Active radicals

Pi is a growing polymer chain with one monomer unit. The chain end remains an active radical and propagates by the sequential addition of monomer. 

Or conversely, active radicals can be obtained by irradiating certain metal—metal bonded species ... 

The growth of long chains ( > 10 ) in the perfectly mixed 1 1 crystals of ethylene with chlorine and bromine at 20-70 K was studied in detail by Wight et al. [1993]. Active radicals were generated by pulse photolysis of CI2 or Br2. The rate constant was found to be /Cc = 8-12s below Tc = 45 K. The chain grows according to the well known radical mechanism including the reactions... 

In systems of LP the dynamic response to a temperature quench is characterized by a different mechanism, namely monomer-mediated equilibrium polymerization (MMEP) in which only single monomers may participate in the mass exchange. For this no analytic solution, even in terms of MFA, seems to exist yet [70]. Monomer-mediated equilibrium polymerization (MMEP) is typical of systems like poly(a-methylstyrene) [5-7] in which a reaction proceeds by the addition or removal of a single monomer at the active end of a polymer chain after a radical initiator has been added to the system so as to start the polymerization. The attachment/detachment of single monomers at chain ends is believed to be the mechanism of equilibrium polymerization also for certain liquid sulphur systems [8] as well as for self-assembled aggregates of certain dyes [9] where chain ends are thermally activated radicals with no initiators needed. 

The process for initiating radical formation in aromatic amine-vinyl monomer systems have been studied by Feng et al. [80-86] who proposed the formation of an aminium radical as the active state of an exciplex as intimate ion-pair and then a cyclic transition state which then would undergo a proton transfer process of deprotonation leading to the formation of active radical species for initiation as follows ... 

Thus, an aminium radical from primary or secondary amine will at last form an amino radical instead of an aminomethyl radical. This amino radical will then serve as the only active radical species to initiate the vinyl polymerization. 

The theory of radiation-induced grafting has received extensive treatment. The direct effect of ionizing radiation in material is to produce active radical sites. A material s sensitivity to radiation ionization is reflected in its G value, which represents the number of radicals in a specific type (e.g., peroxy or allyl) produced in the material per 100 eV of energy absorbed. For example, the G value of poly(vinyl chloride) is 10-15, of PE is 6-8, and of polystyrene is 1.5-3. Regarding monomers, the G value of methyl methacrylate is 11.5, of acrylonitrile is 5.6, and of styrene is >0.69. 

The sulfenic acids have been found to be extremely active radical scavengers showing rate constants of at least 107 m"1 s 1 for the reactions with peroxyl radicals at 333 K17. It has also been suggested that the main inhibiting action of dialkyl sulfoxides or related compounds in the autoxidation of hydrocarbon derives from their ability to form the transient sulfenic acids on thermal decomposition, i.e.17... 

A common biologically active radical is the pentadienyl radical, RCHCHCHCHCHR, where the carbons form a long chain, with R and R, which can be a number of different organic groups, at each end. Draw three resonance structures for this compound that maintain carbon s valence of four. 

Alkyl Side Chains of Aromatic Rings. The preferential position of attack on a side chain is usually the one a to the ring. Both for active radicals such as chlorine and phenyl and for more selective ones such as bromine such attack is faster than that at a primary carbon, but for the active radicals benzylic attack is slower than for tertiary positions, while for the selective ones it is faster. Two or three aryl groups on a carbon activate its hydrogens even more, as would be expected from the resonance involved. These statements can be illustrated by the following abstraction ratios ... 

A number of methods have been used to prepare graft copolymers in the past few decades including both conventional chemical and radiation-chemical methods [20,86,87]. In the latter case, graft copolymerization is usually initiated by creating active radical sites on existing polymer chains. The advantages of radiation-chemical methods are (i) ease of preparation as compared to... 

IR-ATR absorptions at 1730, 1260, and 1019 cm have been observed to rise up to the 50 kGy level beyond which there is a slight drop in the values. With increase in the radiation dose, the generation of active radicals on the EPDM backbone increases and so also that of the carbonyl and ether groups due to aerial oxidation of these radicals as noted in the earlier sections. However, at higher irradiation doses above 50 kGy, the chain scission of the EPDM molecules predominates as a... 

To summarize the description of specificity of adsorption of active radicals on the surface of doped oxide semiconductors, we can conclude that we have a substantial experimental basis to draft the following diagram initially proposed in the study [41] and considered in detail in Chapter 2 ... 

Tram-anular interactions, which would create an active radical site via hydrogen transfer through 98, cannot be invoked to explain the specific loss of a CH3 radical from the ether side chain. This conclusions is based upon the following experimental observations. The radical cation of the tetrafluoro substituted compound 101 eliminates CH3, but loss of CH3 from the para-isomer 102 is not observed. If a transanuiar process according to 97- 98 were operative, then such a reaction is not expected to be suppressed upon substitution of H by F as is known for many examples from the field of photochemistry of fluoro substituted compounds41 (23). 

