Alfa I. Mikhailov

The photolysis of dry benzoyl peroxide (BP) at 77 K in the 480–236 nm range of wavelengths and an ethanol solution is studied via EPR. It is determined that the main photochemical process in irradiating BP at λ = 480–365 nm is the direct photodissociation of the O–O peroxide bonds and C–H bonds of benzene rings to form benzoyloxy radicals and atomic hydrogen, which likely forms via a two-quantum mechanism. The quantum yield of primary intermediates is φ ≈ 2.7 × 10⁻⁵. Upon subsequent irradiation of a sample using light with λ = 300–236 nm, benzoyloxy radicals release carbon dioxide and transform into phenyl radicals with a quantum yield of φ ≈ 3 × 10⁻⁴. At 120–145 K, phenyl radicals react with double bonds of aromatic rings of PB to form phenyl-substituted cyclohexadienyl radicals vanishing at 273 K. Quantum chemical calculations of the radical structures of photolyzed peroxide are made with the density functional approach (B3LYP/6-311g), and good agreement between the experimental and theoretical parameters is noted. Calculations show that in the structure of benzoyloxy radicals, the spin density is distributed almost equally between the two oxygen atoms of the radical, due to the conjugation of the unpaired electron to the lone p electron pair on the oxygen atom of the C=O group. In the photolysis of BP solutions in ethanol at λ ≥ 365 nm, solvent radicals form via sensitized photolysis through the reaction between primary benzoyloxy radicals and solvent molecules.

Molecular mobility of a nitroxyl radical (as a paramagnetic probe) in methylcyclohexane (MCH) solutions of chiral biomimetic gelators—trifluoroacetylated amino alcohols (synthetic analogues of biological molecules)—has been studied by monitoring thermally induced changes in EPR spectra. The phase state of the systems has been examined by the low-temperature scanning calorimetry method. There has been found an unusual concentration and temperature dependence of probe species mobility in diluted solutions (10^–2–10^–3 M) of trifluoroacetylated amino alcohols with a molecular weight of no more than 200. From the temperature-induced changes in EPR spectra of the paramagnetic probe in the temperature range 170–290 K, the rotation activation energies of probe species in biomimetic solutions and in the neat solvent. The minimum radical rotation activation energy (2.1 kcal/mol) is observed in the most concentrated gelator solution, whereas in neat MCH, this energy is 3.7 kcal/mol.

A range of issues related to free-radical processes in polymer-monomer systems is considered. Spectral (EPR, IRS) and other physicochemical methods (calorimetry, chromatography, viscometry) are used to demonstrate that the low-temperature treatment of polymers and monomers with halogens (fluorine, chlorine) is accompanied by the spontaneous formation of free radicals. At F₂ and Cl₂ pressures below 100 Torr, the concentration of radicals reaches 10¹⁷–10¹⁹ spin/g, which can be reproduced in the case of radiolysis only by using doses of several hundred (or thousand) kGy. The formation of radicals through the cleavage of chemical bonds (without external energy impact on the system) is discussed in the framework of the model of low-temperature reactions in polymolecular complexes, involving the simultaneous occurrence of endothermic and exothermic steps in one elementary event with an overall exothermic effect. It is shown that the radicals formed can be used to initiate the polymerization of vinyl and acetylene monomers.

Radiolysis of plant biomass with doses of 200–300 kGy prior to dispersing by chemimechanical methods increases the yield of nanocellulose from cellulosic feedstock by more than a factor of two. Another advantage of radiation pretreatment of initial samples is the effect of radiation sterilization of isolated nanocellulose hydrogels, whereas the hydrogel obtained without preliminary irradiation rapidly suffers from the attack of molds during storage in the laboratory. Photolysis of plant raw material at a wavelength of 253.7 nm has almost no effect on the yield of nanocellulose. The molecular and supramolecular structure of nanoparticles with a size of 200–300 nm remains unchanged on passing to the nanoscale and corresponds to the macromolecular structure of cellulose. Industrial testing of the hydrogel as an additive (2.5%) for an adhesive composite used in the manufacture of wood laminates (plywood) showed an enhancement of the strength characteristics of the products by 15–20%. The increase in strength is mainly due to an increase in the contact area of cohesive bonding through small coiled molecular entities composed of 10–16 Kuhn segments including up to 28 monomer units each.

The mechanisms of photooxidation of the popular commercial polymers polystyrene (PS), polyethylene (PE), and an ethylene-carbon monoxide copolymer (polyketone, PK) differing in the polymer chain structure and the nature and concentration of chromophore groups are considered. In the case of the formation of photosensitive intermediates in polystyrene, taken as an example, the photochain oxidation mechanism was revealed and thoroughly studied, according to which the polymer “burns” into complete oxidation products (CO₂, H₂O) with a degree of conversion of ≥50% and a kinetic-chain length of l = 10³–10⁴ units. The hydroperoxide mechanism plays a minor role in the photooxidation of PS, it is a short-chain process (as in the case of thermal oxidation, l ∼ 10) and does not exceed 1.5% of the total amount of absorbed oxygen. Carbonyl groups, as weak photoinitiators, induce in PE and PK the conventional radical chain mechanism of photooxidation with degenerate branching of kinetic chains on hydroperoxide groups and other oxidations products.

Low-temperature (77 K) radiolysis of solid fullerene C₆₀ and its glassy solutions in isopropylbenzene (IPB) and N-vinylpyrrolidone (N-VP) has been investigated with the use of ESR spectroscopy. Radiationchemical yields for the formation of paramagnetic centers (G _PMC, 1/100 eV) have been determined to be ∼0.001 for pure fullerene and 0.3 and 0.7 for its solutions in IPB and N-VP, respectively. The low value of G _PMC for fullerene indicates a high stability of this form of carbon. The increase in G _PMC in solutions as compared to the pure solvents (0.15 for IPB and 0.3 for N-VP) indicates the sensitizing effect of C₆₀.

The radiation-chemical yield of paramagnetic centres during fullerene C₆₀ low-temperature (77 K) γ-irradiation in a vacuum has been found (G_PMC = 0.001 ± 0.00015 1/100 eV); in solid solutions, fullerene C₆₀ acts as a sensitizer.