Metal 1,1-dithiolates

Another method for slowing oxidation of rubber adhesives is to add a compound which destroys the hydroperoxides formed in step 3, before they can decompose into radicals and start the degradation of new polymer chains. These materials are called hydroperoxide decomposers, preventive antioxidants or secondary antioxidants. Phosphites (phosphite esters, organophosphite chelators, dibasic lead phosphite) and sulphides (i.e. thiopropionate esters, metal dithiolates) are typical secondary antioxidants. Phosphite esters decompose hydroperoxides to yield phosphates and alcohols. Sulphur compounds, however, decompose hydroperoxides catalytically. 

Sulphur compounds, e.g., thiopropionate esters and metal dithiolates (Table la, AO 16 and 17), decompose hydroperoxides catalytically, i.e., one antioxidant molecule destroys several hydroperoxides through the intermediacy of sulphur acids [19,20]. Scheme 6 shows a simplified scheme for the antioxidant mechanism of simple alkyl sulphides. 

Synergism can also arise from cooperative effects between mechanistically different classes of antioxidants, e.g., the chain breaking antioxidants and peroxide decomposers (heterosynergism) [42]. For example, the synergism between hindered phenols (CB—D) and phosphites or sulphides (PD) is particularly important in thermal oxidation (Table 2). Similarly, effective synergism is achieved between metal dithiolates (PD) and UV-ab-sorbers (e.g., UV 531), as well as between HALS and UV-absorbers, (Table 3). 

Irg 1076, AO-3 (CB), are used in combination with metal dithiolates, e.g., NiDEC, AO-30 (PD), due to the sensitized photoxidation of dithiolates by the oxidation products of phenols, particularly stilbenequinones (SQ, see reaction 9C) (Table 3). Hindered piperidines exhibit a complex behavior when present in combination with other antioxidants and stabilizers they have to be oxidized initially to the corresponding nitroxyl radical before becoming effective. Consequently, both CB-D and PD antioxidants, which remove alkyl peroxyl radicals and hydroperoxides, respectively, antagonise the UV stabilizing action of this class of compounds (e.g.. Table 3, NiDEC 4- Tin 770). However, since the hindered piperidines themselves are neither melt- nor heat-stabilizers for polymers, they have to be used with conventional antioxidants and stabilizers. 

Winter, C. S. Oliver, S. N. Rush, J. D. Hill, C. A. S. Underhill, A. E. Third-order Optical Nonlinearities in Metal Dithiolate Complexes. In Organic Molecules for Nonlinear Optics and Photonics Messier, J., Kajzar, F., Prasad, P., Eds. NATO ASI Series E, 194 Kluwer Boston, 1991 pp 383-390. 

Recently, dithio-diolate ligands have been employed for construction of group 4 metal catalysts for the ROP of lactide. These metal dithiolate complexes form mononuclear species of the type [(OSSO)M(OR)2] with an octahedrally coordinated metal center. These fluxional compounds acted as highly active catalysts in the ROP of L- and rac-lactide. Hafnium complexes were also introduced as initiators for the ROP of L-lactide and rac-lactide (vide infra) in very limited cases. To our knowledge, the hafnium derivative 146 displayed the highest activity among the group 4 catalysts reported to date (complete conversion of 300 equiv. of... 

Finally, the first reports120,121) of a conducting sulfur chelate, involving a partially oxidized metal dithiolate, have appeared as have reports of materials based on diben-zotetraaza[14]annulenes172 175 further analogues of the porphyrins. 

Sulfur compounds are known as catalytic hydroperoxide decomposers (PD-C) one antioxidant molecule destroys several hydroperoxide molecules by the action of intermediate sulfur acid moieties.Thioethers and esters of thiodipropionic acid and metal dithiolates are examples of commercial significance (see Table 1, AOs 19-24). 

