Polyacetylenes aromatic

With regards to the mechanical properties of substituted polyacetylenes, aromatic polymers like poly(diphenylacetylene) derivatives are generally hard and brittle, whereas aliphatic polymers with long alkyl chains like poly(2-octyne) are soft and ductile.Considerations of mechanical properties are especially important when polymer membranes or fibers are required for the specific application. 

Oxidative polymerization of trans-bis-deprotected 79 under Hay coupling conditions [54] yielded, after end-capping with phenylacetylene, the high-melting and readily soluble oligomers 80a-e with the poly (triacetylene) backbone [87,106] (Scheme 8). Poly(triacetylene)s [PTAs,-(C=C-CR=CR-C=C) -] are the third class of linearly conjugated polymers with a non-aromatic allcarbon backbone in the progression which starts with polyacetylene [PA,... 

In principle, it should be possible to prepare polyacetylene and its derivatives by coupling reactions of appropriately 1,2-substituted olefins. In practice this route does not appear to have been explored and step-reactions have mostly been applied to prepare aromatic and heteroaromatic polymers. Some of the more important syntheses ae reviewed below others have been reviewed by Feast29). 

The Durham precursor route to polyacetylene is an excellent example of the application of organic synthesis to produce a precursor polymer whose structure is designed for facile conversion to polyacetylene. Durham polyacetylene was first disclosed by Edwards and Feast, working at the University of Durham, in 1980 227). The polymer (Fig. 6 (I)) is effectively the Diels-Alder adduct of an aromatic residue across alternate double bonds of polyacetylene. The Diels-Alder reaction is not feasible, partly for thermodynamic reasons and partly because it would require the polymer to be in the all m-conformation to give the required geometry for the addition to take placed 228). However, the polymer can be synthesised by metathesis polymerization of the appropriate monomer. 

The aromatic residue may be any of a large number of such units but the favourite for academic study has been the perfluoromethylxylene derivative shown, which smoothly eliminates at around room temperature to give a polyacetylene containing 25 % of trans- and 75 % of m-units. After transformation and isomerization at 80 °C, the polyacetylene produced is a continuous dense film. The physical chemistry of the transformation and isomerization reactions has been studied in detail229,230) and the properties of the polyacetylene are reviewed 231). The great advantage of this route is that the precursor is a soluble polymer so that it can be characterized and the physical form of the polyacetylene can be controlled. 

As discussed earlier, substitution onto the polyacetylene chain invariably has a deleterious effect on dopability and conduction properties. At the same time the stability tends to improve. Masuda et al.583) studied a large range of substituted polyacetylenes and found that stability increased with the number and bulkiness of the substituents, so that the polymers of aromatic disubstituted acetylenes were very stable, showing no reaction with air after 20 h at 160 °C. Unfortunately, none of these polymers is conducting. Deitz et al.584) studied copolymers of acetylene and phenylacetylene they found that poly(phenylacetylene) degrades even more rapidly than does polyacetylene and that the behaviour of copolymers is intermediate. Encapsulation of the iodine-doped polymers had little effect on the degradation, which is presumably at least in part due to iodination of the chain. 

Included in such compounds are the fatty acids, polyacetylenes, prostaglandins, macrolide antibiotics and many aromatic compounds, e.g. anthraquinones and tetracyclines. 

Fig. 2. A molecular data storage scheme based on an aromatic molecule (naphthalene) bonded to four gold electrodes by sulfur atoms and polyacetylene wires [37). For the surface an insulator has to be chosen to prevent cross-talk between the electrodes. The variables X, Y and Z could either be chemical substituents or, alternatively, connections to further electrodes. Some parts of the molecule, electrodes and variables are drawn in bright colors, which is meant to indicate an active state during a particular read-out. The darker parts are considered to be inactive.

