Polysulfide

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Two parallel monoatomic sulfur chains grown inside a single-wall carbon nanotube (CNT, a). Zig-zag (b) and straight (c) S chains inside double-wall CNTs.[1]

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 Sn2−. These anions are the conjugate bases of the hydrogen polysulfides H2Sn. Organic polysulfides generally have the formulae RSnR, where R = alkyl or aryl.[2]

Polysulfide salts and complexes

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
Fragment of the solid-state structure of Na2S5. The S52- chain is evident.[3]

In some cases, these anions have been obtained as organic salts, which are soluble in organic solvents.[4]

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 ligands in coordination chemistry. Examples of transition metal polysulfido complexes include (C5H5)2TiS5, [Ni(S4)2]2−, and [Pt(S5)3]2−.[5] Main group elements also form polysulfides.[6]

The compound (C5H5)2TiS5 is an example of a polysulfide complex.

Organic polysulfides

Lenthionine is an organic polysulfide found in shitake 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 R2Sx'. In this formula x 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:

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). 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:

2 RCH=CH2 + H2Sx → (RCH2CH2)2Sx

In reality, homogeneous samples of H2Sx are difficult to prepare.[2]

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. polyphenylene sulfide (C6H4S)n.

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 a step in the processing of several classes of rubbers, including polychloroprene (NeopreneTM), styrene-butadiene, and polyisoprene, 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 which could be fabricated into useful products.

Idealized structure of two strands (blue and green) of natural rubber after vulcanization with elemental sulfur, which forms polysulfide linkages.

Occurrence in 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 are attributed to polysulfides, arising from the exposing the ammonium sulfides to light.[7]

Properties

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

References

  1. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  2. ↑ 2.0 2.1 Ralf Steudel "Sulfur: Organic Polysulfanes" Encyclopedia of Inorganic Chemistry 2007, Wiley-VCH, Weinheim. doi:10.1002/0470862106.ia233.pub2.
  3. ↑ Rosén, E.; Tegman, R. "Preparative and X - ray powder diffraction study of the polysulfides Na2S2, Na2S4 and Na2S5" Acta Chemica Scandinavica 1988, volume 25, pp 3329-3336. doi:10.3891/acta.chem.scand.25-3329
  4. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  5. ↑ Draganjac, M. E.; Rauchfuss, T. B., "Transition Metal Polysulfides: Coordination Compounds with Purely Inorganic Chelate Ligands", Angewandte Chemie International, Edition in English, 1985, vol. 24, 742. doi:10.1002/anie.198507421
  6. ↑ Takeda, N.; Tokitoh, N. and Okazaki, R. (2003) "Polysulfido Complexes of Main Group and Transition Metals", Topics in Current Chemistry, vol. 231, 153–202. ISBN 3-540-40378-7. doi:10.1007/b13184
  7. ↑ Jupiter: Cloud composition. Encyclopaedia Britannica
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