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I am a bit new to the world of chemistry, and after messing around on ChemSpider, I had a thought. Could a polymer like this exist: enter image description here

Due to VSEPR, it would be shaped pretty weirdly and N-N single bonds are known to be atrociously weak, but is there a reason as to why this polymer has not been observed at all? It seems like it should be possible (though very unstable), so I'm not sure why it hasn't been seen at all.

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    $\begingroup$ Polymerization from $\ce{H-N=N-H}$ would be endothermic reaction and the monomer itself is unstable, forming mostly nitrogen as hydrazine. $\endgroup$
    – Poutnik
    Commented Mar 7 at 6:27

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This question touches upon a concept of catenation - an ability of atoms of the same element to make bonds between them that could produce series constituting a chain or ring structure.

Something like that is quite common for nonmetal chemical elements like silicon (polysilanes), phosphorus (red allotrope consisting of interconnected P4 units) or sulfur (S8). In case of chemical elements from 2nd row of periodic table thing are a bit different - we have carbon, which is famous for its catenation abilities, while other elements - nitrogen and oxygen are very limited in this regard. This is often explained by repulsion of lone electron pairs that are present on oxygen and nitrogen atoms, which would destabilize the molecule if present in a series of neighboring atoms (in case of alkanes there are no lone electron pairs on carbon atoms - there are C-H bonds instead). For elements in lower parts of periodic table atom sizes are larger and bonds are longer and thus repulsion of lone electron pairs seem not to be such issue anymore. (Interestingly, boron also makes structures with B-B bonds, but there are no lone pairs on boron atoms and these structures are not linear.)

Going back to nitrogen - there have been attempts on making compounds with a series of nitrogen atoms one after another, i.e. making a chain:

https://chemistry-europe.onlinelibrary.wiley.com/doi/abs/10.1002/chem.202004830

Also, there have been some theoretical studies on the matter:

https://pubs.acs.org/doi/10.1021/acs.jpcb.3c00620

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A magnesium nitride compound with the empirical formula $\ce{MgN4}$, having polymerized nitrogen chains, is known under high pressure.

Under about 50 GPa pressure magnesium can form a pair of nitrides other than the common $\ce{Mg3N2}$[1]. $\ce{[Mg^{2+}]2[N4]^{4-}}$ contains discrete $\ce{N4^{4-}}$ ions, which are trapezoidal in shape with the two end nitrogens cis relative to the central nitrogen-nitrogen bond. This geometry, which would not be expected based on steric considerations, may be stabilized by homoaromatic coupling as the ion has six pi-symmetry electrons. In the other compound, $\ce{[Mg^{2+}][N4]^{2-}}$, the nitrogen forms an infinite chain of overlapping $\ce{N4}$ trapezoids sharing their lateral edges. Including shared electrons along the shared edges, each trapezoid again effectively has access to six pi-symmetry electrons and may be homoaromatic.

Reference

  1. Laniel, D., Winkler, B., Koemets, E. et al. Synthesis of magnesium-nitrogen salts of polynitrogen anions. Nat Commun 10, 4515 (2019). https://doi.org/10.1038/s41467-019-12530-w
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