Sulfur Alone

Although sulfur vulcanization has been studied since its discovery in 1839 by Goodyear, its mechanism is not well understood. Free-radical mechanisms were originally assumed but most evidence points to an ionic reaction [Bateman, 1963]. Neither radical initiators nor inhibitors affect sulfur vulcanization and radicals have not been detected by ESR spectroscopy. On the other hand, sulfur vulcanization is accelerated by organic acids and bases as well as by solvents of high dielectric constant. The ionic process can be depicted as a chain reaction involving the initial formation of a sulfonium ion (XI) by reaction of the polymer with polarized sulfur or a sulfur ion pair. The sulfonium ion reacts with a polymer molecule by hydride 

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Hydrogen sulfide reacts with molten sulfur and depresses the viscosity of the latter, particularly at 130—180°C iafrared spectral studies show that polysulfanes, H2S, form. The average chain length of these polysulfanes is shorter than the equiUbrium chain length of molten sulfur alone at the same temperature consequendy, the viscosity of the molten sulfur is markedly reduced (99,100). 

It would be possible to write an entire book on the oxyacids of sulfur alone. In fact an entire book... 

Vulcanization by heating with sulfur alone is a very inefficient process with approximately 40-50 sulfur atoms incorporated into the polymer per crosslink. Sulfur is wasted by the formation of long polysulfide crosslinks (i.e., high values of m in XHI), vicinal crosslinks (XIV), and intramolecular cyclic sulfide structures (XV). (Structures XIV and XV do not contribute significantly to the physical properties of the polymer.)... 

Formation of /3 C—C bonds is also known, but generally requires the generation of a carbanion a to sulfur and its interaction with a carbonyl centre. Dieckmann cyclization of (136) readily affords ethyl 3-oxo-3,4,5,6-tetrahydro-2F/-thiin-2-carboxylate, while the presence of a sulfone group renders a methyl substituent sufficiently acidic to react as in equation (75) (78JCS(Pl)l32l). Sulfur alone is apparently not sufficiently activating for this method to be useful for the preparation of unfunctionalized molecules. 

The intermediates for sulfur-bake dyes are heated to between 200 and 350° C with sulfur alone, and those for polysulfide bake dyes, with sodium polysulfide. The process is carried out primarily in gas-fired rotating steel drums. 

There is wide variety of vulcanisation agents and methods available for crosslinking rubber materials including peroxide, radiation, urethane, amine-boranes, and sulfur compounds [20]. Because of its superior mechanical and elastic properties, ease in use, and low cost, sulfur vulcanisation is the most widely used. Although vulcanisation with sulfur alone is not practical compared to the accelerated sulfur vulcanisation in terms of the slower cure rate and inferior physical properties of the end products, many fundamental aspects can be learned from such a simply formulated vulcanisation system. The use of sulfur alone to cure NR is typically inefficient, i.e., requiring 45-55 sulfur atoms per crosslink [21], and tends to produce a large portion of intramolecular (cyclic) crosslinks. However, such ineffective crosslink structures are of interest in the understanding of complex nature of vulcanisation reactions. 

Side reactions including cis-to-trans isomerisation and sulfidic cyclisation are observed along with the formation of crosslinks in the BR cured with sulfur alone. In the sulfur-donor vulcanisation of BR, cis-to-trans isomerisation is the predominant feature of the vulcanisation reaction sequence and seems to obey first-order kinetics with respect to the concentration of accelerator. 

Initial studies indicated that sulfur alone was not adequately resistant to moisture penetration. However, it was discovered that the addition of an oil wax sealant over the... 

Phosphaindolizines ([l,3]azaphospholo[l,5- ]pyridines) 40 reacted with hydrogen sulfide and elemental sulfur to produce zwitterionic systems 43. No reaction was observed with sulfur alone. The thiolate was trapped with iodomethane in a couple of cases (to give 47, Scheme 1) <1998HAC445>. An analogous P-diselenate was obtained from 48 using l,3,2,4-diselenadiphosphetane-2,4-diselenide 49, but under different conditions and in the presence of base the C-substituted product 50 was obtained (Scheme 2). 

Kirsten WJ (1979) Automatic methods for the simultaneous determination of carbon, hydrogen, nitrogen, and sulfur, and for sulfur alone in organic and inorganic materials. Anal Chem 51 1173-1179. 

In the absence of Ag or Sn, sulfur alone can decopper lead only if repeated, large doses of sulfur are stirred in, but the large amount of lead sulfided makes the practice of no commercial significance. 

When stirring with sulfur alone, reversion of copper occurs when silver or tin is the catalyst retarding the reaction between sulfur and lead When the reagent is a mixture of pyrites/sulfur (in the ratio 2 1 - 4 1) rather than S alone, no reversion of copper into lead occurs. Blast furnace lead can be rapidly decoppered to very low values, and the operation is faster at temperatures up to at least 450°C. This procedure does not require the presence of either Ag or Sn. With soft lead, when Ag is present, values as low as 1 ppm Cu were achieved experimentally. Hard lead, with no Ag or Sn, could be decopperised to 0.01%Cu in the presence of 2.3% Sb, or to 0.02%Cu with 5.5% Sb. Thus increasing Sb contents make it impossible to decopper to very low values using a moderate quantity of S alone, and difficult to decopper even with S plus FeSa. Using repeated treatments can, however, continue to remove copper to much lower values. 

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