Polysulfide
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Polysulfides are a class of chemical compounds containing chains of sulfur atoms. In their simplest form, polysulfides are anions with the general formula Sn2−.
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[edit] Polysulfide polymers
In the commercial world, however, polysulfide usually refers to a class of polymers with alternating chains of several sulfur atoms and hydrocarbon. 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 repeat units. Polymers containing sulfur atoms separated by hydrocarbon sequences are not 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.
The dihalide species that are reportedly used in the condensation polymerization method are dichloroalkanes (such as 1,2-dichloroethane (ClCH2CH2Cl), bis-(2-chloroethyl)formal (ClCH2CH2OCH2OCH2CH2Cl), and 1,3-dichloropropane (ClCH2CH2CH2Cl)), whereas sodium polysulfides (Na2Sx) are the alkaline polysulfides of choice.
In some cases, polysulfide polymers can be formed by ring-opening polymerization reactions.
Polysulfide polymers are insoluble in water, oils, and many other organic solvents and therefore are often used as sealants. These sealants have been used to fill the joints in pavement, automotive window glass and aircraft structure. A primary reason for using the polysulfide sealants in these applications is to prevent water intrusion.
[edit] Polysulfides in vulcanized rubber
Many commercial elastomers currently have polysulfide 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 adjacent to C=C linkages. Vulcanization is applied to the processing of several classes of rubbers, including polychloroprene (NeopreneTM), 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.
[edit] Inorganic polysulfides
Inorganic polysulfides are common, the most useful one being lime sulfur CaSn, which is used as a pesticide in agriculture. Well defined polysulfide compounds have been widely discussed in the research literature. Examples include polysulfide complexes of the transition metals such as (C5H5)2TiS5, [Ni(S4)2]2−, and Pt(S5)3]2−. Main group elements also form polysulfides.
[edit] References
- Draganjac, M. E. and Rauchfuss, T. B., "Transition Metal Polysulfides: Coordination Compounds with Purely Inorganic Chelate Ligands", Angewandte Chemie International, Edition in English, 1985, vol. 24, 742.
- Takeda, N.; Tokitoh, N. and Okazaki, R., "Polysulfido Complexes of Main Group and Transition Metals", Topics in Current Chemistry, 2003, vol. 231, 153-202.
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