Mechanisms of antioxidant action: Time-controlled stabilisation of polypropylene by transition metal dithiocarbamates

Combinations of iron and nickel dithiocarbamates approach the ‘ideal’ delayed action photo-activators for polypropylene. They give an induction period to photo-oxidation which can be controlled by the amount of the nickel complex and a rapid rate of photo-oxidation at the end of the induction period which is primarily determined by the iron concentration. Combinations of other photo-activators and uv stabilisers show a less ideal behaviour and do not permit such accurate time control.     

Synthesis,characterization, thermal,antimicrobial and antioxidant studies of some transition metal dithiocarbamates

Metal dithiocarbamate complexes of Co(II) [1], Cu(II) [2], Mn(II) [3], Cr(III) [4], and Pd(II) [5] have been synthesized using sodium N-ethyl-N-phenyldithiocarbamate (NaL). The complexes were characterized by elemental analyses, FTIR and UV–vis spectroscopic techniques, magnetic moment, molar conductance and thermal analyses (TGA and DSC). The infrared spectra indicated the coordination of dithiocarbamate through the two sulphur atoms in a symmetrical bidentate fashion. The thermal behavior of these complexes showed that the hydrated complexes lost water molecules in the first step, followed by decomposition of the ligand molecules in the final steps. Mass loss considerations at these final decomposition steps indicate conversion of the complexes to sulphides. The antimicrobial potentials of the complexes were evaluated against some selected bacteria strains (Escherichia coli, Pseudomonas aureginosa, Salmonella typhi and Staphylococcus aureus) and fungi organisms (Aspergillus flavus and Fasiparium oxysporium). The compounds showed a broad spectrum of fungicidal and bactericidal activities which exceeds that of the control drugs at a 100 μg/mL concentration. The antioxidant properties of the ligand and its Cu(II) complex were evaluated in vitro using DPPH assay, and the complex was found to exhibit better radical scavenging ability than the free ligand.     

X-ray spectral study of the nature of electronic interactions in transition metal dithiocarbamates

SK_β spectra are studied for a series of nickel(II), zinc(II), and cadmium(II) dithiocarbamate complexes. Model quantum chemical calculations of the electronic structure of planar chelate rings are reported. It is shown that the metal-ligand interaction forming a coordination bond between the dithiocarbamate ligand and the metal mainly involves the nonbonding n-electrons and the weakly bonding π-electrons localized on the sulfur atoms. Translated fromZhurnal Struktumoi Khimii, Vol. 39, No. 6, pp. 1121–1126, November–December, 1998.     

Mechanisms of antioxidant action: the role of 2-hydroxybenzophenones in photo-oxidizing media

A study of the photo-stability of 2-hydroxy-4-octyloxybenzophenone (HOBP) in the presence of possible photo-sensitizers present in polymers shows that this u.v. stabilizer is readily destroyed by them in the presence of light. HOBP is destroyed by carbonyl compounds and therefore appears to interfere with their photolysis at least partly by a sacrificial quenching process.     

Mechanisms of antioxidant action: The role of mercaptobenzthiazole and its derivatives as antioxidants

Mercaptobenzthiazole (MBT) and its zinc complex (ZMBT) are compared as antioxidants for paraffin oil at 140°. The latter is much more effective than the former at comparable molar concentrations. Autoxidation studies at lower temperatures indicate that MBT is superior to ZMBT both as a hydroperoxide decomposer and as a chain-breaking antioxidant. However, products formed from them by reaction with hydroperoxides are much more effective than either MBT or ZMBT. Both are oxidised to bis-2-benzthiazole disulphide (MBTS) are benthiazole sulphinic acid (BTS) which decomposes to give benzthiazole (BT) and sulphur dioxide. In addition, MBT gives benzthiazole sulphonic acid (BTSO) and ZMBT gives the very stable zinc benzthiazole sulphimate (ZBTS) which also behaves as a reservoir for sulphur oxides. The difference in behaviour at 140° appears to be associated with the relative instability of BTSO compared with ZBTS.     

