Azo Initiator Research Articles

Synthesis of a Poly(p-styrenesulfonate) Brush via Surface-Initiated Polymerization

The synthesis of molecularly thin poly(p-styrenesulfonate) monolayers as an example for a negatively charged polyelectrolyte brush covalently attached to a planar solid surface is described. The polymer monolayers are generated directly at the surface of the substrate by using self-assembled monolayers of an azo initiator and radical chain polymerization of p-styrenesulfonate ethyl ester (ESS) monomer in situ. This “grafting from” approach yields surface-bound poly(p-styrenesulfonate ethyl ester) (PESS) molecules with high molecular weights and with high graft densities of the attached chains. The PESS monolayers can be transformed into a charged poly(p-styrenesulfonate) (PSS) monolayer through a polymer-analogous saponification reaction under mild conditions, and the reaction can be carried out to quantitative conversion. The thickness of the polyelectrolyte brush can be controlled from 2 to more than 35 nm in the dry, collapsed state.

New bifunctional antioxidants—intramolecular synergistic effects between chromanol and thiopropionate groups

We have examined the antioxidant activities, both in tetralin oxidation induced by an azo initiator and in tetralin under thermoxidative conditions of a series of tetramethyl-6-chromanols with a methyl thiomethyl, phenyl thiomethyl, methyl thiopropionate, and thiopropionate group. The antioxidants tested showed almost the same activity for the oxidation of tetralin induced by an azo initiator at 61 °C. At high temperature, we found that tetramethyl-6-chromanols with a thiopropionate group increased the induction period for ortho to the OH group by 7.5-fold and meta to the OH group by 10.7-fold, compared with that for pentamethyl-6-chromanol.

Oxidative Damage to Catalase Induced by Peroxyl Radicals: Functional Protection by Melatonin and Other Antioxidants

Thermal decomposition by the azo initiator 2,2' azobis-(2-amidinopropane) dihydrochloride (AAPH) has been widely used as a water-soluble source of free radical initiators capable of inducing lipid peroxidation and protein damage. Here, in a lipid-free system, AAPH alone (40 mM) rapidly induced protein modification and inactivation of the enzyme catalase (EC 1.11.1.6). Using SDS-PAGE, it was shown that protein band intensity is dramatically reduced after 4 h of incubation with AAPH, leading to protein aggregation. Several antioxidants including melatonin, glutathione (GSH) and trolox prevented catalase modification when used at a 250 w M concentration whereas ascorbate was only effective at 1 mM concentration. All the antioxidants tested reduced carbonyl formation although melatonin was the most effective in this regard. Enzyme inactivation caused by AAPH was also significantly reduced by the antioxidants and again melatonin was more efficient than the other antioxidants used in this study. Results shown here demonstrate that alkyl peroxyl radicals inactivate catalase and reduce the effectiveness of cells to defend against free radical damage; the damage to catalase can be prevented by antioxidants, especially melatonin.

Protection against oxidative protein damage induced by metal-catalyzed reaction or alkylperoxyl radicals: comparative effects of melatonin and other antioxidants

Melatonin is a well-known hydroxyl radical ( ·OH) scavenger that protects DNA and lipids from free radical attack. In this paper, we studied the ability of melatonin to prevent oxidative damage to bovine serum albumin (BSA) induced by two different paradigms: the metal-catalyzed oxidation (MCO) induced by Cu 2+/H 2O 2 and the alkoxyl and alkylperoxyl radicals formed by the azo initiator 2,2′-azobis(2-amidinopropane) hydrochloride (AAPH, 40 mM). The protective effects of melatonin were compared with 6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid (trolox), glutathione (GSH), ascorbate, 3,4′,5-trihydroxy- trans-stilbene (resveratrol, 0.1 μM–4 mM) and mannitol (50 μM–100 mM). Melatonin efficiently prevented protein modification induced by both models, as assayed by polyacrylamide gel electrophoresis and carbonyl content. Both trolox and ascorbate had an obvious pro-oxidant effect in the Cu 2+/H 2O 2 model, whereas both prevented BSA damage induced by AAPH. In the MCO model, the efficacy of GSH in terms of protein protection was higher than melatonin at relatively high concentrations (250 μM–4 mM); however, at lower concentrations (50–250 μM), the efficacy of melatonin was superior to GSH. d-Mannitol (50 μM–100 mM) and resveratrol did not protect BSA from the site-specific damage induced by Cu 2+/H 2O 2. On the other hand, the relative protective efficiency in the AAPH model was melatonin≈trolox>GSH>ascorbate.

