Azo Initiator Research Articles

Use of a trichloromethyl-terminated azo initiator to synthesize block copolymers by consecutive conventional radical polymerization and ATRP

Use of a trichloromethyl-terminated azo initiator to synthesize block copolymers by consecutive conventional radical polymerization and ATRP

Use of a trichloromethyl-terminated azo initiator to synthesize block copolymers by consecutive conventional radical polymerization and ATRP

Use of a trichloromethyl-terminated azo initiator to synthesize block copolymers by consecutive conventional radical polymerization and ATRP

Swelling of a Polyelectrolyte Brush in Humid Air

We present an experimental study on the swelling of polyelectrolyte brushes covalently attached to planar solid surfaces in contact with humid air. Monolayers of poly-N-methyl-[4-vinylpyridinium]iodide (MePVP) with film thicknesses of several hundred nanometers were used in this study. The MePVP brushes were attached to the surfaces of silicon wafers as well as to evaporated silicon oxide films on solid substrates by using self-assembled monolayers of an azo initiator and radical chain polymerization in situ. The film thicknesses of the surface-bound monolayers were measured by optical waveguide spectroscopy (OWS) as a function of the humidity of the environment. The MePVP brushes show strong increases in thickness as well as a strong decrease of the refractive index of the surface-attached layer due to water incorporation caused by the exposure to the humid environment.

Evaluation of chain rigidity of poly(diisopropyl fumarate) from light scattering and viscosity in tetrahydrofuran

Poly(diisopropyl fumarate) (poly(DiPF)) was prepared by radical polymerization in bulk or in benzene with an azo initiator at 60°C, and fractionated with tetrahydrofuran (THF) or methanol as the solvent and water as the non-solvent. The selected nine fractions with a relatively narrow molecular weight distribution were used for the viscometric, light scattering and gel permeation chromatography experiments. The light scattering and viscosity studies using several kinds of solvents revealed that the intrinsic viscosity of poly(DiPF) changed less sensitively to the nature of the solvent used, in contrast with the second virial coefficient which was strongly dependent on the solvent. The Mark–Houwink–Sakurada equation was determined as follows: [η]=5.18×10−4Mw0.98 in cm3/g in THF at 30°C. The persistence length (q) of poly(DiPF) was evaluated from the viscosity data in THF using the Bohdanecký plot. The q and the molecular weight per unit contour length (ML) were 11 nm and 1340 nm−1, respectively. These values were compared with those for typical semiflexible polymers reported in literature.

Neutral and charged polymer brushes at solid surfaces

Neutral and charged polymer brushes covalently attached to planar solid surfaces were generated by using self-assembled monolayers of an azo initiator and radical chain polymerization in situ. The brushes were characterized by FTIR-spectroscopy, optical waveguide-spectroscopy and Ellipsometry. Especially the film thicknesses of surface bound polyelectrolyte (PEL) monolayers were measured by optical waveguide spectroscopy (OWS) as a function of the humidity of the environment. The PEL brushes show strong increases in thickness as well as strong decrease of the refractive index of the surface attached layer due to water incorporation caused by the exposure to the humid environment. Additionally the behavior of neutral as well as charged brushes in contact with solvent was investigated by using multiple-angular-scans of ellipsometry in a total internal reflectance setup. The scaling behavior of the brush height as a function of the graft density of the attached polymer molecules was investigated for the neutral brush as well as for the PEL brush system.

Cinétique de polymérisation radicalaire de (méth)acrylates à chaîne latérale fluorée

The radical polymerization of acrylates and methacrylates bearing a perfluorinated chain with an α,ω-bistrichloromethyled or an α,ω-dibenzylic azo initiator is presented. These initiators were synthesized in two or three steps in 40–50% overall yields. Molecular weights of the fluorinated polymers were determined from various methods by taking into account the behavior of these monomers in the termination step (recombination or disproportionation). These methods consisted in the titration of the chlorine content by elemental analysis or from 1H NMR by the determination of the aromatic end-groups. Although the determination of the chlorine content led to a lack of accuracy, the aromatic initiator enabled one to assess the molecular weights of the fluoropolymers produced. From the characteristic kinetic constants, the kinetics of homopolymerization of these methacrylates contributed to a simulation of these reactions which allowed one to compare the theoretical molecular weights to the experimental ones.

