Coupling Reaction Of Radicals Research Articles

Peculiarity of the Mechanism of Early Stages of Photo-Oxidative Degradation of Linear Low-Density Polyethylene Films in the Presence of Ferric Stearate.

Ferric stearate (FeSt3) is very efficient in accelerating polyethylene (PE) degradation, but there is a lack of exploration of its role in accelerating the early stages of polyethylene photo-oxidative degradation. This study aimed to investigate the effect of FeSt3 on the photo-oxidative degradation of PE films, especially in the early stages of photo-oxidative degradation. The results show that FeSt3 not only promotes the oxidative degradation of PE but also contributes significantly to the early behavior of photo-oxidative degradation. Moreover, the results of the density functional theory (DFT) calculations proved that the C-H in the FeSt3 ligand was more easily dissociated compared with the PE matrix. The generated H radicals participate in the coupling reaction of the primary alkyl macro radicals leading to the molecular weight reduction, thus significantly increasing the initial rate of molecular weight reduction of PE. Meanwhile, the transfer reaction of the dissociation-generated C-centered radicals induced the PE matrix to produce more secondary alkyl macroradicals, which shortened the time to enter the oxidative degradation stage. This finding reveals the mechanism by which FeSt3 promotes the degradation of PE at the early stage of photo-oxidative degradation. It provides guiding significance for the in-depth study of the early degradation behavior in photo-oxidative degradation on polyolefin/FeSt3 films.

Theoretical study on pyrolysis mechanism of decabromodiphenyl ether (BDE-209) using DFT method

Decabromodiphenyl ether (BDE-209), as a brominated flame retardant (BFR), is widely applied to various consumer products due to its superior performance and affordable pricing to improve the flame resistance of materials. To better comprehend the pyrolysis behavior of BDE-209 and the evolution process of main pyrolysis products, the thermal degradation mechanism of BDE-209 was studied using density functional theory (DFT) method at the theoretical level of M06/cc-pVDZ, and thermodynamic parameters were calculated in this paper. Unimolecular degradation was dominated by cleavage of the ether linkage, which results in a high yield of hexabromobenzene, and fission of the ortho-position C–Br bond is the main competitive reaction channel. In the system of BDE-209 + H, the pyrolysis reaction is majorly characterized by debromination, leading to the formation of considerable HBr and low-brominated diphenyl ethers. Additionally, the hydrogen-derived splitting of the ether bond acts as a mainly competitive channel, which is the source of polybromophenols and polybromobenzenes. The formation of polybrominated dibenzofuran (PBDF) derives from the cyclization reaction of ortho-phenyl-type radicals, which are readily generated through the ortho-position Br atom abstraction by H radical. The formation of polybrominated dibenzo-p-dioxin (PBDD) involves the ortho-C-O coupling reaction of polybromophenoxy radicals, debromination reaction, and cyclization reaction. And the total yield of PBDD/Fs was significantly increased when H was involved. Results presented in this work will provide the helpful information for the treatment and reuse of BDE-209-containing waste plastics through using pyrolysis technology.

A loss-of-function of the dirigent gene TaDIR-B1 improves resistance to Fusarium crown rot in wheat.

A loss-of-function of the dirigent gene TaDIR-B1 improves resistance to Fusarium crown rot in wheat.

The pyrolysis of lignin: Pathway and interaction studies

Three different lignin samples (alkali lignin, THF separated lignin) and hemicellulose removed biomass samples were pyrolyzed at various temperatures and heating rate in a wire mesh reactor (WMR). The product distribution and characterisation of liquid product were analyzed and compared to obtain the pyrolysis behavior of lignin and the possible interaction. Different reactivities were observed for the studied components. The pathway of lignin pyrolysis was proposed. Heating rate and temperature played a significant role on the lignin and biomass pyrolysis. At intermediate temperatures of 450–525 °C, the phenols in liquid products were at maximum, while lower and higher temperatures promoted to the formation of dimer compounds in liquid product·THF soluble lignin favored the production of liquid, while THF insoluble lignin mainly formed solid residues. The interaction between THF separated lignin can promote the formation of phenols through the cracking of carboxylic acid, ketone and aldehyde and inhibit the coupling reaction of free radicals into dimer compounds. An interaction between lignin and cellulose was also observed leading to a lower solid residue but a higher yield of liquid. The interaction between lignin and cellulose also promoted the secondary decomposition of guaiacol derivates forming new compounds such as benzene, phenol and p-xylene and inhibited the condensation reaction of lignin.

