Mechanisms of antioxidant action: Evidence for a regenerative cycle during the melt stabilisation of polypropylene by galvinoxyl

Evidence is presented to show that galvinoxyl (G·), a ‘stable’ aryloxyl radical, is converted to the corresponding phenol(GH) in polypropylene during processing. The redox couple (G·/GH) is a catalytic CB-A/CB-D antioxidant and is capable of deactivating 50 kinetic chains, thus effectively protecting polypropylene against oxidative chain scission for up to 20 min at 200°C in a closed mixer. A reciprocal alternation of the concentration of G· and GH is accounted for by changing conditions in the mixer and G· is slowly and irreversibly destroyed by alkylperoxyl radicals.     

Mechanisms of antioxidant action: The catalytic rôle of iodine in the stabilisation of polypropylene

It is shown that I₂ is an effective melt stabiliser for polypropylene at 190°C in a closed mixer but that it is much less effective in an open mixer. The formation of HI and unsaturation in the polymer show that a catalytic antioxidant mechanism (CB-A/CB-D) is involved in the antioxidant activity of I₂ in a closed mixer similar to that occurring in the stabilisation of polyolefins with galvinoxyl and nitroxyl radicals.The processed polymer shows slightly greater uv stability than the control with no additive but is essentially similar to the control in thermal oxidative stability. The volatility of HI in an air oven probably accounts for the fact that it has no CB-D activity under these conditions.     

Mechanisms of antioxidant action: Behaviour of a hindered piperidine and related oxidation products during processing and photo-oxidation of polypropylene

Evidence is presented to show that a commercial hindered piperidine (I(A)) is not an effective melt stabiliser for polypropylene whereas related nitroxyl radicals (II(A)) and (II(B)) and hydroxylamine (III(B)) are highly effective. These results are explained on the basis of an oxidative transformation of the piperidine to the nitroxyl during processing and the involvement of the latter in a cyclical regenerative process in which the nitroxyl acts as a chain-breaking acceptor (CB-A) antioxidant and the derived hydroxylamine as a chain-breaking donor (CB-D) antioxidant.The same CB-A/CB-D cycle operates during photo-oxidation of polymer films containing each of the additives. The nitroxyl radical concentration reaches a stationary concentration in the polymer, irrespective of whether it is added as nitroxyl or as parent amine. The derived hydroxylamines are substantially more effective than the nitroxyl radicals as ultraviolet stabilisers.     

Novel Kinetic Method for Expressing the Ability of Antioxidant to Scavenge Radicals

A method was developed to treat the result from an antioxidant trapping radicals including 2,2'-azinobis (3-ethylbenzothiazoline-6-sulfonate) cationic radical(ABTS^+·), 2,2'-diphenyl-1-picrylhydrazyl radical(DPPH), and galvinoxyl radical. In the presence of a certain concentration of an antioxidant, the decrease of the concentration of a kind of radicals follows the second order exponential function with the increase of the reaction time(t), viz.,[radical]=Ae^-t/a+Be^-t/b+C, the derivation operation of which obtains the differential style, -d[radical]/dt= (A/a)e^-t/a+(B/b)e^-t/b, revealing the relationship between the reaction rate(r=-d[radical]/dt) and the reaction time(t). Thus, the reaction rate at the beginning of the reaction(r₀) can be calculated by assigning t=0 in the equation of -d[radical]/dt-t. Based on the concept of the reaction rate, r=k[radical][antioxidant], the rate constant(k) can be calculated based on r₀ and the initial concentrations of radical and antioxidant, k=r₀/([radical]₀[antioxidant]₀). The k means the rate of a fresh antioxidant molecule to trap a fresh radical. This method was used to treat the interactions of ABTS^+·, DPPH, and galvinoxyl radicals with three homoisoflavonoids, four pyrazoles, and three ferrocenyl Schiff bases. It was found that ferrocenyl group is beneficial for antioxidants to reduce ABTS^+·, and ortho-hydroxyl group is beneficial for antioxidants to donate hydrogen atom in phenolic hydroxyl group to DPPH and galvinoxyl radicals.     

Stabilisation of polymers by catalytic antioxidants

Two types of catalytic antioxidant are now recognised. The first involves the catalytic destruction of hydroperoxides to non-radical products and the second consists of “stable” redox couples with the ability to remove both alkyl (CB-A) and alkylperoxyl (CB-D) radicals in autoxidising systems. The conditions under which both types of antioxidant can operate effectively are reviewed and the limitations to their activity are outlined.     

