Abstract
Combination of C^14^โtagged or inactive benzoquinone with tagged or inactive azobisisobutyronitrile was used to study the kinetics of polymerization of ethyl methacrylate. The course of the reaction was followed by dilatometry, and the resultant polymers were assayed for C^14^ content by liquid scintillation techniques. Molecular weights were obtained by osmometry in order to determine the number of tagged fragments per polymer molecule. A transition from a retarded polymerization to one showing an inhibition period was found at a critical concentration of pโbenzoquinone. This transition was accompanied by a marked change in the C^14^ content of the polymers. The magnitude of chain transfer or copolymerization was directly estimated from the number of tagged fragments incorporated per polymer molecule. These indicated that no more than two molecules of pโbenzoquinone could copolymerize or that a limit could be set of one chain transfer step per kinetic chain. Hence, neither of these processes can be considered as highly significant in the mechanism. The formation of a transient adduct between primary radicals of initiation and benzoquinone is postulated, and the transition from retarded to inhibited polymerization in this system is discussed on this basis. These ideas were extended to a consideration of the role of primary radical capture and radical reactivity for other monomers, such as styrene and vinyl acetate.