Polysulfides

Polysulfides can be obtained from the reaction of a dihalide and Na2S, as shown below for the formation of poly(1,4-phenylene sulfide)  

Poly(1,3-phenylene sulfide) also is known and has a decomposition temperature higher than poly(1,4-phenylene sulfide). The same reaction can take place with alkyl dihalides, such as 1,2-dichloroethane. However, since sodium sulfide is very frequently a mixture of sulfide and polysulfides, the polymer structure is better expressed by the formula -(R-Sx-)n- Other reactions are known to lead to polysulfides. For example, poly(phenylene sulfide) can be obtained from 4-chlorothiophenol by self condensation in the presence of a metal base. 

Some polysulfides contain in the polymer backbone other groups besides -S-. A common type of group is oxycarbonyloxy, such as in the polymer resulting from the reaction of phosgene (chloroformyl chloride or carbonic acid dichloride) with 4,4 -thiobisphenol. This reaction can be considered a polycondensation and can be written as follows  

The polymer is known as poly(oxycarbonyloxy-1,4-phenylenethio-1,4-phenylene) or as poly(4,4 -dihydroxydiphenylsulfide-co-carbonic dichloride) and can be viewed as an alt-copolymer of a sulfide and a carbonate. 

Another polymer containing sulfur in its backbone is obtained from thiophenol/ formaldehyde condensation. Differently from the condensation of phenol and formaldehyde, the main reaction path for this condensation is the following  

Solutions of alkali metal polysulfides have been investigated by UV/vis and Raman spectroscopy.1 2,33 All species S2 in the range n = 2-5 are present in 

Radical anions are also present in solutions of sulfur in oleum and in various polychalcogenide fluxes. However, only one radical ion Sg has been successfully characterized in the solid state, namely in [Ph4P]S6, which can be prepared according to Equation (3) by treating Ph4P]N3 with H2S in the presence of trimethylsilyl azide.35 

Finally, the complexity of the thermal decomposition of disulfides is well illustrated by qualitative results on the pyrolysis of dibenzyl disulfide . At temperatures below 150 °C the major products are dibenzyl, thiobenzophenone and elemental sulfur above 200 C a second reaction sets in, leading to the production of HjS, tetraphenylethane and tetraphenylethylene. It has even been suggested that at temperatures below 140 °C di- and polysulfides dissociate heterolytically into polar persulphenyl intermediates. 

Only qualitaiive details concerning the photolysis of trisulfides are at present available. As expected, the primary process involves S-S fission and the major products are the corresponding di- and tetrasulfides virtually no hydrocarbons were detected . This suggests intital S-S cleavage 

The thermal decomposition of dimethyl trisulfide at 80 °C also results in the formation of di- and tetrasulfides . It was further established that the decomposition proceeds via a radical mechanism, probably homolytic S-S cleavage, since in a weakly polar medium such as benzoyl peroxide and triethylamine, the rate of decomposition is greatly accelerated. 

The thermal racemization of Z)iJ-(l,3-dimethyl-2-butenyl) trisulfide proceeds smoothly at 75 °C with AS = — 7 eu the lack of solvent dependence, the failure of O2 to influence the rate, and the absence of mixed trisulfide products when experiments were carried out in the presence of another trisulfide, led the authors to propose an electron-shift mechanism leading to branched sulfur chains in the intermediate, viz. 

Similarly, cis-trans isomerization was shown to occur in suitably constructed analogs of I with the same overall kinetics. 

Polysulfide polymers have the following general structure  

The history of polysulfides began over 150 years ago. In 1838 chemists in Switzedand reported that the reaction of chloraetherin (1,2-dichloroethane) with potassium polysulfide gaveambivalent a rubbery, intractable, high sulfur semisolid. Subsequently there were reports of similar products obtained by various methods, but the first useful products were developed from studies in the late 1920s. This led to the formation of Thiokol Corp. which began production of the ethylene tetrasulfide polymer Thiokol A in 1928, the first synthetic elastomer manufactured commercially in the United States. One of the first successful applications of Thiokol A [14807-96-6] was for seals where its resistance to solvents justified its relatively high price. 

These new synthetic rubbers were accessible from potentially low cost raw materials and generated considerable woddwide interest. For a time, it was hoped that the polysulfide rubbers could substitute for natural mbber in automobile tires. Unfortunately, these original polymers were difficult to process, evolved irritating fumes during compounding, and properties such as compression set, extension, and abrasion characteristics were not suitable for this application. 

During the 1930s gradual improvements in the product and processing overcame some of the drawbacks of this material. Nonetheless, the applications were limited and Thiokol Corp. struggled to remain solvent. The first year Thiokol reported a profit was in 1941,13 years after its foundation. This was realized when the U.S. Air Force discovered that the aliphatic polysulfides were useful as a fuel-resistant sealant for aircraft tanks and hoses. Polysulfides also began to be used as sealants for boat hulls and decks. 

