Stable free-radical substituent

Thus, Styrene can be more easily induced to polymerize than vinyl acetate. In reverse, the poly(vinyl acetate) free radical is about 1000 times more reactive than the poly(styrene) free radical. In general, monomers more easily activated to polymerize give more stable free radicals, and vice versa. In these cases, the reactivity can, as expected, be influenced by complexation of the substituents, for example, with Lewis acids such as zinc chloride, or aluminum chloride in the case of nitrile or carboxy group containing monomers. 

The instabihty of tert-huty areneperoxysulfonates is increased by the presence of electron-withdrawing substituents on the aromatic ring and decreased by electron-donating substituents. However, even the most stable members decompose violently on warming, as indicated in Table 14. These peroxyesters appear to decompose heterolyticaHy without the formation of free radicals (44). 

Diaroyl peroxides and diacyl peroxides without a-branches are significantly more thermally stable than those with mono- or di-cr-substituents. Tlie primary use of most commercial diacyl peroxides (16. R1 = R2 = alkyl or aryl) is initiation of free-radical reactions. 

The last three entries in Table 13 reflect a marked preference for a-methylene versus methyl proton transfer for electron-withdrawing a substituents. These amines are also unusual in that they react with t in nonpolar solvents and do not display exciplex fluorescence. While this unusual behavior was initially attributed to a free radical hydrogen atom transfer mechanism leading to the formation of exceptionally stable "merostabilized" a-aminoallyl radicals (115), our current view is that the high kinetic acidity of the a-C-H bond of these amines when complexed with t is responsible for their behavior. 

The active center may be a free-radical, ion. or metal-carbon bond (Chapter 6). In any event the propagating species 4-6 will be more stable than its counterpart 4-7 if the unpaired electron or ionic charge can be delocalized across either or both substituents X and Y. When X and/or Y is bulky there will be more steric hindrance to approach of the two substituted C atoms than in attack of the active center on the methylene C as in reaction (4-1). Poly(vinyl lluoride) contains some head-to-head linkages because the F atoms are relatively small and do not contribute significantly to the resonance stabilization of the growing macroradical. 

Autoxidation can be inhibited or retarded by adding low concentrations of chainbreaking antioxidants that interfere with either chain propagation or initiation (286). Chain-breaking antioxidants include phenolic and aromatic compounds hindered with bulky alkyl substituents. Common synthetic chain-breaking antioxidants used in food lipids include butylated hydroxyanisole (BHA), butylated hydroxyto-luene (BHT), ferf-butyUiydroquinone (TBHQ), and propyl gallate (PG). This class of antioxidants react with peroxy free radicals to terminate reaction chains. The antioxidant radical (A ) formed in Equation 5 should be relatively stable and unable to initiate or propagate the oxidation chain reaction. 

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