Carbyne in both its cumulene (H-C=C-C=C-H) and Polyyne (alternating single and triple bonds) forms are very unstable alone. There are ways to synthesize them as pseudocarbynes, however they are not necessarily convenient or simple. Carbyne is quite an elusive carbon allotrope so it may take some tweaking to get the synthesis to work according to how you want it. However, here is an article that I used originally to organically synthesize a chain.