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Is it possible to have an allotrope of carbon with formula $\ce{(=C=)_n}$?

Well, let's actually leave alone the separate question like from which "monomer" would one produce such a thing (can one actually consider it being a polymer? Or a covalent crystal?), because it seems like the most suitable candidate would be dicarbon.

All in all, might it be (under some conditions) energetically favorable for carbon to form a long unbranched chain of itself?

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    $\begingroup$ It's been all over the news recently: ox.ac.uk/news/science-blog/… although it's $\ce{C#C-C#C-C}$ rather than $\ce{C=C=C=C=C}$. $\endgroup$ – orthocresol Aug 23 '19 at 16:17
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    $\begingroup$ Thanks for the link, it was a pleasure to read! However, these guys made cyclocarbon, if I got it correctly. My question though is more about a linear form, reminiscent of a polymer. On the other hand, they tell "After the successful creation of the linear carbon segments ..." - which, maybe, means, that they can make out of them not a ring, but just keep going with linear chain?.. $\endgroup$ – Holographist Aug 23 '19 at 16:25
  • $\begingroup$ I seem to remember sth. about polyacetlyene, which gets first brominated and then dehydrohalogenated. Whether the end product is a cumulene or a polyyne is imo just a question of what end group(s) you have. Cant find a reference now, though. $\endgroup$ – Karl Aug 23 '19 at 17:37
  • $\begingroup$ it depends what you mean by "to have". If you mean that the structure is remotely stable then the answer is NO; if you mean that it is possible to create something, then the answer is still only maybe (and the structure of the bonding might not be what you think). $\endgroup$ – matt_black Sep 4 '19 at 10:42
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Well, this is a rather obvious development (in retrospect, that is). We had an $sp^3$ carbon with tetrahedral bonds, and it made diamond. We had an $sp^2$ carbon with trigonal bonds, and it made graphite. What if we just had a linear $sp$ carbon?

Such thoughts have been around for more than quite a while. Theorists swarmed around that non-existent tree. No wonder that people started trying to actually obtain that linear carbon.

Not knowing anything about you, I still can claim with a good deal of confidence that the first reports of its synthesis are quite probably somewhat older than you (or me, for that matter). Then they were proved... not quite reliable. Some groups agreed with that, some disagreed, a great deal of confusion abounded, then more reports came and things became hairy. So they stayed for the past few decades and will stay for a while, I guess.

Now to the Q&A.

  • Does this compound exist?

· Define "exist", otherwise no meaningful answer would be possible.

  • Is it stable?

· Define "stable", otherwise no meaningful answer would be possible.

  • Is it thermodynamically stable at any P and T?

· Most researchers agree that it isn't.

  • Is it electrically conductive along the strands?

· Most researchers agree that it is.

  • Is it made of double bonds or of alternating single and triple bonds?

· The difference is less than you seem to think, to the point that it might not be a question worth pursuing.

  • What is its crystal system?

· Nobody knows. Chances are there are polymorphs with different crystal systems.

  • Is it super strong?

· Nobody knows, as with any non-existent compound.

  • Should it be called a polymer?

· About as much so as graphite and diamond, which means: No.

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  • $\begingroup$ Great answer, but I have to disagree on your last point: A linear macromolecule of undefined lenght and a repeating structure, thats usually a polymer. $\endgroup$ – Karl Aug 23 '19 at 20:03
  • $\begingroup$ I disagree, but I understand that I might be in a minority on that one. $\endgroup$ – Ivan Neretin Aug 23 '19 at 20:05
  • $\begingroup$ Whats your reasoning for the "non–polymer" verdict? Oh, and you avoided the initial question: "Is this a carbon allotrope?" I also wouldnt want to judge on that. My inclination is no, however. ;) $\endgroup$ – Karl Aug 23 '19 at 20:09
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    $\begingroup$ Of course this thing would be a carbon allotrope (conditional on its existence, that is). As to the polymer question, see here: chemistry.stackexchange.com/questions/38238/… $\endgroup$ – Ivan Neretin Aug 23 '19 at 20:50
  • $\begingroup$ When someone says "we found a new carbon allotrope", Im inclined to retort "no, you just devised a synthetic route for a new molecule which happens to be carbononly". By common definition, that is of course an allotrope. Just feels weird for me. ;) $\endgroup$ – Karl Aug 23 '19 at 21:13

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