Cross coupling of sulfonyl radicals with silver-based carbenes: a simple approach to β-carbonyl arylsulfones.

A coupling reaction between sulfonyl radicals and silver-based carbenes has been well established. This simple radical-carbene coupling (RCC) process provided an efficient approach to a variety of β-carbonyl arylsulfones from sodium arylsulfinates and diazo compounds, and was characterized by wide substrate scope, easy scale-up, simple manipulation, accessible starting materials, and mild reaction conditions.

Activation of ferrate by carbon nanotube for enhanced degradation of bromophenols: Kinetics, products, and involvement of Fe(V)/Fe(IV)

Very recently, several studies have found that homogeneous reducing agents (e.g., sodium thiosulfate (Na2S2O3), and sodium sulfite (Na2SO3)) can activate ferrate to enhance the degradation of selected contaminants. In this work, it was found that heterogeneous carbon nanotube (CNT) could accelerate ferrate (Fe(VI)) for the degradation of bromophenols (BrPs) of environmental concerns and alleviate the appearance of undesired by-products in effluent. Fe(VI) could react with BrPs over a wide pH range of 6–10 with apparent second-order rate constants of 1.8–1850 M−1 s−1. Electrospray ionization-triple quadrupole mass spectrometry (ESI-QqQMS) analysis showed that dibrominated dihydroxylated biphenyls and dibrominated phenoxyphenols were possibly formed via coupling reaction of BrPs radicals generated from Fe(VI) oxidation through one-electron transfer. The presence of CNT could remarkably accelerate the degradation rates of BrPs by Fe(VI) in a wide pH range from 7 to 10. Moreover, the formed undesired polybrominated products during Fe(VI)/CNT oxidation were absorbed on CNT surface and thus removed from treated water. The Fe(VI)/CNT system was capable of selectively oxidizing electron-rich pollutants (e.g., BrPs, and sulfamethoxazole (SMX)), but reluctant to iopamidol (IPM) and nitrobenzene (NB). High-valent metal-oxo intermediates Fe(V)/Fe(IV) formed in situ from the reaction of CNT with Fe(VI) were likely responsible for this activation effect of CNT, which was further confirmed via using methyl phenyl sulfoxide (PMSO) as a probe compound. Comparatively, homogeneous reducing agent Na2S2O3 could enhance Fe(VI) degradation of BrPs at pH 7 and 8, while undesired polybrominated products were detected in effluent from Fe(VI)/Na2S2O3 system. These findings have crucial implications for the development of a promising oxidation process by combination of Fe(VI) and CNT for water and wastewater treatment.

Effect of Temperature on the EPR Properties of Oil Shale Pyrolysates

The reaction involving radical species is a key factor during the pyrolysis process used to form hydrocarbons. In this study, electron paramagnetic resonance (EPR) spectroscopy was used to characterize the properties of free radicals in the pyrolysates of oil shale. Effects of temperature on the yields and the EPR properties of thermal bitumen, shale oil, and semicoke were investigated, with the aim to understand the behavior of free radicals during the oil shale pyrolysis process. This study shows that the free radical concentrations (Ng) of shale oil and semicoke become higher with increasing temperature. The yield and Ng of thermal bitumen as an intermediate product follow the similar trends in the whole temperature range, first increasing and then decreasing. This is attributed to the competing mechnism of thermal bitumen generation and decomposition. The Ng of shale oil is lower than those of semicoke and thermal bitumen due to the coupling reaction of free radicals before the volatiles being condens...

Modular Synthesis of Mid-Chain-Functionalized Polymers by Photoinduced Diene- and Styrene-Assisted Radical Coupling Reaction of Polymer-End Radicals

Photoirradiation of structurally well-defined “living” polymers prepared by organotellurium-mediated living radical polymerization in the presence of dienes or styrenes induced selective polymer-end coupling reaction with the concomitant insertion of the dienes or styrenes with >90% coupling efficiency. The number of inserted dienes or styrenes could be highly controlled to two molecules when acrylic polymers were used. Therefore, various mid-chain-functionalized polymers with well-controlled molecular and macromolecular structure in terms of their molecular weight, molecular weight distribution, functionality, and position were successfully synthesized by employing functionalized dienes or styrenes. The method was applied to the facile synthesis of mid-chain-functionalized telechelic polymers and a 4-miktoarm star polymer with a well-controlled structure.