On the pairing rules for recognition in the minor groove of DNA by pyrrole-imidazole polyamides

Background: Cell-permeable small molecules that target predetermined DNA sequences with high affinity and specificity have the potential to control gene expression. A binary code has been developed to correlate DNA sequence with side-by-side pairings between N-methylpyrrole (Py) and N-methylimidazole (lm) carboxamides in the DNA minor groove. We set out to determine the relative energetics of pairings of Im/Py, Py/Im, Im/Im, and Py/Py for targeting G·C and A·T base pairs. A key specificity issue, which has not been previously addressed, is whether an Im/Im pair is energetically equivalent to an Im/Py pair for targeting G·C base pairs.Results: Equilibrium association constants were determined at two five-base-pair sites for a series of four six-ring hairpin polyamides, in order to test the relative energetics of the four aromatic amino-acid pairings opposite G·C and A·T base pairs in the central position. We observed that a G·C base pair was effectively targeted with Im/Py but not Py/Im, Py/Py, or Im/Im. The A·T base pair was effectively targeted with Py/Py but not Im/Py, Py/Im, or Im/Im.Conclusions: An Im/Im pairing is energetically disfavored for the recognition of both A·T and G·C. This specificity will create important limitations on undesirable slipped motifs that are available for unlinked dimers in the minor groove. Baseline energetic parameters will thus be created which, using the predictability of the current pairing rules for specific molecular recognition of double-helical DNA, will guide further second-generation polyamide design for DNA recognition.     

Conductive characteristics of radical‐bearing polythiophenes using a microcomb‐shaped electrode

The electric conductivity of π‐conjugated and radical‐bearing polymers, i.e., polythiophenes bearing pendant galvinoxyl and phenoxyl radical groups, was measured using a microcomb‐shaped electrode. The electric conductivity was found to be enhanced by the radical content in the polymer. The infrared (IR) and Raman spectroscopies suggested a structural change from an aromatic form to a quinoid one in the polythiophene backbone by the phenoxyl radical generation. The effect of the pendant galvinoxyl radical's unpaired electron on the electric conductivity of the polythiophene was discussed by comparing the conductivity of a radical‐bearing polystyrene and a polythiophene mixed with low‐molecular radical molecules. Copyright © 2007 John Wiley & Sons, Ltd.     

Structural basis of DNA folding and recognition in an AMP-DNA aptamer complex: distinct architectures but common recognition motifs for DNA and RNA aptamers complexed to AMP

Background: Structural studies by nuclear magnetic resonance (NMR) of RNA and DNA aptamer complexes identified through in vitro selection and amplification have provided a wealth of information on RNA and DNA tertiary structure and molecular recognition in solution. The RNA and DNA aptamers that target ATP (and AMP)' with micromolar affinity exhibit distinct binding site sequences and secondary structures. We report below on the tertiary structure of the AMP-DNA aptamer complex in solution and compare it with the previously reported tertiary structure of the AMP-RNA aptamer complex in solution.Results: The solution structure of the AMP-DNA aptamer complex shows, surprisingly, that two AMP molecules are intercalated at adjacent sites within a rectangular widened minor groove. Complex formation involves adaptive binding where the asymmetric internal bubble of the free DNA aptamer zippers up through formation of a continuous six-base mismatch segment which includes a pair of adjacent three-base platforms. The AMP molecules pair through their Watson-Crick edges with the minor groove edges of guanine residues. These recognition G·A mismatches are flanked by sheared G·A and reversed Hoogsteen G·G mismatch pairs.Conclusions: The AMP-DNA aptamer and AMP-RNA aptamer complexes have distinct tertiary structures and binding stoichiometries. Nevertheless, both complexes have similar structural features and recognition alignments in their binding pockets. Specifically, AMP targets both DNA and RNA aptamers by intercalating between purine bases and through identical G·A mismatch formation. The recognition G·A mismatch stacks with a reversed Hoogsteen G·G mismatch in one direction and with an adenine base in the other direction in both complexes. It is striking that DNA and RNA aptamers selected independently from libraries of 10¹⁴ molecules in each case utilize identical mismatch alignments for molecular recognition with micromolar affinity within binding-site pockets containing common structural elements.     

