Nitroxyl radicals of the imidazoline series as agents of pseudoliving polymerization of styrene

Radical polymerization of styrene in the presence of 2,2,4,5,5-pentamethyl-2,5-dihydro- Imidazol-1-oxyl, 2,2-diethyl-4,5,5-trimethyl-2,5-dihydroimidazol-1-oxyl, 2,2,5,5-tetramethyl- 4-phenyl-2,5-dihydroimidazol-1-oxyl, 2,2,5-trimethyl-4,5-diphenyl-2,5-dihydroimidazol- 1-oxyl, and 2-methyl-2,3-diphenyl-1,4-diazaspiro[4.5]deca-3-en-1-oxyl was studied. Effect of substituents in the nitroxyl radical and the nature of initiator on the features of “pseudoliving” polymerization and the molecular-weight characteristics of polystyrene synthesized were considered. Nitroxyl radicals of imidazoline series, like TEMPO and its derivatives, allow one to regulate polymerization of styrene and obtain polymers with relatively low values of polydispersity index.     

Polymerization of styrene in the presence of nitroxide radicals of the dihydroimidazole series

2,2,5,5-Tetramethyl-4-phenyl-2,5-dihydroimidazole-1-oxyl and 2,2,5,5-tetramethyl-4-phenyl-2,5-dihydroimidazole-3-oxide-1-oxyl have been used as regulators of chain growth in the free-radical polymerization of styrene. It has been shown that the compounds of the dihydroimidazole series function as new living polymerization agents and allow the synthesis of polymers with low polydispersity coefficients and desired molecular masses.     

Controlled radical polymerization of styrene in the presence of nitronyl nitroxides

Bulk radical polymerization of styrene in the presence of nitronyl nitroxides (2-(4-substituted phenyl)-4,4,5,5-tetramethyl-4,5-dihydroimidazolyl-1-oxyl 3-oxide) was studied. All nitronyl nitroxides, like other nitroxyl radicals such as 2,2,6,6-tetramethylpiperidine 1-oxyl radical (TEMPO), act as reversible radical scavengers. The efficiency of controlling the polymerization is affected by the substituent at the 4′-position. The efficiency increases with electron donating strength of 4′-substituents, at least at the beginning of the reaction. However, the thermal stability of nitronyl nitroxides decreases in the same order. Thus, TEMPO is more suitable than nitronyl nitroxides for controlled/“living” radical polymerization of styrene.     

Synthesis of Polymethyl Methacrylate in the Presence of 1,3-Diphenyl-1-hydroxytriazene and Nitrobenzene

Radical polymerization of methyl methacrylate in the presence of 1,3-diphenyl-1-hydroxytriazene and nitrosobenzene as sources of stable nitroxyl radicals was studied.     

Polymerization of styrene and methyl methacrylate in the presence of 2,2-diethyl-4,5,5-trimethyl-2,5-dihydroimidazol-1-oxyl

The polymerization of styrene and methyl methacrylate in the presence of 2,2-diethyl-4,5,5-trimethyl- 2,5-dihydroimidazol-1-oxyl has been studied. It has been demonstrated that the nitroxyl radical makes it possible to control chain propagation in the polymerization of styrene and to synthesize polymers with relatively low polydispersity coefficients (<1.4). The polymerization of methyl methacrylate in the controlled mode cannot be performed because of the occurrence of a side reaction related to hydrogen atom transfer.     

XPS spectra of free nitroxyl radicals and their electronic structure

The N(1s) and O(1s) XPS spectra of stable nitroxyl radicals and molecules with a related heterocycle structure: 4,4,5,5-tetramethyl-2-phenyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl, 4,4,5,5-tetramethyl-2-(3-nitrophenyl)-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl, 4,4,5,5-tetramethyl-2-(2-hydroxyphenyl)-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl, 4,4,5,5-tetramethyl-2-(2-hydroxy-3-nitrophenyl)-4,5-dihydro-1Himidazole-3-oxide-1-oxyl, and 4,4,5,5-tetramethyl-2-phenyl-4,5-dihydro-1H-imidazole-3-oxide were studied. The possibility to apply X-ray electron spectra for investigation of the charge electron and spin density distribution on free radical atoms and at their coordination by a metal is considered.     

Synthesis of Polyvinylpyrrolidone under Secondary Inhibition Conditions

A new approach to controlling the molecular weight of polyvinylpyrrolidone in radical polymerization is considered. When the reaction is performed in the presence of a stable nitroxyl radical, 2,2,6,6-tetramethylpiperidyl-1-oxyl, the effect of secondary inhibition is manifested after the end of the induction period. This effect can be used for preparing polymers with prescribed molecular weights.     

