Polymerization inhibition by aromatic compounds

Abstract

AbstractA quantiative study of chain transfer and inhibition reactions makes it possible to determine the ratios kx/Kp where Kp and kx are the rate coefficients for propagation and for chain tranfer, respectively. Since the absolute values of the constants Kp are known for a number of monomers it is possible to determine the absolute values of the constants kx. We have investigated the inhibiting action of some types of aromatic compounds in the polymerization of vinyl acetate, methyl acrylate, acrylonitrile, and methyl methacrylate. In the polymerization of vinyl acetate the inhibiting effect of the aromatic hydrocarbons increases in the order: toluene, diphenyl, naphthalene, phenanthrene, anthracene. The logarithm of kx is linearly related to the logarithm of the methyl affinity. The inhibiting effect increases on incorporating substituents, both electrondonating and electron‐accepting, in the benzene ring. There is no relation, however, between the value of kx and Hammet's σ‐constant. An outstanding position is assumed by nitrobenzene, for which kx is higher by 3–4 orders of magnitude than that of the other substituted benzenes. The incorporation of substitutents in the meta and para positions of the benzene ring follows the Hammet rule, the electron‐donating substituents causing a decrease in the kx values. Similar relations are observed in the polymerization of methyl acrylate, but the absolute values of kx are lower by 3–4 orders of magnitude than in the polymerization of vinyl acetate. Because of this, under the experimental conditions the interaction of the polymethyl acrylate radical with molecules of the transier agent was not revealed (kx < 0.21 1./mole‐sec. at 50°) in the case of many of the substances investigated, for instance of all mono‐substituted benzenes except nitrobenzene. A comparison of the kx values for polyvinyl acetate, polymethyl acrylate, polymethyl methacrylate, and acrylonitrile radicals permits a number of conclusions to be made concerning the influence of the polar factor on radical reactions. The data obtained lead to definite conclusions as to the inhibiting mechanism of aromatic compounds. The primary act is the addition of the polymer radical to the aromatic ring with the formation of a nonreactive radical. In the case of nitro compounds, the polymer radical reacts with the nitro group, probably by transferring to it a β‐hydrogen atom. It has been found that in the polymerization of vinyl acetate in the presence of the inhibitors in question each molecule of the latter terminates two kinetic chains; regeneration of the chains does not take place, and the polymer molecules form as the result of disproportionation between the polymer radical and the radical formed during interaction of the former with the inhibitor molecule.