Hydrolytic stability and hydrolysis reaction mechanism of bis(2,4-di-tert-butyl)pentaerythritol diphosphite (Alkanox P-24)

The excellent processing stability afforded by the commercial phosphate antioxidant, Alkanox P-24 is well known in the literature. However, it is known that Alkanox P-24 is hydrolytically unstable. Enhancement of its hydrolytic stability is therefore a key objective in this work and some binary and ternary blends were developed using other additives that are often used for polymer stabilisation, including the primary antioxidant tetrakis[methylene-3-(3′,5′-di-tert-butyl-4-hydroxyhyphenyl)propionate]methane (Anox 20), acid scavengers calcium stearate (DW) and the hydrotalcite like compound (DHT-4A). An improvement in the hydrolytic stability of Alkanox P-24 was found when it was blended with these additives. A comparison with different physical forms of blends (traditional powders versus recently introduced Non-Dust Blends) was undertaken. Better performance was observed with NDB relative to the free flowing mixed powders. Spectroscopic studies (FTIR, and mass spectrometry) were also undertaken to elucidate the hydrolysis mechanism of the phosphite antioxidant Alkanox P-24. Mechanistic schemes were devised and interpreted. Hydrolysis products of Alkanox P-24 are believed to be involved in the mechanism of stabilisation. In this programme of work, the role of the hydrolysis products was investigated by controlled thermomechanical degradation in an extruder and stabilisation activity evaluated by following the yellowness index and the melt flow rate. The influence of partially hydrolysed Alkanox P-24 on polymer processing was studied. It was found that some active hydrolysis products showed significant antioxidant activity and retarded polymer degradation during processing. Mechanisms for their formation and identity are elucidated.     

Melt stabilisation of Phillips type polyethylene, Part II: Correlation between additive consumption and polymer properties

Phillips type polyethylene stabilised with combinations of 700 ppm phenolic antioxidant and different amounts of various phosphorous stabilisers (sterically hindered aryl phosphite [Hostanox PAR 24], phosphonite [Sandostab P-EPQ], and aryl–alkyl phosphine [PEPFINE]) was processed by six consecutive extrusions. The polymer was characterised by FT-IR spectroscopy, rheological (melt flow index, creep compliance), colour and oxidation induction time measurements. Films were prepared by blowing and their mechanical strength was determined by Elmendorf and dart drop tests. The consumption of the antioxidants was compared to the characteristics of the polymer and to the strength of the films. The consumption rate of both the phenolic and the phosphorous antioxidants is reduced in their combinations compared to single antioxidants. The chemical structure of the polymer is modified considerably in the first extrusion even at high antioxidant levels. The mechanism of stabilisation is determined by the type of the antioxidant(s) in further processing steps. The phenolic antioxidant does not prevent the formation of long chain branches. The phosphonite and the phosphine hinder efficiently hydrogen abstraction from the polymer chain and long chain branching. Their efficiency is similar, but the phosphonite is consumed fast, while the phosphine oxidises slowly. The investigated phosphite is less reactive; the contribution of the phenolic antioxidant to the inhibition reactions is significant in phenol/phosphite combinations, therefore long chain branching increases continuously with increasing number of processing steps.     

Synthesis of an optically active helical poly(1,3-phenyleneethynylene) bearing stable radicals and its chiroptical and magnetic properties

We synthesized an optically active helical poly(1,3-phenyleneethynylene) with pendant galvinoxyl residues and dimethyl(10-(1S)-pinanyl)silyl groups. The hydrogalvinoxyl precursor polymer was given by polymerization of (1,3-diiodophenyl)hydrogalvinoxyl and 1,3-diethynyl-5-[dimethyl(10-(1S)-pinanyl)silyl]benzene using Pd(PPh₃)₄ catalyst (M_w = 1.7 × 10⁵, M_w/M_n = 3.7). In the CD spectrum taken in ethyl acetate solution, clear Cotton effects were observed in the absorption region of the backbone and hydrogalvinoxyl chromophore, indicating an excess of one-handed helical foldamer conformation. The polymer yielded the corresponding polyradical with high spin concentration by treatment of the polymer solution with PbO₂. The Cotton effects appeared in CD spectra of the polymer and polyradical by addition of methanol to the chloroform solution, although the Cotton effects were hardly observed in chloroform. On the other hand, in the MCD spectra of the polymer and polyradical taken in chloroform solution, Faraday effects were observed in the absorption region of the backbone and galvinoxyl chromophore. The static magnetic susceptibility of the chiral polyradical was measured using a SQUID magnetometer, and showed the antiferromagnetic interaction.     

