Possible mediators of the "living" radical polymerization

The stable radicals derived from different compounds were detected in process of styrene autopolymerization. The nitroxide radicals are produced from nitrosocompound, hindered hydroxylamine, nitrophenols and nitroanisoles. The phenoxyl radicals are formed from quinine methides, and naphtoxyl radicals are generated from 2-nitro-1-naphtol. The radicals are identified, the kinetics of their formation and follow-up evolution are studied. These radicals can participate in process of living radical polymerization as the mediators and can effect significantly on kinetics of polymerization and structure of the resulting polymer.     

Formation of DMSO and DMF radicals with minute amounts of base

So far overlooked DMSO and DMF form long-lived radicals in the presence of small amounts of bases, DMF radicals being less stable than DMSO radicals. In solvent mixtures, the presence of DMSO prolonged the lifetime of DMF radicals. The occurrence of radicals may explain previously reported unexpected outcomes of reactions performed in these solvents. The commonly accepted inertness of these solvents towards minor quantities of alkali seems not to be warranted.     

The scope and limitations of 1,3-stannyl shift-promoted intramolecular cyclizations of alpha-stannyl radicals with a formyl group

Alpha-tributylstannyl radicals can be generated from the corresponding bromides or xanthates. These radicals undergo efficient intramolecular 1,5-cyclizations with a formyl group. The resulting beta-stannyl alkoxy radicals proceed through a 1,3-stannyl shift from carbon to oxygen to afford beta-stannyloxy radicals. This novel rearrangement is most likely irreversible and serves as a driving force to promote the cyclizations. Although the cyclization rates can be accelerated when the formyl group carries alpha-dimethyl substituents, unfortunately beta-scission of the alkoxy radicals becomes competitive with the 1,3-stannyl shift. The beta-stannyloxy radicals can be employed in further cyclizations to obtain tandem cyclization products.     

Base hydrolysis of tris(acetylacetonato)metal(III) complexes involving technetium and ruthenium

Irradiation of chloro- and bromoalkanes in solid cis-decalin-d₁₈ results in the selective formation (as major paramagnetic species) of alkyl radicals that are specific or the haloalkane solute, in addition to matrix radicals. The method offers a convenient and universal technique for generating, specific alkyl radicals and for examining their powder ESR spectrum. Examples of the generation of chain-end, penultimate and interior alkyl radicals are given. Computer calculations, in which spectra of penultimate and interior radicals are added, clearly demonstrate that quite extensive amounts of interior radicals can be present in a radical mixture, without considerably affecting the composite spectrum.     

Mechanism of the reaction of radicals with peroxides and dimethyl sulfoxide in aqueous solution

The reactions of methyl and methylperoxyl radicals derived from dimethyl sulfoxide (DMSO) with hydrogen peroxide, peroxymonocarbonate (HCO4 (-)), and persulfate were studied. The major reaction observed for the hydroperoxides was the abstraction of the hydrogen atom by the radicals. The radicals interact with a lone pair of electrons on the peroxide to produce methanol and formaldehyde. Furthermore, the results indicate that in RO2H and RO2R', electron-withdrawing groups cause a considerable increase in the reactivity of the peroxides towards the radicals and not only towards nucleophiles. The HO2 (.)/O2 (.-) and CO3 (.-) radicals react with DMSO to produce methyl radicals. Thus, the formation of the (.)CH3 radicals in the presence of DMSO is not proof of the formation of the (.)OH radicals in the system. These reactions must be considered when radical processes, such as in biological and catalytic systems, are studied. Especially, the plausible role of HCO4 (-) ions in biological systems as a source of oxidative stress cannot be overlooked.     

