Azo Initiator Research Articles

Room temperature polymerization of vinylic monomers by azo initiator

The azo initiator, sodium-{4-(4-hydroxy-phenylazo)-benzenesulfonate (SHPBS)}, has been synthesized by coupling reaction with sodium-(p-sulfonate benzene diazonium chloride) and phenol. Unsymmetrical azo group leading to weak azo bond in case of SHPBS resulted in dissociation at lower temperature (25°C) to produce free radicals. Its versatile solubility (soluble in aqueous and organic solvents), generation of free radicals at 25°C, and its application without mechanical stirring are the added greater advantage over commercially available azo initiators (AIBN, etc.). The SHPBS has been characterized by FTIR, 1H-NMR spectroscopies, and thermal analysis. It has been observed that various monomers such as AM, AA, IPAM, and AMPS which have been polymerized with SHPBS have led to high molecular weight polymers at shorter reaction time. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Dielectric Properties of Free Radical Initiators—Investigation of Thermal Decomposition Products

Studies into the phenomena that explain the dielectric properties of azo initiators (AMBN and V70) at high temperatures are reported in this paper. Previous studies have successfully related the variation in dielectric properties of these species below 110 °C to either phase changes or the thermal production of free radicals. In the latter case the marked increase in their response was attributed to the free radicals having more prominent dipoles than their initiator precursors. This study reports the results of experiments designed to explain their observed dielectric characteristics within the temperature range of 110–150 °C. At these elevated temperatures, the decomposition half-life of the initiators studied should be of the order of few seconds. However, in both this and the previous reports, the dielectric response is found to remain at a significant level for several hours. The two prime explanations for the unexpected duration of the increased dielectric properties are (i) the presence of microwave induced protected radicals or (ii) the dielectric properties of the initiator decomposition products. The observations made in this study were subsequently used to define that the latter of these is the key to the observed phenomenon.

Controlled placement of polystyrene-grafted CdSe nanoparticles in self-assembled block copolymers

Surface functionalization of semiconductor CdSe nanoparticles has been achieved with polystyrene (PS) brushes by “grafting from” technique for further addition to a polystyrene-b-polybutadiene-b-polystyrene (SBS) block copolymer in order to obtain self-assembled composites. For modification of nanoparticle surface 3-glycidoxypropyltrimethoxysilane (GPS) was used at first for the later attachment of the 4,4′-azobis(4-cyanopentanoic acid) azo initiator. Fourier-transform infrared spectroscopy confirmed the presence of GPS and PS on the surface of nanoparticles. Atomic force microscopy was used for morphological characterization of SBS/CdSe nanocomposites. Modification of nanoparticles with PS brushes by radical polymerization improved their affinity with PS block and the dispersion of nanoparticles avoiding agglomeration. CdSe nanoparticle size was measured to be around 2 nm by the use of X-ray diffraction and UV–Vis techniques. Optical properties were characterized using fluorescence measurements.

Function of plastochromanol and other biological prenyllipids in the inhibition of lipid peroxidation—A comparative study in model systems

Lipid peroxidation is an oxidation reaction leading to the generation of lipid hydroperoxides. Here we present comparative data on the inhibition of lipid peroxidation by a variety of biological prenyllipids in liposomes prepared from natural lipid membranes. Lipid peroxidation was initiated by hydrophilic and hydrophobic azo initiators, as well as by singlet oxygen generated via photosensitized reaction of hydrophobic zinc tetraphenylporphine. When lipid peroxidation was initiated in the water phase, tocopherols and plastochromanol-8 were more effective than prenylquinols, such as plastoquinol-9, ubiquinol-10 or α-tocopherolquinol. However, if the peroxidation was initiated within the hydrophobic interior of liposome membranes, long-chain prenyllipids, such as plastoquinol-9 and plastochromanol-8, were considerably more active than tocopherols in the inhibition of the reaction. In the latter system, tocopherols showed even prooxidant activity. The prooxidant activity of α-tocopherol was prevented by plastoquinol, suggesting the reduction of α-tocopheroxyl radical by the quinol. All the investigated prenyllipids were able to inhibit singlet oxygen-mediated lipid peroxidation but the most active were prenylquinols in this respect. Among all the prenyllipids investigated, plastochromanol-8 was the most versatile antioxidant in the inhibition of lipid peroxidation initiated by the three different methods.