The rate of Au(ffl) reduction should have a correlation with the cavitation efficiency at these frequencies. Therefore, the result of Fig. 5.8 suggests that maximum amounts of reductants are sonochemically formed at 213 kHz in the presence of 1-propanol. The existence of an optimum frequency in the sonochemical reduction efficiency would be explained as follows. As the frequency is increased, the number of cavitation bubbles can be expected to increase. This would result in an increase in the amount of primary and secondary radicals generated and an increase in the rate of Au(HI) reduction. On the other hand, at higher frequencies there may not be enough time for the accumulation of 1-propanol at the bubble/solution interface and for the evaporation of water and 1 -propanol molecules to occur during the expansion cycle of the bubble. This would result in a decrease in the amount of active radicals. Furthermore, the size of the bubbles also decreases with increasing frequency. These multiple effects would result in a very complex frequency effect. 

Water soluble impurities and their effect can be easily included in equation (1-4), through which they are going to directly affect the particle nucleation rate, f(t). If one assumes a first order reaction of an active radical with a water soluble impurity (WSI) to give a stable non-reactive intermediate, then one simply has to add another term in the denominator of equation (1-4), of the form kwsr[WSI](t)-kv, and to account for the concentration of WSI with a differential equation as follows ... 

Like all controlled radical polymerization processes, ATRP relies on a rapid equilibration between a very small concentration of active radical sites and a much larger concentration of dormant species, in order to reduce the potential for bimolecular termination (Scheme 3). The radicals are generated via a reversible process catalyzed by a transition metal complex with a suitable redox manifold. An organic initiator (many initiators have been used but halides are the most common), homolytically transfers its halogen atom to the metal center, thereby raising its oxidation state. The radical species thus formed may then undergo addition to one or more vinyl monomer units before the halide is transferred back from the metal. The reader is directed to several comprehensive reviews of this field for more detailed information. 

Antimony oxide is known as a flame retardant synergist when used in combination with halogen compounds. Volatile antimony oxyhalide (SbOX) and/or antimony trihalide (SbX3) are formed in the condensed phase and transport the halogen into the gas phase (3). It has been suggested that antimony is also a highly active radical trap (4). 

The problem of retention of asymmetry of the formed free radical in the fast geminate recombination of radicals was studied by photolysis of the optically active azo-compound PhMeCH—N=NCH2Ph [88,89]. The radical pair of two alkyl radicals was initiated by the photolysis of the azo-compound in benzene in the presence of 2-nitroso-2-methylpropane as a free radical acceptor. The yield of the radical pair combination product was found to be 28%. This product PhMeEtCCH2Ph was found to be composed of 31% 5,5 -(-)(double retention), 48% meso (one inversion), and 21% R.R(+) double inversion. These results were interpreted in terms of the competition between recombination (kc), diffusion (kD), and rotation (kml) of one of the optically active radicals with respect to another. The analysis of these data gave kxo[Pg.126]

The kinetic analysis proves that formation of very active radical from intermediate product can increase the reaction rate not more than twice. However, the formation of inactive radical can principally stop the chain reaction [77], Besides the rate, the change of composition of chain propagating radicals can influence the rate of formation and decay of intermediates in the oxidized hydrocarbon. In its turn, the concentrations of intermediates (alcohols, ketones, aldehydes, etc.) influence autoinitiation and the rate of autoxidation of the hydrocarbon (see Chapter 4). 

Antioxidants possess the reducing activity and can be oxidized by hydroperoxide formed in the oxidizing substance [23,31,32,38,51]. This reaction produces an active radical... 

This reaction is slow due to a high activation energy (see Chapter 15). However, at elevated temperatures and sufficiently high concentrations of antioxidant and hydroperoxide, this reaction becomes fast and, hence, can accelerate the rate of oxidation. As a result, the rate of initiation increases v = vi0 + enkn[ROOH] [InH] (en is the probability of the appearance of active radicals in the bulk). From the other side, this reaction shortens the induction period (T0=/[InH]0/vl0). 

A) Phenols of this group react with peroxyl radicals, hydroperoxide, and dioxygen, while respective phenoxyl radicals can react with RH and ROOH. Reactions of these phenols with R02 most commonly give rise to quinones the breakdown of phenoxyls does not produce active radicals. This group includes all phenols, except 2,6-di-/er/-alky I phenols and alkoxy-substituted phenols. Phenols of this group can inhibit oxidation by mechanisms I-VII. 

Termination of active radicals can occur via three mechanisms carbon-carbon termination, cross termination and recombination of DTC radicals. These reactions appear as ... 

The role of fragments can be performed by valence-active radicals, e.g. CH, CH2, (OH) , NO, N02, (SO4)2, etc. In complex structures this carbon atom usually has not one, but two or three side bonds. The priority significance when calculating based on the principle of adding... 

A flame is quenched in a tube when the two mechanisms that permit flame propagation—diffusion of species and of heat—are affected. Tube walls extract heat the smaller the tube, the greater is the surface area to volume ratio within the tube and hence the greater is the volumetric heat loss. Similarly, the smaller the tube, the greater the number of collisions of the active radical species that are destroyed. Since the condition and the material composition of the tube wall affect the rate of destruction of the active species [5], a specific analytical determination of the quenching distance is not feasible. 

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