Compartmentalized polymerization with oil-soluble initiators. Kinetic effect of single radical formation

The stationary state distribution of radicals in compartmentalized systems initiated by oil-soluble initiators have been calculated for various cases of single radical formation as well as a simultaneous generation of single radicals and pairs of radicals in the particles. The effect of a contribution from radicals produced by initiator dissolved in the aqueous phase has been considered. Desorption and reabsorption of radicals, aqueous phase termination, total rate of radical formation and the water-solubility of the initiator are quantified in terms of dimensionless parameters. The calculations predict that single radicals generated in the particles are kinetically indistinguishable from radicals produced in the aqueous phase over a wide range of variation of the parameters. It is shown that if the rate of generation of single radicals constitutes only about 10 per cent of the overall rate of radical formation in the particles, the former radicals account for the major part of the rate of polymerization. The mechanisms previously proposed to account for the similar kinetic behaviour observed with water-soluble and oil-soluble initiators are discussed. It is concluded that the present calculations support the view that this similarity is mainly due to radicals produced by the water-soluble fraction of the initiator. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35 : 2347–2354, 1997     

Stabilization mechanisms of hindered amines

As a result of studying the interaction of hindered amine stabilizers (2, 2, 6, 6-tetramethylpiperidines) with simple hydroperoxides, peroxy radicals, and acylperoxy radicals, the last two in AIBN-initiated oxidation experiments in chlorobenzene, the following conclusions have been reached:

• 1 Hindered amines have multiple mechanisms of functioning as photostabilizers of polymers.
• 2 Reactions between tetramethylpiperidines and simple hydroperoxides are too slow at moderate temperatures to make a significant contribution to polymer stabilization.
• 3 Reactions between tetramethylpiperidines and alkylperoxy radicals at moderate temperatures occur at varying rates with varying effectiveness for stabilization. With favorable alignment among reaction rates for oxidation propagation and termination, reactions between tetramethylpiperidines and alkylperoxy radicals can play a significant role in oxidation inhibition.
• 4 Hydrocarbon polymer photooxidation proceeds by two major paths - the usually accepted alkyl radical/alkylperoxy radical/hydroperoxide route and the usually neglected aldehyde/acyl radical/acylperoxy radical/peracid route.
• 5 Hindered amine stabilizers are able to participate in inhibiting both photooxidation reactions - they trap acylperoxy radicals, converting them to carboxylic acids and are converted to nitroxyl radicals in the process; the nitroxyl radicals trap alkyl radicals and the hindered amines trap alkylperoxy radicals to inhibit the other oxidation pathway.
• 6 Nitroxyls are regenerated from N-alkyloxy hindered amines in a fast, efficient reactions with acylperoxy radicals and in slow reactions with alkylperoxy radicals. We postulate neither reaction yields peroxides: carboxylic acids and oxidized alkyloxy substituents are obtained from the first reaction; alcohols and oxidized alkyloxy substituents are obtained from the second reaction.

     

The role of hydroperoxides in photo-oxidative degradation of cis-1,4-polybutadiene

Hydroperoxides undergo various types of homolytic reactions on exposure to u.v. radiation. Free radicals formed from the photodecomposition of the hydroperoxide group (OOH) are oxy (HO.) and peroxy (HOO.) radicals which participate in further reactions. In cis-1,4-polybutadiene, they may initiate free radical oxidations. Cleavage of alkoxy (RO.) radicals and crosslinking of polymer radicals through polymer peroxides in the presence of air in solid film nearly balance. Most polymer radicals produced in the absence of oxygen undergo cross-linking but form peroxy radicals (POO.) in its presence. This paper presents results on the photodecomposition of tert-butyl hydroperoxide, cumyl hydroperoxide and 2,5-dimethyl-2,5-dihydroperoxyhexane in cis-1,4-polybutadiene in film and in solution.     

Photochemistry of Thiophen-2(5H)-ones

Mechanistic evidence for the light-induced ring opening of thiophen-2(5H)-ones 1 in alcohols affording α, β-unsaturated mercapto esters 2 is presented. Regio-and stereochemical aspects of the ring closure of alkenylthio (type 3 ) radicals 15 and 17 to S-heterocycles 16 and 18 , of 3-thiahex-5-enyl radicals 4 to (tetrahydrothien-3-yl)methyl radicals 6 and of (2,3-dihydrothien-3-yl)methyl radicals 30 (type 7, but-3-enyl radicals) to cyclopropane-methyl radicals 29 are discussed. Irradiation (λ 350 nm) of 1 in cyclohexane in the presence of 2,3-dimethylbut-2-ene affords [2 + 2] cycloadducts 14 albeit in very low yields.     