Amphiphilically stabilized block copolymer particles via heterophase polymerization in glacial acetic acid

Radical heterophase polymerization of styrene in glacial acetic acid initiated with either 2,2′-azobisisobutyronitrile or poly(ethylene glycol)-azo initiators and in the presence of poly(methyl methacrylate) or poly(ethylene glycol) macromonomers is described for the first time ever. It turned out to be a convenient route to amphiphilically stabilized block copolymer dispersions. These block copolymers, after the polymerization in glacial acetic acid, can be easily transferred to other continuous phases which are selective solvents for one of the different constituent blocks. Electron microscopy results are presented regarding the morphology of the block copolymer particles in glacial acetic acid, water, and in a mixture of tetralin, cis-decalin, and tetrachlormethane. Depending on the particular composition of the block copolymers and the nature of the continuous phase, the changes in the morphology for a given block copolymer can be quite dramatic.

Combined bead polymerization and Cinchona organocatalyst immobilization by thiol–ene addition

In this work, we report an unusually concise immobilization of Cinchona organocatalysts using thiol–ene chemistry, in which catalyst immobilization and bead polymerization is combined in a single step. A solution of azo initiator, polyfunctional thiol, polyfunctional alkene and an unmodified Cinchona-derived organocatalyst in a solvent is suspended in water and copolymerized on heating by thiol–ene additions. The resultant spherical and gel-type polymer beads have been evaluated as organocatalysts in catalytic asymmetric transformations.

CART peptide is a potential endogenous antioxidant and preferentially localized in mitochondria.

The multifunctional neuropeptide Cocaine and Amphetamine Regulated Transcript (CART) is secreted from hypothalamus, pituitary, adrenal gland and pancreas. It also can be found in circulatory system. This feature suggests a general role for CART in different cells. In the present study, we demonstrate that CART protects mitochondrial DNA (mtDNA), cellular proteins and lipids against the oxidative action of hydrogen peroxide, a widely used oxidant. Using cis-parinaric acid as a sensitive reporting probe for peroxidation in membranes, and a lipid-soluble azo initiator of peroxyl radicals, 2,2′-Azobis(2,4-dimethylvaleronitrile) we found that CART is an antioxidant. Furthermore, we found that CART localized to mitochondria in cultured cells and mouse brain neuronal cells. More importantly, pretreatment with CART by systemic injection protects against a mouse oxidative stress model, which mimics the main features of Parkinson's disease. Given the unique molecular structure and biological features of CART, we conclude that CART is an antioxidant peptide (or antioxidant hormone). We further propose that it may have strong therapeutic properties for human diseases in which oxidative stress is strongly involved such as Parkinson's disease.

High molecular weight acrylonitrile–butadiene architectures via a combination of RAFT polymerization and orthogonal copper mediated azide–alkyne cycloaddition

α-Functional nitrile butadiene rubber (NBR) building blocks were employed in the copper mediated 1,3-dipolar Huisgen coupling upon addition of 1,4-bis(azidomethyl)benzene (4). Polymer–polymer coupling afforded linear polymers with molecular weights ranging from 2500 g mol−1 to 97 000 g mol−1 and polydispersities from 1.1 to 1.6. The α-functional NBR building blocks were obtained via the reversible addition–fragmentation chain transfer (RAFT) copolymerization of acrylonitrile (AN) and 1,3-butadiene (BD) at 100 °C, utilizing the high temperature azo initiator 1,1′-azobis(cyclohexane-1-carbonitrile) and chlorobenzene or acetone as solvents. A novel alkyne-functional trithiocarbonate 2 was synthesized in 64% yield via the N,N′-dicyclohexylcarbodiimide mediated coupling of 2-((dodecylsulfanyl)carbono-thioyl)sulfanyl propanoic acid (DoPAT, 1) and propargyl alcohol. 2 was shown to be an efficient controlling agent for the controlled/living radical copolymerization of acrylonitrile and 1,3-butadiene. The use of copper mediated azide–alkyne cycloaddition was extended towards the side-chain modification of acrylonitrile–butadiene rubbers as well as applied in the synthesis of branched and cross-linked NBR structures. For this purpose an acrylonitrile-1,3-butadiene–propargyl methacrylate (PMA) terpolymer of 3900 g mol−1 with a PDI of 1.3 was synthesized by a DoPAT-mediated RAFT polymerization. Herein, monomers were employed in the ratio of 56 : 35 : 9 (BD : AN : PMA). The ability of the terpolymer to undergo side-chain modification was demonstrated upon addition of 1-undecane azide. Cross-links were established via addition of 1,4-bis(azidomethyl)benzene. The current study provides the first successful approach to employ an orthogonal conjugation technique on this technically important class of synthetic rubbers.

