Polyacetylene decomposition

In TGA studies on the decomposition of iodine-doped polyacetylene, at high heating rates (30°C/min), decomposition becomes explosive at the m.p. of iodine, 113°C. This was attributed to exothermic reaction of liquid iodine with polyacety-lene. 

The tendency towards explosive decomposition noted for dihalo-2,4-hexadiyne derivatives appears to be associated more with the co-existence of halo- and acetylene functions in the same molecule, than with its being a polyacetylene. Haloacetylenes should be used with exceptional precautions [1], Explosions may occur during distillation of bromoacetylenes when bath temperatures are too high, or if air is admitted to a hot vacuum-distillation residue [2], Precautions necessary in isolating and handling such compounds on the small (1 g) scale are detailed [3], Individually indexed compounds are ... 

Polyacetylenes intended as intermediates in synthesis of, especially, [18] annulene have been the major growth area in academic explosion from the 5th to the 6th edition of this handbook. No injuries and negligible damage have resulted, doubtless as much due to the care of the researchers as the often limited power of the compounds. Annulenes do not yet seem to have resulted even when they should be the initial decomposition product, implying that they will not be very stable themselves. Related compounds are now deliberately being caused to explode in hopes of preparing carbon nanostructures. 

Fig. 57 a and b. Change of the diffraction pattern of cis-polyacetylene upon heating from room temperature with 11 °C min" The cis/trans isomerization is indicated by the disappearance of the 201 reflection at about 150 °C. Above about 290 °C a broad peak indicates decomposition... 

The reaction of dienophiles with polyacetylene (33) was investigated for limiting the decomposition of the polyene. Previously, intermolecular Diels-Alder reactions involving ds bonds were believed to form cross-links and render polyacetylene intractable, and that their elimination would lead to soluble products (34). No reaction of maleic anhydride or benzoquinone with polyacetylene was observed. In contrast, a second study (35) described the successful formation of the Diels-Alder adduct of maleic anhydride with remnant cis bonds in thermally isomerized samples of fran -polyacetylene. These data were used to support the notion that the thermal isomerization process used to convert cw-polyacetylene to the trans isomer does not go to completion. Unfortunately, the effect of this transformation on the tract-ability or stability of the polymer was not noted. 

A number of other unsaturated poiyhydrocarbons have practical applications. These include poiy(phenyl acetylene) and poly((E,E)-[6.2]paracyclophane-1,5-diene), which have been studied as photoconducting polymers. The thermal decomposition of polyacetylenes and of poly((E,E)-[6.2]paracyclophane-1,5-diene) generates fragments summarized in Table 7.1.8 [19]. 

Table 7.1.8. Summary regarding thermal decomposition of polyacetylene and of poly((E,E)-[6.2]paracyclophane-1,5-diene) [19],...

Riekel [50] investigated the irreversible transformation of ew-polyacetylene into th trans-modification. The sample was heated from room temperature to 350 °C wii a rate of 11 °C/min and the WAXS was recorded continuously. The result is shov in Fig. 24. At 150 °C the (201) crystal reflection vanishes, indicating the cisjtrans transition. Above 290 °C a decomposition takes place, leading to the observed bro peak. 

Hexayne 70 is one of the highest melting (m.p. 240 C with decomposition) and least soluble macrocycles we have encountered in this work. It survives sublimation under vacuum at 160 °C and is barely soluble enough in cyclohexane to give a UV spectrum. Unlike most of the other macrocyclic polyacetylenes, it shows poor solubility even in benzene. Unfortuantely, the UV... 

Compounds in this category are bicyclo[3.3.0]octene (15), tricyclo[5.2.1.0 ]dec-3-ene (16), exo-tricyclo[5.2.1.0 ]dec-8-ene (17), benzvalene (18), and delta-cyclene (19). Of these, 15 and 16 do not undergo ROMP with conventional catalysts (Ofstead 1972), but 17 does so (Oshika 1968). The ROMP of 18 proeeeds smoothly using tungsten carbene initiators, and films of the polymer 18P can be cast directly from the reaction mixture (Swager 1988, 1989). The polymer has a tendency to cross-link and to decompose spontaneously once isolated in dry form, so is best handled in solution, especially as the decomposition can be explosive. The DSC thermogram of the polymer shows an exotherm at 153°C, attributed mainly to isomerization to polyacetylene 20, eqn. (3), and a second exotherm at... 

The isomerization temperature of linear polyacetylene is reported as 155 The higher temperature process is assumed to be due to the exo to endo rearrangement and/or the thermal decomposition reaction. Poly(l,6-heptadiyne) homologues also showed similar two exothermic peaks in the DSC thermogram, as shown in Table 14. 

Structuring polymer occurs in the temperature range 280-380°. But 72 per cent of initial weight of polyacetylene losses at 720°C. The main products of the decomposition of polyacetylene are benzene, hydrogen and... 

Polyacetylene (PA) is a good model for studying the thermal decomposition of the polyene sequences formed from PVC by HCl loss. ... 

Ito, T., H. Shirakawa, and S. Ikeda. 1975. Thermal cis-trans isomerization and decomposition of polyacetylene. J Polym Sci Polym Chem 13 1943. 

Table II. Onset Ibmperaiure of Thermal Decomposition of Disubstituted Polyacetylenes in Air...

A variety of routes have been proposed for the synthesis of polyacetylene. These can be classified into four categories as summarized in Table 7.1. The first is via the catalytic polymerization of acetylene. The second is noncatalytic polymerization. So far, spontaneous polymerization of acetylene has been reported under high pressure. The third type of route is the catalytic polymerization of monomers other than acetylene. The fourth is a so-called precursor route in which linear conjugated polyene chains are formed either by decomposition or by isomerization of soluble precursor polymers. 

One method of achieving orientation in a conducting polymer is the preparation of a prepolymer which is tractable. P.D. lowsand presented results on "Durham" polyacetylene (J. Feast) which is prepared by the thermal decomposition of a percursor polymer. A 20 1 ratio ofi to obtained for the polyacetylene prepared in this way. 

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