Efficiency and mechanism of phosphorous antioxidants in Phillips type polyethylene

The stabilising efficiency of three phosphorous secondary antioxidants of different chemical structures (phosphonite, phosphite and phosphine) was compared in a Phillips type polyethylene. The polymer was processed by six consecutive extrusions in the presence of 700 ppm primary antioxidant and 700 ppm phosphorous compound. The consumption of the secondary antioxidant was followed quantitatively by Fourier transform infrared (FTIR) spectroscopy. The properties of the polymer were characterised by FTIR spectroscopy, colour and rheological measurements, as well as by the determination of its residual thermo-oxidative stability. The results of the experiments proved that the chemical reactions occurring in the first extrusion of the polymer are different from those taking place in the further processing operations. The rate of antioxidant consumption and the chemical reactions of the polymer are strongly affected by the type of the phosphorous secondary antioxidant. The analysis of the results indicated that the three stabilisers must act according to different mechanisms. The investigated phosphine showed the best melt stabilising efficiency, while phosphonite was found to protect the polymer most effectively from discoloration.     

Free radical scavenging ability of some 5-aminosalicylic acid derivatives

Theories explaining the clinically proven efficacies of 5-aminosalicylic acid and its derivatives sulfasalazine and olsalazine in the treatment of inflammatory bowel disease have invoked the neutralization or scavenging of reactive free radicals. This is a preliminary study comparing the relative scavenging abilities of these 5-ASA derivatives towards the stable free radicals 2,2-diphenyl-1-picrylhydrazyl (DPPH) and galvinoxyl with several compounds including phenols, acetaminophen and vitamins C, K and E which have well documented antioxidant and radical scavenging capabilities both in solution and in biological systems.     

Novel ninhydrin adduct of catechin with potent antioxidative activity

The reaction of ninhydrin with (+)-catechin in the presence of TMSOTf resulted in condensation product 1, which consists of a 2:1 mixture of epimers at the C-2 position. The antioxidative radical-scavenging activity of 1 against the galvinoxyl radical, acting as an oxyl radical, was significantly enhanced compared to (+)-catechin. Our results offer a new method for chemical modification of a natural phenolic antioxidant.     

Water-insoluble cyclodextrin polymer crosslinked with citric acid for paraquat removal from water

The anionic water-insoluble cyclodextrin polymer (polyCTR-β-CD) was crosslinked between β-cyclodextrin (β-CD) and citric acid (CTR) at 180?°C during 30?minutes to eliminate paraquat (PQ) from water. The reaction yield was equal to 70.2%, the ionic exchange capacity corresponded to 3.29?mmol·g^?1 and the β-CD content was 0.29?mmol·g^?1. Then, samples were characterized by SEM, ATR-FTIR, TGA, BET and stereoscopic microscope. Adsorption experiments were investigated with different factors such as pH of the solution, contact time, initial concentration of paraquat and adsorption temperature. The relevant pH was equal to 6.5 and the optimal contact time was 120?minutes to attain adsorption equilibrium. At 30?°C, the adsorption capacity was increased (9.4, 17.4 and 20.8?mg·g^?1) when the initial concentration of paraquat was raised (25, 50 and 200?mg·L^?1 respectively). Adsorption kinetics was appropriated to the pseudo-second-order model and adsorption isotherm was fitted to the Langmuir model. For thermochemistry parameters at different temperatures, the negative ΔG° showed a spontaneous adsorption process, the negative ΔH° indicated an exothermic process and the positive ΔS° exhibited an increase disorder. Finally, the reusability of the insoluble polymer was reached 78.3% after four regeneration cycles in methanol.     

Evaluation of the melt stabilization performance of hydroxytyrosol (3,4-dihydroxy-phenylethanol) in polypropylene

The use of hydroxytyrosol (3,4-dihydroxy-phenylethanol) as a potential alternative to synthetic compounds in the melt stabilization of polyolefins is considered. Hydroxytyrosol was found to play a role in enhancing the oxidative stability of olive oil, and a similar reduction in polyolefins’ thermo-oxidative degradation during processing is expectable. Rheological tests (melt flow index and viscosity vs. shear rate) showed the good antioxidant performance of hydroxytyrosol during polypropylene processing as was also demonstrated by the increase in apparent activation energies and oxidation induction parameters after addition to polypropylene (0.1 wt%). Results were compared to those obtained for a commercial synthetic phenolic antioxidant and for a natural compound widely used in polymer stabilization (α-tocopherol). The main conclusion of this work is the good performance of hydroxytyrosol in polypropylene stabilization during processing and consequently the possibility of its use in formulations with improved resistance to oxidative degradation.     

