Low-molecular radicals

This is followed by attaching (accepting) the resulting low-molecular radical to the diamagnetic surface center containing two-coordinated silicon atom ... 

Therefore, by using the appropriate molecules, one can generate various low-molecular radicals r (H, D, OH, OD, CH3, etc.) and subsequently obtain the silicon-centered radicals containing chemically different substituents. The EPR parameters of some of the PCs obtained by this method are presented in Table 7.7. 

Low-molecular radicals OH and H were also detected under UV-irradiation of the surface (=Si-0)2Si -0H radicals (our unpublished data). Thus, in this case the decomposition with the production of the low-molecular radical is also typical for the electronically excited state of the... 

In both examples considered it was only possible to detect the primary products of photo decay of the surface defects because they represented low-molecular radicals and could easily leave the place of their birth and can be trapped by especially chosen acceptors. 

DOSGs are efficient acceptors of low-molecular free radicals [51,73] (reactions 2-4) in Table 7.12). The formation of the corresponding products occurs, most likely, in two steps (except the H atom). The low-molecular radical first adds to one of the oxygen atoms of the cycle, which leads to the cleavage of the 0-0 bond and the formation of the oxy radical ... 

The formation of paramagnetic sites in the reactions of DOSGs with saturated hydrocarbon molecules indicate that, in one of the steps, the system is transformed into the biradical state. The 0-0 bond of the cycle is the most probable precursor for formation reactive in such processes. It can be assumed that, in both cases, the process occurs through the biradical state. However, in the first case, the low-molecular radical escapes from the reaction zone through the gas phase, and paramagnetic sites are formed. In the second case, the spatial separation of free radical sites is impossible, and they combine or disproportionate to form non-paramagnetic reaction products. 

This method using grafting to one silicon atom of SC makes it possible to approach spatially two chemically different groups r and rt and obtain free radical structures of the (=Si-0-)2Si(r )(r) type. This leads to more diverse intermediates on the solid surface and allows one to study new chemical processes involving these intermediates, including intramolecular reactions between r and /y groups. The low-molecular radicals necessary for the first step of SC modification can conveniently be obtained from the saturated H-r molecules, and appropriate defects on the silica surface can be used as... 

Let us discuss several specific examples to demonstrate the possibility for obtaining various intermediates and study of their reactivity. The next example illustrates another important advantage of the method possibility for the control of the processes of formation of low-molecular radicals during chemical reactions and obtaining the data on their nature. It was mentioned in Section 4 that one can obtain samples that have diamagnetic groups... 

Elementary stages of tribochemical transformations. Tribochemical reactions in polymers usually follow a free-radical mechanism [78,80]. As a result of mechanical interactions pol3mier or low-molecular radicals appear, taking part in substitution, addition, decomposition or other reactions. On breakage of a macromolecule, two primary radicals with free valence at the chain ends are formed. In the vinyl series these radicals are CH2 and CXY (where X and Y are hydrogen atoms and other side substitutes) in... 

Before application of the above kinetic data for reactions of low molecular radicals to reactions of macroradicals in polyoldins, a difference between reaction coiKlitions during determination of the values of the above rate constants and those in cross-linking should be accounted for. 

Termination reactions also occur between low molecular radicals such as hydroxyl (HO ) and hydroperoxy (HOj) and other available radicals (R ) ... 

These results provide additional evidence that the effect of PHB on the photo-oxidation of VAVAC is not associated with changes in the activity of the chromophore groups of VAVAC in the areas of partial compatibility of the phases and near the interface boimdary. Therefore it can be assumed, that this effect is largely due to the formation of active low-molecular radicals in PHB-phase during its photo oxidation. These radicals diffuse in the phase of VAVAC and initiate its oxidation. These radicals, apparently, are the radicals OH, because the water, along with CO2, is the main product of photo oxidation of PHB [7]. 

My faculty colleagues of the Institute also bring great expertise in the areas of anionic, cationic, and radical polymerization to the transformation of low-molecular-weight hydrocarbons into macromole-... 

Until 1981, mass spectrometry was limited, generally, to the analysis of volatile, relatively low-molecular-mass samples and was difficult to apply to nonvolatile peptides and proteins without first cutting them chemically into smaller volatile segments. During the past decade, the situation has changed radically with the advent of new ionization techniques and the development of tandem mass spectrometry. Now, the mass spectrometer has a well-deserved place in any laboratory interested in the analysis of peptides and proteins. 

The cyanoacryhc esters are prepared via the Knoevenagel condensation reaction (5), in which the corresponding alkyl cyanoacetate reacts with formaldehyde in the presence of a basic catalyst to form a low molecular weight polymer. The polymer slurry is acidified and the water is removed. Subsequendy, the polymer is cracked and redistilled at a high temperature onto a suitable stabilizer combination to prevent premature repolymerization. Strong protonic or Lewis acids are normally used in combination with small amounts of a free-radical stabilizer. 

An important side reaction in all free-radical nitrations is reaction 10, in which unstable alkyl nitrites are formed (eq. 10). They decompose to form nitric oxide and alkoxy radicals (eq. 11) which form oxygenated compounds and low molecular weight alkyl radicals which can form low molecular weight nitroparaffins by reactions 7 or 9. The oxygenated hydrocarbons often react further to produce even lighter oxygenated products, carbon oxides, and water. 

Oxidation of LLDPE starts at temperatures above 150°C. This reaction produces hydroxyl and carboxyl groups in polymer molecules as well as low molecular weight compounds such as water, aldehydes, ketones, and alcohols. Oxidation reactions can occur during LLDPE pelletization and processing to protect molten resins from oxygen attack during these operations, antioxidants (radical inhibitors) must be used. These antioxidants (qv) are added to LLDPE resins in concentrations of 0.1—0.5 wt %, and maybe naphthyl amines or phenylenediamines, substituted phenols, quinones, and alkyl phosphites (4), although inhibitors based on hindered phenols are preferred. 

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