Interaction of chlorine dioxide with nitroxyl radicals

The formation of charge transfer complexes between chlorine dioxide and nitroxyl radicals (2,2,6,6-tetramethylpiperidin-1-oxyl, 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl, 4-oxo-2,2,6,6-tetramethylpiperidin-1-oxyl, 4-methoxy-2,2,6,6-tetramethylpiperidin-1-oxyl, 4-acetylamido-2,2,6,6-tetramethylpiperidin-1-oxyl, 2,2,5,5-tetramethyl-4-phenyl-3-imidazolin-1-oxyl, and bis(4-methoxyphenyl) nitroxide) in acetone, acetonitrile, n-heptane, diethyl ether, carbon tetrachloride, toluene, and dichloromethane was found by spectrophotometry at –60—+20 °C. The thermodynamic parameters of complex formation were determined. The radical structure affects its complex formation ability. The charge transfer complex is transformed into the corresponding oxoammonium salt.     

Reaction products of nitronyl nitroxyl radicals with acids

Study of the structures of compounds generated by the reactions of 4,4,5,5-tetramethyl-2-phenyl-4,5-dihydro-1H-imidazole-1-oxyl 3-oxide with trifluoromethanesulfonic and picric acids demonstrated that these reactions produced salts of disproportionation products of nitronyl nitroxyl.     

A cation-directed switch of intermolecular spin-spin interaction of guanosine derivatives functionalized with open-shell units

The guanosine derivative 1 functionalized with the persistent radical unit 4-carbonyl-2,2,6,6-tetramethylpiperidin-1-oxyl in solution has no particular intermolecular spin-spin interactions; however, in the presence of potassium ions this compound can form a D4-symmetric octameric assembly [1(8)K]+ in which the nitroxyl moieties show a weak electron spin-spin exchange interaction. Since the relative geometry of the radicals is the outcome of K+-directed self-assembly, the spin-spin interaction can be suppressed by removing the alkaline ion.     

Structural and hydrodynamic characteristics of polystyrene synthesized in the presence of conjugated dinitrones

A series of branched high-molecular-weight alkoxyamines (HAAs) based on polystyrene of different molecular weight were synthesized using nitroxide radicals generated in situ in the presence of conjugated dinitrones (N,N-dimethylglyoxaldinitrone, N,N-di-tert-butylgly-oxaldinitrone, and N,N-diphenylglyoxaldinitrone). Structural features of the products obtained were studied by MALDI-TOF mass spectrometry. Modification of the synthesized HAAs in the presence of azobisisobutyronitrile, carbon tetrabromide, dodecyl mercaptane, 4,5,5-trimethyl-2,2-diethyl-2,5-dihydroimidazole-1-oxyl, and 3,5-di-tert-butylbenzoquinone, as well as thermolysis of the HAAs in the presence of atmospheric oxygen showed that the nitroxide fragments are located within the polymer chain irrespective of the initial structure of the conjugated dinitrone. The molecular weight characteristics and conformational properties of the nitroxide-containing linear macromolecules and polymers were studied by static and dynamic light scattering and by viscometry. In most cases, the calculated values of the ρ-parameter (R_g/R_h) and the Zimm viscosity factor of the branched polystyrene samples synthesized using conjugated dinitrones are lower than those of linear analogues.

     

A Tr Esr study of the quenching of photoexcited dioxouranium (VI) salts by stable nitroxyl free radicals

TR ESR spectroscopy was applied to the study of the quenching of excited dioxouranium (VI) (uranyl) nitrate and sulfate by stable nitroxyl radicals of the 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) family. Photoexcitation of uranyl in solutions of alcohols of moderate viscosity (η = 3-10 cP) in the presence of TEMPO leads to CIDEP signals of TEMPO due to a radical triplet pair mechanism (RTPM). Polarized nitroxyls were also observed in solutions of polyelectrolyte sodium poly(styrenesulfonate), NaPSS, in the presence of the nitroxyl with a positively charged trimethylammonium group. Photolysis of uranyl salts in solutions of alcohols leads to the generation of free radicals of alcohols. No CIDEP of these radicals was observed, distinguishing U₂ ^2+* from its organic analog, the triplet benzophenone. The probable reason for the lack of polarization in uranyl photoreduction reactions is the difficult access of free radicals to the U atom of the solvated radical UO₂₊ (V); this atom bears the unpaired electron. The role of polyelectrolytes in the enhancement of the quenching of excited states is discussed. Results are in agreement with the statement that photoexcited uranyl has a triplet multiplicity.     

Novel copolymers via nitroxide mediated controlled free radical polymerization of vinyl chloride

Controlled free radical polymerization (CFRP) of vinyl chloride (VCM) and copolymerization with several comonomers have been studied in aqueous suspension. Therefore di-tert-butylnitroxide and three novel nitroxyl radicals were used as mediating agents. Copolymerization of VCM with styrene, partly combined with acrylonitrile, maleic acid anhydride and maleic acid imide as well as methyl methacrylate, n-butyl methacrylate, butyl acrylate and butadiene have been achieved, demonstrating an efficient route for novel vinyl chloride copolymer architecture.     