Properties of novel gel particles with projections and preparations of composite particles with gold nanoparticles

Mono-dispersed gel particles with projections were prepared by dispersion polymerization with ethyleneglycol dimethacrylate and a polyethyleneglycol (PEG) block macro azo initiator in H2O/ethanol solutions. The effects of molecular weight of PEG blocks and polymerization conditions on the morphology and some properties were examined for the gel particles. The diameters and the extent of coagulation were different with the polymerization conditions. Relatively large specific areas and large swelling ratio with H2O were obtained, and these values were related with the polymerization conditions. By mixing the gel particles with gold nanoparticles in solutions, composite particles were formed, which were composed of gold nanoparticles adsorbed on the gel particles. Interaction between gold nanoparticles and reactive azo groups remained in the gel particles concerned for the formation of the composite particles.

A Detailed investigation of the experimental conditions for the reversible addition fragmentation chain transfer-mediated copolymerization of acrylonitrile and butadiene

The synthesis of acrylonitrile-butadiene rubbers (NBRs) via trithiocarbonate-mediated reversible addition fragmentation chain transfer (RAFT) polymerization of acrylonitrile (ACN) and 1,3-butadiene (BD) in solution under azeotropic conditions (38/62) was investigated for a broad range of common solvents: N,N-dimethylacetamide (DMAc), chlorobenzene, 1,4-dioxane, tert-butanol, isobutyronitrile, toluene, trimethylacetonitrile, dimethyl carbonate, acetonitrile, methyl acetate, acetone, and tert-butyl methyl ether. The gravimetrically determined conversions for the free radical polymerizations of ACN/BD after 22 h at 100 °C were in the range of 15% for methyl acetate to 35% for DMAc. The origin of the differences in conversion is attributed to the unequal decomposition behavior of the employed azo initiator 2,2′-azobis(N-butyl-2-methylpropionamide) (1) in the solvents under investigation, as determined by ultraviolet–visible (UV–vis) spectroscopy. Relative decomposition of 1 in solution (0.1 mol L−1) at 100 °C was calculated from the UV–vis spectra for selected solvents. 90% of 1 in DMAc was decomposed after 22 h, 83% in tert-butanol, 57% in 1,4-dioxane, 53% in isobutyronitrile, 45% in chlorobenzene, and 21% in toluene. The evolution of molecular weight with conversion using the initiator 1 was in accordance with the theoretically expected values, regardless of the solvent studied. Moreover, the RAFT-mediated copolymerization of ACN/BD in DMAc with azo initiators 1, 1-[(1-cyano-1-methylethyl)azo]formamide (2) and 1,1′-azobis(cyclohexanecarbonitrile) (3) was investigated. A strong deviation from the linear evolution of molecular weight due to a fast decomposition of these initiators – congruent with high primary radical delivery rates – at the selected temperature was observed when using 2 and 3. The deviation was not observed when using 1. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012

Grafting of silica with a hydrophilic triol acrylamide polymer via surface-initiated “grafting from” method for hydrophilic-interaction chromatography

