Other Free Radical Reactions

TABLE 4.2 Values of Chain Transfer to Monomer Constant C]y[ for Some Common Monomers  

All values are taken from Reference [6] except for vinyl chloride [7]. 

Values for chain transfer constants to initiator are listed in Table 4.3. Note that these values are generally larger than those of transfer to monomer however, the effect of this reaction is attenuated by the fact that the initiator is present in very small amount with respect to the monomer, as the rate of transfer to initiator is [P][I]. 

4 Chain Transfer to Solvent and Chain Transfer Agents 

When polymerization takes place in a solvent, it is important to be aware of possible chain transfer to solvent reactions. On the other hand, there are many instances, especially in industrial processes, in which it is convenient to include in the polymerization recipe some species (called chain transfer agent) that have a 

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For a review of this and other free-radical reactions of organomercury compounds, see Barluenga, J. Yus, M. Chem. Rev., 1988, 88, 487. 

Ketone and the formed a-ketoperoxyl radical are polar molecules. Hence the polar effect influences the reactivity of the ketones and the peroxyl radicals. Polar solvents also influence the reactions of peroxyl radicals with ketones as well as other free radical reactions. 

It can be seen that the steric effect is profound in radical reactions of Ar2OH with peroxyl and methyl radicals. It will be shown later that the steric effect exists in other free radical reactions of Ar2OH. The AES values of the reactions of alkyl radicals with Ar2OH are considerably higher than those for phenols reacting with oxygen-centered radicals. The steric effect can also manifest itself in the inverse reactions of sterically hindered phenoxyl radicals Ar20 with various molecules (see later). 

Another important rearrangement is that of cyclopropylmethyl radicals to the corresponding homoallyl radicals. This is an exceptionally fast reaction (t1/2 10 8) and has been used as a radical clock to determine the rates of other free-radical reactions.95 Cyclopropylcarbene also undergoes rearrangement, leading to cyclobutene.96... 

Eventually the reaction chains are broken by termination reactions. Other free radical reactions also take place to a lesser extent leading to the formation of CH4 and some higher hydrocarbons among the products. 

Barnard, D., Bateman, L., Cole, E.R. and Cunneen, J.I. (1958) Sulphoxides and thiolsulphinates as inhibitors of autoxidation and other free radical reactions. Chem. Ind., pp. 918—919. 

Charge-transfer polymerization is characterized by a highly efficient propagation step that tends to exclude other free-radical reactions, except those that can compete in an efficient manner. 

Some unusual organizational decisions have been made, too. SrnI reactions and carbene reactions are treated in the chapter on polar reactions under basic conditions. Most books on mechanism discuss SrnI reactions at the same time as other free-radical reactions, and carbenes are usually discussed at the same time as carbocations, to which they bear some similarities. I decided to place these reactions in the chapter on polar reactions under basic conditions because of the book s emphasis on teaching practical methods for drawing reaction mechanisms. Students cannot be expected to look at a reaction and know immediately that its mechanism involves an electron-deficient intermediate. Rather, the mech-... 

In addition to the selective oxidation reactions above, a number of other free-radical reactions are summarized herein. Tanko and Blackert (267,268) report the free-radical side-chain bromination of toluene and ethylbenzene in SCCO2 using bromide radicals initiated photochemically from molecular bromine. They report the production of the corresponding benzylic bromides in high yield with selectivities essentially identical to that observed in a conventional chlorinated... 

Why do you suppose that such high pressures are needed for LDPE production They are not needed for the other free-radical reactions we have seen. 

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