Soluble substituted polyacetylenes

The examination of conjugated, soluble, substituted polyacetylenes remains in its early stages. There are still a number of interesting synthetic targets which have not been approached. One example is fluorinated polyacetylene. Theoretical reports indicate that the material should behave very differently from normal polyacetylene [143-147]. For example, a recent report suggests that poly(difluoroacetylene) will be nonplanar, and that poly(fluoro-acetylene), if polymerized head-to-tail, will be most stable in the cis configuration [148]. 

Unlike polyacetylene, substituted polyacetylenes are amorphous, electrically insulator soluble polymers.413 They are highly stable and not sensitive to oxidation. Since the substituents exert a strong steric effect, the polyene backbone is not copla-nar, and as a result, only limited conjugation is possible. 

Extending the metathesis polymerization methodology to other cyclooctate-traene derivatives, provides a convenient route to a variety of substituted polyacetylene derivatives. For example, soluble conjugated polyacetylene derivatives can be prepared through the ROMP of trimethylsilylcyclooctatetraene (40) Eq. (41) [60 a d]. 

Many substituted polyacetylenes (e.g., all the polymers in Table 25) are completely soluble in low-polarity solvents such as toluene and chloroform. Their solubility can be attributed to the presence of substituents and their amorphous structure. When inspected in more detail, aliphatic and silicon-containing polyacetylenes are soluble in aliphatic solvents such as hexane but insoluble in 1,2-dichloroethane, whereas aromatic polyacetylenes show the opposite solubility property. 

The high solubility and high stability of substituted polyacetylenes are the two most important properties which are not seen with polyacetylene. Consequently, stable membranes can be easily obtained by casting solutions of substituted polyacetylenes. This will greatly facilitate their application. Here, we refer to several functions of substituted polyacetylenes, which might be applied to oxygen enrichment of air, separation of ethanol-water mixtures, and so on. 

Many properties of polyacetylenes with bulky substituents are substantially different from those of polyacetylene. For example, the substituted polyacetylenes are soluble because of the interaction between the substituents and solvent. Furthermore, such polymers are usually only lightly colored and are stable in air at room temperature. These properties arise from twisted conformations assumed by the main chain because of the presence of substituents. The electrically insulating and nonparamagnetic properties of substituted polyacetylenes are attributable also to the same cause. 

Poly(2a) and poly(2b) are partly insoluble in toluene. In contrast, poly(2c) and poly(2d) are totally soluble in toluene, because they have a long, flexible alkyl chain. The molecular weights of these polymers are fairly high, 5 X 10 -3 X 10 Free-standing films can be obtained from poly(2c) and poly (2d) by solution casting (usually, a molecular weight of over 10 is necessary for the formation of free-standing film from a substituted polyacetylene). All the poly(2)s are yellow solids. 

A growing array of different terminal and internal alkynes have been polymerized [8]. Many polyalkynes are air-stable, soluble materials, and not highly conjugated. As new catalysts allow the polymerization of alkynes with an increasing variety of substituents, an exploration of what properties unsaturated polymers have to offer is warranted. In general, substituted polyacetylenes may or may not be colored, and tend to be more rigid than saturated polymers. Selected materials are described below and compiled in Table 10-1. 

However the material obtained was an unprocessable powder. Unlike substituted polyacetylenes, polyacetylene is insoluble, infusible and unstable in air. The discovery of a technique to synthesize the polymer in the form of a free-standing film and the use of electron donors and acceptors to dope it to have metallic conductivity produced intense interest in the polymer in the last two decades. A wide variety of catalyst systems has been described for the polymerization of acetylene. Besides the route via acetylene polymerization, polyacetylene can also be obtained by a two-step route which involves the synthesis of soluble polymer precursors, which are converted to polyacetylene via thermal elimination and transition-metal-catalysed isomerization, as well as by polymerization of cyclooctatetraene , by dehydrochlorination of poly(vinyl chloride) and by dehydration of poly(vinyl alcohol)... 

Most substituted polyacetylenes obtained with group 5 and 6 transition-metal catalysts are soluble in common organic solvents with relatively low polarities such as toluene, chloroform, and tetrahydrofuran (Table 11). Aliphatic polyacetylenes are also soluble in... 

Another system of interest is substituted polyacetylene, first because the backbone is simple and close to the ethylene structure, second because numerous soluble polyacetylenes with either long alkyl chains or bulky substituents are available, so that a systematic study can be performed. Due to the structure of the backbone and the great variety of linear polymers, a direct comparison with a similar saturated polymer was thu.s possible, providing additional information on the actual effect of the conjugated backbone. 

In general, the stability and solubility of conductive polymers are improved by substitution on the polymer backbone, whereas their dopability and electrical conductivity are badly affected. For example, the phenyl substituted polyacetylene showed fairly good stability, however, the material was not dopable to... 

To increase the processability and provide various functionalities of polyacetylene, a study on the synthesis and characterization of substituted polyacetylenes has been extensively investigated.The introduction of functional substituents into polyacetylene causes a drastic change in various properties of the polymers, because of their solubility, fusibility, and interesting chemical, optical, and other properties. 

Additional improvements in preparations of polyacetylene came from several developments. One is the use of metathesis polymerization of cyclooctatetraene, catalyzed by a titanium alkylidene complex. The product has improved conductivity, though it is still intractable and unstable. By attaching substituents it is possible to form soluble and more stable materials that can be deposited from solution on various substrates. Substitution, however, lowers the conductivity. This is attributed to steric factors introduced by the substituents that force the double bonds in the polymeric chains to twist out of coplanarity." Recently, a new family of substituted polyacetylenes was described. These polymers form from ethynylpyridines as well as from ethynyldipyridines. The polymerization reaction takes place spontaneously by a quatemization process ... 

Like other substituted polyacetylenes, these materials are fairly stable in air and are soluble in polar solvents, also in water. The conductivity, however, is improved over previously reported substituted polyacetylenes to within the range of semiconductors. 

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. 

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. 

Since polymers of substituted PA have good solubility and good air stability, they make good membranes. Even though substituted PA do not possess very high conductivity, some of them exhibit excellent gas and liquid permeability. These two factors combined imply that substituted polyacetylenes could potentially be used for the oxygen enrichment of air and the separation of ethanol-water mixtures [111]. 

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