Mercaptide groups

Cost bilizers. In most cases the alkyl tin stabilizets ate particularly efficient heat stabilizers for PVC without the addition of costabilizers. Many of the traditional coadditives, such as antioxidants, epoxy compounds, and phosphites, used with the mixed metal stabilizer systems, afford only minimal benefits when used with the alkyl tin mercaptides. Mercaptans are quite effective costabilizets for some of the alkyl tin mercaptides, particularly those based on mercaptoethyl ester technology (23). Combinations of mercaptan and alkyl tin mercaptide ate currendy the most efficient stabilizers for PVC extmsion processes. The level of tin metal in the stabilizer composition can be reduced by up to 50% while maintaining equivalent performance. Figure 2 shows the two-roU mill performance of some methyl tin stabilizers in a PVC pipe formulation as a function of the tin content and the mercaptide groups at 200°C. 

Reaction of Coordinated Mercaptide Group with Alkyl Halides... 

The synthesis of a large number of metal complexes of mercaptoamines by Busch and coworkers (6, 15, 17, 24) has opened the way for detailed study of the reactions of the coordinated mercaptide function. Mercaptoamine complexes provide the first extensive series of complexes of known structure in which the effect of varying structural parameters on the reactivity of the mercaptide group can be subjected to investigation. 

Analytical data on the soluble products isolated from chloroform are in excellent agreement with the composition 1 Ni+2 1 monoalkylated ligand 1 I or Br. The magnetic moment of this methylated complex was found to be 1.89 Bohr magnetons per nickel (II). The molar conductivities of the methylated and benzylated complexes in methanol at 25° C. are 75.4 and 68.4 ohm-1, respectively. These values approximate those expected for uni-univalent electrolytes in this solvent. The formulation of these alkylated compounds as dimeric electrolytes (structure VII) does not appear to be totally consistent with their physical properties. One or both halide ions may be bound to the metal ion. These results lead to the easily understood generalization that terminal sulfur atoms alkylate more readily than bridged mercaptide groups. 

The susceptibility of the mercaptide groups in these Schiff base complexes to ligand reaction was evaluated by treating these compounds with methyl iodide and benzyl bromide in chloroform solution. The pure compounds isolated from these reaction mixtures were of the composition NiL.2RX, where L represents the tetradendate Schiff base and RX represents the alkyl halide added. These reactions proceed smoothly and the stoichiometry of the products implies that both sufur atoms are reactive. The products are monomeric in dichloroethane. They exhibit magnetic moments consistent with octahedral structures, and they behave as di-univalent electrolytes in coordinating, polar solvents. 

Allylic Chloride vs. tert-Chloride Reactivity. There is some question in the literature as to whether the allylic chloride moiety or ferf-chloride group is more responsible for the thermal instability of poly (vinyl chloride) (I, 2). To shed some light on this problem we compared the relative reactivities at 100 °C. in chlorobenzene of 4-chloro-2-pentene and 2-chloro-2-methylbutane with dibutyltin -mercaptopropionate. Data are summarized in Table I. The half-time for the reaction of the allylic chloride with the stabilizer mercaptide group was less than 15 minutes, whereas the half-time for the tert-chloride was nearly 20 times longer. The greater reactivity of the allyl chloride suggests that it is the more important functionality in polymer degradation. However, these results on rates of chlorine substitution are not necessarily an exact measure of thermal instability. 

Typically, substitution reactions occur by attack of a nucleophilic reagent on a benzyl carbon present in the form of a carbonium ion or a methine group in a quinonemethide structure. Several representative substitution reactions are illustrated in Fig. 1.5. At moderate temperatures ( 100°C) and under mildly alkaline conditions, benzylic hydroxyl groups in phenolic units are converted to thiols by reaction with bisulfide (Q, Fig. 1.5). At higher temperatures and alkalinities, e.g., under kraft pulping conditions, the mercaptide group undergoes a series of transformations in which the sulfur is ultimately eliminated. 

Studies of the reactivity of coordinated mercaptide groups show that terminal groups exhibit a greater tendency to act as nucleophiles than do bridging groups H). Kinetic investigations suggest that there may be a pre-equilibrium coordination of the alkyl halide to the central metal ion... 

Scheme 3.3.3 Proposed mechanism of exchange of chloride atoms in PVC by mercaptide groups of organ-otin stabilizers coordination ofallylic chlorine by organotin mercaptide with subsequent rearrangement to a non-allylic structure...

The sulfite ion prevents the reversal of the equilibrium by splitting of the sulfur from the thiothiol . .S- R- H + SO3 S- R-SH + S2O3 These polymers crosslink by oxidative coupling of the mercaptide groups ... 

These polymers cross-link by oxidative coupling of the mercaptide groups ... 

The cyclic disulphide shown above has been collected in significant amounts from products purposely cured to give a substantial proportion of mercaptide groups in the polymer backbone [4]. 

The lithium mercaptide groups are then hydrolyzed with acids. 

Another source of thermal instability arises from metal incorporation in the polymer via reaction of mercaptan with metal oxides. Formation of mercaptide groups can be minimized by incorporation of small amounts of sulfur. 

An excess of sodium polysulfide would produce a polymer of low molecular weight with terminal mercaptide groups (-SNa or -SSNa)... 

Since the mercaptide group is regenerated, the interchange can continue until a major part of the polymer is volatilized [16]. Pyrolysis of cured polysulfide polymers based on bis(2-chloroethyl)formal has so been studied under GC-MS conditions by Rama Rao et aL [81]. Apart fr( n the cyclic monomer, appreciable amounts of other compounds are also formed. The breaking of formal bond was found to be the initiation step. 

A few examples are known of the replacement by mercaptide groups of the nitro group from 4-nitropyridine l-oxide 07, 987, 994 and also of its replacement by a sulphonic acid group . ... 

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