Main Question or Discussion Point
How strong a long chain of carbon carbon double bonds will be compared to nano tubes?
I think this is more a consequence of the Peierls instability of long chains with equidistant bonds.Cumulenes are actually less stable than conjugated alkenes (C-C=C...) where a single and double bond alternate. This occurs because the valence π orbitals in a cumulene are at right angles to one another and so do not allow much delocalization. For conjugated species, however, the π orbitals are oriented such that an electron can delocalize over a large distance, lowering the energy of the system and stabilizing it.
In the long-chain limit, cumulenes buckling to polyalkynes is due to Peierls distortion, but you don’t see the same strong bifurcation in CC bond lengths in polyalkenes like you do with polyalkynes. I couldn’t say for certain why that’s the case off the top of my head. (It’s also interesting that you see isomerization in short chain cumulenes as well. I have an idea for why this occurs, but no hard evidence.)I think this is more a consequence of the Peierls instability of long chains with equidistant bonds.
I don’t have access to the full paper right now, but I doubt it. For One, I imagine the nanotube (already a very strong material) would swamp any effect you’d see from the carbyne. Also, as far as actually using the material in bulk for its mechanical properties, the limiting factor is always going to be the weakness of the noncovalent intertube interaction. It’s the same reason the whole “elephant on a pencil” argument for graphene’s strength is so misleading.