The use of polymers in heterophase polymerizations

Polymers can play an important role in heterophase polymerizations either as initiators, or as stabilizers, or as dispersion media. The use of polymeric radicals derived from poly(ethylene glycol) (PEG) to start the polymerization as well as to stabilize the particles leads to the formation of block copolymer particles. The block copolymer architecture depends on the number of radicals per PEG chain as well as on the termination mode of the growing polymer chain. PEG azo initiators resulting in one radical per PEG chain and styrene as monomer lead to PEG-polystyrene-PEG triblock copolymers whereas methyl methacrylate and PEG chains with a radical on both ends lead to poly(methyl methacrylate)-PEG-poly(methyl methacrylate) triblock copolymers. In both cases the particle morphology deviates from the ordinary solid sphere morphology. Block copolymers with one poly(styrene sulfonate) block are effective stabilizers and lead to different particle decorations depending on the length as well as on the degree of sulfonation of the polyelectrolyte block. Fully sulfonated poly(styrene sulfonate) blocks with a contour length of 113 nm lead to electrosterically stabilized particles with a polyelectrolyte brush corona thickness of approx. 60 nm (`porcupine' particles). The same poly(styrene sulfonate) block with a degree of sulfonation of approx. 50% results in `ringlet' particles due to the formation of loops and multiple adsorption points. The `porcupine' particles posses an extraordinary salt stability associated with the entropic elasticity of the polyelectrolyte chain. The use of a homologous series of PEG as a dispersion medium and PEGA initiators leads to polymer in polymer dispersions where the molecular weight ratio of the free PEG chains and the confined PEG chains determines the particle properties. The average particle size is smaller and the polydispersity of the particle size distribution is larger, the higher the molecular weight of the molecules in the dispersion medium compared to the confined PEG chains stabilizing the particles. This is a new way to control particle as well as polymer properties in heterophase polymerizations.

Formation of the aldehydic choline glycerophospholipids in human red blood cell membrane peroxidized with an azo initiator.

The production of phospholipid hydroperoxide and aldehydic phospholipid was examined in human red blood cell (RBC) membranes after peroxidation with 2,2-azobis(2-amidinopropane)dihydrochloride (AAPH) or xanthine/xanthine oxidase (XO/XOD/Fe3+). Both radical-generation systems caused a profound decrease in the amount of polyunsaturated fatty acid (PUFA) in choline glycerophospholipid (CGP) and induced formation of peroxidized CGP in RBC membranes to different extents. No consistent generation of peroxidized lipids from CGP was evident after peroxidation with XO/XOD/Fe3+, which caused the apparent decomposition of phospholipids and the formation of large amounts of thiobarbituric acid-reactive substance (TBARS). On the other hand, CGP hydroperoxide was formed as a primary product of peroxidation with AAPH. Aldehydic CGP was also detected as a secondary product of hydroperoxide decomposition in AAPH-peroxidized RBC membranes. Aldehydic CGP was preferentially generated from arachidonoyl CGP rather than from linoleoyl CGP in AAPH-peroxidized membranes. AAPH mainly oxidized CGP to hydroperoxide and aldehydic phospholipids. The sum of hydroperoxide and aldehyde of CGP corresponded to the loss of CGP due to peroxidation by AAPH. This result indicates that CGP was mainly converted into these two oxidized phospholipids in AAPH-peroxidized RBC membranes.

Radical grafting from glass fiber surface: Graft polymerization of vinyl monomers initiated by azo groups introduced onto the surface

This paper describes the radical graft polymerization of vinyl monomers from glass fiber surface initiated by alkylazo groups introduced onto the fiber surface. The introduction of azo groups onto the glass fiber surface was achieved by reaction of isocyanate groups which were previously attached onto the surface with two kinds of azo initiators, 4,4′-azobis(4-cyanopentanoic acid) (ACPA) and 2,2′-azobis(2-cyanopropanol) (ACP). The amounts of surface azo groups introduced by ACPA and ACP were both determined to be 1.3 × 10−5 mol g−1 by nitrogen analysis. The radical graft polymerization of methyl methacrylate (MMA) was found to be initiated in the presence of the glass fiber having surface azo groups. During the polymerization, part of resultant poly(MMA) grafted onto the fiber surface through propagation of the polymer from the surface radicals produced by the decomposition of the azo groups. The percentage of grafting of poly(MMA) reached 48.1% after 24 h. The graft polymerizations of other monomers, such as styrene, N-vinylcarbazole, and acrylic acid, were also initiated by the surface azo groups, and the corresponding polymer effectively grafted onto the surface. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 2121–2128, 1999

