thumb|220px|The compound [[Titanocene pentasulfide| is an example of a polysulfide complex]]
Polysulfides are a class of chemical compounds derived from anionic chains of sulfur atoms. There are two main classes of polysulfides: inorganic and organic. The inorganic polysulfides have the general formula . These anions are the conjugate bases of polysulfanes . Organic polysulfides generally have the formulae , where R is an alkyl or aryl group.
Polysulfide salts and complexes
thumb|left|220px|Fragment of the solid-state structure of . The chain consists of the yellow-colored atoms.
The alkali metal polysulfides arise by treatment of a solution of the sulfide with elemental sulfur, e.g. sodium sulfide to sodium polysulfide:
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 -conductive membranes that are unreactive toward sodium, sulfur, and sodium sulfide.
Polysulfides are ligands in coordination chemistry. Examples of transition metal polysulfido complexes include titanocene pentasulfide|, , and . Main group elements also form polysulfides.
Organic polysulfides
thumb|130px|left|[[Lenthionine is an organic polysulfide found in shiitake mushrooms]]
In commerce, the term "polysulfide" usually refers to a class of polymers with alternating chains of several sulfur atoms and hydrocarbons. They have the formula . In this formula n indicates the number of sulfur atoms (or "rank"). Polysulfide polymers can be synthesized by condensation polymerization reactions between organic dihalides and alkali metal salts of polysulfide anions:
Dihalides used in this condensation polymerization are dichloroalkanes such as 1,2-dichloroethane, bis(2-chloroethoxy)methane (), and 1,3-dichloropropane. The polymers are called thiokols. In some cases, polysulfide polymers can be formed by ring-opening polymerization reactions.
Polysulfide polymers are also prepared by the addition of polysulfanes to alkenes. An idealized equation is:
In reality, homogeneous samples of are difficult to prepare.
Properties
Polysulfides, like sulfides, can induce stress corrosion cracking in carbon steel and stainless steel.