Determination of in‐cage and out‐of‐cage recombination of initiator radicals in solution polymerization using labeled initiators

The amount of in‐cage and out‐of‐cage recombination of initiator radicals has been measured by recovering the recombination products of simultaneous decomposition of an unlabeled and a deuterium‐labeled initiator. The occurrence of recombination products generated from both an unlabeled and a deuterium‐labeled initiator fragment, which can be monitored in a mass spectrometer, indicate out‐of‐cage recombination. It turned out that in the course of decomposition of dibenzoylperoxide in benzene, biphenyl was not produced by direct recombination of the phenyl radicals originally generated, but rather by interaction with benzene. Upon decomposing azoisobutyronitrile in various solvents, however, both in‐cage recombination and out‐of‐cage recombination were observed. When small amounts of styrene were added to such systems, the initiator radicals tended rather to react with styrene than to undergo out‐of‐cage recombination.