helical sense is not fixed and the preferential helicity is interconvertible by external, physical, chemical or electrical stimuli. 14,23 There are more reports on polymers, oligomers and some organic molecules with single helix morphology. [24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42] But the formations of double helices from synthetic molecules are relatively rare when compared with single helix formation.…”