Radical traps concentration effect

Finally, Nudelman and coworkers examined the effects of temperature, reagent concentration, reaction time, a radical trap, light and solvent on the formation of radical byproduct, in the reaction of PhLi with fi-cinnamaldehyde. It was claimed that the PhLi dimer is the reacting species with the aldehyde and that the reaction is initiated by an FT from the PhLi dimer to the cinnamaldehyde. MO calculations at the MNDO level of theory were claimed to be consistent with the participation of a dimer species. ... 

Competition between metal ion-induced and radical-induced decompositions of alkyl hydroperoxides is affected by several factors. First, the competition is influenced by the relative concentrations of the metal complex and the hydroperoxide. At high concentrations of the hydroperoxide relative to the metal complex, alkoxy radicals will compete effectively with the metal complex for the hydroperoxide. Competition is also influenced by the nature of the solvent (see above). Contribution from the metal-induced reaction is expected to predominate at low hydroperoxide concentrations and in reactive solvents. The contribution from the metal-induced decomposition to the overall reaction is readily determined by carrying out the reaction in the presence of free radical inhibitors, such as phenols, that trap the alkoxy radicals and, hence, prevent radical-induced decomposition.129,1303 Thus, Kamiya etal.129 showed that the initial rate of the cobalt-catalyzed decomposition of tetralin hydroperoxide, when corrected for the contribution from radical-induced decomposition by the... 

Noyes has made numerical solutions of the case of special interest at higher pressures, that of the second-order recombination of radicals in the gas phase competing with first-order wall recombination. The interesting result here may be stated in terms of the effective thickness over which the wall exerts an appreciable effect. Noyes finds that r , where D is the diffusion coefficient, /c, the constant rate of initiation of radicals, and fc<2 is the rate constant for homogeneous second-order termination of radicals. Within the shell of thickness Vw near the wall, it may be assumed, if the surface is an efficient radical trap, that the concentration of radicals is essentially zero, while outside this shell the concentration of radicals may be assumed to correspond to that which is unperturbed by the walls. 

Some monomer show a more or less anticipated decrease in polymer deposition rates based on the concept that a pulsed discharge decreases the initiation rate, but some monomers show dramatically increased deposition rates. The most significant effect of pulsed discharge, however, can be seen in the concentration of free radicals trapped in plasma polymers (dangling bonds), which reflects the unique mechanisms of polymer formation in plasmas. 

If the particular reaction studied is the unimolecular decomposition of a free radical, such as (3), then the use of a trap will enable the effective concentration of the radical to be measured. A radical trap will indicate the presence or absence of a free radical reaction and may sometimes provide evidence for a partly or entirely molecular reaction. Rate data for free radical reactions are derived assuming the occurrence of a steady state concentration of radicals. The time required to produce a steady state concentration of methyl radicals in the pyrolysis of AcH is shown for various temperatures in Fig. 1. Realistic values for rate coeflBcients may be obtained only if the time of product formation is long compared to the time to achieve the steady state concentrations of the radicals concerned. Thus deductions from the results from the bromination of isobutane , neopentane , and toluene have been criticised on the grounds that a steady state concentra-... 

Here /g is the intensity of incident monochromatic radiation, I is the intensity of radiation at a distance I cm, and e is the decadic molar extinction coefficient of an absorbing species (concentration, c mole. 1 ). This law is strictly valid only if molecular interactions are unimportant at all concentrations. Deviations occur for a variety of reasons this means that the validity of the law should be checked under the particular experimental conditions. An initial determination of the absorption spectrum of the compound under investigation is obligatory. This produces immediate qualitative information, particularly about the usefulness of the source of radiation. Banded, diffuse or continuous spectra give direct information about the complexity and variety of primary processes that may occur. Further information will be gained from the effect of radical traps such as Oj or NO, and of various energy transfer agents. 

Radical trapping. To allow for stabilizaton by this mechanism, another reaction (number 49) was included to allow easy abstraction of a hydrogen atom from an additive (QH) by a peroxy radical to form a hydroperoxide and a harmless adduct. With the same value of the rate constant as for energy transfer and for concentrations as low as 10 M, the photooxidation process was efficiently slowed. Figure 9 shows the linear dependence of the time to failure (5% oxidation) as the concentration of QH is altered. Note that the trap is consumed in the process and the apparent induction time is associated with its removal. The stabilization is less effective for higher intensity (and probably higher temperature) because the faster photo (or thermal) decomposition of ROOH continues the degradation process. 

When the cure site concentration is k t constant and the peroxide and radical-trap ooncentratims vary, the cure rate is proportional to both of these variables. The state of cure, however, depends on the triallylisocyanurate concentration and only up to a point on the peroxide. Increasing the peroxide concentrations above the 3 dir level causes no change in the state of cure. In order to establish the effect of the rate of the peroxide decomposition on the curing characteristics of these polyners, the samples were cured at veurious ten ratures. Table 2 shows the half-life of two peroxides at various cure temperatures eis well as the CDR data obtained at these same temperatures. The results indicate that the rate of cure, as expressed by the t 2 and t 90 values, is greatly affected by the cure temperature. However, the state of the cure as expressed by values remains nearly constant, which indicates that a chain mechanism is involved and that the peroxide acts as the initiator. 

Thus, the probability of monomolecular chain termination first of all depends on the concentration of traps which, in turn is determined by the conformation of a propagating polymeric chain. From this point of view any additive that does not contain a functional group able to induce polymerization, displacing the monomer from the conformation volume of propagating polymeric chain, increases the traps concentration. In other words, the probability that the active center of a radical will be blocked from the functional groups of a monomer is increased. The stronger this effect, the longer the chain, namely its conformation volume. 

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