Mechanisms of antioxidant action: The behaviour of a bound antioxidant as an antiozonant

A rubber reactive antioxidant, 4-(mercapto acetamido)diphenylamine (I, MADA) was found to be more effective as an antiozonant when prereacted with the rubber chain (MADA-B) than when present as a conventional compounding ingredient (MADA-A) at the same molar concentration. In bound form after extraction MADA-B is as effective as an unextracted commercial antiozonant, iso-propylaminodiphenylamine (IPPD), at the same molar concentration. This result casts some doubt on the currently accepted view that antiozonants must migrate to the surface of the rubber in order to exert their protective effect.     

Infrared-spectroscopic study of amino-substituted nitrilimines and their photochemical transformations in an argon matrix

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Mechanisms of antioxidant action: The role of copper salts in the photostabilization of paint media

Verdigris, a copper pigment containing basic copper acetate hydrate, is an effective light stabilizer for linseed oil based paint media but gives rise to discolouration over a long period. The antioxidant process is shown to result in the formation of conjugated chromophores in the medium. A catalytic antioxidant mechanism involving redox reactions of the copper ions is proposed to account for the efficiency of the pigment as a light stabilizer.     

Mechanisms of antioxidant action: The synthesis of bound antioxidant concentrates in nitrile rubber

A phenolic thiol antioxidant and two thiol amides have been reacted with nitrile-butadiene rubber both in the latex and during melt processing to give masterbatch concentrates. BHBM (I) reacts to give higher bound antioxidant levels at 20% masterbatch concentration than the amides in the latex but MADA (II) gives the highest level of binding by the mechanochemical procedure (80% masterbatch concentration). The primary initiation process is mechanochemical but secondary peroxide initiated adduct formation occurs during the later stages.     

Mechanisms of antioxidant action: Transformations of 4-hydroxy-2,2,6,6-tetramethylpiperidinoxyl (HTMPO) in polypropylene during processing

When 4-hydroxy-2,2,6,6-tetramethyl piperidinoxyl (HTMPO) is processed in polypropylene in a closed mixer, almost 50% is converted to other products during the first few minutes whilst the applied torque in the mixer is high. There is associated formation of unsaturation and this fact, in conjunction with the almost complete regeneration of nitroxyl within five minutes, suggests that the corresponding hydroxylamine (HTMPOH), which can be qualitatively identified, is the major transformation product. A study of the UV stability of PP films fabricated from polymer processed for varying times shows that UV stability is related to the quantity of the redox couple (HTMPO + HTMPOH) remaining in the polymer. This is considerably reduced by severe processing. The redox capable has almost no thermal antioxidant (oven aging) activity.     

Mechanisms of antioxidant action: transformations involved in the antioxidant function of the nickel dithiolates

The mechanism of the antioxidant function of the nickel O,O-dialkyl dithiophosphates is examined in relation to that of related nickel dithiolates. A common mechanistic pathway is identified and evidence is presented to suggest that two distinct catalytic processes are involved, the first involving homolytic peroxide breakdown and the second an ionic process. It is proposed that the initial stage of the reaction leads to the formation of the corresponding disulphides which then undergo further oxidation to sulphur acids which are responsible for the ionic catalytic decomposition of hydroperoxides and the dehydration of derived alcohols.     

Mechanisms of antioxidant action: Formation of antioxidant adducts with rubbers through sulphur by a mechanochemical procedure

It is shown that antioxidants containing a thiol group can be mechanochemically reacted with both natural rubber (NR) and styrene-butadiene-rubber (SBR). Oxygen inhibits the reaction and the optimum temperature is in the region of 70°C. Increase in the antioxidant concentration in the rubber leads to increasing levels of adduct formation. Evidence is presented to show that although a substantial part of the adduct is formed during mechanochemical treatment, about 20% of the total binding occurs during the vulcanisation reaction.     

Theoretical study of first-row transition metal porphyrins and their carbonyl complexes