The most significant improvement came in the early 1940s when a method for preparing thiol-terminated liquid polysulfides was developed. Cure of the liquid polysulfides could be accomplished by oxidative coupling. Thus, in effect, a mbber could be compounded without the need of heavy mixing equipment. One of the first large-scale applications of the liquid polysulfides was as a binder for solid rocket fuel. From about 1946 until 1958, these binders were used in various rocket systems and the aliphatic polysulfides achieved commercial success. The switch to predominately liquid-fueled rockets in 1958 ended this phase of the polysulfide business. 

Polysulfide polymers have the foUowiag general stmcture  

Some of the early Thiokol soHd mbbers are stiU made and used in printing roUs, solvent-resistant spray hose, gaskets, and gas-meter diaphragms. Many of the polysulfide products have been in use since the 1940s with an exceUent track record. Continuing improvements in technology keep these products competitive. 

The commercial polysulfldes are made from bis-chloroethylformal (formal) as shown later in equation 11. In some products trichloropropane is added as a branching agent. Table 1 shows typical properties of polysulfldes available from Morton International. These products were acquired from Thiokol Corp. in 1983. 

The liquid polymer is converted to the rubbery state by reagents that react with mercaptan (-SH) and side groups of the polymer segments by oxidation, addition or condensation to effect sulfide (-S-S-) bond formation. The oxidation reactions are exothermic and accelerated by an alkaline environment. The most commonly employed oxidizing agents which are suitable for curing liquid polymers are cobalt or manganese or lead octoate, p-quinonedioxime and di- or tri-nitrobenzene. Epoxy resin also reacts with liquid polysulfide polymers by addition in the presence of an aliphatic or aromatic amine and polyamide activator as shown in Equation 5.8  

The reaction of the epoxy group with the mercaptan ( as with an amine or amide group. 

Major polymer applications sealants, rocket propellant binders, electrical potting compounds, additives to epoxy, ftiel hoses and tubing, insulating glass, fuel-contact sealants 

Important processing methods vulcanization, moisture or chemical curing of premixed compounds 

Typical fillers calcium carbonate, carbon black, zinc oxide, calcium oxide and hydroxide 

Methods of filler pretreatment drying for moisture cured systems 

Special considerations calcium oxide is used as acid scavenger and desiccant zinc oxide is a curing agent in vulcanization processes calcium oxide and hydroxide are used as acid scavengers 

Descriptions of metal sulfides already covered include  

The group 1 and 2 metal sulfides possess the antifluorite and NaCl lattices respectively (see Section 5.11), and appear to be typical ionic salts. However, the adoption of the NaCl lattice (e.g. by PbS and MnS) cannot be regarded as a criterion for ionic character, as we discussed in Section 

The blue paramagnetic [82] ion is an analogue of the superoxide ion and has been detected in solutions of alkali metal sulfides in acetone or dimethyl sulfoxide. Simple salts containing [82] are not known, but the blue colour of the silicate mineral ultramarine is due to the presence of the radical anions [82] and [83] (see Box 15.4). 

Polysulfide ions [8 ] are not prepared by deprotonation of the corresponding polysulfanes. Instead, methods of synthesis include reactions 15.18 and 15.41, and that of H2S with S suspended in NH4OH solution which yields a mixture of [NH4]2[S4] and [NH4]2[S5]. 

UV-VIS spectrum, [82] absorbs at 370 nm and [S3] at 595 nm. In artificial ultramarines, this ratio can be controlled, so producing a range of colours through from blues to greens. 

Like H2O, the hydrides of the later elements in group 16 have bent structures but the angles of 90° (Table 16.4) are significantly less than that in H2O (105°). This suggests that the E—H bonds (E = S, Se or Te) involve p character from the central atom (i.e. little or no contribution from the valence 5 orbital). 

In aqueous solution, the hydrides behave as weak acids (Table 16.4 and Section 7.5). The second acid dissociation constant of H2S is 10 and, thus, metal sulfides are hydrolysed in aqueous solution. The only reason that many metal sulfides can be isolated by the action of H2S on solutions of their salts is that the sulfides are extremely insoluble. For example, a qualitative test for H2S is its reaction with aqueous lead acetate (equation 16.40). 

Sulfides such as CuS, PbS, HgS, CdS, Bi2S3, AS2S3, Sb2S3 and SnS have solubility products (see Sections 7.9 and 7.10) less than 10 and can be precipitated by H2S in the presence of dilute HCl. The acid suppresses ionization of H2S, lowering the concentration of S in solution. Sulfides such as ZnS, MnS, NiS and CoS with solubility products in the range 10 to 10 ° are precipitated only from neutral or alkaline solutions. 

Polysulfanes are compounds of the general type H2SJC where x 2 (see structure 16.8). Sulfur dissolves in aqueous solutions of group 1 or 2 metal sulfides (e.g. Na2S) to yield polysulfide salts, (e.g. Na2Sjc). Acidification of such solutions gives a mixture of polysulfanes as a yellow oil, which can be fractionally distilled to yield H2S (x = 2 ). An alternative method of synthesis, particularly useful for polysulfanes with X 6, is by condensation reaction 16.41. 

