The mechanism of molecular weight limitation in the anionic polymerization of 1 , 3-cyclohexadiene has been studied using nmr and gas chromatographic techniques to detect and characterize benzene as a low molecular weight reaction byproduct of polymerization. This benzene is thought to occur by aromatization of cyclohexadienyl anions formed originally by abstraction of allylic protons from the monomer by the propagating polymeric carbanion. Such aromatization is part of a degradative chain transfer reaction sequence which limits molecular weight and conversion. Polycyclohexadiene endgroups were studied by ultraviolet spectroscopy which indicated that aromatization of the intermediate cyclohexadienyl anion occurred in complete preference to the alternative possibility of chain reinitiation by the cyclohexadienyl anion, a process leading to molecular weight, but not conversion, limitation. The latter alternative requires one cyclohexadienyl terminal group to be present for each polymer chain reinitiated; if present, such groups should be readily detectable in the ultraviolet spectrum but were not observed.