A novel hydrophilic polymer-coated silica sorbent has been prepared using the radical "grafting from" polymerization method through surface-bound azo initiators for hydrophilic-interaction chromatography (HILIC). The azo groups were introduced to the surface of silica gel through the reaction with amino groups on the surface of silica gel with 4,4'-azobis(4-cyanopentanoic acid chloride) (ACVC). The resultant azo-immobilized silica gel served as surface initiator to polymerize hydrophilic triol acrylamide monomer N-acryloyltris(hydroxymethyl) aminomethane (NA) in methanol to get hydrophilic polymer-coated silica sorbent. The obtained poly(NA)-coated silica (pNA-sil) was characterized by Fourier transform infrared spectroscopy (FT-IR), elemental analysis (EA), and nitrogen sorption porosimetry (NSP). Then the pNA-sil was packed into the stainless-steel column and evaluated in high-performance liquid chromatography (HPLC). Good chromatographic performance for the separation of peptides and nucleosides was obtained under HILIC mode. The results indicated that the pNA-sil stationary phase behaved as mixed-mode retention mechanisms of hydrophilic and ionic interactions. Furthermore, the pNA-sil phase was used to separate tryptic digest of β-casein and our results showed that more than 12 peptides peaks were resolved and well distributed within the elution window. Finally, the pNA-sil stationary phase was demonstrated to possess remarkable reproducibility and stability. Taken together, the pNA-sil stationary phase prepared in the current study offers a potential application in proteomics study.

Quantitative Analysis of the Effect of Azo Initiators on the Structure of α-Polymer Chain Ends in Degenerative Chain-Transfer-Mediated Living Radical Polymerization Reactions

The effects of azo initiators on the structures of the α-polymer chain ends in organotellurium-, organostibine-, organobismuthine-, and organoiodine-mediated living radical polymerizations (TERP, SBRP, BIRP, and IRP, respectively) and reversible addition–fragmentation chain transfer radical polymerization (RAFT) were quantitatively analyzed for the first time. These polymerization methods predominantly, or exclusively, follow the degenerative chain transfer (DT) mechanism for the activation and deactivation of dormant and radical species. An external radical species is required to initiate the polymerization under DT conditions, and the influx of radical species decreases the α-end-group fidelity. The effect was examined for polymerizations of styrene (St), n-butyl acrylate (BA), N-vinylpyrrolidone (NVP), methyl methacrylate (MMA), and isoprene (Ip) in the presence of different chain transfer agents (CTAs) and azo initiators, such as 2,2′-azobis(isobutyronitrile) (AIBN), 1,1′-azobis(cyclohexane-1-carbonit...

Blood Pressure-Lowering Response to Amlodipine as a Determinant of the Antioxidative Activity of Small, Dense HDL3

High-density lipoproteins (HDLs) exert multiple antiatherogenic activities including protection of low-density lipoproteins (LDLs) from oxidative stress. Beneficial effects of calcium channel blockers on cardiovascular disease may in part be related to the reduction of oxidative stress, potentially enhancing the antioxidative activity (AOX) of HDLs. This study aimed to assess the effect of 1 month's treatment with amlodipine on HDL AOX in hypertensive subjects. This was a prospective trial of amlodipine 10 mg/day administered for 1 month in primary-care patients with hypertension (n = 28), 46% of whom were obese and 57% of whom displayed the metabolic syndrome. The main outcome measure was HDL AOX, assessed as the capacity of small, dense HDL3c particles to attenuate LDL oxidation induced in vitro by an azo initiator (AAPH). Mean (± SD) systolic (SBP) and diastolic (DBP) BP were reduced by amlodipine by 22.1 mmHg (± 13.2) and 10.4 mmHg (± 7.5), respectively (p < 0.001). Body mass index, waist circumference, and plasma levels of triglycerides, cholesterol, and fasting blood glucose did not change significantly. Amlodipine treatment did not modify HDL3c AOX in the whole study population; changes in AOX were, however, positively correlated with SBP (r = 0.37, p = 0.05 for maximal diene concentration; r = 0.34, p = 0.08 for LDL oxidation rate). When the population was divided into two subgroups according to the BP response to amlodipine (change in SBP below or above the median), HDL3c AOX was significantly improved in hyper-responders (BP-lowering response >22/10 mmHg) as compared with hypo-responders (BP-lowering response <22/10 mmHg: mean [± SD] change in the LDL oxidation rate in the presence of HDL3c, -6.8% [± 11.2] vs +1.9% [± 5.2], respectively, p = 0.04; maximal diene concentration, -8.6% [± 13.0] vs +1.9% [± 8.2], respectively, p < 0.05). By contrast, neither plasma concentrations of oxidized LDL, a marker of systemic oxidative stress, nor the chemical composition of HDL3c were modified between the subgroups. In hypertensive patients, amlodipine treatment enhanced HDL AOX in subjects who had a BP reduction that exceeded the median response. This effect appears to be secondary to the hypotensive effect, rather than to the direct antioxidant properties, of the drug.

