Polysulfide bridges

The final product may contain sulfur in cyclic or pendent groups or may contain polysulfide bridges as suggested in the following scheme ... 

Vulcanisation involves the crosslinking of the polymer chains by mono-, di-or polysulfide bridges rather like the disulfide crosslinks in natural proteins (see Chapter 4, p. 51). The process modifies the properties of the resultant polymer, and makes the product more suitable for the manufacture of footwear, hoses, raincoats, tyres and so on. 

Later in 2010, Movassaghi et al. proposed an alternative synthesis for (+)-chaetocin A (677) as well as syntheses for the epipolythiodiketopiperazines (+)-chaetoxin C (693) and (+)-12,12 -dideoxychetracin A (694) (432). These were the first reports on the preparation of high-order polysulfides. Studies have shown that the polysulfide bridge is important for the biological activity of these compotmds and that potency increases with the number of sulfur atoms incorporated (433). Therefore, the versatile construction of this scaffold might lead to highly active substrates. 

Elementary sulfur or compounds that can be used as a source of sulfur form together with suitable additives at higher temperatures thio-ether-, disulfide- or polysulfide-bridges in and between chains. This vulcanisation method is primarily suitable for those elastomers that have unsaturated bonds. The rubber produced by this method has good mechanical characteristics. However, a disadvantageous chemical characteristic of rubber vulcanised with sulfur is that additives can leach into the product. An example is the release of thiol compounds, which are incompatible with some mercury compounds. 

Sulfur Dyes. These dyes are synthesized by heating aromatic amines, phenols, or nitro compounds with sulfur or, more usually, alkah polysulfides. Unlike most other dye types, it is not easy to define a chromogen for the sulfur dyes (qv). It is likely that they consist of macromolecular stmctures of the phenothiazone-thianthrone type (72), in which the sulfur is present as (sulfide) bridging links and thiazine groups (1). 

Bridging polysulfide ligands feature in the complexes [Ru2(//-8 )(/u-82CNMe2)(82CNMe2)-(CO)2(PPh3)2] ( = 5 or 6). In the solid state, the structure of each complex is unsymmetrical as... 

Nickel (continued) polysulfide complex, 31 98 half chair conformation, 31 115 porphyrin complexes, 32 13 in proteins, 47 284-285 quadruply bridged dimers, 40 211-214 steric structures, 40 190-194 quaternary phosphonium salts of, 6 31-32 redox chemistry probes of structure, 32 243-245 steric and electronic requirements, 32 242-243... 

The controlled formation of disulfide bridges in peptide synthesis has been studied extensively in the past, but only recently have polysulfides been identified as unexpected side products in these reactions. 

Semi-EV cures represent a compromise between conventional and EV cures. Although semi-EV cures do yield polysulfide cross-links, they tend to minimize formation of inefficient moieties such as sulfur bridging with itself, accelerator-terminated sulfur linkages, etc. This cleaner usage of sulfur is the reason for their compromise properties between conventional and EV cures. 

Sulfur-rich binary P-S anions of the type [S2P(p-S )2PS2] form monocyclic structures with sulfide or disulfide bridging units ( = 1, 2) (11.39). Another type of cyclic P-S anion with the general formula [(PS2) ] is formed in the reaction of white phosphorus with polysulfides in a non-aqueous medium these anions have homocyclic structures with two exocyclic sulfur atoms attached to each phosphorus atom, e.g. [P4Ss] (11.40). ... 

For = 2 we observe that there are traces of sulfur unreacted we do not observe any thiolate formation but only coupled compounds with 3.6 average number of sulfur atoms in the bridge of the dlalkylpolysulfldes The number of sulfur atoms in the polysulfides varies from 1 to 4, and one observes even traces of a higher polysulfide. 

The chemical and fuel resistance of polysulfides makes them useful as sealants and coatings for secondary containment areas, where they prevent chemicals, solvents, fuels, etc, from seeping into the ground in the event of spillage or a storage tank leak. Polysulfide-based coatings and sealants are also used for bridges, air fields, and road construction. 

Dinuclear complexes with polysulfide ions as bridging ligands 542... 

The Schiff bases with two benzylidenimine residues bridged by polysulfide chains of different lengths can act as potentially bidentate ligands. These compounds have been obtained by reaction of sulfur with benzylamine in the presence of PbO. They have been characterized by X-ray, NMR, IR and chromatographic techniques.49 The structural analysis of three of them, having n = 2,3 and 4 and R = H, showed that the overall geometry is dominated by the conformation of the polysulfide chain.50... 

Sulfur dyes are water-insoluble, macromolecular, colored compounds which are produced by bridging aromatic amines, phenols, and amino phenols with sulfur and/or sodium polysulfide [16], These dyes are of little interest for dyeing paper. Only C.I. Sulphur Black 1 (36), the most important dye of all in terms of volume, is used for special paper dyeings. 

The chemical shifts due to the monosulfidic crosslinks are influenced not only by the position on the monomeric unit to which it belongs, but also by the position of the carbon atom of the monomeric unit on the other side of the bridge. The shielding parameters of monosulfide substitution on the individual carbons of the isoprene unit have also been determined. It is shown that resolvable polysulfidic crosslink resonances exist in all positions of the backbone carbons while monosulfidic crosslinks appear only between C-l and C-4 carbons with detectable intensity [18]. 

The 13C chemical shifts were assigned in more detail for monosulfidic and polysulfidic crosslinks occurring in the accelerated sulfur vulcanisation of NR [18]. The NR was cured with a pure thiuram formulation (TMTD alone) in order to predominantly prepare monosulfidic bridges in the network. The distortionless enhancement by polarisation transfer (DEPT) experiments, in which the carbons with different level of protonation can be distinguished [22-24], were performed for the NR cured with extended levels of sulfur. Based on the DEPT results and previously reported model compound results [20], the chemical shifts of the resonances occurring in the spectra were assigned. 

There are large numbers of compounds with the simple sulfide ion (S2 ) as a ligand comparable to O2- that are mononuclear like S=WC13 or bi- or polynuclear with sulfur bridges. These and related species derived from polysulfide ions (S2 ) have been intensively studied in part because sulfur bridged species occur in Nature in ferredoxins and related compounds. 

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