Antioxidative and free radical scavenging effects of ecdysteroids from Serratula strangulata.

The antioxidative and free radical scavenging effects of four ecdysteroids, 20-hydroxyecdysone (E1), 25-deoxy-11,20-dihydroxyecdysone (E2), 24-(2-hydroxyethyl)-20-hydroxyecdysone (E3), and 20-hydroxyecdysone-20,22-monoacetonide (E4), isolated from the Chinese herb Serratula strangulata have been investigated in vitro. These ecdysteroids could protect human erythrocytes against oxidative hemolysis induced by a water-soluble azo initiator 2,2'-azobis(2-amidinopropane hydrochloride) (AAPH). They could also inhibit the peroxidation of rat liver microsomes induced by hydroxyl radicals, as monitored by the formation of thiobarbituric acid reactive substances (TBARS), and prevent radical-induced decrease of membrane fluidity as determined by fluorescence polarization. They reacted with galvinoxyl radicals in homogeneous solution, and the pseudo-first-order rate constants were determined spectrophotometrically by following the disappearance of galvinoxyl radicals. Compounds E1 and (or) E3 were the most active in both antioxidative and radical-scavenging reactions.

Chemical studies on antioxidant mechanism of garcinol: analysis of radical reaction products of garcinol with peroxyl radicals and their antitumor activities

Antioxidant actions of garcinol ( 1 ), a polyisoprenylated benzophenone, purified from Garcinia indica fruit rind, are believed to contribute to its chemopreventive activity. However, the mechanisms of its antioxidant reactions remain unclear. The objective of this study was to characterize the reaction products of garcinol with peroxyl radicals generated by thermolysis of the azo initiator azo-bis-isobutyrylnitrile (AIBN). Structure elucidation of these products can provide insights into specific mechanisms of antioxidant reactions. Four reaction products ( 2– 5 ) were isolated and identified. Their structures were determined on the basis of detailed high field 1D and 2D spectral analysis. The identification of these products provides the first unambiguous proof that the double bond of the isopentenyl group is a principal site of the antioxidant reaction of 1 . The induction of apoptosis in human leukemia HL-60 cells, the inhibition of NO generation, and the inhibition of LPS-induced iNOS gene expression by Western blot analysis by 1 and its four oxidation products ( 2– 5 ) were investigated.

Antioxidant flavonoids from leaves of Polygonum hydropiper L.

Ten flavonoid compounds were isolated from the dried leaves of Polygonum hydropiper L. (Laksa leaves), and identified as 3- O-α- l-rhamnopyranosyloxy-3′,4′,5,7-tetrahydroxyflavone; 3- O-β- d-glucopyranosyloxy-4′,5,7-trihydroxyflavone; 6-hydroxyapigenin; 6″- O-(3,4,5-trihydroxybenzoyl) 3- O-β- d-glucopyranosyloxy-3′, 4′, 5, 7-tetrahydroxyflavone; scutillarein; 6-hydroxyluteolin; 3′,4′,5,6,7-pentahydroxyflavone; 6-hydroxyluteolin-7- O-β- d-glucopyranoside; quercetin 3- O-β- d-glucuronide; 2″- O-(3,4,5-trihydroxybenzoyl) quercitrin; quercetin. Evaluation of the antioxidative activity, conducted in vitro, by using electron spin resonance (ESR) and ultraviolet visible (UV–vis) spectrophotometric assays, showed that these isolated flavonoids possess strong antioxidative capabilities. Measurement of the Trolox equivalent antioxidant capacity (TEAC) values, against ABTS (2,2′-azinobis(3-ethyl-benzo-thiazoline-6-sulphonic acid) radicals and phenyl- tert-butyl nitrone (PBN) azo initiator (AI) also showed strong anti-oxidative activity. The most powerful of the antioxidants was 2″- O-(3,4,5-trihydroxybenzoyl) quercitrin (galloyl quercitrin). A combination of two flavonoid compounds was tested for synergistic anti-oxidative capacity, but no significant improvement was observed.

Antioxidant effects of resveratrol and its analogues against the free-radical-induced peroxidation of linoleic acid in micelles.