Enzymatic polymerization of aniline in the presence of different inorganic substrates

The effect of different inorganic substrates in the structure of polyaniline synthesized by enzymatic oxidation was studied. The polymer characterization was done by electronic absorption and X-ray photoelectron spectroscopy. The substrates studied were: controlled pore glass, mordenite, zeolite Y, zeolite MCM-41, Wollastonite, silica gel, fuming silica and short glass fibers type E. Polyaniline was synthesized in the presence of the substrates under acidic aqueous conditions, using hydrogen peroxide as oxidizer and HRP or SBP enzymes as catalyst. The composition of the substrates strongly affected the degree of electronic conjugation of the synthesized polyaniline, whereas the pore size and the enzyme type apparently had no effect. The chemical structure of polyaniline enzymatically synthesized was more sensitive to the substrate composition than that chemically synthesized. Apparently substrates containing alkaline ions, such as sodium and calcium, promoted the formation of the branched, non-conductive polyaniline form. The effect of the substrates on the polyaniline structure can be explained considering the local pH effect of the templates surface on the coupling reaction of aniline radicals.

Electrochemical Reduction of Nitrobenzene in Ionic Liquid BMimBF<SUB>4</SUB>-H<SUB>2</SUB>O

Nitrobenzene was electroreduced to azobenzene in ionic liquid BMimBF_4-H_2O on Pt electrode at room temperature. The concentrations of nitrobenzene and water in BMimBF_4 had complicated influence on the peak potential and peak current of cyclic voltammogram. The results from cyclic voltammetry and constant-potential electrolysis experiments showed that the reaction was a two-molecule and three-step electrochemical process with 8 electrons transferred. In the reaction, anion radical arose in first step, 2 electrons were transferred in second step and accompanied with coupling reaction of radicals, 2 electrons were transferred in third step, and azobenzene was produced.

Covalent attachment of poly(ethylene glycol) on multi-walled carbon nanotubes

A new 'graft-onto' method to attach poly(ethylene glycol) (PEG) onto multi-walled carbon nanotubes (MWNTs) has been developed. The method is based on the coupling reaction of radicals formed at the chain end of PEG onto the surface of MWNTs. The polymeric radicals are generated by atom (halogen) transfer reaction between chloroacetyl-terminated PEG and transition metal catalysts. The method allows direct covalent attachment of PEG to pristine MWNTs without pretreatment that could alter their original structure. The resulting PEG-grafted MWNTs showed improved dispersion stability in isopropanol and methanol.

Switched external magnetic field CIDNP studies of coupling reaction of carbon-centered radicals with TEMPO

CIDNP in a switched external magnetic field (SEMF) is applied to study reactions of carbon-centered radicals with the persistent nitroxyl radical TEMPO (2,2,6,6-tetramethylpiperidinyl-N-oxyl) in benzene. The carbon-centered radicals were generated by laser pulse photolysis of the corresponding ketone precursors RC(O)R with well-known photochemistry in the presence of TEMPO. Theoretical background of SEMF CIDNP was adapted to the system under study and the analytical solution of the corresponding set of kinetic equations was found. In the case of low initial concentration of radicals R the SEMF CIDNP kinetics are expected to be monoexponential with decay equal to kc[Y] + 1/T1, where kc — coupling rate constant between alkyl and nitroxyl radicals, [Y] — concentration of nitroxyl radicals, and T1 — electron-nuclear relaxation time of transient radicals. Analysis of SEMF CIDNP kinetics with varied concentrations of nitroxide allowed us to derive the corresponding kc values, which were compared with the results of laser flash photolysis experiments found in the literature. Good agreement between different methods is achieved.

Photochemical Polymerization of Oligothiophenes

Abstract Photochemical polymerization of oligothiophenes was investigated from steady-state and laser-flash photolysis. Photo-irradiation to bithiophene or terthiophene solutions containing p-dinitrobenzene or CCl4 gave their α-linked polymers. It was concluded that the coupling reaction of oligothienyl radicals formed through the photoinduced electron transfer from the excited oligothiophenes to the electron acceptor resulted in a formation of polymeric products.

