Cyclic monosulfide

With an unaccelerated sulfur-natural-rubber system, the poor crosslinking efficiency results m sulfur being incorporated into the rubber network as long polysulfide crosslinks, cyclic monosulfides, and vicinal crosslinks, which are very close together and act physically as a single cross-link (see Fig. 2). 

The peak at 33 ppm is assigned to the trans structure of 1,4-BR. An increasing intensity at 33 ppm peak with cure in both sulfur-cured and accelerated sulfur-cured BR postulates the occurrence of cis-to-trans chain isomerisation in these systems. The resonances at 38 and 50 ppm are assigned to cyclic monosulfide and polysulfidic crosslink structures. The expected monosulfidic junctions are not detected in this study possibly due to the low concentration of these species [33]. 

FIGURE 1.31 Typical chemical groupings in a sulfin-vulcanized natural rubber network, (a) Monosulfide crosslink (b) disulfide cross-link (c) polysulfide cross-link (x=3-6) (d) parallel vicinal cross-links ( = 1-6) attached to adjacent main-chain atoms (e) cross-links attached to common or adjacent carbon atoms (f) intrachain cyclic monosulfid (g) intrachain cyclic disulfide (h) conjugated diene (i) pendant sulfide group terminated by moietyZ derived from accelerator. 

Fig. 12. Sulfur-based cure system designs where conventional systems are polysulfidic, EV systems are mono- to disulfidic, and semi-EV are di- to polysulfidic. A is pendent sulfide group terminated with accelerator residue, B monosulfidic cross-link, C disulfidic crosslink, D polysulfidic cross-link (x > 20), E cyclic monosulfidic cross-link, F cyclic polysulfidic cross-link, and Acc accelerator residue.

Sulfur vulcanization, as first discovered by Goodyear in 1839 and Hancock in 1843, leads to poor rubber qualities and to wastage of sulfur atoms through formation of three types of inefficient or useless structures long polysulfidic crosslinks 10, intrachain cyclic monosulfides 11, and vicinal crosslink pairs 12 which act as a single crosslink [116-118]. 

As in the other polycondensation reactions, competition between linear polymerization and ring formation also occurs here and it is strongly dependent on the structure of both the dihalide and inorganic polyailfide. Cyclic monosulfides are favoured when the dihalide monomer has four or five carbon atoms between the halide terminals [18]. 

Consequently, sulfur dissolves in polysulfide solutions much faster than in equimolar monosulfide solutions [73]. In this context it is of interest that the analogous decaselenium dianion Scio has been prepared and structurally characterized in solid [PPN]2Seio [74]. This anion is however bi-cyclic. 

Another way to describe this non-cyclic phosphino methanide derivatives consists of theuse of monodentate complexes with themono-oxide [203] or monosulfide [204] dppm giving the methanide or methanediide derivatives (14) after addition of the appropriate amounts of [Au(acac)PPh3[. A mixed-valence Au(I)-Au(III) can be isolated by coordination of the fragment AuPPh3 to the free sulfur atom in the mono-oxide starting material [204]. 

More recently, this method has been extended to preparation of a variety of disulfonium dications from both acyclic and cyclic bis-sulfides, including very labile dications not observed when other methods were used.78 Thus, simple acyclic S-S dications were prepared by an intermolecular reaction of a monosulfide, a monosulfoxide and triflic anhydride.79 In the first step, reaction of triflic anhydride with dimethylsulfoxide generates a highly electrophilic80 complex 50 (dimethyl sulfide ditriflate).81 The latter reacts with dimethyl sulfide to give labile tetramethyldisulfonium dication 51 identified by NMR spectroscopy.79 In a similar manner, bis-(tetramethylene)disulfonium dication 52 is obtained from tetrahydrothiophene and its S-oxide (Scheme 17). 

The most widely used dihalide is 1,2-dichloroethane. The use of polyhalides (e.g., 2% 1,2,3-trichloropropane) results in the formation of branched or crosslinked products. Sodium tetrasulfide (Na2S4) is generally used as the polysulfide since it contains scarcely any of the monosulfide which reacts with dihalides to form cyclic by-products with unpleasant odors. 

Oxidation potentials determined by cyclic voltammetry in MeCN are considerable higher for monosulfides than those for the disulfides (XII) with w = = 3, w = 3, = 4,... 

