Mechanistic Studies of Some Phosphite Antioxidants

Abstract

A number of esters of phosphorous acid were shown to be effective inhibitors for hydrocarbon oxidation catalysed by azobisisobutyronitrile and by hydroperoxides after an initial rapid, but short-lived, pro-oxidant stage. Cyclic compounds derived from catechol were particularly effective, exhibiting autoretardation even at low concentrations. The reactions of some phosphites with cumene hydroperoxide were examined kinetically and by subsequent product studies, and their kinetic parameters determined. Common to all compounds studied was an initial second order reaction which showed features consistent with a redox process. This reaction led to stoichiometric reduction of hydroperoxide with concomitant formation of phosphate. This phosphite/hydroperoxide system showed features in common with the better known amine/peroxide systems, including the ability to initiate the polymerisation of vinyl monomers, and to absorb oxygen. After this stage the linear phosphates formed were inert to hydroperoxide whilst the (5-membered) cyclic compounds underwent further reaction(s) with peroxide to yield an unidentified species which was an extremely powerful catalyst for destruction of hydroperoxide by a pseudo-first order process. Product studies and solvent effects suggested a Lewis acid for this unknown compound. The effects of ring strain were not marked in these reactions nor in their rates of hydrolysis which were briefly examined. This contrasts strongly with the large rate enhancements often encountered in other cyclic systems. The phosphites examined did not inhibit the polymerisation of styrene initiated by azobisisobutronitrile. The catechol phosphates were shown to be as effective as the (patented) phosphites in stabilising polypropylene against thermooxidative degradation, and both types of compounds were shown by induction=period measurements to compare favourably with a commercial synergistic stabiliser composition.