Preparation and characterization of a molecularly imprinted polymer by grafting on silica supports: a selective sorbent for patulin toxin

A new molecularly imprinted polymer (MIP) has been prepared on silica beads using the radical "grafting from" polymerization method for selective extraction of minor contaminant mycotoxin of patulin (PTL). After the introduction of amino groups onto the silica surface with 3-aminopropyltriethoxysilane, azo initiator onto the silica surface was achieved by the reaction of surface amino groups with 4,4'-azobis(4-cyanopentanoic acid). The scale-up synthesis of MIP was then carried out in the presence of 6-hydroxynicotinic acid as template substitute, functional, and cross-linking monomers. The prepared sorbent was characterized using FT-IR spectroscopy, scanning electron microscopy, elemental analysis, and the adsorption-desorption selectivity, and the capacity characteristic of the polymer was investigated by a conventional batch adsorption test and Scatchard plot analysis. The results indicated that coated polymers had specific adsorption to PTL as compared with its co-occurring 5-hydroxymethyl-2-furaldehyde (hydroxymethylfurfural (HMF)), at the same bulk concentration for solution of PTL and HMF, the maximum absorbance in the solid-phase extraction (SPE) method to PTL were 93.97% or 0.654 μg/mg while to HMF they were 76.89% or 0.496 μg/mg. Scatchard analysis revealed that two classes of binding sites were formed in PTL-MIP with dissociation constants of 3.2 × 10(-2) and 5.0 × 10(-3) mg/mL and the affinity binding sites of 8.029 and 1.364 mg/g, respectively. The recoveries of PTL were more than 90% for the developed MISPE and around 75% for the traditional liquid-liquid extraction in spiked apple juice samples. It was concluded that the method is suitable for the scale-up synthesis of PTL-MIP grafted on silica, and the polymer can be effectively applied as SPE coupled with high-performance liquid chromatography (HPLC) for the determination of PTL in apple juice or other related products.

All-Acrylic Film-Forming Colloidal Polymer/Silica Nanocomposite Particles Prepared by Aqueous Emulsion Polymerization

The efficient synthesis of all-acrylic, film-forming, core-shell colloidal nanocomposite particles via in situ aqueous emulsion copolymerization of methyl methacrylate with n-butyl acrylate in the presence of a glycerol-functionalized ultrafine silica sol using a cationic azo initiator at 60 °C is reported. It is shown that relatively monodisperse nanocomposite particles can be produced with typical mean weight-average diameters of 140-330 nm and silica contents of up to 39 wt %. The importance of surface functionalization of the silica sol is highlighted, and it is demonstrated that systematic variation of parameters such as the initial silica sol concentration and initiator concentration affect both the mean particle diameter and the silica aggregation efficiency. The nanocomposite morphology comprises a copolymer core and a particulate silica shell, as determined by aqueous electrophoresis, X-ray photoelectron spectroscopy, and electron microscopy. Moreover, it is shown that films cast from n-butyl acrylate-rich copolymer/silica nanocomposite dispersions are significantly more transparent than those prepared from the poly(styrene-co-n-butyl acrylate)/silica nanocomposite particles reported previously. In the case of the aqueous emulsion homopolymerization of methyl methacrylate in the presence of ultrafine silica, a particle formation mechanism is proposed to account for the various experimental observations made when periodically sampling such nanocomposite syntheses at intermediate comonomer conversions.

Plasmon-Mediated Photopolymerization Maps Plasmon Fields for Silver Nanoparticles

Visible light exposure of films containing silver nanoparticles (AgNPs) shows that the enhanced field around AgNPs in a thin film containing an azo free radical initiator (AIBN) and a triacrylate selectively cross-links the triacrylate within the plasmonic region around the particles. The cross-linked polymer is less soluble than its precursor and behaves as a solubility switch. After the film is developed with ethanol, polymer-encapsulated nanoparticles are preserved on the surface. The 8-10 nm polymer structure that encapsulates the particles effectively maps and preserves the morphology of the plasmon field in AgNP-controlled nanostructures.