The antioxidant effect of resveratrol (3,4',5-trihydroxy-trans-stilbene) and its analogues, that is, 4-hydroxy-trans-stilbene (4-HS), 3,5-dihydroxy-trans-stilbene (3,5-DHS), 4,4'-dihydroxy-trans-stilbene (4,4'-DHS), 3,4-dihydroxy-trans-stilbene (3,4-DHS), 3,4,5-trihydroxy-trans-stilbene (3,4,5-THS) and 3,4,4'-trihydroxy-trans-stilbene (3,4,4'-THS), against the peroxidation of linoleic acid has been studied in sodium dodecyl sulfate (SDS) and cetyltrimethyl ammonium bromide (CTAB) micelles. The peroxidation was initiated thermally by a water-soluble azo initiator 2,2'-azobis(2-methylpropionamidine) dihydrochloride (AAPH), and the reaction kinetics were studied by monitoring the formation of linoleic acid hydroperoxides. The synergistic antioxidant effect of these compounds with alpha-tocopherol (vitamin E) was also studied by following the decay kinetics of alpha-tocopherol and the reaction intermediate, the alpha-tocopheroxyl radical. Kinetic analysis of the antioxidant process demonstrates that these compounds are effective antioxidants in micelles used either alone or in combination with alpha-tocopherol. The antioxidative action involves trapping the propagating lipid peroxyl radical and reducing the alpha-tocopheroxyl radical to regenerate alpha-tocopherol. It was found that the antioxidant activity of resveratrol analogues depends significantly on the position of the hydroxyl groups, the oxidation potential of the molecule and the reaction medium. Molecules with ortho-dihydroxyl and/or para-hydroxyl functionalities possess high activity.

Synthesis and characterization of the copolymer of dodecyl methacrylate/styrene containing an amidine functional group

AbstractA copolymer of dodecyl methacrylate/styrene (DMA/St) containing the amidine functionality was synthesized and characterized. The one‐step solution copolymerization process was carried out using a mixed solvent, high temperature, and a functional azo initiator, 2,2′‐azobis[N,N′‐dimethylene isobutyramidine]. Copolymers with different compositions (DMA/St 10/1 to 1/2), molecular weight (Mn 2000–9000), and functionality (1.0–2.0) were prepared and characterized by NMR, FTIR, DSC, and elemental analysis. The molecular weight of the copolymers could be controlled by the amount of the initiators. Copolymer composition depends on the feed molar ratio of comonomers. The amidine functionality of the copolymers was determined by elemental analysis. It was found that 72% of the polymer chain has one functionality. The Tg of the copolymer depends on the feed molar ratio. This copolymer could be used as a potential dispersant for lubricant oil. Published 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 1684–1691, 2002

Synthesis and swelling behavior of a weak polyacid brush

We describe the synthesis and the swelling behavior of a weak polyacid brush attached to a solid surface. Monolayers of poly methacrylic acid are generated by using self-assembled monolayers of an azo initiator and radical chain polymerization of methacrylic acid monomer at the surface of a planar substrate in situ. The thickness of the resulting surface-attached polyelectrolyte brush can be adjusted between 5 and 400 nm in the dry, collapsed state. The swelling behavior in water as a function of pH and salt concentration is investigated by multiple-angle null-ellipsometry. At high salt concentrations the brush thickness decreases due to electrostatic screening. At low concentration, however, an increase of thickness with ion concentration is found.

Perfluorinated Polymer Monolayers on Porous Silica for Materials with Super Liquid Repellent Properties

We report on the preparation of water- and oil-repellent surfaces using surface-attached monolayers of perfluorinated polymer molecules. A covalent attachment of the polymer molecules to the substrate surfaces is achieved by generation of the polymer chains through starting a surface-initiated radical-chain polymerization of a fluorinated monomer. To this, self-assembled monolayers of azo initiators are attached to SiO2 substrates, which are used to kick off the polymerization reaction in situ. The growth of the fluorinated polymer brushes and the characterization of the obtained surfaces by Fourier transform infrared spectroscopy, surface plasmon spectroscopy, X-ray photoelectron spectroscopy, and contact angle measurements is described. It is shown that perfluorinated polymer films can be grown with controlled thicknesses on flat and even on porous silica surfaces, essentially without changing the surface roughness. The combination of the low surface energy coating and the surface porosity allows generation of materials which are both water and oil repellent.

Inhibitory effects of Keishi-bukuryo-gan on free radical induced lysis of rat red blood cells.

Keishi-bukuryo-gan (KBG) prevents the progression of atherosclerosis in cholesterol-fed rabbits by its antioxidative effect. The present study was to test our hypothesis that ingestion of KBG would protect red blood cell (RBC) membranes from free radical induced oxidation if polyphenolic antioxidants in KBG could be absorbed and circulated in the blood. When incubated with a RBC suspension, KBG and four of five herb medicines constituting KBG provided strong protection for RBC membranes against haemolysis induced by 2,2-azo-bis (2-amidinopropane) dihydrochloride (AAPH), an azo free radical initiator. The inhibitory effect was dose dependent at concentrations of 100-1000 microg/mL. Furthermore, the ingestion of 200 mg of KBG was associated with a significant decrease in susceptibility of RBC to haemolysis in rats.

