Kinetics of the inhibition of styrene polymerization by nitro compounds

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Abstract
A kinetic study has been made of the inhibiting action of s‐trinitrobenzene and its derivatives on the initiation of styrene polymerization. The effect of −CH3, −OH, −OCH3, −NH2, −Cl, −CO2C2H5 as substituents was investigated. The nitro compounds were found to be weak inhibitors of styrene polymerization. They bring about a well defined inhibition period during which the reaction mixture usually darkens. During the inhibition phase the rate of polymerization gradually increases, but the rate of polymerization afterwards is lower than during the noninhibited process. It concerns the fact that the initial inhibitor transforms to a retarder about 20 times weaker. In the light of this the basic equations of inhibition followed by retardation have been derived. Such are, for example, equations for the polymerization rate, for the kinetics of inhibitor consumption, and also a formula for determining the duration of the inhibition phase on the basis of dilatometric data. Procedures have also been developed for determining the inhibition and retardation constants and the stoichiometric coefficient indicating number of radicals deactivated by one molecule of inhibitor. The inhibition constants (as well as retardation constants) have been shown to obey the Hammett rule in the case of substituents of small volume (−H, −CH3, −OCH3, −Cl). The inhibition reactivity of the compounds is the higher, the higher the electron acceptor properties of the substituent. In the case of the −CO2C2H5 group the activity is below that expected from the Hammett rule, which is evidently the result of a decrease in conjugation. In the case of OH and NH2 substituents the activity is much above that given by the rule, because the hydrogen bond has an additional electron acceptor effect on the nitro group.

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