Polysulfide

Polysulfides are a class of chemical compounds containing chains of sulfur atoms. There are two main classes of polysulfides: anions and organic polysulfides. Anions have the general formula S_n²⁻. These anions are the conjugate bases of the hydrogen polysulfides H₂S_n. Organic polysulfides generally have the formulae RS_nR, where R = alkyl or aryl.

Polysulfide salts

The alkali metal polysulfides arise by treatment of a solution of sulfide, e.g. sodium sulfide, with elemental sulfur:

S²⁻
+ n S → S²⁻
_n+1

Alkali metals other than Na⁺ can be used. In some cases, these anions have been obtained as organic salts, which are soluble in organic solvents.

The energy released in the reaction of sodium and elemental sulfur is the basis of battery technology. The sodium-sulfur battery and the lithium sulfur battery require high temperatures to maintain liquid polysulfide and Na⁺-conductive membranes that are unreactive toward sodium, sulfur, and sodium sulfide.

Polysulfides are common ligands in coordination chemistry. Example of transition metal polysulfido complexes include (C₅H₅)₂TiS₅, [Ni(S₄)₂]²⁻, and [Pt(S₅)₃]²⁻. Main group elements also form polysulfides.

Organic polysulfides

In commerce, the term "polysulfide" usually refers to a class of polymers with alternating chains of several sulfur atoms and hydrocarbons. The general formula for the repeat unit is –[(CH₂)_m–S_x]_n– , where x indicates the number of sulfur atoms (or rank), and n indicates the number of repeating units. Polymers containing sulfur atoms separated by hydrocarbon sequences are usually not classified polysulfides, e.g. polyphenylene sulfide (C₆H₄S)_n.

Polysulfide polymers can be synthesized by condensation polymerization reactions between organic dihalides and alkal metal salts of polysulfide anions:

n Na₂S₅ + n ClCH₂CH₂Cl → [CH₂CH₂S₅]_n + 2n NaCl

Dihalides used in this condensation polymerization are dichloroalkanes (such as 1,2-dichloroethane, bis-(2-chloroethyl)formal (ClCH₂CH₂OCH₂OCH₂CH₂Cl), and 1,3-dichloropropane. In some cases, polysulfide polymers can be formed by ring-opening polymerization reactions. The polymers are called thiokols.

Polysulfide polymers are insoluble in water, oils, and many other organic solvents. Because of their solvent resistance, these materials find use as sealants to fill the joints in pavement, automotive window glass, and aircraft structures.

Polysulfides in vulcanized rubber

Many commercial elastomers contain polysulfides as crosslinks. These crosslinks interconnect neighboring polymer chains, thereby conferring rigidity. The degree of rigidity is related to the number of crosslinks. Elastomers therefore have a characteristic ability to "snap back" to their original shape after being stretched or compressed. Because of this memory for their original cured shape, elastomers are commonly referred to as rubbers. The process of crosslinking the polymer chains in these polymers with sulfur is called vulcanization. The sulfur chains attach themselves to the "allylic" carbon atoms, which are adjacent to C=C linkages. Vulcanization is applied to the processing of several classes of rubbers, including polychloroprene (Neoprene^TM), styrene-butadiene, and polyisoprene, which is natural rubber. Charles Goodyear's discovery of vulcanization, involving the heating of polyisoprene with sulfur, was revolutionary because it converted a sticky and almost useless material into an elastomer which could be fabricated into useful products.

Occurrence on gas giants

In addition to water and ammonia, the clouds in the atmospheres of the gas giant planets contain ammonium sulfides. The reddish-brownish clouds, which have been exposed to prolonged sunlight are attributed to polysulfides.

Properties

Polysulfides, as sulfides, can induce stress corrosion cracking in carbon steel and stainless steel.