Self reactions

Bimolecular reactions involve two particles in their essential step. In the so-called self-reactions they are of the same species ... 

Overpressure of Store drum at proper temperature material in drum, Keep drum away from heat source due to external heat input or self reaction is complete before drumming heating. Allow adequate freeboard for material Provide adequate sprinkler protection Thermally initiated venting (e.g., melt-out bungs) CCPS G-3 CCPS G-15 CCPS G-22 CCPS G-29... 

Most radicals are transient species. They (e.%. 1-10) decay by self-reaction with rates at or close to the diffusion-controlled limit (Section 1.4). This situation also pertains in conventional radical polymerization. Certain radicals, however, have thermodynamic stability, kinetic stability (persistence) or both that is conferred by appropriate substitution. Some well-known examples of stable radicals are diphenylpicrylhydrazyl (DPPH), nitroxides such as 2,2,6,6-tetramethylpiperidin-A -oxyl (TEMPO), triphenylniethyl radical (13) and galvinoxyl (14). Some examples of carbon-centered radicals which are persistent but which do not have intrinsic thermodynamic stability are shown in Section 1.4.3.2. These radicals (DPPH, TEMPO, 13, 14) are comparatively stable in isolation as solids or in solution and either do not react or react very slowly with compounds usually thought of as substrates for radical reactions. They may, nonetheless, react with less stable radicals at close to diffusion controlled rates. In polymer synthesis these species find use as inhibitors (to stabilize monomers against polymerization or to quench radical reactions - Section 5,3.1) and as reversible termination agents (in living radical polymerization - Section 9.3). 

The decomposition of an initiator seldom produces a quantitative yield of initiating radicals. Most thermal and photochemical initiators generate radicals in pairs. The self-reaction of these radicals is often the major pathway for the direct conversion of primary radicals to non-radical products in solution, bulk or suspension polymerization. This cage reaction is substantial even in bulk polymerization at low conversion when the medium is essentially monomer. The importance of the process depends on the rate of diffusion of these species away from one another. 

The proportion of useful radicals generated from common dialkyldiazenes is never quantitative typically it is the range 50-70% in media of low viscosity i.e. in low conversion polymerizations).3 88 89 The main cause of this inefficiency is loss of radicals through self-reaction within the solvent cage. 

Alkylperoxy radicals are generated by the reactions of carbon-centered radicals with oxygen and in the induced decomposition of hydroperoxides (Scheme 3.82). Their reactions have been reviewed by Howard452 and rate constants for their self reaction and for their reaction with a variety of substrates including various inhibitors have been tabulated.453... 

Before any chemistry can take place the radical centers of the propagating species must conic into appropriate proximity and it is now generally accepted that the self-reaction of propagating radicals- is a diffusion-controlled process. For this reason there is no single rate constant for termination in radical polymerization. The average rate constant usually quoted is a composite term that depends on the nature of the medium and the chain lengths of the two propagating species. Diffusion mechanisms and other factors that affect the absolute rate constants for termination are discussed in Section 5.2.1.4. 

Termination by self-reaction of propagating radicals is a diffusion-controlled process even at very low conversion.1 The evidence for this includes the following ... 

Even though the rate of radical-radical reaction is determined by diffusion, this docs not mean there is no selectivity in the termination step. As with small radicals (Section 2.5), self-reaction may occur by combination or disproportionation. In some cases, there are multiple pathways for combination and disproportionation. Combination involves the coupling of two radicals (Scheme 5.1). The resulting polymer chain has a molecular weight equal to the sum of the molecular weights of the reactant species. If all chains are formed from initiator-derived radicals, then the combination product will have two initiator-derived ends. Disproportionation involves the transfer of a P-hydrogen from one propagating radical to the other. This results in the formation of two polymer molecules. Both chains have one initiator-derived end. One chain has an unsaturated end, the other has a saturated end (Scheme 5.1). 

The self reaction of substituted phenylethyl radicals (1) has been widely investigated.92 96 The findings of these studies are summarized in Table 5.2. Unless R2 is very bulky (eg. r-butyl, see below ), combination is by far the dominant process with the value k /k typically in the range 0.05-0.16. Thus, a small amount of disproportionation is always observed. 

The self-reactions of 2-carboalkoxy-2-propyl radicals (8-10) have been examined.89 Ia> 104 The results of these studies are reported in Table 5.3. Combination is slightly favored over disproportionation. The value of kjkw for 8 was found to be essentially independent of temperature. 

Bizilj et at reported that disproportionation is more important for oligomeric radicals. While combination products were unequivocally identified, analytical difficulties prevented a precise determination of the disproportionation products. Accordingly, they were only able to state a maximum value of kt[Pg.256]

The self reaction of primary alkyl radicals gives mainly combination.118 For primary alkyl radicals [CH3(CH2)nCH2 ], kjkxa is reported to lie in the range 0.12-0.14, apparently independent of chain length (n=0-3).1,8,119... 

The reaction between the PMMA and PS model radicals (4 and 5, generated from the unsymmetrical azo-compound 3) has been studied as a model for crosstermination in MMA-S copolymerization (Scheme 7.13).178,179 The value for tcross reaction was 0.56. In disproportionation, transfer of hydrogen from the PS model 5 to the PMMA radical 4 was ca 5.1 times more prevalent than transfer in the reverse direction (from 4 to 5). The value of kJklc(90°C) is between those of Atd/ tc(90oC) for the self-reaction of these radicals... 