Although polysulfide chains with 2-8 members have been structurally characterized in the solid state, those with five or more sulfur atoms can only be isolated as salts of large monocations such as Cs and [Na(15-crown-5)] ( = 6),21,22 PPI1J+ ( = 7)23 [NR4, HJ+ (n = 6-8, R = alkyl or aryl 

Solutions of alkali metal polysulfides have been investigated by UV/vis and 

The structure of H2S2 (16.18) resembles that of H2O2 (Fig. 16.10) with an internal dihedral angle of 90.5° in the gas phase. All polysulfanes are thermodynamically unstable with respect to decomposition to H2S and S. Their use in the preparation of cyclo-S species was described in Section 16.4. 

The characterisation of crosslinked and cured polymers is hindered by their intractability and insolubility, and, in recent years, examination of the thermal degradation behaviour of such polymers is widely recognized to provide characterising information [30, 31]. The nature and composition of the degradation species is a function of the chemical composition and molecular order in the substrate as well as the degradation conditions. 

As stated previously, the curing of thiol-terminated liquid polysulfide polymers involves the oxidation of the mercaptan to disulfide using higher valency metal oxides or p-quinonedioxime. Consequently, the cured polymers will contain reduced forms of the metal ions or p-phenylenediamine heterogeneously dispersed in them. 

These products may have an effect on the nature and composition of the thermal degradation products of the polymers. In addition, other reagents nsed in the curing formulations may also affect the prodnct distribution. Hence, a systematic study on the effect of various salts on the thermal degradation of liquid polysulfide polymer was undertaken first. 

Twenty percent (w/w) of the salt was mixed thoronghly with the liqnid polymer, and the resultant mixture was pyrolysed at 420 °C. The nature and composition of the pyrolysates obtained are given in Table 1.1. As was shown in earlier studies [34], the major components of the font peaks are peak 1 1,3-oxathiolane (I) peak 2 2-mercaptomethyl oxirane (II) peak 3 l,3-dioxa-6-thiocane(III), and peak 4  

60-120 C, 15 Cymin flow rate 40 ml/min. (Reprinted with permission from T.S. Radhakrishnan and M.R. Rao, Journal of Applied Polymer Science, 1987, 34, 5, 1985. 1987, John Wiley and Sons [36]) 

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Polysulfide rubbers possess excellent resistance to weathering and oils and have very good electrical properties. 

Polysulfide elastomer Polysulfide polymers Polysulfide process Polysulfidepulping Polysulfide rubbers Polysulfides... 

Polysulfide sealants Polysulfide units Polysulfonates Polysulfone... 

Polythiodipropionic acids and their esters are prepared from acryUc acid or an acrylate with sulfur, hydrogen sulfide, and ammonium polysulfide (32). These polythio compounds are converted to the dithio analogs by reaction with an inorganic sulfite or cyanide. 

Potting. Potting of wire insulated with Tefzel has been accompHshed with the aid of a coating of a coUoidal siHca dispersion. The pots produced with a polysulfide potting compound meeting MIL-S-8516C Class 2 standards exhibit pullout strengths of 111—155 N (25—35 Ibf). 

Lead dioxide is electrically conductive and is formed ia place as the active material of the positive plates of lead-acid storage batteries. Because it is a vigorous oxidizing agent when heated, it is used ia the manufacture of dyes, chemicals, matches (qv), pyrotechnics (qv), and Hquid polysulfide polymers (42) (see Polypous containing sulfur). 

Mercuric Sulfide. Mercuric s A ide[1344 8-5] HgS, exists ia two stable forms. The black cubic tetrahedral form is obtaiaed when soluble mercuric salts and sulfides are mixed the red hexagonal form is found ia nature as cinnabar (vermilion pigment). Both forms are very insoluble in water (see Pigments, inorganic). Red mercuric sulfide is made by heating the black sulfide in a concentrated solution of alkah polysulfide. The exact shade of the pigment varies with concentration, temperature, and time of reaction. 

Soluble sulfides such as sodium sulfide, potassium sulfide, and calcium polysulfides have been used to precipitate mercury salts from alkaline solutions. When this procedure is used, exercise of caution is requked to maintain the pH within a given alkaline range so as to prevent evolution of H2S. Because the solubiUty of mercuric sulfide in water is 12.5 flg/L at 18°C or 10.7 ppb of mercury, use of this method for removal of mercury is adequate for most purposes. However, the presence of excess alkah, such as sodium hydroxide or sodium sulfide, increases the solubiUty of mercuric sulfide as shown ... 

The dinuclear ion Mo2(S2) g (F - prepared from the reaction of molybdate and polysulfide solution (13) is a usehil starting material for the preparation of dinuclear sulfur complexes. These disulfide ligands are reactive toward replacement or reduction to give complexes containing the Mo2S " 4 core (Fig. 3f). 

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