The equilibrium geometry, relative energies, normal mode frequencies, and electron and spin density distributions for first-row transition metal porphyrins M(P) (M is a transition metal in the oxidation state +2, P = C₂₀H₁₂N₄) and their five-and six-coordinate carbonyl complexes M(P)CO and M(P)(CO)(AB) (AB = CO, CN^?, CS) in different spin states have been calculated by the density functional theory B3LYP method with the 6-31G and 6-31G* basis sets. The energies of binding of the CO group to M(P) molecules D(M-CO) have been estimated. The calculated properties change as a function of the metal, the number of carbonyl groups (shown for Fe(P) as an example), and the multiplicity. Calculations show that, for five-coordinate complexes M(P)CO with M = Ti and V, high-spin states and significant D(M-CO) energies are typical. For Fe(P)CO, a singlet with a small D(M-CO) energy is preferable. For Cr(P)CO and Mn(P)CO (which also have small D(M-CO) energies), the states with different spins, which strongly differ in geometry and electronic structure, are close in energy, within 0.1–02. eV. The energy of binding of CO to M(P)CO (M = Cr, Mn, Fe) is considerably higher than the energy of binding of CO to M(P), which is evidence that the transformation of five-coordinate metalloporphyrins into six-coordinate ones is energetically favorable. The behavior of the D(M-CO) energies is interpreted using a qualitative model that considers not only the effects of participation (or nonparticipation) of “active” $ d_{x^2 - y^2 } The equilibrium geometry, relative energies, normal mode frequencies, and electron and spin density distributions for first-row transition metal porphyrins M(P) (M is a transition metal in the oxidation state +2, P = C₂₀H₁₂N₄) and their five-and six-coordinate carbonyl complexes M(P)CO and M(P)(CO)(AB) (AB = CO, CN⁻, CS) in different spin states have been calculated by the density functional theory B3LYP method with the 6-31G and 6-31G* basis sets. The energies of binding of the CO group to M(P) molecules D(M-CO) have been estimated. The calculated properties change as a function of the metal, the number of carbonyl groups (shown for Fe(P) as an example), and the multiplicity. Calculations show that, for five-coordinate complexes M(P)CO with M = Ti and V, high-spin states and significant D(M-CO) energies are typical. For Fe(P)CO, a singlet with a small D(M-CO) energy is preferable. For Cr(P)CO and Mn(P)CO (which also have small D(M-CO) energies), the states with different spins, which strongly differ in geometry and electronic structure, are close in energy, within 0.1–02. eV. The energy of binding of CO to M(P)CO (M = Cr, Mn, Fe) is considerably higher than the energy of binding of CO to M(P), which is evidence that the transformation of five-coordinate metalloporphyrins into six-coordinate ones is energetically favorable. The behavior of the D(M-CO) energies is interpreted using a qualitative model that considers not only the effects of participation (or nonparticipation) of “active” , and , d _xz , and d _yz AO in bonding of M to the P ring and axial ligands, but also the fraction of the total bond energy consumed for the preparation (promotion) of those “valence states” of the M(P) molecules that are realized in M(P)CO and M(P)(CO)(AB) complexes. For the series of compounds Fe(P)(CO)₂ − Fe(P)(CO)(CS) − Fe(P)(CS)₂ − Fe(P)(CO)(CN⁻) in the singlet, triplet, and ionized states, the trans influence of axial ligands in low-spin metalloporphyrins is shown to follow the same qualitative scheme as is typical of octahedral transition metal complexes: in mixed-ligand complexes (as compared to the symmetric ones), the stronger bond becomes shorter and even stronger, while the weaker bond becomes longer and even weaker. It is assumed that the same scheme will persist for more complicated low-spin six-coordinate metalloporphyrins in the states with the vacant AO and occupied d _xz and d _xz AOs involved in bonding with both axial ligands with the filled shell. Original Russian Text ? O.P. Charkin, A.V. Makarov, and N.M. Klimenko, 2008, published in Zhurnal Neorganicheskoi Khimii, 2008, Vol. 53, No. 5, pp. 781–794.     

Mechanisms of antioxidant action: Transformations involved in the antioxidant function of zinc dithiocarbamate: Part II

The initial transformation products from reactions of zinc dialkyl dithiocarbamates with hydroperoxides have been identified by spectrophotometric and kinetic methods. It is shown that reactions of the zinc complex with tertiary butyl hydroperoxide, unlike those of the corresponding nickel complexes, do not give rise to disulphide as the first transformation product. The initial product in the case of the zinc complex was found to be the zinc dialkyl dithiopercarbamate, ZnDMSO. The evidence presented rules out the involvement of the further oxygenated product, the zinc dialkyl dithiocarbomylsulphinate, ZnDMSO₂.     