Radical grafting from glass fiber surface: Graft polymerization of vinyl monomers initiated by azo groups introduced onto the surface

This paper describes the radical graft polymerization of vinyl monomers from glass fiber surface initiated by alkylazo groups introduced onto the fiber surface. The introduction of azo groups onto the glass fiber surface was achieved by reaction of isocyanate groups which were previously attached onto the surface with two kinds of azo initiators, 4,4′-azobis(4-cyanopentanoic acid) (ACPA) and 2,2′-azobis(2-cyanopropanol) (ACP). The amounts of surface azo groups introduced by ACPA and ACP were both determined to be 1.3 × 10−5 mol g−1 by nitrogen analysis. The radical graft polymerization of methyl methacrylate (MMA) was found to be initiated in the presence of the glass fiber having surface azo groups. During the polymerization, part of resultant poly(MMA) grafted onto the fiber surface through propagation of the polymer from the surface radicals produced by the decomposition of the azo groups. The percentage of grafting of poly(MMA) reached 48.1% after 24 h. The graft polymerizations of other monomers, such as styrene, N-vinylcarbazole, and acrylic acid, were also initiated by the surface azo groups, and the corresponding polymer effectively grafted onto the surface. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 2121–2128, 1999

Polymers grafted on mica by radical chain growth from the surface

Mica is a popular model substrate for surface studies due to its availability in sheet form and its molecularly flat surface. As mica has ion-exchangeable surface ions, a variety of organic moities bearing cationic groups can readily be attached to the surface via ion exchange at very high surface densities (∼1 per 50 Å 2). We wished to investigate the preparation of polymers grafted to mica by free radical polymerization initiated from the surface. Mica powders with ultrahigh specific surface areas (∼100 m 2 g −1) were treated with aqueous solutions of a peroxide radical initiator bearing one cationic group to exchange the mica surface ions with initiator cations. The resulting materials were used to initiate the polymerization of styrene. Following reaction, it was found that polystyrene chains were bound to the surface. The individual chains could be imaged in the form of ‘molecular droplets’ using scanning electron microscopy (SEM). The observed kinetics of bound chain growth support the view that chain growth proceeds by propagation of surface free radicals into the liquid monomer phase, i.e. ‘growth from the surface’. The results obtained with the monocationic, surface-bound peroxide initiator strongly differ from those obtained with a dicationic surface-bound azo initiator, where growth has been shown to occur ‘to the surface’. At sufficiently high density of attached chains, the droplets overlap and thin films result.

Radical copolymerization of methyl α-acyloxyacrylates with some vinyl monomers

Radical copolymerization of some methyl α-acyloxyacrylates and vinyl monomers has been studied using an azo initiator in benzene at 60°C. It is found that the Q and e values of methyl α-acyloxyacrylates are in the ranges 0.47-1.65 and 0.51-0.99, respectively, and the relative reactivity of the acrylates towards the propagating radicals of styrene in copolymerization is correlated to the modified Taft equation considering the steric and electronic effects. Copolymerization of methyl α-acetoxyacrylate and vinyl monomers shows that the relative reactivity of the vinyl monomers for the attack of poly (methyl α-acetoxyacrylate) radical tends to increase with increasing Q values of the vinyl monomers.

Polymerization of Styrene with Peroxide Initiator Ionically Bound to High Surface Area Mica

Mica powders with ultrahigh specific surface areas (ca. 100 m2/g) were treated with aqueous solutions of peroxide radical initiators bearing a single cationic group to exchange the mica surface ions with initiator cations. The stability of the initiators, the adsorption isotherms on the mica, and the adsorption equilibrium constants were measured. The resulting materials initiate the polymerization of styrene at various conditions, forming bound polymer chains in amounts up to ca. 30 wt % polymer (ca. 55 vol %) with respect to the mica. The bound chains could be removed from the surface by extraction with good solvents for poly(styrene) only when the solvent contained ions for ion exchange. On the basis of kinetic evidence, the mechanism of grafting appears to be propagation of free radicals from the surface into the monomer solution, i.e., “growth from the surface”. The results differ strongly from those previously obtained with a dicationic surface-bound azo initiator, where grafting was shown to occur ...

