Inhibition and retardation of polymerization of methyl methacrylate

The kinetics of the polymerization of methyl methacrylate in the presence of p‐benzoquinone were investigated. The course of the reaction was followed by dilatometry and with tracer methods. Alternate experiments were performed C14‐tagged azobisisobutyronitrile initiator and inactive p‐benzoquinone and, alternatively, radioactive initiator and inactive retarder being used. These data, coupled with number average molecular weights determined by osmometry, served to determine the number of fragments of initiator and retarder incorporated per polymer molecule. It was found that the reaction was retarded at low concentrations of p‐benzoquinone but showed a definite transition to inhibition at higher concentrations. Analysis of the data showed that the number of initiator fragments incorporated per polymer molecule was independent of retarder concentration, but that the number of retarder molecules per polymer molecule increased. The results indicate that the rate of chain initiation decreases with retarder concentration until inhibition is observed. The data are interpreted on the basis of interaction of benzoquinone with primary radicals generated by the initiator. The results also furnish distinct evidence against the acceptance of copolymerization or chain transfer as significant steps in the reaction mechanism.