Monosulfide initiators

Certain yY V-dialkyl dithiocarbamates e.g. benzyl A At-diethyl dilhiocarbamate 

Since the experiment is no longer reliant on the dithiocarbamyl radical to both initiate and terminate chains (c/ Section 9,3.2.1), lower reaction temperatures may be used (where the dithiocarbamyl radical is slower or unable to add monomer) and better control over the polymerization process can be obtained. The transfer constants for the benzyl dithiocarbamates in polymerization of acrylic and styrcnic 

The use of mono-, di- and multifunctional initiators provides scope for designing polymer architectures. The use of 14, 18 and 19 in the production of block or star polymers has been demonstrated. Homopolymers of 20 or copolymers of 20 with S or MMA have been successfully used in photoinitiated 

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The S-S linkage of disulfides and the C-S linkage of certain sulfides can undergo photoinduced homolysis. The low reactivity of the sulfur-centered radicals in addition or abstraction processes means that primary radical termination can be a complication. The disulfides may also be extremely susceptible to transfer to initiator (Ci for 88 is ca 0.5, Sections 6.2.2.2 and 9.3.2). However, these features are used to advantage when the disulfides are used as initiators in the synthesis of tel ec he lies295 or in living radical polymerizations. 96 The most common initiators in this context are the dithiuram disulfides (88) which are both thermal and photochemical initiators. The corresponding monosulfides [e.g. (89)J are thermally stable but can be used as photoinitiators. The chemistry of these initiators is discussed in more detail in Section 9.3.2. 

Synonym(s) Initiating explosive lead styphnate lead trinitroresorcinate styphnate of lead Sulfuric acid lead(2+) salt lead (II) sulfate Lead monosulfide lead(2+) sulfide Lead (II) sulfide plumbous sulfide natural galena Lead tetraethide TEL tetraethyllead tetraethylplumbane... 

Diagenesis of Microbially Reduced Sulfur. Postdepositional transformations play an important role in controlling the extent of recycling of microbially reduced S. Pore water profiles from many freshwater systems clearly show that H2S is a short-lived intermediate in sulfate reduction which does not accumulate in sediments (14.16 41-431. However, the conventional paradigm for sulfur diagenesis, in which H2S is initially immobilized by iron monosulfides that later are diagenetically altered to pyrite and elemental S (e.g., 2Q)> does not apply to all freshwater systems. Instead, organic S and CRS (chromium reducible S, which is believed to represent pyrite + S° after preliminary acid distillation to remove AVS), are important initial endproducts of dissimilatoiy reduction. 

In marine and lacustrine muds, the initial sulfide phase precipitated during early diagenesis is mackinawite (FeS09) which is subsequently converted to greigite (Fe3S4) and pyrite (FeS2) (85-89). This reaction path leads to the formation of framboidal pyrite (88.90). However, in salt marsh sediments under low pH and low sulfide ion activity conditions, direct precipitation of pyrite by reaction of ferrous iron with elemental sulfur without the formation of iron monosulfides as intermediates has been reported (85-87.89.91.92). This reaction is one possible pathway for the precipitation of pyrite as single crystals (89). 

In these clastic sediments the dominant form of sulfur is pyritic, while organic sulfur is usually present only in trace amounts. For this reason, much work on sulfur in these sediments focuses on pyrite formation and its crystallization has been studied in detail by Berner (IT), Sweeney and Kaplan (12). Rickard (13). Rickard (14) and others. Under saline and hypersaline conditions precipitation of monosulfides may be the initial step. Sulfur is then added to these precipitates, converting them to pyrite. Laboratory studies indicate that if griegite is present in the original precipitate, sulfurization may produce framboidal aggregates (12). Conversion may depend on chemical factors such as H2S concentrations (9). In contrast, in conditions that are undersaturated with respect to monosulfides, but supersaturated with respect to pyrite, pyrite may form directly and rapidly from... 

Bromothiazole (1) reacts with thiourea in alcohol to yield a mixture of dithiazolyl monosulfide (2) and A-4-thiazoline-2-thione (3) (Scheme 1) (4-6). Treatment of 2-bromo-4-methvlthiazole with potassium hydrogen sulfide in alcohol is reported to result in the formation of bis(4-methyl-2-thiazolylisulfide (7). which probably results from the reaction between the initially formed 2-mercaptothiazole and the initial 2-bromo-4-methylthiazole. 

With the exception of Equation (10), all these pathways involve a precursor iron monosulfide phase. Direct precipitation of pyrite from solution (Equation (10)) is strongly inhibited. This is due to the difficulty of direct nucleation of pyrite, leading to very large supers aturation with respect to pyrite in experimental and natural solutions (Schoonen and Barnes, 1991a). Experimental studies have thus focused on the role of one or more iron monosulfide precursors to pyrite, which have long been recognized as intermediates in sedimentary pyrite formation (see review by Morse et al., 1987). Poorly crystalline mackinawite is the initial product of reaction of H2S with aqueous or solid... 

The pure monosulfide can be prepared nearly quantitatively from (9H-9-BBN)2 with elemental sulfur. The yellow solutions of both reactants in mesitylene are mixed and heated to 130-140°. The liberation of gas (Hj) starts slowly at about 90°. The initially yellow solution is decolorized slowly. After more than 20 h at 130° a colorless clear solution is obtained. Only the monosulfide ( B, (5 83.7) and the quantitative amount of gas (Hj) are obtained. The stoichiometric ratio of the two reactants according to the above equation must be realized exactly, because the disulfide ( B, d 78.4) is an intermediate of the Sg degradation with the BHjBC reagent. Despite the precipitation of the slightly soluble disulfide, one obtains solutions with... 

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. 

In the development of maximum cure state, the initial cross-link stmctnre, which may be high in polysulfides, goes through a rearrangement process in which the Zn-accelerator complexes, which promote curing, extrude sulfur from the initial cross-links and reutilize the extruded sulfur to form additional cross-links (29). After the maximum cure state is established with a desired proportion of polysulfide, disulfide, and monosulfide cross-links for compoimd performance, the network cross-link structures will continue to evolve with additional heat introdnced... 

Iron monosulfide, FeS, is produced in soils and sediments primarily through dissimilatory microbial reduction of sulfate to sulfide, which subsequently reacts with available iron to precipitate FeS (5, 4). The mineral mackinawite, often in poorly crystalline form (3, 23-25), is the initial FeS precipitate in the transformation of iron minerals by sulfate-reducing bacteria (3). For example, when the sulfate-reducing bacterium Desulfovibrio desulfuricans was grown at pH 8 in cultures containing a Fe(II)/Fe(III) oxyhydroxide and synthetic geothite (FeOOH), mackinawite was the predominant iron sulfide phase present after six and nine months, respectively (26). Even at lower pH values, mackinawite was the only iron sulfide phase detected after two weeks of microbial activity, and still a minor phase after that. 

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