Most of the UV-absorbers (UVA) used commercially fall into two main classes of compounds, the 2-hydroxybenzophenones, e.g., AO 28, Table 1, and the 2-hydroxybenzotriazoles, e.g., AOs 29-32, Table 1, with the 2-hydroxy group being essential for their activity. These UVAs operate by additional mechanisms too, for example, by removing initiating radicals (e.g., alkoxyl radicals) in a weak chain breaking-donor (CB-D) mechanism.UV absorbers, such as AO 28, also synergize effectively with peroxide decomposers, e.g., metal dithiolates, see Table 4. 

Table 2 Effect of different metal dithiolates on the photostability (embrittlement time, EMT) of PP processed at 190°C (For structures, see Table 1)...

Carbon disulfide reacts with a wide range of bases to produce ligands which readily coordinate to metals. New synthetic work including the formation of aryl xanthates, disulfide products, and oxidized metal dithiolates is reviewed. The sulfur addition and abstraction reactions are discussed, along with the photobleaching reaction of [Ni(n-butyldtc)a] Bridged mercaptide complexes of nickel triad elements are also described. 

L. Alcacer and A. H. Maki, Electrically conducting metal dithiolate-perylene complexes, J. Phvs. Chem. 78 215 (1974). 

Phosphites can react with ROOH to give alcohols and phosphates, avoiding the decomposition of the hydroperoxide to free radicals. They include the tristearyl, diphenyl or tri-isodecyl phosphite, and di-isodecyl pentaerythritol diphosphite. Besides phosphites, sulphur compounds can be used, such as thioesters, thioethers, thiodipropionates (including the dilauryl and distearyl thiodipropionates) and the metal dithiolates, e.g., iron dithiocarbamate. 

Figure 2 Simplified antioxidant mechanism of metal dithiolates (dithio-phosphates, dithiocarbamates, xanthates) exemplified with nickel dialkyl dithiophosphate.

Many sulfur-containing PD antioxidants, especially the metal dithiolates, are very effective long-term thermal stabilizers (also melt and photo antioxidants see later). This versatility in antioxidant action is the direct consequence of their highly effective PD activity (54,105,107,136-140). Moreover, the effectiveness of dithiolate antioxidants, within any one series depends strongly on their solubility in the polymer. For example, in the case of nickel dialkyldithiophosphate (NiDRP, AO 23, Table 3) series, the lower molecular weight members of the series are less soluble, and less effective than the higher members (see Fig. 3) (141). The zinc dialkyl dithiocarbamate (ZnDRC, AO 21) series too show similar behavior (142). 

Table 9. Effect of Different Metal Dithiolates on the Photostability (Embrittlement Time, EMT) of PP Processed at 190°C ...

One approach that meets the above requirements involves the use of transition metal dithiolates for time-controlled stabilization. These can fulfil their... 

Preventive antioxidants (sometimes referred to as secondary antioxidants), on the other hand, interrupt the second oxidative cycle by preventing or inhibiting the generation of free radicals. The most important preventive mechanism is the non-radical hydroperoxide decomposition, PD. Phosphite esters and sulphur-containing compounds, e.g. AO 12-18 in Table 1, are the most important classes of peroxide decomposers. The simple trialkyl phosphites (e.g. Table 1, AO 12) decompose hydroperoxides stoichiometrically (PD-S) to yield phosphates and alcohols, see reaction 4. Sulphur compounds, e.g. thioethers and esters of thiodipropionic acid and metal dithiolates (Table 1, AO 15-18, 31, 32), decompose hydroperoxides catalytically (PD-C) whereby one antioxidant molecule destroys several hydroperoxides through the intermediacy of sulphur acids, see reaction 5. References 1 and 2 give detailed discussion on antioxidant mechanisms. 

The antioxidant stage of peroxidolytic stabilizers is frequently preceeded by a prooxidant stage which varies in intensity (see Section 19.3.2.2). In the case of metal dithiolates, however, (during both thermal and photooxidation), the prooxidant stage is not observable at the low stabilizer concentrations used normally in polymers. The radical scavenging ability of metal dithiolates may be responsible for the absence of this stage. 

It is now well established that the metal dithiolates are themselves not responsible for the heterolytic decomposition of hydroperoxides but rather their transformation products which are formed by oxidation in the presence of hydroperoxides. They are, therefore, precursors of the... 

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