When acrylonitrile is added to the Belousov-Zhabotinsky reaction, periodic polymerization is observed. (Pojman, J. A.; Leard, D. C.; West, W. W. J. Am. Chem. Sec. 1992, 114, 8298). The polymer's structure and the amount of bromine in the polymer sample were determined by C-13 NMR and elemental analysis, respectively. Carbon-13-labeled malonic acid indicates that polymerization of acrylonitrile is initiated by malonyl radical. The bromous radicals are believed to terminate the polymer leaving an alcohol group on the polymer end. Further analysis of the polymer sample confirms that the sample is partially brominated. Numerical simulations based on a modified FKN mechanism are found to be in good agreement with experimental findings. Simulations indicate that periodic termination of the polymer chain by bromine dioxide, not the periodic initiation by malonyl radical, is the dominant cause of the periodic polymerization. Similar results were found in the malonyl-controlled variant of the BZ reaction.