An AM1 study of the coupling reaction of radicals with the acetone enolate ion and some related nucleophiles

AbstractWe here report an AM1 study for the reaction of different types of radicals with the acetone enolate ion 1 and other ambident nucleophiles. The theoretical reactivity order determined is phenyl > methyl > tert‐butyl ≅ bicyclo‐[2.2.1]hept‐1‐yl. The activation energy for the coupling of aliphatic radicals is ascribed mainly to nonbonded interactions. Considering possible solvent effects, we propose the same order of reactivity in solution. The coupling reaction of phenyl radical with 1 is an exothermic process with nonactivation energy in the gas phase. Any energy barrier in solution is here ascribed to desolvation of the anion. The regiochemistry of the coupling of radicals with ambident nucleophiles such as 1, phenoxide, or phenylamide anions is also explained on a theoretical basis. © 1992 John Wiley & Sons, Inc.

Product analysis of the photo-oxygenation of tert-butylguaiacylcarbinol sensitized by 4-hydroxy-3-methoxy acetophenone

The product formation in the photosensitized reaction of a lignin model compound tert-butylguaiacylcarbinol in the presence of acetoguaiacone (4-hydroxy-3-methoxyacetophenone) or polymer-bound acetoguaiacone was studied in dimethoxyethane solutions saturated with air. Volatile products were carried off and trapped. Peroxidic compounds were quantitatively analysed by iodometric titration and the other products from the reaction mixture were fractionated using chromatographic methods. The following products were identified: formaldehyde, isobutene, pivalaldehyde and the dimer 2,2′-dihydroxy-3,3′-dimethoxy-5,5′-bis(1-hydroxy-2,2-dimethylpropyl)biphenyl. The reaction mixture was found to be very complex but nevertheless the quantitative analysis showed that more than 50% of the products were characterized. It is concluded that at least one-quarter of the products results from the oxidative bond cleavage between the side-chain and the aromatic ring, yielding primarily pivalaldehyde and acidic compounds formed by further cleavage of the aromatic ring. Another one-quarter is a result of a coupling reaction of intermediate phenoxyl radicals, leading to biphenyl structures. The mechanism of the reaction is discussed in terms of the formation of products.

Study on the Modes of Thermal Decomposition of Several Azo-Type Initiators

Modes of thermal decomposition reactions of several azo-type initiators were studied in the presence or absence of a transition-metal complex. Some metal complexes examined cause dramatic influence upon the relative ratio of decomposition products. Particularly, a cobalt complex prevents the coupling reaction of radicals formed from azo compounds, with increased formation of noncoupling products being observed. Futhermore no difference was observed in the mole ratios of an unsymmetrical coupling product to the sum of symmetrical products in the decomposition of an unsymmetrical azo compound with or without a complex, Co(II)(sal)2((R)-CHXDA). This phenomenon was satisfactorily explained by assuming that primary radicals immediately formed interact with the metal complex and are stabilized prior to subsequent reactions.

Chain Length Dependence of Coupling Reaction of Polystyryl Radicals

Chain Length Dependence of Coupling Reaction of Polystyryl Radicals

The Synthesis of Glycidylferrocenes and Related Compounds

Abstract Ferrocene derivatives with one or two glycidyl substituents, desired for the formation of polymeric ferrocene compounds, were synthesized by the epoxidation of chlorohydrin compounds obtained from the products of the addition of ferrocenyllithium to epichlorohydrin at −40–−78°C. In the course of this synthesis, mono- and di-chlorohydrin compound and a disubstituted ferrocene with a 3-chloro-2-hydroxypropyl and a glycidyl substituent were isolated. All of them are stable liquid compounds. The investigation of the relationship between the reaction conditions and the yield of glycidylferrocenes revealed that the larger molar ratio of each reactant to ferrocene at each reaction step causes a higher conversion into glycidylferrocenes. In the epoxidation reaction of chlorohydrins with caustic alkali, a small amount of ethers, formed by the reaction of epoxides and the solvent ethanol, was obtained under some reaction conditions. In all cases, a small amount of biferrocenyl was isolated; the formation of this compound was considered to be attributable to the Wurtz-type coupling reaction of ferrocenyl radicals. The structures of all these products were confirmed by their infrared absorption spectra and, in part, by the NMR spectra.