Synthesis and Characterization of Silica/Poly (Methyl Methacrylate) Nanocomposite Latex Particles through Emulsion Polymerization Using a Cationic Azo Initiator

Following a previous work (J. L. Luna-Xavier et al., Colloid Polym. Sci.279, 947 (2001)), silica–poly (methyl methacrylate) (PMMA) nanocomposite latex particles have been synthesized in emulsion polymerization using a cationic initiator, 2,2′-azobis (isobutyramidine) dihydrochloride (AIBA), and a nonionic polyoxyethylenic surfactant (NP30). Silica beads with diameters of 68, 230, and 340 nm, respectively, were used as the seed. Coating of the silica particles with PMMA was taking place in situ during polymerization, resulting in the formation of colloidal nanocomposites with a raspberry-like or a core–shell morphology, depending on the size and nature of the silica beads. The amount of surface polymer was quantified by means of ultracentrifugation and thermogravimetric analysis as extensively described in the first article of the series (see above reference). The influence of some determinant parameters such as the pH of the suspension, the initiator, silica, monomer, or surfactant concentration on the amount of coating polymer and on the efficiency of the coating reaction was investigated in details and discussed in light of the physicochemical properties of the seed mineral. Electrostatic attraction between the positive end groups of the macromolecules and the inorganic surface proved to be the driving force of the polymer assembly on the seed surface at high pH, while polymerization in adsorbed surfactant bilayers (so-called admicellar polymerization) appeared to be the predominant mechanism of coating at lower pH. Optimal conditions have been found to reach high encapsulation efficiencies and to obtain a regular polymer layer around silica.

Radical cyclisation onto pyrazoles: synthesis of withasomnine

A novel synthetic protocol for the synthesis of [1,2- b]-fused bicyclic pyrazoles has been developed using radical cyclisation. The protocol uses cyclisation of pyrazole-1-(ω-alkyl) radicals generated from 1-[ω-(phenylselenyl)alkyl]-pyrazole precursors. The pyrazole natural product, withasomnine (3-phenyl-5,6-dihydro-4 H-pyrrolo[1,2- b]pyrazole), and larger ring analogues have been synthesised in good yield using the protocol. A Bu 3SnH-mediated oxidative cyclisation mechanism is facilitated by azo or Et 3B radical initiators acting as oxidants of the intermediate π-radicals.

Opposite effects of Mn(III) and Fe(III) forms of meso-tetrakis(4-N-methyl pyridiniumyl) porphyrins on isolated rat liver mitochondria.

The relevance of porphyrins as therapeutic drugs targeted to mitochondria has been widely recognized. In this work, we studied the action of meso-tetrakis porphyrins (TMPyP) on respiring rat liver mitochondria. Mn(III)TMPyP exerted a protective effect against lipid peroxidation induced by Fe(II) or the azo initiator 4,4'-azobis(4-cyanopentanoic acid) (ABCPA), which partition in the hydrophobic phospholipid moiety, and 2,2'-azobis(2-amidinepropane)dihydrochloride (ABAP), which partitions in the aqueous phase. In contrast, Fe(III)TMPyP itself induced an intense lipid peroxidation, accompanied by mitochondrial permeability transition. Both mesoporphyrins studied promoted a release of mitochondrial state-4 respiration, in the concentration range of 1.0-20 microM. Based on the relative effects of Mn(III)TMPyP against ABAP and ABCPA-induced lipid peroxidation, we believe that meso-tetrakis porphyrins must concentrate preferably at membrane-water interfaces.

Inhibitory effects of Choto-san (Diao-teng-san), and hooks and stems of Uncaria sinensis on free radical-induced lysis of rat red blood cells

The present study is designed to test our hypothesis that the ingestion of Uncaria sinensis (US), the main medicinal plant of Choto-san (Diao-teng-san, CS), would protect red blood cell (RBC) membrane from free radical-induced oxidation if polyphenolics in US could be absorbed and circulated in blood. When incubated with RBC suspension, Choto-san extract (CSE) and Uncaria sinensis extract (USE) exhibited strong protection for RBC membrane against hemolysis induced by 2,2-azo-bis (2-amidinopropane) dihydrochloride (AAPH), an azo free-radical initiator. The inhibitory effect was dose-dependent at concentrations of 50 to 1000 microg/mL. Ingestion of 200 mg of USE was associated with a significant decrease in susceptibility of RBC to hemolysis in rats. Furthermore, caffeic acid, an antioxidative hydroxycinnamic acid, was identified in rat plasma after administration of URE.

