Radical transfer

When the polymer is charged, the repulsive forces of the charges prevent an approach of the radicals, and the lifetime of the radicals increases dramatically. In the case of poly(acrylic acid), for example, the decay of the poly(acrylic) acid radicals is fast and follows the same kinetics as any radical derived from neutral polymers as long as the polymer is fully protonated (low pH) (Ulanski et al. 1996c). With increasing pH and concomitant dissociation of the polymer, however, the polymer assumes a rod-like shape, its segments become less flexible, and repulsive forces increasingly prevent their approach. Some radicals survive even for hours under such conditions. 

Interestingly, the corresponding peroxyl radicals have a noticeably shorter lifetime (Ulanski et al. 1996c). This has been explained by the fact that these radicals do not have to approach one another as closely, and thus the repulsive forces are less effective (Ulanski et al. 1997). 

A polymer usually has more than one functional group, more than one type of radical is formed upon -OH-attack. These radicals may undergo radical transfer 

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FIGURE 24.21 A mechanism for the methylmalonyl-CoA mntase reaction. In the first step, Co is rednced to Co dne to homolytic cleavage of the Co —C bond in cobalamin. Hydrogen atom transfer from methylmalonyl-CoA yields a methylmalonyl-CoA radical that can undergo rearrangement to form a snccinyl-CoA radical. Transfer of an H atom regenerates the coenzyme and yields snccinyl-CoA. 

Peroxide initiators may also undergo primary radical transfer to produce unsaturated end groups, which may result in a less stable polymer. In the case of benzoyl peroxide, an additional possibility is initiation by phenyl radicals to give a polymer with terminal phenyl groups [Eq. (17)] ... 

B. Formation of MAIs by Cationic Chain Polymerization—Cation Radical Transfer... 

Regarding anion radical transfer, low-molecular weight azo compounds were used as terminating agents in anionic polymerizations. An interesting example is the addition of a living polystyrene chain to one nitrile group of AIBN [71]. The terminal styryl anion is likely to form... 

Atom or radical transfer reactions generally proceed by a SH2 mechanism (substitution, homolytie, bimolecular) that can be depicted as shown in Figure 1.6. This area has been the subject of a number of reviews.1 3 27 97 99 The present discussion is limited, in the main, to hydrogen atom abstraction from aliphatic substrates and the factors which influence rate and specificity of this reaction. 

A simple unifying theory to explain rate and specificity in atom abstraction reactions has yet to be developed. However, as with addition reactions, it is possible to devise a set of guidelines to predict qualitatively the rate and outcome of radical transfer processes. The following are based on those suggested by Tedder 2... 

Since the benzene emission in the thermal decomposition of benzoyl peroxide results from radical transfer by the phenyl component of a benzoyloxy-phenyl radical pair, phenyl benzoate produced by radical combination within the same pair should appear in absorption. A weak transient absorption has been tentatively ascribed to the ester (Lehnig and Fischer, 1970) but the complexity of the spectrum and short relaxation time (Fischer, personal communication) makes unambiguous assignment difficult. Using 4-chlorobenzoyl peroxide in hexachloro-acetone as solvent, however, the simpler spectrum of 4-chlorophenyl 4-chlorobenzoate is clearly seen as enhanced absorption, together with... 

Each radical transfers an electron to the colloidal particles. One colloidal particle can accept and store a large number of electrons until the aqueous solvent is reduced. Under stationary conditions, a certain number x of electrons reside on a particle, thus producing a negative potential sufficiently high for Hj evolution (n = agglomeration number) ... 

Hatfield, R. D. Ralph, J. Grabber, J. H. A potential role of sinapyl p-coumarate as a radical transfer mechanism in grass lignin formation. Planta 2008, 228, 919-928. 

Quiclet-Sire B, Zard SZ (2006) The Degenerative Radical Transfer of Xanthates and Related Derivatives An Unusually Powerful Tool for the Creation of Carbon-Carbon Bonds. 264 201-236... 

More recent work 8> shows that the S—N bond can be cleaved by hydroperoxides and that aromatic sulphonyl azides only undergo free radical thermal decomposition if a source of radicals is provided. Some light on the nature of the radical transfer agent has recently been shed by the observation 14> that dodecyl azides are formed (2.3%) in the thermolysis of mesitylene-2-sulphonyl azide (3) at 150 °C in w-dodecane under nitrogen. It seems likely that a dodecyl radical is produced by hydrogen abstraction by the triplet nitrene (5) [mesitylene-2-sulphonamide was also formed (1.1%)] which then attacks undecomposed sulphonyl azide... 

In this regard, it should be noted at this point that one of the products identified by CGC/MS from these pyrolysis reactions was SbBr3- Furthermore, the data presented concerning the importance of the polymer substrate in the degradation of the DBDPO and the proposed chain radical transfer mechanism [7] would suggest that the condensed phase chemistry could be much more important in antimony oxide/organohalogen flame retardant systems than had been previously thought. 

As for any chain reaction, radical-addition polymerization consists of three main types of steps initiation, propagation, and termination. Initiation may be achieved by various methods from the monomer thermally or photochemically, or by use of a free-radical initiator, a relatively unstable compound, such as a peroxide, that decomposes thermally to give free radicals (Example 7-4 below). The rate of initiation (rinit) can be determined experimentally by labeling the initiator radioactively or by use of a scavenger to react with the radicals produced by the initiator the rate is then the rate of consumption of the initiator. Propagation differs from previous consideration of linear chains in that there is no recycling of a chain carrier polymers may grow by addition of monomer units in successive steps. Like initiation, termination may occur in various ways combination of polymer radicals, disproportionation of polymer radicals, or radical transfer from polymer to monomer. 

Hydrogen Radical Transfer Catalyzed by a Coenzyme Bn-Dependent Mutase... 

Table 11.10 Transmission coefficients and their effect on KIE for H/D radical transfer at 278 K catalyzed by a coenzyme B -dependent mutase (Dybala-Defratyka, A., Paneth, P., Baneijee, R. and Truhlar, D. G., Proc. Natl. Acad. 5c/.104, 10774 (2007) Chowdhury, S. and Baner-jee, R.,./. Am. Chem. Soc. 122, 5417 (2000)) ...

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