The mechanisms for Ni(dppe)Cl-2-catalyzod chain-growth polymerization of 4-bromo-2,5-bis(hexyloxy)phenyl magnesium chloride and 5-bromo-4-hexylthiophen-2-ylmagnesium chloride were investigated. Rate studies utilizing IR spectroscopy and gas chromatography revealed that both polymerizations exhibit a first-order dependence on the catalyst concentration but a zeroth-order dependence on the monomer concentration. P-31 NMR spectroscopic studies of the reactive organometallic intermediates suggest that the resting states are unsymmetrical Ni-II-biaryl and Ni-II-bithiophene complexes. In combination, the data implicate reductive elimination as the rate-determining step for both monomers. Additionally, LiCl was found to have no effect on the rate-determining step or molecular weight distribution in the arene polymerization.