Metodyka badania polimeryzacji rodnikowej. Cz. II. Oznaczanie stalej szybkosci termicznego rozpadu inicjatorow azowych metoda wolumetryczna i spektrofotometryczna

The relative degree (a) of thermal decomposition of initiators was defined as a measure of the process rate. This definition (eqn. 2) was introduced into the differential equation (1) of decomposition kinetics, which is of order x ≥ 1 with respect to initiator, to simplify eqn. (7) that describes the decomposition rate and the rate constant. By eqn. (7), if the initiator decomposition rate is unrelated to initiator's initial concentration, the reaction is first-order. The rate of decomposition can be followed in terms of any physical parameter that varies with the instantaneous value of a. Equation (7) incorporating the definition (2) was adjusted to the physical parameters characteristic of the volumetric method (measurement of nitrogen volume evolving on thermal decomposition of azo initiators, eqns. 12-14 & 18) and of the spectrophotometric method (measurement of absorbance, eqns. 20-22). Authors' studies were used to discuss the merits and demerits of these methods as applied to measure the thermal decomposition of AIBN and its derivatives in N,N-dimethylformamide (DMF) as solvent (Figs. 2-10). The volumetric results were affected neither by inhibitor nature nor by concentration; for systems containing both a monomer and an inhibitor, in addition to an initiator and solvent, the spectrophotometric method (UV-VIS) proved to be useless.

Synthesis of Polymer Brushes on Silicate Substrates via Reversible Addition Fragmentation Chain Transfer Technique

Reversible addition fragmentation chain transfer (RAFT) has been used to synthesize polymer brushes. Styrene, methyl methacrylate, and N,N-dimethylacrylamide brushes were prepared under RAFT conditions using silicate surfaces that were modified with surface-immobilized azo initiators. Films with controlled thicknesses were produced. RAFT was also used to synthesize PS-b-PDMA and PDMA-b-PMMA block copolymer brushes that displayed reversible surface properties upon treatment with block-selective solvents.

Free‐radical crosslinking copolymerization of 2,2′‐azobis[N‐(2‐propenyl)‐2‐methylpropionamide] with vinyl benzoate resulting in polymeric azo initiators

Abstract2,2′‐Azobis[N‐(2‐propenyl)‐2‐methylpropionamide] (APMPA) with two carbon–carbon double bonds and an azo group was copolymerized with vinyl benzoate (VBz) at 60 °C, resulting in azo groups containing VBz/APMPA prepolymers and crosslinked polymers as soluble and insoluble polymeric azo initiators, respectively. The polymerization characteristics of APMPA as a novel diallyl monomer were clarified with the rate and degree of polymerization and the monomer reactivity ratios. The gelation behaviors in VBz/APMPA crosslinking copolymerizations were examined in detail with a comparison of the actual gel point and the theoretical gel point calculated according to Stockmayer's equation with the tentative assumption of equal reactivity for both vinyl groups belonging to VBz and APMPA. The effectiveness of the resulting branched or crosslinked poly(VBz‐co‐APMPA)s as soluble or insoluble polymeric azo initiators, respectively, at providing graft polymers through the cleavage of azo groups at an elevated temperature was examined by the polymerization of allyl benzoate at 120 °C. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 40: 317–325, 2002

Solid-phase acylating reagents in new format: macroporous polymer disks.

Acylation resins in a new monolithic format have been prepared by the functionalization of polyethylene-encased porous poly(chloromethylstyrene-co-divinylbenzene) disks. These disks have been obtained from a monolithic rod prepared by polymerization in a cylindrical glass mold, then cut into a disk format. A free radical azo initiator 4,4'-azobis(4-cyanovaleric acid) attached to available chloromethyl functionalities at the surface of the pores was used to initiate graft polymerization of 4-acetoxystyrene or chloromethylstyrene from the surface. Addition of a small percentage of divinylbenzene to the polymerization mixture leads to the formation of a layer of swellable reactive polymer gel at the surface of the macropores. This both prevents the undesirable increase in flow resistance through the monolith and improves the yield of grafting. The final reaction steps involve formation of an active phenolic moiety grafted to the disks and its reaction with acid anhydride. The use of grafted disks as acylating resin to transform various amines to amides in flow-through operations is demonstrated in a variety of solvents including alcohols. The acylation ability of the depleted disks can easily be recovered, and the disks can be reused many times.