Helical structures polyacetylenes

It has been shown that thermotropic N -LC materials demonstrate supramo-lecular helical ordering that leads to CPL with high dissymmetry factors [49-51], On the other hand, it would be appealing to investigate lyotropic N -LC systems as alternative circularly polarized optical materials for use in optoelectronic devices and displays. The effects of the solvent, solution concentration, and chiral dopant employed in the lyotropic N -LC system would be of particular interest, especially in relation to the helical structure of the polymer LC phase and its chiroptical properties. It was reported that di-substituted polyacetylene (di-PA) adopting a poly(diphenylacetylene) (PDPA) structure with alkyl side chains exhibits lyotropic LC behavior [18, 19]. The PDPA structure, with phenyl moieties... 

Scanning electron microscope (SEM) images of polyacetylene films show that multidomains of spiral morphology are formed (Figure 3.4a), and each domain is composed of helical structure of bundle of fibrils with one-handed screwed direction (Figure 3.4b). The multidomain-type fibril morphology of polyacetylene seems to replicate the chiral nematic LC during interfacial acetylene polymerization. 

FIGURE 3.9 Hierarchical helical structures from primary to higher order in helical polyacetylene. 

Helical polyacetylene was synthesized in asymmetric reaction field consisting of chiral nematic LC. The chiral nematic LC was prepared by adding a chiroptical binaphthyl derivative as a chiral dopant to a mixture of two nematic LCs. Acetylene polymerizations were carried out using the catalyst Ti(0-n-Bu)4— EtsAl dissolved in the chiral nematic LC solvent. The polyacetylene film consisted of clockwise or counterclockwise helical structure of fibrils in SEM. Cotton effect was observed in the region of ir — ir transition of the polyacetylene chain in CD spectrum. The high-electrical conductivities of 1.5 1.8 x... 

Stereoregular ds-transoidal polyacetylenes can take a regular helical conformation. The main-chain stereoregularity is important in inducing a single-handed helical conformation. Rhodium catalysts such as [Rh(norbor-nadiene)Cl]2 are often used to obtain a stereoregular polymer. The helical structure of polyacetylenes is... 

Thermosensitive, and amphiphilic polymer bmshes optically active polymers consist of helical poly(iV-propargylamide) main chains and thermosensitive poly(iV-iso-propylacrylamide) (PNlPAm) side chains, were prepared via a novel methodology combining catalytic polymerization, atom transfer radical polymerization (ATRP), and click chemistry. The characterization of GPC, FT-IR, and H-NMR measurements indicated the successful synthesis of the novel amphiphilic polymer brashes. For the confirmation of helical structure of the polymers backbones and the optical activity of the final brashes used UV-Vis and CD spectra. The polymer with optically active cores (helical polyacetylenes) and thermosensitive shells (PNlPAm) brashes self-assembled in aqueous solution forming core/shell structured nanoparticles [137]. 

Polyacetylenes (for reviews on polyacetylenes and poly(phenylacetylene)s see [1, 2]) are a versatile family of helical polymers that have attracted the attention of a number of research groups in recent years, mostly due to their capacity to adopt helical structures and, correspondingly, to display axial chirality and due to the properties associated with this structural feature. 

Studied the conformations of these polymers using the DPT calculations [19]. He observed that the twisted helical structure is energetically more stable than the cis-trans planar structure because of the intramolecular electrostatic interaction. The bond length alternation for the unsubstituted phosphazene is half of that for polyacetylene. They found that the Jt-bond is due to the negative hyperconjugation from the lone pair of electrons on N to the a bond of P-X, where X is any halogen atom. 

A lot of work, mainly synthesis and elementary characterizations, have been performed on substituted polyacetylenes. It is somewhat surprising that almost no companion structural studies are ava able. The lack of structural data does not allow an accurate knowledge of the internal conformation of the polymer. Nevertheless these systems appear very interesting, the existence of a possible cis-cisoid conformatitm may involve a well defined helical structure such that we can speculate a relation between the specific elastic behavior of an helical-spring like stmcture and the optical properties of the conjugated backbone. 

Helix is the most common higher-order structure of synthetic polymers such as peptides, polymethacrylates, polychloral, polyisocyanides, polyisocyanates, and polysilanes. Polyacetylenes bearing appropriate substituents also form a helix. Substituted helical polyacetylenes are promising candidates for enantioselective permeable materials, polarization-sensitive electro-optical materials, asymmetric electrodes, and hence their synthesis is currently under intensive research. This section overviews the synthesis and properties of helical polyacetylenes recently reported. 

The exponent a in the intrinsic viscosity-molecular weight relationship ([rj] = K.M ) of a polymer is associated with the expansion of the polymer in solution, and hence with the conformation and stiffness of the polymer (Table 24). The a values of tobacco mosaic virus, Kevlar and helical poly(a-amino acids) are close to 2, which means that they take rigid-rod structures. The a values of vinyl polymers are usually 0.5-0.8, indicating randomly coiled structures. In contrast, the a values of substituted polyacetylenes are all about unity. This result indicates that these polymers are taking more expanded conformations than do vinyl polymers. This is atrributed to their polymer-chain stiffness stemming from both the alternating double bonds and the presence of bulky substituents. 

As another example of a helical polyacetylene, the single-handed helical polyacetylene fibril, whose structure was studied by SEM, was prepared by the polymerization of acetylene within a chiral nematic liquid crystalline phase.192... 

It has been generally accepted that polyacetylene has a planar structure, irrespective of cis and trans forms, due to ir-conjugation between the sp hybridized carbon atoms in the polymer chain [1,4]. If it were possible to modify such a planar structure of pofyacetylene into a hehcal one [13], one might expect novel magnetic and optical properties [14]. Here, we report polymerization of acetylene in asymmetric reaction field constructed with chiral nematic LCs, and show that pofyacetylene films formed by helical chains and fibrils can be synthesized [15]. Polymerization mechanism giving hehcal structure from primary to higher order and hierarchical spiral morphology is discussed. 

Yashima, E. Maeda, K. Helicity induction on optically inactive polyacetylenes and polyphosphazenes. In Synthetic Macromolecules with Higher Structural Order, Kahn, T. I. M., Ed. ACS Symposium Series 812 American Chemical Society Washington, DC, 2002 pp 41-53. 

Nakako, H. Nomura, R. Tabata, M. Masuda, T. Synthesis and structure in solution of poly[(-)-menthyl propiolate as a new class of helical polyacetylene. Macromolecules 1999, 32, 2861-2864. 

A group of new amphiphilic macromolecules comprised of hydrophobic polyacetylene backbone and hydrophilic pendant groups of naturally occurring species such as amino acids, saccharides, and nucleosides are synthesized. The polymers exhibit solvatochromism. The macromolecular chains show helical confommtions that depend on the molecular structures of the pendants, solvent, temperature, pH, and additives. 

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