Nitroxide-mediated oxidation of cellulose using TEMPO derivatives: HPSEC and NMR analyses of the oxidized products

Regenerated cellulose (viscose rayon) was oxidized using NaBr, NaClO and 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO) or one of ten related nitroxyl radicals in water at pH 10–11. The C6 primary hydroxyl groups in rayon were oxidized to carboxyl groups in most cases, thus giving water-soluble products. However, the oxidation times required for complete dissolution of the products varied substantially, depending on the nitroxyl radical used. Weight average degrees of polymerization (DPw) of the oxidized products were determined by means of high performance size exclusion chromatography (HPSEC) using pullulan standards. All the products had bimodal HPSEC distribution patterns, probably reflected by the solid-state structure of viscose rayon. When 4-acetamido-TEMPO and 4-carboxy-TEMPO were used, cellouronic acids having almost homogeneous chemical structures with higher DPw than for TEMPO were obtained quantitatively within 30 min. The oxidations using 4-amino-TEMPO, 4-carboxy-PROXYL and 4-carbamoyl-PROXYL gave cellouronic acids having the highest DPw, although reaction times of more than 4 h were required, and some side reactions occurred on the products.     

Radical Polymerization of Methyl Methacrylate in the Presence of Aromatic Nitroso Compounds

Features of radical polymerization of methyl methacrylate in the presence of various aromatic nitroso compounds (nitrosobenzene, nitrosodurene, 2,4,6-tribromonitrosobenzene) as potential sources of stable nitroxyl radicals controlling the chain propagation were studied.     

Electron spin resonance spectra and hyperfine coupling constants of the α-tocopheroxyl and 2,2,5,7,8-pentamethyl-chroman-6-oxyl radicals derived from vitamin E and its model and deuterated model compounds

Highly resolved electron spin resonance spectra of the α-tocopheroxyl or 2,2,5,7,8-pentamethylchroman-6-oxyl radicals were obtained from the oxidation of α-tocopherol (vitamin E) or its model compound, 2,2,5,7,8-pentamethylchroman-6-ol, with 2,2-diphenyl-1-picrylhydrazyl or superoxide ion. Complete assignments of the hyperfine coupling constants of these radicals were made on the basis of the spectra of the deuterated isotopomers of the 2,2,5,7,8-pentamethylchroman-6-oxyl radical. The large coupling constant due to the methyl group at C-5 in the α-tocopheroxyl radical may have implications for the high reactivity toward radicals of C-5 in α-tocopherol.     

Structure and characteristics of chitosan cobalt-containing hybrid systems,the catalysts of olefine oxidation

Cobalt-containing hybrid organo-inorganic materials based on the chitosan-SiO₂, chitosan-Al₂O₃, and chitosan-cellulose systems were obtained. The surface structure and processes that occur during the formation of metal-containing materials, the catalytic properties of which were studied in the oxidation reactions of alkene, were investigated by EPR spectroscopy using a stable pH-sensitive nitroxyl radical, 4-dimethylamino-2-ethyl-5,5-dimethyl-2-(pyridin-4-yl)-2,5-dihydro-1H-imidazole-1-oxyl, as the adsorbed probe molecules.     

Polymerization of Styrene in the Presence of Nitroxyl Radicals Generated Directly in the Course of the Polymer Synthesis (in situ)

The features of radical polymerization of styrene in the presence of nitroxyl radicals generated directly in the course of the polymer synthesis (in situ) by irreversible reaction of stable organic compounds 2-methyl-2-nitrosopropane, C-phenyl-N-tert-butylnitrone, and nitrosodurene with propagating polymer radicals were studied.     

Interpretation of X-ray photoelectron spectra of free nitroxyl radicals

N1s and O1s X-ray photoelectron spectra of stable nitroxyl radicals (substituted piperidine-1-oxyl, 2-imidazoline-1-oxyl, and 2-imidazoline-3-oxide-1-oxyl) and also diamagnetic derivatives with a {ONCNO} fragment are measured. It is shown that the structure of N1s and O1s X-ray photoelectron spectra of nitroxyls is determined by charge transfer processes between different parts of the molecule to screen the X-ray hole and the spin-spin interaction of unpaired electrons on the HOMO and 1s level.     

Investigation of the structure of chitosan hybrid systems by pH-sensitive nitroxyl radical

Hybrid organo-inorganic materials based on the chitosan-SiO₂, chitosan-Al₂O₃, and chitosancellulose systems are prepared. The structure of the surface is studied by means of EPR spectroscopy using the stable pH-sensitive nitroxyl radical, 4-dimethylamino-2-ethyl-5,5-dimethyl-2-pyridine-4-yl-2,5-dihydro-1H-imidazole-1-oxyl as adsorbed probe molecules, and the processes that occur during the formation of hybrid materials are considered.