In the case of sulfur-vulcanized NR the oxidation reaction is much more complex than in raw NR because the various different types of crosslink e.g. polysulfidic, disulfidic, monosulfidic, cyclic sulfides, conjugated dienes and trienes, etc) present in the network structure may affect oxidation in some way or another. Most types of sulfur vulcanizates initially harden on ageing before degradation occurs. This hardening is associated with the crosslinking associated with oxidative reactions of sulfur species in the network taking place before chain scissions take place. 

Figure 5.15 Separation of accelerators by thin-layer chromatography. Solvent benzene - ethylacetate - acetone (100 5 1) development distance 15 cm indicators (a) 4 N hydrochloric acid (b) 0.5% ninhydrin in ethanol containing 10% acetic acid and 0.5% cadmium acetate. 1 = tetramethylthiuram monosulfide 2 = tetrabutylthiuram monosulfide 3 = tetramethylthiuram disulfide 4 = tetraethylthiuram disulfide 5 = dipentamethylenethiuram tetrasulfide 6 = cyclic thiuram 7 = piperidinium pentamethylene dithiocarbamate 8 = zinc dimethyldithiocarbamate 9 = zinc diethyldithiocarbamate 10 =...

A novel one-pot synthesis of sulfur-containing polymers including poly(monosulfide)s (polythioethers), poly(disulfide)s, polythioesters, and poly-thiourethanes from a five-membered cyclic dithiocarbonate (5-phenoxjunethyl-l,3-oxathiolane-2-thione) and diamines has been examined. Polythioethers with Mn = 2600-17,700 were obtained by condensation of in situ forming dithiol with o ,a -dibromo-p-xylene [623-24-5] (25). A synthesis by using a bifimctional five-membered cyclic dicarbonate and benzylamine [100-46-9] has also been reported (26). 

Figure 1.15 Schematic representation of all sulfide link t5 pes in a networked rubber (a) polysulfide links (b) disulphide links (c) monosulfide links (d) cyclic sulfides (e) conjugated triene (f) hanging groups originated from the accelerator.

Thermal reaction of the cyclic trisulfide with tetrafluoroethylene yields l,2,S-trithiepane. This compound polymerizes with triethylamine at — 40°C to form a disulflde polymer which aW contains a monosulfide linkage ... 

Penczek and Duda studied the copolymerization of sulfur with organic sulfides (cf Reference 31 and references listed therein). These copolymerizations, when performed below the floor temperature of sulfur, can be regarded as the equilibrium copolymerization because homodepropagation of sulfur counterbalance the homopropagations, and the equilibrium is established determining the average length of blocks of sulfur atoms of the sulfur-terminated anionic active species of copolymerization. However, when alkylene monosulfides are copolymerized v th sulfur, cyclic trisulfide is formed as the by-product and then it functions as a comonomer of copolymerization. Besides, homopolymerization of monosulfides is practically irreversible. Therefore, a simpler equilibrium copolymerization system is that when trisulfide is used instead of monosulfide in copolymerization with elemental sulfur. 

The second reaction is thermal decomposition (e.g. reaction 2), forming cyclic mono- and disulfides and conjugated dienes and trienes in the rubber backbone, and zinc sulfide. Monosulfide crosslinks were found to be resistant to such decomposition up to about 170°C. 

The first category is the desulfurization of polysulfides to di- and monosulfidic crosslinks. This pathway is affected by the Zn-accelerator complex (found in accelerated sulfur vulcanization). The routes in the other category are characterized as thermal decomposition, in which the crosslinks and the sulforation species decompose into conjugated species, cyclic sulfides, shorter sulfur crosslinks, and main-chain modifications. 

The first use of high resolution solid-state C-NMR to study sulfur-crosslinked carbon structure was by Zaper and Koenig [62]. An increase in linewidths and decrease in the resolution are observed with increase in cure time. A dominant new resonance is observed at 58 ppm which is assigned to the polysulfide or vicinal crosslink. Other new resonances appear at 47 and 129 ppm which are assigned to monosulfide and trans-NR species, respectively. Broad resonances are observed in the 18 21 ppm and 30 33 ppm regions which are attributed to the products of chain scission and cyclic structures, respectively. 

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