Decay kinetics of aryloxy and semiquinone radicals in the presence of copper ions

The decay kinetics of aryloxy and semiquinone radicals in the presence of copper ions in aqueous solutions has been studied by means of the flash photolysis technique. The radicals are involved in electron transfer reactions and those leading to the formation of intermediate complexes with copper ions. The complexes of p-benzosemiquinone anion radicals and 2-hydroxyphenoxy radicals with cupric ions decay in bimolecular self-reactions at a much slower rate than the original radicals. The increased stability of the complexes compared with the initial radicals is attributed to partial delocalization of the unpaired electron over the electron shell of copper and to steric hindrances in the self-reactions of complexes.     

Cyclopolymerization. II. Electron Spin Resonance Studies of the Free-Radical Reactions of Some Diolefins

ESR spectra have been recorded for a number of radicals generated in a flow cell by interaction of diallylmalonic acid, diallyl ether, diallylamine, and related compounds with amino, hydroxyl, and phenyl radicals in aqueous solution. The results indicate that the radicals initially formed by homolytic addition at one of the double bonds in the diolefins rapidly undergo cyclization to afford product radicals containing five-membered rings.     

Esr of pterin and lumazine radicals in solution

Cationic tetrahydrolumazine radicals and cationic tetrahydropterin radicals were detected by electron spin resonance when 5-alkyl-5,6,7,8-tetrahydrolumazines and 5-alkyl-5,6,7,8-tetrahydropterins were oxidized with hydrogen peroxide in formic acid. The hyperfine interactions of both types of radicals are essentially the same.Two consecutive radical species were observed during the oxidation of 3,5,8-trialkyl-5,6,7,8-tetrahydrolumazines in formic acid. They were identified as cationic tetrahydrolumazine radicals and cationic dihydrolumazine radicals.The ESR spectra of neutral trihydro- and monohydro-lumazine radicals, which have not been obtained before, were recorded during the oxidation of 5-alkyl-5,6,7,8-tetrahydrolumazines in chloroform. Starting from 5-butyl-1,3-dimethyl-5,6,7,8-tetrahydrolumazine three different radicals were observed.The spectra were interpreted in terms of hyperfine coupling constants and nuclear spins of the atoms involved.     

Competition between mono-and bimolecular reactions of artemisinin alkoxyl radicals

The competition between intramolecular and bimolecular reactions of alkoxyl radicals formed from artemisinin was theoretically analyzed. The enthalpies of these reactions were calculated. The activation energies and rate constants of reactions of intramolecular hydrogen atom transfer, decyclization, and decomposition of alkoxyl radicals of artemisinin and several its derivatives, as well as the activation energies and rate constants of reactions of these radicals with the C-H, S-H, and O-H bonds in biological substrates and their analogs were calculated by the intersecting parabolas method The fastest reactions of artemisinin alkoxyl radicals were identified. The full kinetic scheme of transformation of these radicals was proposed. Artemisinin radicals with the free valence on the carbon atom are predominantly formed due to the transformation of the artemisininoxyl radicals. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1502–1510, September, 2006.     

5-Exo versus 6-endo cyclization of primary aminyl radicals: an experimental and theoretical investigation

The cyclization of neutral primary pent-4-enylaminyl radicals was investigated experimentally and theoretically. Unlike the corresponding secondary aminyl radicals, primary pent-4-enylaminyl radicals underwent efficient cyclization to afford the pyrrolidine and/or piperidine products in good to high yields. While the simple pent-4-enylaminyl radical gave predominately the 5-exo cyclization product, 4-chloropent-4-enylaminyl radicals led to the formation of the corresponding 6-endo cyclization products in excellent regioselectivity. Theoretical calculations revealed that the 5-exo cyclization rate of primary aminyl radicals is about 3-4 orders of magnitude higher than that of secondary aminyl radicals.     

