Peroxidolytic activity

Phosphites are known to act by a preventive mechanism, i.e. preventing the formation of initiating radicals from hydroperoxides by reducing the latter to alcohols, see reaction 4 [20]. In addition to their peroxidolytic activity (PD), sterically hindered aromatic phosphites, e.g. Ultranox U626, act also by chain breaking (CB) mechanism. These phosphites react with the propagating alkylperoxyl (ROO ), reactions 5, and alkoxyl (RO) radicals, reactions... 

Due to their peroxidolytic activity, phosphites are normally used in combination with hindered phenols in order to deactivate hydroperoxides formed as a consequence of the antioxidant function of hindered phenols, see reaction 1. The enhanced melt stability of tocopherol-containing PP extruded in the presence of a phosphite is clearly illustrated in Fig. 5a with an optimum melt stabilising performance at a 1 2 w/w ratio in favour of the phosphite. 

Amongst the simplest peroxidolytic antioxidants are the alkyl and aryl phosphites which also act as good melt stabilizers in PP (Table 1). Phosphites reduce hydroperoxides to alcohols with a 1 1 stoichiometry and are therefore referred to as stoichiometric peroxide decomposers (PD-s), Figure 5(a). In addition to their stoichiometric peroxidolytic activity, some phosphite esters also behave as catalytic peroxide decomposers (PD-c) in addition to having some chain-breaking (CB) activity ... 

Scheme 5 Peroxidolytic and chain breaking activity of phosphorus antioxidants.

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