Specificity Observed in the Reactions of Oxygen-Centered Radicals with Various Monomers at

The relative importance of these processes depends strongly on the particular 

In contrast to most other oxygen-centered radicaLs e.g. benzoyloxy (3.4.2.2.1), hydroxy (3.4.2.3)1, t-butoxy radicals and other r-alkoxy radicals (3.4.2.1.2) show relatively high regiospecificity in reactions with carbon-carbon double bonds (Table 3.8). Nonetheless, significant amounts of head addition are observed with the halo-olcfins, simple alkcncs, vinyl acetate and methyl acrylate. Head addition is generally not observed with 1,1-disubstituted monomers. The exception is vinylidene niioride where head addition predominates (Section 

The relative amounts of double bond addition, hydrogen abstraction and p-scission observed are dependent on the reactivity and concentration of the particular monomer(s) employed and the reaction conditions. Higher reaction temperatures are reported to favor abstraction over addition in the reaction of /-butoxy radicals with AMS and cyclopentadiene. However, the opposite trend is seen with isobutylene.  

Pioneering work by Walling established that the specificity shown by t-butoxy radical is solvent dependent. Work on the reactions of /-butoxy radicals with a scries of a-mcthylvinyl monomers has shown that polar and aromatic solvents favor abstraction over addition, and (3-scission over either addition or abstraction. Recently, Weber and Fischer and Tsentalovich ct u/. reported absolute rale constants for p-scission of /-butoxy radicals in various solvents. These studies indicate that p-scission is strongly solvent dependent while abstraction is relatively insensitive to solvent. 

even if /-amyloxy radicals (101) show similar specificity for addition i. v abstraction to /-butoxy radicals, abstraction will be of lesser importance. The reason is that most /-amyloxy radicals do not react directly with monomer. They undergo [3-seission and initiation is mainly by ethyl radicals. Ethyl radicals are much more selective and give addition rather than abstraction. This behavior has led to /-amyl peroxides and peroxyesters being promoted as superior to the corresponding /-butyl derivatives as polymerization initiators.  

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Table 3.8 Specificity Observed in the Reactions of Oxygen-Centered Radicals with Various Monomers at 60 °C...

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