Fabrication of block copolymer brushes on hollow sphere surface via reverse iodine transfer polymerization

The block copolymer brushes grafted from hollow sphere surface via reverse iodine transfer polymerization (RITP) were investigated in this work. A sufficient amount of azo initiator was introduced onto hollow sphere surface firstly. Then the monomer methyl methacrylate (MMA) was polymerized via surface-initiated reverse iodine transfer polymerization (RITP) using azo group modified hollow sphere as initiator. The microstructure of the samples was characterized by FT-IR, 1H NMR, respectively. Results indicated that the poly(methyl methacrylate) (PMMA) with end functionality of alkyl iodine group had grafted from hollow sphere surface. TEM observations showed that the average diameter of hollow core was central at 1.3–1.4μm and the average wall thickness increased from 103nm to 138nm and 172nm after grafting polymerization of MMA and Tb complex, respectively. The closely linear plots of molecular weight (Mn) versus conversion, linear kinetic plots for the free polymer formed in solution and the ability to extend the chains by sequential addition of monomer indicated that the RITP was a controlled process with a “living” characteristic.

Syntheses of an azo-group-bound silica initiator and silica-polystyrene composites

In this article, we present a facile method for the synthesis of an azo-group-bound silica (SiO2–azo) initiator. The azo groups were introduced onto the surface of silica (SiO2) nanoparticles via facile condensation between 4,4′-azobis-4-cyanopentanoic acid and the alkyl–hydroxyl groups - immobilized on the SiO2 nanoparticle surface under ambient conditions. The polystyrene (PS) chains were grafted onto the SiO2 nanoparticle surface by in situ polymerization with the resulting SiO2–azo as an initiator, and then, the SiO2–PS composite was prepared. The syntheses and properties of the SiO2–azo initiator and the composite were characterized by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, thermogravimetric analysis, gel permeation chromatography, and differential scanning calorimetry techniques. The results confirm that the SiO2–azo initiator and the composite were synthesized successfully. Styrene was polymerized with the initiation of SiO2–azo, and the resulting PS domain accounted for 48.6% of the total amount of composite. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Synthesis and characterization of novel PEG-tethered PMAA hydrogels based on a PEG macromolecular azo initiator

A new route was proposed to synthesize polyethylene glycol-tethered polymethacrylic acid (viz., PEG-tethered PMAA or PEG-t-PMAA) hydrogels based on a macromolecular azo initiator (MAI), PEG-attached azo initiator. The preparation of the MAI was accomplished by coupling PEG with 2,2-azobis[2-methyl-N-(2-hydroxyethyl)propionamide] (AMHP) using 1,1-methylene bis-(4-isocyanato-cyclohexane) (H12MDI) as bridging molecules. The structure and morphologies of the resulting hydrogels were characterized by FTIR and SEM. The pH and ionic strength responses were investigated by swelling measurements, depending on molecular weight (MW) of PEG, molar ratios of isocyanate-terminated PEG to AMHP and mass percentage of the as-prepared MAIs. The PEG-t-PMAA hydrogels synthesized by 100 wt% PEG1000-attached azo initiators and a molar ratio 2 of -NCO-terminated PEG to AMHP exhibit more remarkable pH and ionic strength response than any other hydrogels. The driving force of swelling is believed to be mainly due to the porosity and dissociation balance of PMAA gels as well as gel compositions, while the deswelling behavior may be correlated with the decrease in osmotic pressure caused by the so-called “charge screening effect“ of the cations. The PEG-t-PMAA hydrogels are expected to find specific applications in oral drug delivery and tissue engineering.

A new generation of ultralight thermochromic indicators based on temperature induced gas release

We present here a new generation of thermochromic composites that require a minimal amount of materials for a significant optical effect. Polymer nanofibers were loaded with azo initiators that release nitrogen upon heating, and were subsequently coated with a thin gold film. The local release of nitrogen gas was shown to change the local topography of the fibers, while keeping the fiber morphology intact. The optical properties of the non-wovens could be controlled by the heating time, the heating temperature and the initial thickness of the film.