Polyacetylenes monosubstituted

The discovery of junipal focused the attention of Sorensen, who had been investigating the occurrence of polyacetylenes in Com-positae, on the possibility that these acetylenes were accompanied by thiophenes. From Coreopsis grandiflora Hogg ex sweet, 2-phenyl 5-(1-propynyl) thiophene (240) was isolated and its structure confirmed by synthesis of the tetrahydro compound, 2-phenyl-5-n-propyl-thiophene. From the root of tansy, the cis and trans isomers of methyl 5-(l-propynyl)-2-thienylacrylate (241) have been isolated. The total synthesis of trans (241) was achieved by reacting junipal with methylcarbethoxy triphenylphosphonium bromide (Wittig reaction) Several monosubstituted thiophenes, (242), (243), and... 

A route to processible polyacetylene, devised initially using classical initiators (Scheme 1i) 576-578 has been developed using well-defined molybdenum initiators to prepare conjugated polymers.579-585 They have also been employed to prepare polyacetylene via the polymerization of cyclooctate-traene, COT,586 and by the isomerization of poly(benzvalene).587 588 Substituted, and hence soluble, polyacetylene derivatives may be synthesized by polymerizing monosubstituted COT substrates.589-591... 

R. Hidayat, S. Tatsuhara, D.W. Kim, M. Ozaki, K. Yoshino, M. Teraguchi, and T. Masuda, Time-resolved study of luminescence in highly luminescent disubstituted polyacetylene and its blend with poorly luminescent monosubstituted polyacetylene, Phys. Rev. B, 61 10167-10173, 2000 and references therein. 

These polymers are generally white and have intrinsic viscosities which are sometimes nearly proportional to the MW, for example the polymer of EtC=CPh695. This indicates that the polymer molecules have a fairly rigid but twisted backbone with very little conjugation of the double bonds, unlike polyacetylene and polymers of linear monosubstituted... 

In Table 19 are collected typical monomers that polymerize with group 5 and 6 transition metal catalysts to produce high-molecular-weight (Mw > 1 x 10s) polyacetylenes. Among them, tert-butylacetylene and 3-(trimethylsilyl)-l-octyne are monosubstituted acetylenes, while the others are disubstituted ones. It is noteworthy that all of these monomers are considerably sterically crowded. By judicious choice of polymerization conditions, the polymer yield becomes fair to quantitative in every case. The Rtw s of the polymers reach ca. 3 x 10s 2 x 106. 

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. 

Monosubstituted acetylene polymers with relatively small steric effect, naturally, show properties intermediate between those of the two extremes in Table 25. For example, poly(l-hexyne) and poly(phenylacetylene) are dark brown to yellow, more or less sensitive to air, and somewhat paramagnetic. In the following discussion, individual properties of not these polyacetylenes but those with large steric effect will be mostly described. 

While polyacetylene is an electrical semiconductor, substituted polyacetylenes are virtually insulators (Table 13). Among them, the aliphatic polymers possess specific conductivities as low as 10 S cm" while aromatic monosubstituted acetylene polymers have values in the order of ca 10 S cm"h The low conductivities of substituted polyacetylenes are attributable to their twisted main-chain conformation induced by the bulky substituents. The unpaired-electron densities of most disubstituted acetylene polymers are below a detection limit of 1 x 10 spin In contrast, poly(phenylacetylene)... 

Gorman, C.B., Ginsburg, E.J., Grubbs, R.H., 1993. Soluble, highly conjugated derivatives of polyacetylene from the ring-opening metathesis polymerization of monosubstituted cyclooctatetraenes synthesis and the relationship between polymer structure and physical properties. I. Am. Chem. Soc. 115, 1397-1409. 

Tabata, M., T. Sone, and Y. Sadahiro. 1999. Precise synthesis of monosubstituted polyacetylenes using Rh complex catalysts. Control of solid structure and ir-conjugation length. Macro Chem Phys 200 265. 

FIGURE 47.19. Examples of EL polyacetylenes, (a) Monosubstituted (b) disubstituted. Reprinted with permission from Synth Met 1997 91 283. 1997 Elsevier Science. 

Polyacetylenes and vinyl acetylenes are known to show some selectivity in their reactions [109, 110]. Raphael [111] showed that terminal acetylenes could be protected as their sodium salts during reduction of disubstituted acetylenes using sodium in liquid ammonia. This procedure enabled him to reduce a disubstituted acetylenic bond in the presence of a terminal acetylene in the stereospecific synthesis of trans-undeca-7-en-l-yne (18) from undeca-l,7-diyne (19). This method of protection of monosubstituted acetylenes was used during reduction of the enone system (20) with lithium in liquid ammonia [112]. Raphael made a study of other possible methods of protecting terminal acetylenes and found that the best of these was the bromoacetylene [113]. 

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