Unsubstituted polyacetylene, like many other conductive polymers, is an intractable material and thus its processing into useful shapes and morphologies is limited. One solution to the processing problems has been the use of soluble precursor polymers that can be transformed into conductive polymers. Application of ROMP in the formation of soluble polyacetylene precursors was elegantly pioneered by Feast and coworkers [61]. Using this approach, a precursor polymer is synthesized by the ROMP of a cyclobutene derivative. Once synthesized, the precursor polymer can undergo a thermally promoted, retro-Diels Alder reaction to split off an aromatic fragment and produce polyacetylene, Eq. (42). 

The hyperbranched polymers are carbon-rich macromolecules and show excellent thermal stabilities. The thermal properties of the hb-PAs are described below as an example. Their thermal stabilities were evaluated by TGA. Figure 3 shows TGA thermograms of some hb-PAs and Table 4 lists their thermal analysis data. The hb-FAs were thermally very stable for instance, hb-P66 lost merely 5% of its weight at a temperature as high as 595 °C. All the polymers, except for hb-F(44-Vl) and hb-F(59-Vl), carbonized in > 50% yields on pyrolysis at 800 °C, with hb-P(45-V) graphitized in a yield as high as 86% (Table 4, no. 3). The thermal stabilities of the hb-PAs are similar to that of Unear pol-yarylenes such as PPP but different from those of Unear polyacetylenes such as PH and PPA. The dramatic difference in the thermal stability is mainly due to the structural difference PPP is composed of thermally stable aromatic rings (Td 550 °C) [108-112], whereas PPA and PH are comprised of labile polyene chains, which start to decompose at temperatures as low as 220 and 150 °C, respectively [113]. The excellent thermal stabilities of the hb-FAs... 

V-UV Application First Excited State of Linear Polyenes. The first electronic absorption band of perfect linear aromatic polyenes (CH)X, or perfect polyacetylene shifts to the red (to lower energies) as the molecule becomes longer, and the bond length alternation (BLA) would be zero. This was discussed as the free-electron molecular orbital theory (FEMO) in Section 3.3. If this particle-in-a-box analysis were correct, then as x > oo, the energy-level difference between ground and first excited state would go to zero. This does not happen, however first, because BLA V 0, next, because these linear polyenes do not remain linear, but are distorted from planarity and linearity for x > 6. 

In the examples of our work on organic molecular and polymeric solids that follow, first some contributions to the UPS line widths in condensed molecular solids are discussed for two prototype systems, anthracene and isopropyl benzene then the UPS of two.aromatic pendant group polymers, polystyrene and poly(2-vinyl pyridine), are discussed and compared with some spectra concerning the simplest linear conjugated polymer, polyacetylene. 

The polymers whose geometric structures have been quantitatively evaluated are polyacetylene 89), poly(ferf-butylacetylene)19), poly(isopropylacetylene)14), and poly-(phenylacetylene)88). In the case of polymers from aromatic monosubstituted acetylenes, qualitative evaluation of geometric structure is possible by means of IR spectroscopy, differential thermal analysis, and X-ray diffraction 66,90). In contrast, no information has been obtained on the geometric structure of disubstituted acetylene polymers. This is due to the fact that their main chain comprises fully substituted ethylene units, the difference between cis and trans structures being small. 

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 instability of polyacetylene is notorious, that is, it is easily oxidized in air at room temperature. On the other hand, the substituted polyacetylenes shown in Table 27 are much more stable96. In general, the stability of substituted polyacetylenes increases with increasing number and/or bulkiness of substituents -f-CH = C(n-alkyl)- < -fCH=CPh, -f CMe=C( -alkyl)+n < CH=C(t-Bu), -f-CMe= CfSiMes)- < -j-C(n-alkyl) = CPh-, -(-CC CPh. Especially, the polymers of aromatic disubstituted acetylenes (e.g., (CMe=CPh, -fCCl=CPh ) are extreme-... 

Base-catalysed prototropic rearrangements of cyclic polyacetylenes have often been used to synthesize annulene and dehydroannulene systems. In such reactions, transannular carbon-carbon bond formation between proximate triple bonds occasionally takes place to give polycyclic compounds. Treatment with a base of cyclic polyacetylenes 57 and 59 gave benzenoid aromatic compounds 58 and 61, with the desired dehydro[16] and [I8]annulenes, respectively. ... 

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