Synthesis of Oxazoline-Terminated Polystyrene via Controlled-Radical Polymerization

Two novel bis(1,3-oxazoline-2-yl)-functionalized azo initiators were prepared, and their half-lives were determined by UV spectroscopy. These initiators were used in conjunction with 2,2,6,6-tetramethylpiperidin-1-yloxy (TEMPO) to afford controlled-radical styrene polymerization. Polymerization kinetics, molar mass versus conversion dependence and polydispersities with varying [TEMPO]/[initiator] ratios were examined, and the degree of endgroup functionalities was determined by NMR, FT-IR, endgroup titration and size exclusion chromatography (SEC) data. Well-defined narrow-distributed mono(1,3-oxazolin-2-yl)-terminated polystyrenes with molar mass varying between 1000 and 50 000 with polydispersities of 1.2−1.3 were obtained. The reactivity of the oxazoline moiety was proven by reacting monooxazoline-terminated polystyrene with monocarboxy-terminated polystyrene. Increases in molar mass and the formation of ester amide coupling groups, resulting from the reaction of oxazoline with carboxylic acid endgroup...

Chain-breaking fused heterocyclic antioxidants: antioxidant activities of phenothiazines compared to related compounds

The antioxidant activities of phenothiazines and related diphenylamines were evaluated in the oxidation of tetralin at 60°C and linoleic acid micelles in aqueous dispersion at 37°C induced by an azo initiator. The antioxidant activities of phenothiazines were much higher than those of the structurally comparable diphenylamines in both systems. The results are interpreted in terms of electron donating capacity and stabilization of the aminyl radical by the sulfur atom. Semiempirical molecular orbital AM1 and PM3 calculations were applied to study hydrogen abstractions of antioxidants in the chain process of autoxidation. The results indicate that the rates of oxidation during the induction period were correlated with the dissociation energy of the N–H bond.

Preparation and Characterization of a Polyelectrolyte Monolayer Covalently Attached to a Planar Solid Surface

A novel concept for the synthesis of molecularly thin polyelectrolyte layers covalently attached to planar solid surfaces is introduced. The polymer monolayers are generated directly at the surface by using self-assembled monolayers of an azo initiator and radical chain polymerization in situ. This “grafting from” approach yields surface-bound polymer molecules with a high molecular weight and with high graft densities of the attached chains (“brush”-like state). As an example for the preparation of a positively charged polyelectrolyte “brush” at first a monolayer of poly(4-vinylpyridine) (PVP) was generated at the surface, which was subsequently quarternized to yield a poly[4-vinyl-N-n-butylpyridinium] bromide (BuPVP) monolayer. The thickness of this resulting cationic monolayer can be controlled in a wide range, starting from 2 nm to more than 1000 nm in the solvent-free state.

Latex Blends of Fluorinated and Fluorine-Free Acrylates: Emulsion Polymerization and Tapping Mode Atomic Force Microscopy of Film Formation

Latex based upon fluorinated acrylates such as perfluorooctylethyl methacrylate (FMA) and perfluorooctylethyl acrylate (FA) and fluorine-free acrylates such as n-butyl methacrylate (BMA) was prepared by means of emulsion polymerization in the presence of octadecyltrimethylammonium bromide (C18TAB) and 2,2‘-azobis(2-amidinopropane) dihydrochloride (V50) as water-soluble azo initiator. Reaction calorimetric studies revealed that acetone addition, preferably 15−25 wt %, was the key to achieve high monomer conversion and to produce stable latex with average particle sizes varying between 50 and 300 nm. Monomer conversion was increased, and latex particle size was reduced when 2 wt % n-butyl acrylate (BA) was added together with fluorine-containing monomers. The reaction enthalpy of FMA was determined to be 62 kJ/mol with respect to 75 kJ/mol for the more reactive FA. Stable poly(perfluorooctylethyl methacrylate)/poly(n-butyl methacrylate) (PFMA/PBMA) latex blends were formed. According to tapping mode atomic force microscopy, PBMA formed a dense film at 65 °C containing PFMA nanoparticles dispersed in the PBMA matrix. Annealing at 100 °C caused accumulation of PFMA at the surface and formation of a gradient film containing a surface layer with high fluorine content. Such latex blends are of interest in water-, oil-, and soil-repellent coatings.

