Polysulfides are a class of chemical compound
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...

s containing chains of sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...

 atoms. There are two main classes of polysulfides: anions and organic polysulfides. Anions have the general formula Sn2−. These anions are the conjugate bases of the hydrogen polysulfides H2nSn. Organic polysulfides generally have the formulae RSnR, where R = alkyl or aryl
In the context of organic molecules, aryl refers to any functional group or substituent derived from an aromatic ring, be it phenyl, naphthyl, thienyl, indolyl, etc....


Polysulfide salts

The alkali metal polysulfides arise by treatment of a solution of sulfide
A sulfide is an anion of sulfur in its lowest oxidation state of 2-. Sulfide is also a slightly archaic term for thioethers, a common type of organosulfur compound that are well known for their bad odors.- Properties :...

, e.g. sodium sulfide
Sodium sulfide
Sodium sulfide is the name used to refer to the chemical compound Na2S, but more commonly it refers to the hydrate Na2S·9H2O. Both are colorless water-soluble salts that give strongly alkaline solutions...

, with elemental sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...

+ n S → S

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
Sodium-sulfur battery
A sodium–sulfur battery or liquid metal battery is a type of molten metal battery constructed from sodium and sulfur . This type of battery has a high energy density, high efficiency of charge/discharge and long cycle life, and is fabricated from inexpensive materials...

 and the lithium sulfur battery
Lithium sulfur battery
The lithium–sulfur battery is a rechargeable galvanic cell with a very high energy density. By virtue of the low atomic weight of lithium and moderate weight of sulfur, Li–S batteries are relatively light; about the density of water. They were demonstrated on the longest and highest-altitude...

 require high temperatures to maintain liquid polysulfide and Na+-conductive membranes that are unreactive toward sodium, sulfur, and sodium sulfide.

Polysulfides are common ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...

s 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–Sx]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
Poly(p-phenylene sulfide)
Polyphenylene sulfide is an organic polymer consisting of aromatic rings linked with sulfides. Synthetic fiber and textiles derived from this polymer are known to resist chemical and thermal attack. PPS is used to make filter fabric for coal boilers, papermaking felts, electrical insulation,...


Polysulfide polymers can be synthesized by condensation polymerization reactions between organic dihalides and alkali 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
The chemical compound 1,2-dichloroethane, commonly known by its old name of ethylene dichloride , is a chlorinated hydrocarbon, mainly used to produce vinyl chloride monomer , the major precursor for PVC production. It is a colourless liquid with a chloroform-like odour...

, bis-(2-chloroethyl)formal (ClCH2CH2OCH2OCH2CH2Cl), and 1,3-dichloropropane. In some cases, polysulfide polymers can be formed by ring-opening polymerization
Ring-opening polymerization
In polymer chemistry, ring-opening polymerization is a form of chain-growth polymerization, in which the terminal end of a polymer acts as a reactive center, where further cyclic monomers join to form a larger polymer chain through ionic propagation...

 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 sealant
A sealant may be viscous material that has little or no flow characteristics and stay where they are applied or thin and runny so as to allow it to penetrate the substrate by means of capillary reaction...

s to fill the joints in pavement, automotive window glass, and aircraft structures.

Polysulfides in vulcanized rubber

Many commercial elastomers contain polysulfides as crosslinks
Cross-links are bonds that link one polymer chain to another. They can be covalent bonds or ionic bonds. "Polymer chains" can refer to synthetic polymers or natural polymers . When the term "cross-linking" is used in the synthetic polymer science field, it usually refers to the use of...

. 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
Curing (chemistry)
Curing is a term in polymer chemistry and process engineering that refers to the toughening or hardening of a polymer material by cross-linking of polymer chains, brought about by chemical additives, ultraviolet radiation, electron beam or heat...

 shape, elastomers are commonly referred to as rubber
Natural rubber, also called India rubber or caoutchouc, is an elastomer that was originally derived from latex, a milky colloid produced by some plants. The plants would be ‘tapped’, that is, an incision made into the bark of the tree and the sticky, milk colored latex sap collected and refined...

s. The process of crosslinking the polymer chains in these polymers with sulfur is called vulcanization
Vulcanization or vulcanisation is a chemical process for converting rubber or related polymers into more durable materials via the addition of sulfur or other equivalent "curatives." These additives modify the polymer by forming crosslinks between individual polymer chains. Vulcanized material is...

. 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
Chloroprene is the common name for the organic compound 2-chlorobuta-1,3-diene, which has the formula CH2=CCl-CH=CH2. This colorless liquid is the monomer for the production of the polymer polychloroprene, a type of synthetic rubber...

Neoprene or polychloroprene is a family of synthetic rubbers that are produced by polymerization of chloroprene. Neoprene in general has good chemical stability, and maintains flexibility over a wide temperature range...

TM), styrene-butadiene, and polyisoprene
Isoprene , or 2-methyl-1,3-butadiene, is a common organic compound with the formula CH2=CCH=CH2. Under standard conditions it is a colorless liquid...

, which is natural rubber. Charles Goodyear
Charles Goodyear
Charles Goodyear was an American inventor who developed a process to vulcanize rubber in 1839 -- a method that he perfected while living and working in Springfield, Massachusetts in 1844, and for which he received patent number 3633 from the United States Patent Office on June 15, 1844Although...

'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
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

 and ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...

, the clouds in the atmospheres of the gas giant
Gas giant
A gas giant is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in the Solar System: Jupiter, Saturn, Uranus, and Neptune...

 planets contain ammonium sulfides. The reddish-brownish clouds, which have been exposed to prolonged sunlight are attributed to polysulfides.


Polysulfides, as sulfides, can induce stress corrosion cracking
Stress corrosion cracking
Stress corrosion cracking is the unexpected sudden failure of normally ductile metals subjected to a tensile stress in a corrosive environment, especially at elevated temperature in the case of metals. SCC is highly chemically specific in that certain alloys are likely to undergo SCC only when...

 in carbon steel and stainless steel
Stainless steel
In metallurgy, stainless steel, also known as inox steel or inox from French "inoxydable", is defined as a steel alloy with a minimum of 10.5 or 11% chromium content by mass....

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