Rigid Backbone Polymers. 2. Polyisocyanates and Their Liquid-Crystal Behavior

Aharoni, Shaul M.

doi:10.1021/ma60067a021

Cited by 187 publications

(99 citation statements)

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“…Using this technique, the helical structure of the thermotropic LC 12 40 has been determined to be a 7/3 helix. Starting with the pioneering research by Aharoni 50 and Millich 51 in the late 1970s on the LC formations of polyisocyanates and polyisocyanides, 51,52 various main-chain LC helical polymers with either static or dynamic helical characteristics have been reported. 53 In contrast to the stiff helical polyisocyanates and polysilanes, helical polyacetylenes were considered to be too flexible to show an Synthesis and structures of helical polymers E Yashima LC phase because of their main-chain stiffness.…”

Section: Helical Structure Determination Of Liquid Crystalline Helicamentioning

confidence: 99%

Synthesis and structure determination of helical polymers

Yashima

2010

Polym J

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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...

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Section: Helical Structure Determination Of Liquid Crystalline Helicamentioning

confidence: 99%

Synthesis and structure determination of helical polymers

Yashima

2010

Polym J

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“…Because of their unique structural properties, polyisocyanates have received much attention in various fields such as chiral recognition materials, optical switches, liquid crystal materials, and etc. [1][2][3] Among them, poly(n-hexyl isocyanate) (PHIC) is the best characterized polyisocyanate in solution. 4 We have recently reported the preparation of 4-vinylbenzyloxy-and 2-methacryloyloxyethoxy-ended rodlike PHIC macromonomers and their fundamental radical copolymerization behavior.…”

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confidence: 99%

Preparation of α-, ω-End-Functionalized Poly(n-hexyl isocyanate) Heterotelechelics

Lien

Kikuchi

Narumi

et al. 2008

Polym J

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“…24 Aharoni and Walsh 19 • 20 and Conio et a/. 22 studied the phase separation of the solutions of these A single sample or combinations of more than two samples with different molecular weights, all polydisperse, were used and significant molecular weight fractionation was found to occur on phase separation.…”

Section: Data For Other Rod/ike Polymersmentioning

confidence: 99%

Multi-Phase Equilibrium in Aqueous Solutions of the Triple-Helical Polysaccharide, Schizophyllan

Itou

Teŕamoto

1984

Polym J

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ABSTRACT:Ternary phase diagrams were determined at 25oC for aqueous solutions of a rodlike polysaccharide schizophyllan, using three fractionated samples of different axial ratios x: sample T-3, x=223; sample 36.8; 18.3. Two ternary systems UR-28+ T-3 +water and U-110+ T-3+water were investigated. The phase diagram of either system was occupied by a wide biphasic region, where a dilute isotropic phase and concentrated cholesteric phase coexisted at equilibrium. The phase diagram of the system U-110 + T-3 +water contained two addftional regions, one where two cholesteric phases and one isotropic phase coexisted at equilibrium and the other where two cholesteric phases coexisted. A remarkable molecular weight fractionation took place on phase separation, and the larger x component was almost excluded from the dilute phase (or phases). All these features of the phase diagrams were consistent with the theoretical prediction by Abe and Flory for athermal ternary solutions. Quantitative agreement between theory and experiment was far from satisfactory, although the disagreement was considerably diminished when the interaction parameter was incorporated into the Abe-Flory theory.KEY WORDS Phase Equilibrium I Ternary System I Rodlike Polymer I Polysaccharide I Molecular Weight Fractionation I Triphase Separation I Cholesteric Liquid Crystal I Schizophyllan I Recently, we have been interested in aqueous solutions of the polysaccharide, schizophyllan, 1 -4 which exists as a trimer forming a rigid straight helix in water. 5 • 6 One purpose of these studies has been to investigate the phase behavior of solutions of rodlike polymers with schizophyllan as a model substance. Aqueous schizophyllan shows various interesting characteristics of rodlike polymers. Indeed, it forms a cholesteric mesophase above a certain critical concentration, and there is a concentration range where the isotropic and cholesteric phases coexist at equilibrium. 1 -3 Detailed experimental data for this equilibrium compare favorably with existing theories of rodlike polymers 7 -g in qualitative terms, although a quantitative agreement between theory and experiment has not yet been achieved. 3 It was shown in a preliminary paper 10 that an aqueous solution of two fractionated samples at appropriate composition separated into three phases at equilibrium: an isotropic phase and two cholesteric phases. This may be the first evidence for the theory of Abe and Flory 11 on ternary systems of two rodlike polymers in a single solvent. This theory as well as many others is formulated for monodisperse polymers, but most of the experimental systems consist of polydisperse polymers. As has been predicted theoretically 7 -9 • 11 -15 and confirmed experimentally, 3 • 16 -22 the phase behavior of a given system depends on the molecular weight of the polymer solute, and hence on the molecular weight distribution for a polydisperse solute. Therefore, any experimental test of the theories cannot be conclusive unless due attention is paid on the samples' polydispersity...

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Rigid Backbone Polymers. 2. Polyisocyanates and Their Liquid-Crystal Behavior

Cited by 187 publications

References 11 publications

Synthesis and structure determination of helical polymers

Synthesis and structure determination of helical polymers

Preparation of α-, ω-End-Functionalized Poly(n-hexyl isocyanate) Heterotelechelics

Multi-Phase Equilibrium in Aqueous Solutions of the Triple-Helical Polysaccharide, Schizophyllan

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