On the mechanism of polymerization retardation by hydroquinone

Abstract The retardation of methyl methacrylate polymerization by hydroquinone, 1,4‐napthalenediol, and benzoquinone was studied by an accurate dilatometric method. The degree of retardation was found to be very small for the phenols in the absence of air and very sensitive to the presence of impurities. Kinetic analysis of the experimental results indicates that the intermediate radicals formed in the reaction between the chain radicals and the phenols disappear from solution principally by reinitiating polymerization. Comparison with results obtained for retardation by benzoquinone indicates that semiquinone radicals are probably not formed in the initial reaction of the phenols with the chain radicals. It is suggested that the intermediate radicals produced in phenolic retardation are addition complexes between the phenols and the chain radicals which may involve hydrogen bridging. The breakdown of these complexes then causes reinitiation of polymerization.