Antioxidant activity of palm oil carotenes in peroxyl radical-mediatedperoxidation of phosphatidyl choline liposomes

The antioxidant efficacy of α-carotene and comparison with β-carotene in multilamellar liposomes prepared from egg yolk phosphatidyl choline (EYPC) exposed to the lipid soluble 2,2′-azobis (2,4-dimethyl valeronitrile) (AMVN) was investigated. Lipid peroxidation was measured as thiobarbituric acid reacting substances (TBARS)at 532 nm or as hydroperoxide formation at 234 nm after separation of phosphatidyl choline hydroperoxide (PCOOH) by high-pressure liquid chromatography (HPLC). Lutein and zeaxanthin, the hydroxyl derivatives of α- and β-carotenes, and the chain breaking antioxidant α-tocopherol were also included in the study.AMVN being a lipid soluble, non polar azo initiator penetrates into the hydrophobic interior of the phospholipid bilayer, forming peroxyl radicals which peroxidate the phospholipid leading to PCOOH accumulation. All the carotenoids tested at 1 mol% relative to EYPC significantly suppressed the formation of PCOOH compared to control samples.In this system, α-carotene retarded PCOOH formation better than β-carotene. Similarly, lutein was a better antioxidant than is zeaxanthin. But lutein and zeaxanthin were more effective antioxidants than α- and β-carotenes, respectively. After 1 h of incubation of the carotenoid with AMVN, α-, β-carotene, lutein and zeaxanthin limited PCOOH formation by 77%, 68%, 85%and 82%, respectively, while α-tocopherol elicited 90%reduction.AMVN incubated with EYPC for 2 h induced the formation of TBARS compared to control (P <0.001). α-Carotene significantly suppressed the TBARS formation by 78% whilst β-carotene, lutein, zeaxanthin and α-tocopherol elicited 60%, 91%and 80% reductions, respectively. Increasing the concentration of the carotenoid >1 mol% to EYPC did not significantly increase protection of the membrane against free radical attack.Our findings suggest that α-carotene is a better antioxidant than is β-carotene in phosphatidyl choline vesicles. It may, therefore, be useful in limiting free radical mediated peroxidative damage against membrane phospholipids in vivo.

Poly(styrene) Latex Carrying Cerium(IV)-Initiated Terminally Attached Cleavable Chains: Analysis of Grafted Chains and Model of the Surface Layer

Ce(IV)-initiated grafting of N(2-methoxyethyl-acrylamide (MEA) to a shell of poly(styrene-co-2-hydroxyethyl acrylate) on polystyrene latex has been performed and the molecular weight distribution of the graft determined by size exclusion chromatography (SEC) on grafted chains cleaved from the surface. Cationic poly(styrene) beads were covered with a shell containing poly(styrene-co-2-hydroxyethyl acrylate) utilizing an azo initiator. The density of the surface hydroxyl groups was assessed by saponification of the ester bonds followed by conductometric titration of the resulting carboxylic residues. Ce(IV) redox initiation of MEA from the surface hydroxyl groups produced chains terminally attached to the latex surface. MEA consumption kinetics were monitored by HPLC of the supernatant on a reverse phase column. Competing solution polymerization of MEA occurred, probably due to leakage of shell copolymer from the beads. The terminally attached chains were characterized by quantitative SEC following cleavage from the surface by saponification, allowing a model of the tethered surface to be developed.

Autoxidation of Binary Mixtures of Phosphatidyicholine and Phosphatidylethanolamine in Aqueous Dispersions Initiated by a Lipid-Soluble Initiator

The aerobic oxidation of binary mixtures of phosphatidylcholine (PC) and phosphatidylethanolamine (PE) in aqueous dispersions was carried out at 37°C initiated by a lipid-soluble azo initiator. The relative rates of the oxidation of PC and PE was found to depend on the number of bisallylic hydrogens in the phospholipids irrespective of the nitrogen moiety used. The increase in the mole ratios of PE to PC resulted in a decrease in the oxidizabilities of the lipids in aqueous dispersions, probably due to an increase in the microviscosity.