Peroxidolytic antioxidants

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. 

Organotin stabilizers containing sulfur (e.g. 45) or their transformation products 96, 97 are considered as hydroperoxide-decomposing antioxidants (Al-Malaika, 1989). Within this stabilizing function, thiol 97 is oxidized by hydroperoxide in disulfide 99 and sulfenic acid 100, having peroxidolytic properties. 

Scheme 5 Peroxidolytic and chain breaking activity of phosphorus antioxidants.

Al-Malaika, S. Chakraborty, K.B. Scott, G. Peroxidolytic antioxidants metal complexes containing sulfur ligands. In Developments in Polymer Stabilisation Scott, G., Ed. Applied Science London, 1983 Vol. 6, 73-120. 

Scott, G. Peroxidolytic antioxidants sulphur antioxidants and autosynergistic stabilizers based on... 

Stabilisers against the effects of light on polymers are essentially light-stable antioxidants. Many of the peroxidolytic and chain-breaking anti-... 

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 ... 

Figure 6 Simplified antioxidant mechanism of thiodipropionate esters as peroxidolytic antioxidants.

Table 7. Effect of Some Peroxidolytic Antioxidants as Melt Stabilizers for PP at 270°...

The second antioxidant mechanism, the peroxidolytic or peroxide decomposing process (PD) removes the hydroperoxides (POOH) that are the main source of initiating radicals by reactions that do not produce free radicals. 

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 has been known for a long time that, in the case of sulfur-containing antioxidants, the effective peroxidolytic catalysts are not the parent compounds but their oxidation products (see Section 19.4.2.2.i). The realization that melt-processing operations exert a profound effect on the light stability of polymer articles containing them is, however, relatively recent. The main reason for this is the fact that earlier mechanistic studies were mainly conducted in model compounds or hydrocarbon oils. 

Scott G (1983) Peroxidolytic Antioxidants Sulphur Antioxidants and Autosynergistic Stanilisers based on Alkyl and Aryl sulphides in Developments in Polymer Stabilisation, ed. G.Scott, Appl. Sci. Publ, Chapter 2. 

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