Long-chain polyynes

Cataldo F. Submerged electric arc between graphite electrodes a one-pot tool for the synthesis of long chain polyynes in solution. Tetrahedron Lett. 2004, 45, 141. 

Yet another serious problem that makes the preparation of perfect carbyne structures (long-chain polyynes or cumulenes) an almost hopeless endeavor is the presence of intrinsic defects in the original halogen-containing polymers. The polymers produced by common polymerization technologies always contain some defect structures such as head-to-head and "tail-to-tail links [32-34] as well as branching sites [35,36]. For instance, poly(vinylidene fluoride) was shown to contain up to 6 mol.% [32,33] and poly(vinylidene chloride) even 12.5% of abnormal links [34], which hamper exhaustive dehydrohalogenation (Scheme 12.9). 

Much current research is centering on polyynes—linear carbon chains of sp-hybridized carbon atoms. Polyynes with up to eight triple bonds have been detected in interstellar space, and evidence has been presented for the existence of carbyne, an allotrope of carbon consisting of repeating triple bonds in long chains of indefinite length. 

Although short-chain CAjjlL ( = 4-6) polyynes are observed from Cgo, long-chain C2 H2 (n = 7 and 8) polyynes could not be detected. In general, the n-distribution of CL, L has a sharp peak at = 4 and rapidly decreases with increasing n. 

What about the claimed synthesis of carbyne reported several times in literature and the uncertain detection of this allotrope in certain meteorites One important distinction is between carbyne and carbynoid structures. Carbyne remains an ideal structure for the representation of the pure sp-hybridized carbon allotrope. An almost infinite chain of carbon must be a solid, insoluble in any solvent under standard conditions, as they are the other well known allotropes graphite and diamond. On the other hand the carbynoid structures represent any carbon solid which contains polyyne chains, even very long chains, or domains, but stabilized in a matrix of other differently hybridized carbon atoms. Therefore, many publications on the synthesis of carbyne must be interpreted and rationalized in terms of the carbynoid structure concept. 

Vuitton V, Scemama A, Gazeau MC, Chaquin P, Benilan Y. IR and UV spectroscopic data for polyynes prediction for long carbon chain compounds in Titan s atmosphere. Adv. Space Res. 2001, 27, 283. 

Dinuclear complexes with a bridging polyynediyl ligand and diphosphine ligands with long methylene chains between the two P atoms show a structure in which the alkylene chains are double-helixed around the polyyne ligand... 

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