STUDIES OF PRIMARY RADICAL TERMINATION IN VINYL POLYMERIZATION . POLYMERIZATION OF STYRENE INITIATED BY TRITIUM-LABELED AZOBISISOBUTYRONITRILE . MEASURE OF TRANSFER CONSTANT OF AZOBISISOBUTYRONITRILE

It is generally agreed that primary radical termination (PRT) is one of the most important causes of deviations of the kinetics of vinyl polymerizations from the kinetic laws predicted from the “simple” system of equations. (Primary radical termination is the reaction in which primary radicals from the initiator terminate growing chains rather than adding to monomer to initiate chain growth.) One of the systems in which PRT has been studied in the greatest detail is the polymerization of styrene initiated by azobisisobutyronitrile (AIBN). We have attempted to determine the number of end groups due to primary radical termination in this system using a radiotracer technique. To do this, it is necessary to know the transfer constant of AIBN; this transfer constant has been reported to be zero by four groups of workers. Data are here reported on the polymerization of styrene at 60° in the presence of a constant amount of tritium-labeled AIBN and a variable amount of inactive AIBN. The data are treated according to the equation R* =kdf(I*) + C[(I*)/(M)]RP,3 where R* is the rate of incorporation of AIBN-t into the polymer, (I*) is the concentration of labeled AIBN, (M) is that of styrene, kd is the rate constant for decomposition of AIBN, f is its efficiency, C is the transfer constant of AIBN, and Rp is the rate of formation of polymer. A plot of R* vs. Rp can yield values of kdf and C. In contrast with the previous reports, our data yield an apparent value for C which is substantial. Possible systematic errors in the tracer method for measuring the transfer constant of AIBN are discussed; for example, copolymerization of the AIBN would lead to an apparent transfer constant. However, experiments have been done which appear to rule out copolymerization and other alternative explanations. Thus it appears that AIBN does have an appreciable transfer constant. The implications of this for studying PRT by this method are discussed. © 1969, American Chemical Society. All rights reserved.