= Solution
<polysulfur> has a chain of S–S <sigma bonds> with filled lone-pair orbitals. Its occupied states do not supply a partly filled delocalized band, so the chain is an <electrical insulator>.
In <polythiazyl>, an approximately planar S–N chain permits overlap of orbitals perpendicular to the backbone. In an elementary <pi bond> <electron> picture, <nitrogen> supplies one <electron> and <sulfur> two to this manifold: \b[three pi <electrons> per SN unit], rather than a completely filled, isolated two-centre bonding pair. The extended states are therefore not all fully occupied. More realistic descriptions include the crystallographic repeat, interchain interactions and overlapping bands crossing the <Fermi level>; the odd <electron> count of a chosen chemical repeat is suggestive, not by itself a proof of metallicity.
The essential distinction is \b[occupied, localized bonding/lone-pair states in <sulfur> chains versus available extended states at the <Fermi level> in crystalline <sulfur> nitride]. The measured high conductivity along the <polymer> fibres and its metallic temperature dependence provide the experimental basis, alongside the structural evidence for the chain. Conductivity depends on order and defects as well as chemical composition.<Synthesis and structure of the polymeric metal, (SN)x, and its precursor, S2N2 by Alan G. MacDiarmid and others>[J. Chem. Soc., Chem. Commun., pp. 476–477; X-ray structure and chain-direction metallic conductivity]{ref}
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