Nickel dithiocarbamates hydroperoxide decomposition

Increase the oxidation rate of polymers, e.g. metal ions which increase the hydroperoxide decomposition rate. Photodegradation and thermal degradation are enhanced by transition metal ion containing pro-oxidants, such as iron dithiocarbamate (as opposed to nickel dithiocarba-mate, which acts as a photo-antioxidant). 

The nickel dithiocarbamates (Vllb) are hydroperoxide decomposers [21]. Unlike the zinc analogues, they are light stable and effective UV screening agents below 340 nm [30]. They react rapidly with hydroperoxides as they are formed in the photo-oxidizing polymer to give sulphur acids which are catalysts for hydroperoxide decomposition [21] and are themselves converted to nickel sulphate (see Scheme 1.5). This chemistry leads to the eventual depletion of the metal thiolate in the polymer and at this point (the end of the induction period), the rate of photooxidation reverts to that of the polymer containing the uncomplexed transition metal ion. 

In view of the conclusion that hydroperoxides are important photo-initiators it is important to evaluate the relative contribution of uv screening, peroxide decomposition, triplet quenching and kinetic chain-breaking to the mechanism of action of typical nickel complex steibilisers. Nickel dibutyl dithiocarbamate (III, R=n Bu, M = Ni) was selected as representative of the peroxide decomposers... 

Hydroperoxides are potent photoinitiators and their decomposition into inactive products is an important method of stabilization in polyolefins for it prevents generation of radicals by reactions such as (4), (5), (14), and (15). Decomposition of hydroperoxides can be achieved by a reaction with phosphite esters or nickel chelates or by a catalytic action by a range of compounds including dithiocarbamates and mercaptoben-zothiazoles. Decomposition of hydroperoxides formed during processing is important for the stabilization ofPVC. 

Metal dialkyl dithiocarbamates are oxidized to sulfur acids, which act as ionic catalysts for the non-radical decomposition of hydroperoxides. When the metal is nickel or another transition metal, they also function as UVAs. Dialkyl dithio-propionates are oxidized by hydroperoxides, as shown in Scheme 9.20. 

Undoubtedly more important than the rate of decomposition of hydroperoxides is the type of product obtained. Table 5 lists the major products obtained by completely decomposing TBHP with various sulfur compounds. With nickel and zinc dithiocarbamates... 

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