Polysulfides are a class of

chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...

s derived from

anion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...

ic chains of

sulfur Sulfur ( American spelling and the preferred IUPAC name) or sulphur ( Commonwealth spelling) is a chemical element; it has symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms ...

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 In organic chemistry, an alkyl group is an alkane missing one hydrogen. The term ''alkyl'' is intentionally unspecific to include many possible substitutions. An acyclic alkyl has the general formula of . A cycloalkyl group is derived from a cy ...

or aryl group.

Polysulfide salts and complexes

The alkali metal polysulfides arise by treatment of a solution of the

sulfide Sulfide (also sulphide in British English) is an inorganic anion of sulfur with the chemical formula S2− or a compound containing one or more S2− ions. Solutions of sulfide salts are corrosive. ''Sulfide'' also refers to large families o ...

with elemental

, 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

ligand In coordination chemistry, a ligand is an ion or molecule with a functional group that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's el ...

s in

coordination chemistry A coordination complex is a chemical compound consisting of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of chemical bond, bound molecules or ions, that are in turn known as ' ...

. Examples of transition metal polysulfido complexes include , , and . 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. 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. 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. Polymers containing one or two sulfur atoms separated by hydrocarbon sequences are usually not classified polysulfides, e.g. poly(''p''-phenylene) sulfide .

Polysulfides in vulcanized rubber

Many commercial

elastomer An elastomer is a polymer with viscoelasticity (i.e. both viscosity and elasticity) and with weak intermolecular forces, generally low Young's modulus (E) and high failure strain compared with other materials. The term, a portmanteau of ''ela ...

s 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 return to their original shape after being stretched or compressed. Because of this memory for their original cured shape, elastomers are commonly referred to as

rubber Rubber, also called India rubber, latex, Amazonian rubber, ''caucho'', or ''caoutchouc'', as initially produced, consists of polymers of the organic compound isoprene, with minor impurities of other organic compounds. Types of polyisoprene ...

s. 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 a step in the processing of several classes of rubbers, including poly chloroprene ( Neoprene), styrene-butadiene, and poly isoprene, which is chemically similar to 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 that could be fabricated into useful products.

Occurrence in gas giants

In addition to

water Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...

and

ammonia Ammonia is an inorganic chemical compound of nitrogen and hydrogen with the chemical formula, formula . A Binary compounds of hydrogen, stable binary hydride and the simplest pnictogen hydride, ammonia is a colourless gas with a distinctive pu ...

, the clouds in the atmospheres of the

gas giant A gas giant is a giant planet composed mainly of hydrogen and helium. Jupiter and Saturn are the gas giants of the Solar System. The term "gas giant" was originally synonymous with "giant planet". However, in the 1990s, it became known that Uranu ...

planets contain ammonium sulfides. The reddish-brownish clouds are attributed to polysulfides, arising from the exposure of the ammonium sulfides to light.

Properties

Polysulfides, like sulfides, can induce stress corrosion cracking in carbon steel and

stainless steel Stainless steel, also known as inox, corrosion-resistant steel (CRES), or rustless steel, is an iron-based alloy that contains chromium, making it resistant to rust and corrosion. Stainless steel's resistance to corrosion comes from its chromi ...

References