Types of Inhibitors and Retarders

As with chain-transfer agents, one observes that the inhibition constant for any particular compound varies considerably depending on the reactivity and polarity of the propagating 

TABLE 3-9 Retarder/Inhibitor Chain-Transfer Constants 6 

The most useful class of inhibitors are molecules that react with chain radicals to yield radicals of low reactivity. Quinones such as benzoquinone and chloranil (2,3,5,6-tetrachloro-benzoquinone) are an important class of inhibitor. The behavior of quinones is quite complex [Eastmond, 1976a,b,c George, 1967 Small, 1975 Yamamoto and Sugimoto, 1979 Yassin and Risk, 1978a, 1978b]. Two major types of products are obtained—quinone and ether—formed by reaction at the C and O atoms of a quinone, respectively. Attack of a propagating radical at oxygen yields the aryloxy radical XXVIII, which can terminate by 

Polar effects appear to be of prime importance in determining the effect of quinones. p-Benzoquinone and chloranil (which are electron-poor) act as inhibitors toward electron-rich propagating radicals (vinyl acetate and styrene) but only as retarders toward the electron-poor acrylonitrile and methyl methacrylate propagating radicals. A further observation is that the inhibiting ability of a quinone toward electron-poor monomers can be increased by the addition of an electron-rich third component such as an amine. Thus the presence of triethylamine converts chloranil from a very weak retarder to an inhibitor toward methyl methacrylate. 

Oxygen is a powerful inhibitor, as seen from the very large z values. Oxygen reacts with radicals to form the relatively unreactive peroxy radical that reacts with itself or another 

TABLE 3-9 Retarder/Iohibitor Chain-Transfer Constants  

Contrary to the general impression, phenol and aniline are poor retarders even toward highly reactive radicals such as the poly(vinyl acetate)-propagating radical (Table 3-8). Phenols with electron-donating groups (and, to some extent, similarly substituted anilines) 

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