Melt stabilisation of Phillips type polyethylene, Part I: The role of phenolic and phosphorous antioxidants

The role of a phenolic and three phosphorous (phosphite, phosphonite and phosphine) antioxidants in the melt stabilisation of polyethylene was studied in a Phillips type polyethylene by multiple extrusions. The polyethylene was stabilised with a single antioxidant at 700 ppm and with phenolic/phosphorous antioxidant combinations containing 700 ppm of each component. The functional groups (methyl, vinyl, vinylidene, trans-vinylene and carbonyl) of polyethylene and the residual amount of phosphorous antioxidants were analysed quantitatively by FT-IR methods developed in our laboratory. The rheological characteristics, the colour and the residual thermo-oxidative stability of the polymer were determined and compared. Blown films were prepared and their mechanical strength measured by the Elmendorf and Dart-drop tests. The comparison of the different characteristics revealed that the chemical reactions taking place during the first processing of the nascent polymer powder, as well as the chemical composition of the antioxidants determine the reactions taking place in further processing operations. The changes in the characteristics of stabilised polyethylene during processing are controlled by the phosphorous stabiliser. The effect and final result depend on the chemical structure of the given antioxidant. The phenolic antioxidant itself does not hinder the formation of long chain branches. It reduces the rate of oxidation of the various phosphorous stabilisers, but does not modify the mechanism of stabilisation of the phosphonite and the phosphine. The reactions of the phosphite are significantly modified by the presence of a phenolic antioxidant.     

BREAKTHROUGH CHEMISTRY FOR PROCESSING STABILIZATION OF POLYPROPYLENE

Traditional stabilization systems for polypropylene are typically based on a binary combination of a phenolic antioxidant and a phosphorus based melt processing stabilizer. The phenolic antioxidant provides melt processing stability as a hydrogen atom donor and free radical scavenger; it also provides the polymer with some desired level of short-term or long-term thermal stability, simply for storage or throughout the lifetime of the final article. The phosphorus based melt processing stabilizer, usually a phosphite or phosphonite, functions as a hydroperoxide decomposer above the melting point of the polymer, during melt compounding and processing. Both chemistries work together synergistically to help maintain the original molecular architecture of the polymer.

Recently, a new class of additives, 3-aryl-benzofuranones (lactones) has been introduced and adopted commercially. Lactones are highly efficient at scavenging both carbon and oxygen centered radicals during melt processing of polyolefins. Investigations have been underway to examine the impact of low concentrations of lactone in regard to enhancing the performance of traditional stabilization systems. The presumed stabilization mechanism of a lactone and the optimum composition of ternary stabilizer blends for polypropylene are discussed.     

The antioxidant role of α-tocopherol in polymers. I. The nature of transformation products of α-tocopherol formed during melt processing of LDPE

The antioxidant role of α-tocopherol (vitamin E) in low-density polyethylene (LDPE), and the nature of its transformation products formed during extrusion of the polymer are investigated. The melt stabilizing effectiveness of α-tocopherol was found to be very high, higher than that of the commercial hindered phenol antioxidants, Irganox 1076 and 1010, after single and multiple extrusion. The high antioxidant activity of α-tocopherol as a melt stabilizer is due, at least in part, to its transformation products. The importance of the processing history and the parent antioxidant concentration on the transformation products is discussed. Transformation products of α-tocopherol were analyzed after each of the four extrusion passes of the polymer. These were fractionated, analyzed, and characterized by HPLC and spectroscopy, respectively. The main products formed are diastereoisomers of dimers and trimers, as well as aldehydes; the relative concentration of each was shown to depend on processing severity (number of extrusion passes) and the initial concentration of α-tocopherol. The dihydroxydimer was found to be formed at a high concentration relative to the other products and proportional with the initial concentration of tocopherol. Based on both the identity and distribution of transformation products, a mechanism is proposed for the melt stabilization effect of α-tocopherol in LDPE. © 1994 John Wiley & Sons, Inc.