Analysis of the products from the thermal decomposition of the mixed azo compound 6 showed that in the cross-reaction of radicals 5 and 7 ld/A tt(90oC) is 0.61.179 This study also found that in disproportionation, hydrogen transfer from 5 to 7 is ca 2.2 times more frequent than transfer from 7 to 5. Both self-reactions involve predominantly combination (Scheme 7.14). The values of Ar1j/Aru.(80°C) are 0.16 and 0.05 for radicals 5 (Section 5.2.2.1.1) and 7 (Section 5.2.2.1.3) respectively. It is clear that values of kJkK for homotermination cannot be used as a guide to the value for kjkyt in cross-termination. 

The value of ,<)// ,- 80°C) in the cross-reaction between radicals 4 and 8 has been examined.175 This system is a model for cross-termination in MMA-BMA eopolymeri/alion. The value of kjkw (1.22) is similar to that found for the self-reaction of 8(1.17) and much larger than that for the self-reaction of 4 (0.78). There is a small preference (m 1.4 fold) for the transfer of hydrogen from the butyl ester (8) to the methyl ester (4). 

The polymerizations (a) and (b) owe their success to what has become known as the persistent radical effect."1 Simply stated when a transient radical and a persistent radical are simultaneously generated, the cross reaction between the transient and persistent radicals will be favored over self-reaction of the transient radical. Self-reaction of the transient radicals leads to a build up in the concentration of the persistent species w hich favors cross termination with the persistent radical over homotermination. The hoinolermination reaction is thus self-suppressing. The effect can be generalized to a persistent species effect to embrace ATRP and other mechanisms mentioned in Sections 9.3 and 9.4. Many aspects of the kinetics of the processes discussed under (a) and (b) are similar,21 the difference being that (b) involves a bimolecular activation process. 

The signal-to-noise ratio is usually too low to be useful unless the full light intensity is used. To circumvent this difficulty, it can be assumed, provided the radicals are unhindered, that all the self-reactions will occur at the same rate that is ku = k 2 = k22. Moreover, this rate will be at the diffusion-controlled limit, about 6 x 109 L mol-1 s 1 in aqueous solutions at room temperature, and in the range 109 to... 

TABLE 7.7 Percent Yield of Methylene and Ether Linkages of 2-Hydroxylmethyl-4,6-Dimethylphenol Self-reaction, 1 1 with 2,4-Xylenol, and 1 1 with 2,6-Xylenol... 

In 1986 we proposed a cycle that involves the self reaction of chlorine monoxide radicals, without requiring free oxygen atoms to regenerate the chlorine atoms (14) ... 

A definition of clean air based upon NOj concentrations is more difficult to obtain, because of the ubiquitous natural source in lightning (56,154), and because NO may influence HO concentration even at the lowest tropospheric NO levels (a few ppt) that have been observed. An operational definition might require that the rate of the H02 self-reaction R33 be much faster than its reaction with NO, R13. At NO concentrations below about 1 ppb, [H02 ] is relatively independent of [NO ], while [HO ] may increase with [NO ]. Above this level, both [HO2 ] and [HO ] fall with increasing [NOJ. These [HOJ dependencies on [NO ] are shown in Figure 4 (58) and discussed further in the last section. 

R02./R02 Recombinations. Another area of uncertainty is the peroxyl radical recombination reactions described above, which become especially significant when the NO concentration is low. This can occur late in the photooxidation of polluted air undergoing transport, as in some rural environments (60,85) and in clean air. Although reactions of H02 with itself (R33) are reasonably well understood (their rate depends upon total pressure and upon water vapor concentration), reactions of H02 with R02 species and the R02 self reaction are much less well quantified. Since these serve as important radical sink processes under low NO. conditions, their accurate portrayal is important for accurate prediction of HO, concentrations. 

Parameter setup for Example 8.2a. Self-reaction kinetics... 

Explosions involving flammable gases, vapours and dusts are discussed in Chapter 5. In addition, certain chemicals may explode as a result of violent self-reaction or decomposition when subjected to mechanical shock, friction, heat, light or catalytic contaminants. Substances containing the atomic groupings listed in Table 6.7 are known from experience to be thermodynamically unstable, or explosive. They include acetylides and acetylenic compounds, particular nitrogen compounds, e.g. azides and fulminates, peroxy compounds and vinyl compounds. These unstable moieties can be classified further as in Table 6.8 for peroxides. Table 6.9 lists a selection of potentially explosive compounds. 

Platz J, OJ Nielsen, J Sehested, TJ Wallington (1995) Atmospheric chemistry of l,Ll-trichloroethane UV absorption spectra and self-reaction kinetics of CCljCHj and CCI3CH2O2 radicals, kinetics of the reactions of the CCljCHjOj radical with NO and NOj, and the fate of alkoxy radical CCI3CH2O. J Phys Chem 99 6570-6579. 

A characteristic reaction of free radicals is the bimolecular self-reaction which, in many cases, proceeds at the diffusion-controlled limit or close to it, although the reversible coupling of free radicals in solution to yield diamagnetic dimers has been found to be a common feature of several classes of relatively stable organic radicals. Unfortunatly, only the rate constants for self-termination of (CH3)jCSO (6 x 10 M s at 173 K) and (CH3CH2)2NS0 (1.1 X 10 M s at 163K) have been measured up to date by kinetic ESR spectroscopy and consequently not many mechanistic conclusions can be reached. 

Chemical structure and reactivity. A wide variety of chemicals exist that are thermodynamically unstable. These chemicals easily react, usually with a large heat effect. Most of these chemicals can undergo violent self-reaction or decomposition initialized by mechanical shock, friction, or heat. An incomplete list of dangerously reactive groups is given below ... 

---