T. König

Chain-breaking antioxidant activity of phosphite esters

Abstract The inhibition of the autoxidation of hydrocarbons and polypropylene by aliphatic, aromatic, sterically hindered and cyclic phosphites has been studied by means of volumetric and 31P-NMR techniques. The antioxidant activity of phosphites depends on the rate of their reactions with peroxyl radicals and on the way they react with alkoxyl radicals. Only those phosphites which react by substitution to give free aryloxyl radicals are effective as chain-breaking antioxidants. The reaction modes of various phosphites with various peroxyl and alkoxyl radicals have been studied in some model reactions and the relationship between structure, reaction mechanism and antioxidant activity has been elucidated.

Antioxidant action of organic sulphites—I. Esters of sulphurous acid as secondary antioxidants

The efficiency of esters of sulphurous acid as secondary antioxidants has been studied. Several aliphatic as well as aromatic open-chain and cyclic sulphites were synthesized and reacted with tert-butyl-, cumene- and α-tetralin hydroperoxides. All sulphites investigated decompose hydroperoxides in a nonradical way. This reaction proceeds in two steps: in a slow redox reaction and a rapid catalytic decomposition of the hydroperoxides. The open-chain alkyl sulphites and the cyclic sulphite of 2,2′-thiobis-(6-tert-butyl-4-methylphenol) show especially high efficiency. In comparison with trivalent phosphorus compounds, esters of sulphurous acid react more slowly in the stoichiometric reaction step, but are also able to decompose hydroperoxides catalytically. Besides these sulphuric acid esters, SO2, SO3, and H2SO4 also take part as catalysts in the reaction. Finally, the rate constants of the reductive and catalytic step of the hydroperoxide decomposition were determined.

Organophosphorus antioxidants part IX—Inhibition of the oxidation of hydrocarbons by hindered aryl phosphites

Abstract Sterically hindered aryl phosphites inhibit the AIBN-initiated oxidation of cumene and tetralin at 65°C by trapping peroxyl radicals to give hindered aryloxyl radicals. The stoichiometric factors of inhibition depend on the oxidisability of the hydrocarbon and on the phosphite concentration, decreasing from one at high concentrations to zero at low concentrations. An apparent critical antioxidant concentration is observed. These peculiarities are due to the oxidation of the phosphites by hydroperoxides formed in the reaction. Kinetic schemes which take into account this reaction and the participation of peroxyl radicals derived from the initiator, allow one to evaluate consistent rate constants for the reaction of alkylperoxyl radicals with phosphites. These are of the order of 10 2 to 10 3 litres/mol/s at 65°C depending on the structures of the phosphite and of the hydrocarbon substrate.

Organophosphorus antioxidants—VIII. Kinetics and mechanism of the reaction of organic phosphites with peroxyl radicals☆

Abstract Trialkyl phosphites react with cyanoisopropylperoxyl radicals, generated by thermolysis of azobis(isobutyronitrile) in the presence of oxygen, to give the corresponding phosphates with rate constants of the order of 10 3 M −1 sec −1 at 65°C. Phenyl phosphites are oxidized also. A small amount of cyanoisopropyl phosphite is formed by substitution of the phosphite by alkyloxyl radicals leading to phenoxyl radicals. Sterically hindered aryl phosphites react with cyanoisopropylperoxyl radicals to yield the corresponding phosphates and alkoxyl radicals which in a second step react with phosphite by substitution releasing a sterically hindered phenoxyl radical. Therefore, sterically hindered phosphites are capable of acting as chain-terminating primary antioxidants. Because the rate constants of reaction of these phosphites with peroxyl radicals are only in the range of 10 2 M −1 sec −1 and 100 times smaller than those of phenols, phosphites should be less active as primary antioxidants than phenols.

Phosphorus-containing antioxidants: Part V—Mechanism of hydroperoxide decomposition by 4,8-di-tert-butyl-2,10-dimethyl-6-phenoxy-dibenzo[d, g]-1,3,6,2-dioxathiaphosphocin☆

Abstract The mechanism of the hydroperoxide decomposing effect of 4,8-di-tert-butyl-2,10-dimethyl-6-phenoxy-dibenzo[d,g]-1,3,6,2-dioxathiaphosphocin (IIa), a cyclic phosphite derived from 2,2′-thiobis(6-tert-butyl-4-methyl-phenol), was determined using tert-butylhydroperoxide (TBH) at 30°C and 90°C and 2-phenyl-2-propylhydroperoxide (cumyl hydroperoxide, CHP) at 30°C and 75°C. Analytical data obtained by means of iodometric titration, 31 P NMR spectroscopy and HPLC methods reveal changes in the composition of the reaction mixture used and the formation of transformation products from IIa. Mechanistic details of the antioxidant activity of IIa were obtained using model compounds containing sulphide and phosphate functions in the same molecule (i.e. IIIa) and phosphate functions (i.e. IIId), respectively. The formation and high antioxidant activity of S- and P-acids in the mechanism of transformation of IIa have been considered.

