Polysulphides

Polysulphides. The reactions of lithium and sodium with sulphur in liquid ammonia have been studied. The thermal behaviour Of the polysulphides was also investigated by means of thermogravimetric and differential thermal analyses. The enthalpies of formation of both the sulphides and polysulphides of lithium and sodium have been determined in 0.1 N-H2SO4 by reaction calorimetry. The crystal structures of two compounds, and both containing 

The preparation of the polysulphides K2S (n = 3, 4, 5, or 6) in ethanol and in liquid ammonia have been described, and lattice parameters calculated from powder diffraction data. ° A crystal-structure determination of the polysulphides Ba2Ss and BaS3 has shown that Ba2S3 contains a sulphide ion and a S ion with a S—S distance of 2.32 A. In BaS3 the polysulphide anion is S , with S—S = 2.074 A, and Z.SSS is 114.8°. 

In 1975 the Hercules company announced the preparation of a polymer very similar in structure to the discontinued Penton. The new polymer is poly-(1,2-dichloromethyl)ethylene oxide  

It may be prepared in two stereo-regular forms, cis- and trans-. The cii-polymer, which crystallises in zig-zag form, has a of 235°C, whilst the fran -polymer, which crystallises in helical form, melts at the much lower temperature of 145°C. Tensile strengths of both forms are reportedly similar to that of Penton whilst the tensile modulus of 2300 MPa is about twice as high. Unfortunately the material is rather brittle with an impact strength only about half that of polystyrene although this may be improved by orientation. 

In 1928 J. C. Patrick attempted to produce ethylene glycol by reacting ethylene dichloride with sodium polysulphide. In fact a rubbery polymer was formed by the reaction  

This polymer became the first commercially successful synthetic rubber, with the trade name Thiokol A. 

Other commercial products were produced using different dihalides as indicated in Table 19.8. 

Thiacyclophanes have attracted interest not only as useful intermediates in the synthesis of cyclophanes and annulenes, but also for their stereochemical and mass spectral behaviour. The synthesis of new thiacyclophanes utilized established methods, and JiT-ray analysis provided a convenient method of determining their crystal and molecular structure.  

Tokunaga, K. Akiba, and N. Inamoto, Bull. Chem. Soc. Japan, 1972, 45, 506. 

Other commercial products were produced using different dihalides as 

---

Prepared by reduction of 4-nitrophenol or 4-nitrosophenoi. Can be diazotized and used as a first component in azo-dyes. Chief outlet is for sulphur dyes in which it is fused with sodium polysulphides. L/sed as a photographic developer. 

Formed from polysulphides and acid followed by cracking. Decompose to H2S and S. 

Sodium polysulphides, Na2Sj( (x = 2,4, 5) are formed from Na in liquid NHj plus S. 

However, the sulphide ion can attach to itself further atoms of sulphur to give polysulphide ions, for example Sj , Sj , and so these are found in solution also. Further, the sulphite ion can add on a sulphur atom to give the thiosulphate ion, S203 which is also found in the reaction mixture. 

It is convenient to include under Aromatic Amines the preparation of m-nitroaniline as an example of the selective reduction of one group in a polynitro compound. When wt-dinitrobenzene is allowed to react with sodium polysulphide (or ammonium sulphide) solution, only one of the nitro groups is reduced and m-nitroanUine results. Some sulphur separates, but the main reaction is represented by ... 

When an alkyl aryl ketone is heated with yellow ammonium polysulphide solution at an elevated temperature, an aryl substituted aliphatic acid amide is foimed the product actually isolated is the amide of the ci-aryl carboxylic acid together with a smaller amount of the corresponding ammonium salt of the oarboxylio acid. Thus acetophenone affords phenylacetamide (50 per cent.) and ammonium phenylacetate (13 per cent.) ... 

The conversion of a carbonyl compound by ammonium polysulphide solution into an amide with the same number of carbon atoms is known as the Willgerodt reaction. The procedure has been improved by the addition of about 40 per cent, of dioxan or of pyridine to increase the mutual solubility of the ketone and aqueous ammonium polysulphide the requisite temperature is lowered to about and the yield is generally better. 

A further improvement is embodied in the Klndler variation of the Willgerodt reaction this consists in heating the ketone with approximately equal amounts of sulphur and a dry amine instead of aqueous ammonium polysulphide. The principal product is a thioamide, and hydrolysis with acid or alkali affords the carboxylic acid, usually in good yield. 

