Factors affecting rate free-radical chain

The efficiency of a particular amine must depend not only on the rate of the initial hydrogen abstraction, but also on the nature and subsequent reactions of the radical produced. The free radical produced by H transfer may well be stabilised by resonance and may be insufficiently reactive to start a new oxidation chain [40], particularly when the amino group is surrounded by bulky substituents [9]. If the radical does react, then the subsequent rate and nature of the reaction will depend upon the intermediates and on the relative importance of chain termination and chain transfer reactions. Some formal grouping of the factors affecting the efficiency of a given inhibitor and the kinetics of the inhibited reaction is possible. 

Woodall et al. suggested that the different reactivities of the carotenoids against free radicals can be partly attributed to the differences in electron distribution along the polyene chain of different chromophores that would alter the susceptibility of free radical addition to the conjugated double bond system. However, other factors must be also considered. Stearic hindrance, hydrogen abstraction from the allylic position to the polyene chain (C-4 of P-carotene and its derivatives, end of lycopene), would reduce radical scavenging activity. In addition, the stability of the polyene radical is important in determining the rate of the loss of carotenoids and hence it affects their antioxidant activity. 

The hrst section covers the basic principles and characteristics necessary for polymer preparation by polymerization, being either (a) stepwise polymerization of bifunctional monomers by polycondensation, stepwise polyaddition and ringopening processes, or (b) chain polymerization of vinyl monomers by free radical, cationic, anionic, and coordination addition processes. Both of these polymerization techniques are used for polymer preparation from monomer. The goal of the polymerization technique is to obtain polymers with specific structures and properties -this generally requires specialized polymerization conditions. Also described are the factors affecting the rates of homo- and copolymerizations and the reactivity ratios of different comonomers. 

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