Inhibition and Retardation
Free-radical polymerizations are subject to inhibition and retardation from side reactions with various molecules [54]. Such polymerization suppressors are classified according to the effect that they exert upon the reaction. Inhibitors are compounds that react very rapidly with every initiating free radical as it forms. This prevents any polymerization reaction from taking place until the inhibitor is completely consumed in the process . The reactions of inhibitors with initiating radicals result in formations of new free radicals. The newly formed free radicals, however, are too stable to initiate chain growths. As a result, well-defined induction periods exist. After the inhibitors are used up, polymerizations proceed at normal rates. Retarders are compounds that also react with initiating radicals. They do not react, however, as energetically as do the inhibitors, so some initiating radicals escape and start chain growth. This affects the general rate of the reaction and slows it down. There is no induction period and retarders are active throughout the course of the polymerization. The efficiency of an inhibitor depends upon three factors: (1) the chain transfer constant of an inhibitor with respect to a particular monomer, (2) the reactivity of the inhibitor radical that forms, (3) the reactivity of the particular monomer. Phenols and arylamines are the most common chain transfer inhibitors. The reaction of phenols, though not fully elucidated, is believed to be as follows [153]
Quinones are effective inhibitors for many polymerization reactions. The reaction occurs either at an oxygen or at a ring carbon [153–156]:
The reaction, however, is not always strict inhibition. Thus, for instance, hydroquinone acts as an efficient inhibitor for the methyl methacrylate radical but only as a retarder for the styrene radical [155]. Hydroquinone is often employed as an inhibitor; it requires, however, oxygen for activity [156, 157]:
Oxygen, however, can also act as a comonomer in a styrene polymerization:
It causes marked retardation, however, in the polymerizations of methyl methacrylate [158]. The same is true of many other free-radical polymerizations. The ability of phenols to inhibit free-radical polymerizations appears to increase with the number of hydroxyl groups on the molecules [157]. The locations of these hydroxyl groups on the benzene rings in relationships to each other is important. For instance, catechol is a more efficient inhibitor than is resorcinol [158]. Aromatic nitro compounds can act as strong retarders. Their effect is proportional to the quantity of the nitro groups per molecule [160, 161].
Figure 3.1 illustrates the effect of inhibitors and retarders on free-radical polymerization [162]. The equation that relates rate data to inhibited polymerizations is
Fig. 3.1 Illustration of the effects of inhibitors and retarders. (A) Normal polymerization rate, (B) effect of a retarder, (C) effect of an ideal inhibitor, and (D) effect of a non ideal inhibitor. The time between A and C is the induction period caused by an ideal inhibitor where Z is the inhibitor or the retarder in chain-growth termination:
To simplify the kinetics it is assumed that Z• and ~MnZ• do not initiate new chain growth and do not regenerate Z upon termination.