Triphenyl phosphine adducts of platinum(IV) and palladium(II) dithiocarbamates complexes: a spectral and in vitro study

Triphenyl phosphine adducts of dithiocarbamate complexes of platinum(IV) and palladium(II) of the type [Pt(L)2PPh3Cl2] and [Pd(L)2PPh3] [L: morpholine dithiocarbamate (L1), aniline dithiocarbamate (L2) and N-(methyl, cyclohexyl) dithiocarbamate (L3)] were prepared and characterized by elemental analysis, electronic, IR, 1H NMR and 13C NMR spectral studies. Thermal studies of the complexes were carried out. In vitro antitumor activity has been screened towards human adenocarcinoma cell lines and showed significant inhibition even at very low concentration.     

Mechanisms of antioxidant action: Chemical binding of a thiol antioxidant into polypropylene by means of uv irradiation

3,5-Di-t-butyl-4-hydroxybenzyl mercaptan (BHBM) became chemically combined to polypropylene to the extent of 68% of the antioxidant present when films containing BHBM were exposed to uv irradiation. In addition to the radical coupling reaction of the thiyl radical derived from BHBM with the polymer, a second technologically less useful process also occurs, leading to the formation of a derived stilbenequinone. The latter absorbs strongly both in the visible and in the near uv and consequently slows down the subsequent loss of the bound antioxidant from the polymer.     

Structure and photochemical behaviour of 3-azido-acrylophenones: a matrix isolation infrared spectroscopy study

(Z)-3-Azido-3-methoxycarbonyl-2-chloro-acrylophenone (MACBP) has been synthesized, isolated in low temperature argon and xenon matrices and studied by FTIR spectroscopy, complemented by DFT(B3LYP)/6-311++G(d,p) calculations. The molecule was characterized both structurally and spectroscopically, and its photochemistry used to probe the mechanism of photo-induced conversion of 3-azido-acrylophenones into oxazoles. In situ UV irradiation (λ = 235 nm) of matrix-isolated MACBP yielded as primary photoproduct a 2H-azirine, which undergoes subsequent photoisomerization to methyl 4-chloro-5-phenyl-1,3-oxazole-2-carboxylate. In a competitive process, a ketenimine is also formed upon photolysis of MACBP. The reported results indicate that this ketenimine must be formed from the starting 3-azido-acrylophenone via a Curtius type concerted rearrangement.     

Mechanisms of antioxidant action: Mechanochemically bound antioxidants to polyethylene and polypropylene

It is shown that a thiolamide antioxidant, 4-mercaptoacetamido diphenylamine (MADA) can be reacted with both polypropylene and polyethylene in a mechanochemical process. The resulting bound antioxidant concentrates (MADA-B) can be used for the thermal and photostabilisation of the parent polymers. The adduct is very stable to solvent extraction and is also very effective as a high temperature antioxidant for polypropylene subjected to water leaching.The uv stabilising mechanism of MADA-B is believed to be related to the regenerative process involved in the mechanism of stabilisation by hindered nitroxyl radicals.     

Mechanisms of antioxidant action: autosynergistic antioxidants containing chain-breaking and peroxidolytic functions

Antioxidant activities, both in model substrate oxidation and in polypropylene under technological conditions, have been examined for a series of alkyl (3,5-di-tert-butyl-4-hydroxybenzyl) sulphides (I). They were found to be intrinsically much more active than typical commercial chain-breaking antioxidants in a closed system. This higher activity is attributed to an additional antioxidant function resulting from the presence of the sulphur atom. The latter is the precursor for a highly active peroxidolytic species which leads to autosynergism. In a thermal-oxidative (air-oven) test I (R, C₂—C₁₈) increases in activity with decreasing volatility but effectiveness as melt stabilizer during the processing of polypropylene and as u.v. stabilizer appears to depend more on the physical state of the additive in the polymer than on its volatility.     

Mechanisms of antioxidant action: Radical addition reactions of the dialkyl dithiophosphoric acids and their derivatives to natural rubber

It is shown that the dialkyldithiophosphoric acids and their ammonium salts can be readily reacted with natural rubber (NR) latex to give adducts containing up to 50% of the compound added. The free acids tend to coagulate the latex at relatively low concentrations but the use of the neutral ammonium salt permits relatively high loadings of adduct to be obtained in NR.The same additives and a related disulphide were also reacted mechanochemically with NR at temperatures between 110°C and 140°C but the adduct yields were somewhat lower than in the latex procedure.