Substituted polyacetylene-based

Also shown in Table lO-l is an (alkylcyclohexylaryloxy)-substituted polyacetylene [77]. Polymers of this general structure have been found to display liquid-crystalline behavior. In contrast to vinyl-based liquid-crystalline polymers, the geometric isomerism of the main-chain double bonds plays a role in determining the type of phase that is found. Advincula et al. have examined Langmuir films of polyacetylenes at the air-water interface [78]. Polyacetylene derivatives are unusual in that the polymer backbone itself acts as a chromophore therefore, in studies such as these, UV-visible spectroscopy can be a sensitive probe of polymer conformation. 

Typical functions of substituted poly acetylenes are based on their (i) high gas permeability and (ii) electronic and photonic properties. The former originates from the rigid main chain and bulky substituents. Though electrical insulators, substituted polyacetylenes are more or less conjugated polymers, and this feature has been utilized to develop their electronic and photonic functions such as photoconductivity, electrochromism, optical nonlinearity and ferromagnetism. 

B.A. San Jose, S. Matsushita, K. Akagi, Lyotropic chiral nematic liquid crystalline aliphatic conjugated polymers based on di-substituted polyacetylene derivatives that exhibit high dissymmetry factors in circularly polarized luminescence. J. Am. Chem. Soc. 134, 19795-19807 (2012)... 

When conducting the ROMP of norbornene or cyclooctadiene in miniemulsions [82], two approaches were followed (i) addition of a catalyst solution to a miniemulsion of the monomer and (ii) addition of the monomer to a miniemulsion of Grubbs catalyst in water. With the first approach it was possible to synthesize stable latexes with a high conversion, whereas for the second approach particles of >400 nm were created, without coagulum, but with 100% conversion. Subsequently, a water-soluble ruthenium carbene complex [poly(ethylene oxide)-based catalyst] was prepared and used in the direct miniemulsion ROMP of norbornene [83], whereby particles of 200-250 nm were produced. The catalytic polymerization of norbornene in direct miniemulsion was also carried out in the presence of an oil-soluble catalyst generated in situ, or with a water-soluble catalyst [84] the reaction was faster when using the oil-soluble catalyst. Helical-substituted polyacetylene could be efficiently polymerized in direct miniemulsion to yield a latex with particles that ranged between 60 and 400 nm in size, and which displayed an intense circular dichroism [85] that increased as the particle size decreased. The films were prepared from dried miniemulsion latexes that had been mixed with poly(vinyl alcohol) (PVA) in order to conserve the optical activity. 

Thanks to the tremendous progress in the transition metal-catalyzed polymerization of substituted acetylenes as described in the previous sections, it is now possible to access various acetylene-based polymers having desired flrst-order structures. This, in combination with highly advanced organic synthetic technology, provides novel fimctional materials based on polyacetylenes, and the following surveys examples of the design and synthesis of functional substituted polyacetylenes. 

Optically Active Substituted Polyacetylenes. The repulsion between the pendants in substituted polyacetylenes twists the main chain, which discourages the studies on the synthesis of acetylene-based pol5uner magnets. Recently, this main-chain torsion has been extensively applied to the synthesis of chiral... 

Helical Poly(phenylacetylenes). The most widely studied helical-substituted polyacetylenes are based on poly(phenylacetylene) with chiral ring substituents. Pol5unerization of chiral phenylacetylenes was first reported in 1995 (72). 4-(-)-Menthoxycarbonylphenylacetylene (41) was subjected to the polymerization with several transition metal catalysts such as [(nbd)RhCl]2 and WCle. The resultant Rh-based polymer shows a large optical rotation and intense CD effects in the electric absorption region of the main chain. The polymer, thus, exists in a helical conformation with an excess of one-handed screw-sense. Poly(phenylacetylene) with small chiral pendants, poly(42), in contrast, displays poorer chiroptical properties. Interestingly, an increase in temperature steeply increases the optical rotation of poly(41) if the polymer is produced with a W catalyst. Such a drastic enhancement of chiroptical properties is not observed in the case of Rh-based poly(41). 

Polyacetylenes are the most typical and basic r-conjugated polymers, and can ideally take four geometrical structures (trans-transoid, trans-cisoid, cis-transoid, cis-cisoid). At present, not only early transition metals, but also many late transition metals are used as catalysts for the polymerization of substituted acetylenes. However, the effective catalysts are restricted to some extent, and Ta, Nd, Mo, and W of transition metal groups 5 and 6, and Fe and Rh of transition metal groups 8 and 9 are mainly used. The polymerization mechanism of Ta, Nd, W, and Mo based catalysts is a metathesis mechanism, and that of Ti, Fe, and Rh based catalysts is an insertion mechanism. Most of the substituted polyacetylenes prepared with W and Mo catalysts provide trans-rich and cis-rich geometries respectively. Polymers formed with Fe and Rh catalysts selectively possess stereoregular cis main chains. 

As with any prospective new application we reasoned that optimization of physical and chemical properties would be required in order to generate practically useful electrically conductive polymers. We were concerned about mechanical properties, flexibility, conductivity levels, solubility, processability, oxidative stability, etc. Based upon the perceived requirement of a conjugated polyene structure, substituted polyacetylenes were the obvious way to introduce substituents for the purpose of tailoring these characteristics. Unfortunately the literature provided ample evidence of the sluggish nature of substituted polyacetylenes toward polymerization. 

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