Polysulphide rubbers. Ethylene dichloride and excess of sodium tetrasulphide when heated together give a polymeric polysulphide, Thiokol A, with properties resembling those of rubber ... 

Prepare a saturated solution of sodium sulphide, preferably from the fused technical sodium polysulphide, and saturate it with sulphur the sulphur content should approximate to that of sodium tetrasulphide. To 50 ml. of the saturated sodium tetrasulphide solution contained in a 500 ml. round-bottomed flask provided with a reflux condenser, add 12 -5 ml. of ethylene dichloride, followed by 1 g. of magnesium oxide to act as catalyst. Heat the mixture until the ethylene dichloride commences to reflux and remove the flame. An exothermic reaction sets in and small particles of Thiokol are formed at the interface between the tetrasulphide solution and the ethylene chloride these float to the surface, agglomerate, and then sink to the bottom of the flask. Decant the hquid, and wash the sohd several times with water. Remove the Thiokol with forceps or tongs and test its rubber-like properties (stretching, etc.). 

Although the small molecule most commonly split out is water this is not necessarily the case. In the formation of polysulphides from dihalides and sodium polysulphide, sodium chloride is produced. 

Figure 11.15. Typical chemical groupings in a sulphur-vulcanised natural rubber network, (a) Monosulphide cross-link (b) disulphide cross-link (c) polysulphide cross-link (j = 3-6) (d) parallel vicinal cross-link (n = 1-6) attached to adjacent main-chain atoms and which have the same influence as a single cross-link (e) cross-links attached to common or adjacent carbon atom (f) intra-chain cyclic monosulphide (g) intra-chain cyclic disulphide (h) pendent sulphide group terminated by moiety X derived from accelerator (i) conjugated diene (j) conjugated triene (k) extra-network material (1) carbon-carbon cross-links (probably absent)...

Of the many other aliphatic polyethers, polythioethers and polysulphides prepared in the laboratory, some have become commercially available. Those of interest to the plastics industry are dealt with later in this chapter. 

Further variations in the properties of the polysulphides were also achieved by the following means ... 

By varying the value of the ranking x in the polysulphide Na2S. The tetrasulphide is necessary to obtain a rubber with ethylene dichloride, but disulphides may be used with other dichlorides. 

By reduction in the degree of polymerisation. To produce processable rubbers the original polymers are masticated with substances such as benzothiazole disulphide and tetramethylthiuram disulphide. The more severe degradation techniques to produce liquid polysulphides are mentioned below. 

The general method of preparation of the polysulphides is to add the dihalide slowly to an aqueous solution of sodium polysulphide. Magnesium hydroxide is often employed to faeilitate the reaction, which takes 2-6 hours at 70°C. 

Early examples of such branched polysulphides, e.g. Thiokol FA, are believed to possess hydroxyl end groups and are coupled by means of zinc compounds such as the oxide, hydroxide, borate and stearate by a mechanism which is not understood. Later elastomers, e.g. Thiokol ST, have been modified by a restricted reductive cleavage (see below) and this generates thiol (mercaptan) end groups. These may be vulcanised by oxidative coupling as illustrated below with lead peroxide ... 

The applications of polysulphide rubbers are due to their excellent oil and water resistance and their impermeability to gases. Because of other factors, including their unpleasant odour, particularly during processing, they are much less used than the two major oil-resistant synthetic rubbers, the polychloroprenes and the nitrile rubbers. 

The sodium salt of the polysulphide is converted back to the free thiol on coagulation with acid. 

The low molecular weight polysulphides have found somewhat greater use. Of general structure HS—R—SH and with molecular weights of approximately 1000 they will react with the epoxy group to cause chain extension but not cross-linking. The normal hardeners must therefore be employed in the usual amounts (Figure 26.20). 

The polysulphides used are relatively mobile liquids with viscosities of about 10 poise and are thus useful as reactive diluents. They may be employed in any ratio with epoxide and produets will range from soft rubbers, where only polysulphides are employed, to hard resins using only epoxide. 

The more the polysulphide the higher will be the dielectric constant and the lower the volume resistivity. There will be reduction in tensile strength and heat distortion temperature but an increase in flexibility and impact strength. 

---