Rate constant peroxy radical with

Oxygen has two possible interactions during the polymerization process [94], and these reactions are illustrated in Fig. 2. The first of these is a quenching of the excited triplet state of the initiator. When this quenching occurs the initiator will absorb the light and move to its excited state, but it will not form the radical or radicals that initiate the polymerization. A reduction in the quantum yield of the photoinitiator will be observed. The second interaction is the reaction with carbon based polymerizing radicals to form less reactive peroxy radicals. The rate constant for the formation of peroxy radicals has been found to be of the order of 109 1/mol-s [94], Peroxy radicals are known to have rate constants for reaction with methyl methacrylate of 0.241/mol-s [100], while polymer radicals react with monomeric methyl methacrylate with a rate constant of 5151/mol-s [100], This difference implies that peroxy radicals are nearly 2000 time less reactive. Obviously, this indicates that even a small concentration of oxygen in the system can severely reduce the polymerization rate. 

Oxidation rate constant k, for gas-phase second order rate constants, k0H for reaction with OH radical, kN03 with N03 radical and kQ3 with 03 or as indicated data at other temperatures see reference photooxidation tA = 125 d to 22 yr in water, based on measured rate constant for reaction with hydroxyl radical in water (Dorfman Adams 1973 Anbar Neta 1967 quoted, Howard et al. 1991) k 360 Mr1 Ir1 for singlet oxygen, k 1.0 M-1 Ir1 for peroxy radical at 25°C (Mabey et al. 1982) k = (0.09 0.02) 1VP1 s-1 for 5-10 mM to react with ozone in water using 50-1000 mM of f-BuOH as scavenger at pH 2 and 20-23°C (Hoignd Bader 1983) koe(obs.) = 0.15 x 10-12 cm3 molecule-1 s-1 at 296 K (Becker et al. 1984 quoted, Carlier et al. 1986) koe = 0.21 x 10-12 cm3 molecule-1 s-1 at room temp. (Zetzsch 1982)... 

This reaction is facilitated by formation of the stabilized RSSO radical that is isoelectronic with the stabilized polysulfide radical, RS3. The analogous sulfenic acids are effective radical scavengers reacting with peroxy radicals with a rate constant of KFM"1 sec1 at 60°C (16). The S-S bond in the thiolsulfinate is weak, and the corresponding bond in the thiosulfoxyl radical should be considerably less stable. Thus, the thiosulfoxyl radical may function as a source of sulfur oxides ... 

The reaction of peroxy radicals with ketone is that between two dipolar particles in a polar medium. The role of the medium in methyl ethyl ketone oxidation has been studied in detail [152—157]. The rate coefficient, ftp, decreases with dilution of methyl ethyl ketone by a non-polar solvent (benzene, n-decane, etc.). The change of fep is caused by the nonspecific solvation of reacting particles and activated complexes. The relationship between ftp and the dielectric constant, e, is expressed by the Kirkwood equation... 

Rate constants for the reactions of P-hydroxyalkyl peroxy radicals with NO are essentially identical to those for the reaction of NO with > C2 alkyl peroxy radicals formed from alkanes. 

OH free radicals react with almost all amino-acids. For aliphatic residues, rate constants are correlated with the strength of the X-H bond(X = S, C or N) (1). Thus the reaction is relatively slow with glycine (k = 1.7 x 10 mol 1 s ) and fast with the -SH function of cysteine (k = 1.9 x 10 mol M s i). The thiyl radical formed upon oxidation of cysteine, whose spectral properties are in table 3, is formed but a carbon-centered radical is also present (50, 51). In the presence of oxygen, thiyl radical fixes O2 giving peroxy radicals (52). These radicals may photoisomerize into sulfonyl radicals RS02 (53). In small molecules, disulfide groups can also be oxidized. This reaction was not demonstrated in proteins, but cannot be neglected. A disulfide radical cation is formed (54). Final compounds are not known. 

Once formed, an alkyl radical reacts rapidly with O2 to form the corresponding peroxy radical. With high-pressure limiting rate constants in the range of 10" cm s" the atmospheric lifetime of an alkyl radical is expected to be on the order of 1 /rs or less, and no other removal mechanism... 

Other ROj Reactions. There is a limited number of rate constant measurements for the reactions of HCFC-based peroxy radicals with HO2 and O3. Whereas the removal of peroxy radicals is dominated by reaction with NO in NO,-rich environments, such as the urban atmosphere, the reaction with HO2 can become important in areas with low NO, levels. Preliminary rate constants exist for the reaction of HO2 with two halogenated ethylperoxy radicals. For CF3CCI2O2, Hayman et al. [90] report a rate constant of (1.9 0.3) x 10 cm s at 298 K. The measurements of Maricq et al. [118] provide a rate constant of (1.8 S) x cm s for the reaction between CFjCFHOj and HO2. The room temperature rate constant of 4.3 x 10 cm s is in good agreement with the value of 4 X 10 cm s reported by Hayman [119] for this reaction. 

Addition reactions of peroxy radicals with olefins (Equation 4.63) have often been described (Mayo, 1958 Hamberg and Gotthammar, 1973). Among the stable products are epoxides, possibly formed by elimination of alkoxy radicals. The structural constraints on epoxide formation are quite stringent and the overall rate constants for their formation can vary by three or more orders of magnitude. 

The most striking feature of the results is the dramatic increase of the rate constant under substitution of an H atom in the alkyl peroxy radicals with halogen atom, an... 

TABLE 1 Structure, name, abbreviation used terpenephenois, their stoichiometric factor (i) and rate constants of TP with ethylbenzene peroxy radical k). 

Under these conditions, a component with a low rate constant for propagation for peroxy radicals may be cooxidized at a higher relative rate because a larger fraction of the propagation steps is carried out by the more reactive (less selective) alkoxy and hydroxy radicals produced in reaction 4. 

These ESR spectra are in good agreement with ESR spectra of ozonized PP published previously (30) The rapid formation of peroxy radicals indicates that ozone reacts with PP without induction period. In the initial stage of reaction the hydroperoxide groups (POOH) concentration increases and the rate of POOH formation is linearly dependent on the ozone concentration (Fig.2). After prolonged ozonization the concentration of POOH remains almost constant. 

Peroxy radical recombination appears to be the most important source of the electronic excitation energy emitted during hydrocarbon autoxidation. In addition to the above-mentioned energetic considerations, this is clear from the following experimental facts the termination rate for secondary peroxy radicals is 103 times faster than for tertiary peroxy radicals due to their having no a-hydrogen 14> the termination rate constant decreases by 1.9 with a-deuteration 39 40>. 

Oxidation rate constant k, for gas-phase second order rate constants, k0H for reaction with OH radical, kN03 with N03 radical and k0j with 03 or as indicated, data at other temperatures see reference k = 4 x 107 M-1 h-1 for singlet oxygen and k = 5 x 103 M-1 h-1 for peroxy radical (calculated, Mabey et al. 

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