The fluorescence lifetimes of the methyl-substituted benzyl,triphenylmethyl and diphenylmethyl radicals

Time resolved fluorescence of some methyl-substituted benzyl, triphenylmethyl, and diphenylmethyl radicals trapped in rigid solvents at low temperature has been observed. These radicals were excited by pulsed N₂ laser at 337 nm. It is found that these radicals exhibit very long fluorescence lifetime. The long lifetimes of these radicals seem to indicate that the first doublet-doublet electronic transitions of the radicals have a forbidden character.     

EPR characterization of new phosphavinyl σ radicals

A number of radicals have been added to 1,3-bis(2,4,6-tri-tert-butylphenyl)diphospha-allene at low temperature in cyclopropane solution. Addition of thiyl and alkoxyl radicals occurs regiospecifically to one of the phosphorus atoms leading to new phosphavinyl radicals, isoelectronic with vinyl radicals, which have been characterised by means of EPR spectroscopy. The results of ab initio calculations are in agreement with the experimentally determined spectral parameters and suggest that these radicals are bent, with a PCP bond angle of ca. 150°. In contrast, silyl and germyl radicals appear to add to the carbon atom of the phosphaallene.     

Radical formation in pulse radiolysis of organic solutions of 9,10-dihydrophenanthrene

Formation and decay of neutral radicals from 9,10-dihydrophenanthrene (DHPT) in irradiated organic solutions was studied by pulse radiolysis method. Major species of the neutral radicals are H-adducts (H-DHP ) in ethanol solution of DHPT, whereas they are 9-hydrophenanthryl radicals (9-HP ) in 1,2-dichloroethane(DCE) solution of DHPT. In ethanol solution, H-DHP is generated through proton transfer to the anion radical of DHPT and through reaction between atomic hydrogen and DHPT. In the DCE solution, 9-HP is generated mainly through deprotonation from cation radicals of DHPT. Reaction between solvent radicals and DHPT plays a minor role for the formation of 9-HP .     

ESR application to radiation chemistry of polymers

Important results obtained in our group in the field of ESR application to the study of irradiated polymers are summarized. They are the analysis of the decay reaction of the free radicals, spur-like trapping of the free radicals and the related discussions. A diffusion controlled bimolecular reaction scheme was a good way of analyzing the data of the decay reaction. Power saturation phenomenon of ESR spectra of the free radicals showed a circumstance of the spur-like trapping of the free radicals in irradiated polyethylene. The phenomenon of spur-like trapping was quite consistent with the interpretation of the decay reaction of the free radicals.     

Investigation of the anthracene-nitroxide hybrid molecule as a probe for hydroxyl radicals.

A new method for the determination of hydroxyl radicals is proposed. The method is based on the use of a hybrid molecule consisting of a fluorescent chromophore, anthracene, and a nitroxide radical. In the hybrid molecule, the nitroxide quenches the fluorescence of anthracene strongly. The reaction of hydroxyl radicals with dimethyl sulfoxide generates quantitatively methyl radicals, which then combine with the nitroxide moiety of the hybrid molecules to result in an increase in the fluorescence intensity. The fluorescence increase is proportional to the concentration of hydroxyl radicals. The proposed method is capable of detecting hydroxyl radicals generated in the Fenton system. It is a simple and sensitive technique for the determination of hydroxyl radicals.     

Photooxidative degradation of cellulose: Reactions of the cellulosic free radicals with oxygen

Photooxidative degradation of cellulose resulted in decreases of degree of polymerization (DP) and α-cellulose content, concurrently producing chromophoric groups; namely, carbonyl, carboxyl, and hydroperoxide groups within the polymer. Electron spin resonance (ESR) studies revealed that cellulosic carbon free radicals readily reacted with oxygen molecules at 143–160 K to produce peroxy radicals, whereas cellulosic oxygen free radicals were inert toward oxygen molecules throughout the photooxygenation reactions. At 77 K it is feasible that only photoexcited oxygen molecules reacted with cellulosic carbon free radicals to produce peroxide radicals. These radicals were themselves stabilized at 273 K by abstraction of hydrogen atoms from cellulose to produce polymer hydroperoxides. Simultaneously, new radical sites, which exhibited three-line ESR spectra, were generated in cellulose.