S. I. Kuzina

Formation of free radicals in the low-temperature fluorination of polymers

The formation of free radicals in the process of direct fluorination of natural and synthetic polymers at temperatures close to 77 K was studied by ESR. The maximum concentrations of radicals, (1017–5·1018 spin g−1) and their complete oxidation were observed in the temperature interval from 77 to 250 K at a fluorine pressure of 30 Torr. The initiation of chain halogenation, which consists of homolytic breaking of chemical bonds to yield free-radical intermediates. was examined in the framework of the multi-center synchronous transitions model.

The photo-oxidation of polymers—1. Initiation of polystyrene photo-oxidation

Abstract The initiation of polystyrene photo-oxidation has been studied and the quantum yield of initiation (ϕR·) for u.v.-irradiation at 315 K in the presence of atmospheric oxygen has been found to be 10−3. Using ESR spectra, five distinct free radicals have been found; their nature depends on the wave length of the light and the irradiation conditions (temperature and presence of oxygen).

Photo-oxidation of polymers 4. The dual mechanism of polystyrene photo-oxidation: a hydroperoxide and a photochain one

Abstract By comparing the quantum yields of initiation, O 2 absorption and formation of intermediate and final products a conclusion has been made that the polystyrene photo-oxidation process involves two mechanisms: a photochain (∼90%) and a hydroperoxide (∼1%) one. The photochain mechanism is based on the reaction of excited peroxide macroradicals dissociation (the kinetic chain length is 10 2 –10 3 units), leading directly to formation of the main final products, H 2 O and CO 2 , whose yield is comparable with the quantity of the absorbed oxygen. The ordinary hydroperoxide mechanism, by which up to 10% of the radicals formed in the initiation stage are converted, includes an RO 2 • reaction with a macromolecule and hydroperoxide formation. A subsequent ROOH decomposition results in the scissions of the polymer chains and the formation of high-molecular oxidation products (carbonyls, alcohols, ethers).

Direct and initiated halogenation of carbon nanomaterials at low temperatures

Cryochemical reactions of the direct and initiated (by photolysis and radiolysis) halogenation of carbon nanomaterials (C60 fullerene, nanotubes, and nanofibers) at 77–240 K were investigated by the ESR, IR spectroscopy, and elemental analysis techniques. A high reactivity of C60 in reactions with fluorine and chlorine with the formation of corresponding derivatives was shown. High concentrations of radical intermediates indicating the radical chain halogenation of C60 were detected (the kinetic chain length for the chlorination process reaches 104–106 units). The amount of chlorine attached to fullerene is ∼35% and practically does not depend on the initiation mode (UV or γ-irradiation at doses up to 350 kGy). The mechanism of the cryochemical halogenation of C60 is considered within the limits of the model of multicenter synchronous transitions in a molecular complex consisting of several reactant molecules including molecular fluorine or chlorine and ensuring a net exothermic effect. The amount of chlorine added to nanotubes and nanofibers did not exceed 2.5–8%, thereby indicating a low reactivity of these materials under cryogenic conditions.

Influence of radiolysis on the yield of nanocellulose from plant biomass

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.

Chain and photochain mechanisms of photooxidation of polymers

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.

Spontaneous formation of free radicals at low temperatures

Abstract Spontaneous formation of free radicals at low temperatures (77–200 K) was investigated during fluorination and chlorination of polymers (polystyrene, lignin) and low-molecular weight phenols, as well as during interaction of sodium hydroxide with lignin.

The oxidation and thermal transformations of macrooradicals in gamma irradiated cellulose

The secondary reactions of the oxidation and thermal transformations of gamma irradiated (at 77 K) and plasticized (with water) cellulose radicals were studied by 3 cm-and 2 mm-band EPR spectroscopy. The radiolysis of cotton cellulose was found to produce the H-C*=O formyl radical, and heating the irradiated samples to 190–200 K resulted in the formation of the ROO* peroxide radical. The EPR spectra of microcrystalline cellulose recorded at room temperature contained an individual triplet (αβH = 2.5–2.7 mT) with an additional quadruplet structure (splitting 0.5–0.7 mT) from three γ-hydrogens. This triplet was interpreted as a signal of the primary radical at C4. The main direction of thermal transformations of primary radicals was synchronous reactions of the dehydration of the polycarbohydrate complex accompanied by the dissociation of the C-H, C-OH, and C-C bonds and elimination of H2O, H2, CO, and CO2 with successive formation of allyl and then polyene radicals, which were a source of the growth of polyconjugated systems in macromolecules.

Low-temperature radiation telomerization of tetrafluoroethylene in hexafluoroisopropanol

The kinetics of low-temperature (180–270 K) postradiation telomerization of tetrafluoroethylene in hexafluoroisopropanol has been investigated. The efficiency of the process is determined by the initial concentration of the monomer in the solvent and the dose of preliminary irradiation. It has been shown that the telomerization reaction proceeds via the radical mechanism during the devitrification of the system. Active centers that initiate the process and their yield at 77 K have been studied by the ESR technique. Some properties of the product telomers have been studied.

Free radicals and photo-oxidation of an alternating copolymer of ethylene and carbon monoxide

Abstract Reactions of formations of free radicals on photolysis of low-molecular alternating ethylene-carbon monoxide copolymer and its photo-oxidation are investigated. It is found that the photolysis occurs by the Norrish 1 type mechanism with terminations of the polymer chain and formation of acyl and alkyl terminal radicals (quantum yield ϕR≃2×10−4 at 77 K and ∼10−3 at 300 K). The copolymer photo-oxidation is characterized by high efficiency. In the case of doses of up to ∼5 × 1017 quanta/cm2, there is a correspondence between the amount of absorbed O2 and the formation of gaseous products of oxidation, H2O and CO2 (ϕO2≃ϕgas≃7). At doses >1018 quanta/cm2 the yield of gaseous products is retarded, and the yield of O2 absorption decreases by an order of 10. The gaseous products of the photolysis and photo-oxidation involve (among H2O and CO2) also carbon monoxide, methane, ethane, propane, ethylene, acetone and hydrogen peroxide (ϕ=10−2–10−4).