Antioxidant action of organic sulfites—II. Esters of sulfurous acid as primary antioxidants

Abstract The effect of esters of sulfurous acid as primary antioxidants was examined. Different aliphatic, aromatic, open-chain and cyclic sulfites were synthesized. The reactions of organic sulfites with RO 2 and RO radicals, the chain carriers of the autoxidation of hydrocarbons and polymers, were simulated by means of the thermal decomposition of azobisisobutyronitrile (AIBN) in the presence of oxygen and of di-tert-butylperoxalate (DTBPO). The reactivity of organic sulfites with 2-cyanoisopropylperoxyl radicals is low. Only aromatic sulfites are able to trap peroxyl radicals; however, they are not very effective primary antioxidants. The reactions of the organic sulfites with tert-butoxyl radicals generally lead to an increase in the rate of decomposition of DTBPO, as determined from rate constants measured at 50 °C. A decomposition of DTBPO induced by liberated tert-butyl radicals in the presence of alkyl sulfites is very probable. Alkyl sulfites and aromatic sulfites with aliphatic groups act mainly as hydrogen donors in reactions with alkoxyl and peroxyl radicals.

Antioxidant action of phosphite, sulfite and borate esters—A comparison

Abstract The antioxidative activities and mechanisms of action of alkyl and aryl sulfites, borates and boronates have been elucidated and compared with those of phosphites. Sulfurous and boric acid esters decompose hydroperoxides in a non-radical way, as do phosphites. Sulfites are oxidized to sulfates which readily hydrolyze to give hydrogen sulfates and sulfuric acid which decompose hydroperoxides catalytically. Borates also act as catalytic hydroperoxide decomposers. Phenylboronates initially give phenyl borates by O-insertion. Aryl sulfites and borates, in addition, exhibit chain-breaking antioxidative activity, whereas the alkyl derivatives do not inhibit the oxidation of hydrocarbons and polymers. Aryl sulfites are less efficient than phenols. Aryl borates, however, attain the activity of the underlying phenols. Obviously, they hydrolyze in the course of oxidation, so acting as a ‘depot’ for phenols. Even more effective are the aryl esters of phenylboronic acid; they considerably surpass the antioxidative activity of the corresponding phenols. In the oxidation of polypropylene at 180°C, the esters of bisphenol 2246 are antioxidatively active in the following order:

Organoboron antioxidants: Part 1—Boric acid derivatives as primary antioxidants

Organoboron compounds, particularly boric acid esters and arylboronic acid derivatives, are known to be good antioxidants. In this paper, the inhibiting activities of various boron compounds are compared with those of phenolic antioxidants and phosphites. Whereas in the autoxidation of polypropylene, aryl borates give rise to approximately the same induction periods as the constituent phenols, aryl phenylboronates are, to some extent, considerably better. For the calculation of kinetic data, particularly the rate constants (k7) of the reaction with peroxyl radicals and the stoichiometric factor (f), investigations into the inhibiting activity of boron compounds in the initiated oxidation of cumene at low temperatures are used. The rate constant k7 of aryl phenylboronates is approximately ten times greater than that for structurally analogous phosphites, whereas aryl borates can be regarded as ‘depots’ for phenols.

Antioxidant action of organic sulphites III. Antioxidative effect of esters of sulphurous acid in hydrocarbons and polypropylene

The influence of aliphatic, aromatic, cyclic and acyclic esters of sulphurous acid on the AIBN initiated oxidation of the hydrocarbons cumene and tetraline at 65°C as well as of polypropylene at 180°C was examined. AIBN initiated oxidation of cumene and tetraline in presence of alkyl sulphites took place without an induction period, but the reaction rate was reduced. Aromatic sulphites caused occurrence of induction periods and considerably reduced oxygen uptake rates. The thermooxidative decomposition of hydrocarbons and polymers was retarded. The rate constants of the reaction of organic sulphites with peroxyl radicals were determined and compared with those of the appropriate phenols. During thermooxidation of polypropylene films and the processing of polypropylene, the stabilizing effect of sterically hindered aromatic sulphites was similar to those of the commercially produced antioxidant BHT. They acted also as lightstabilizers and reduced the UV ageing of polypropylene.