The mechanisms for Ni(dppp)Cl-2-catalyzed chain-growth polymerization of 4-bromo-2.5-bis(hexyloxy)phenylmagnesium chloride and 5-bromo-4-hexylthiophen-2-ylmagnesium chloride were investigated. A combination of rate and spectroscopic studies revealed that transmetalation is the rate-determining step of the catalytic cycle for both monomers. P-31 NM R spectroscopic studies revealed that a Ni(II) aryl halide and a Ni(II) thienyl halide are the catalyst resting states. In addition, LiCI was found to alter the arene polymerization rates. These results are different than those previously obtained with an alternative catalyst (Ni(dppe)Cl-2) and suggest that the ligand has a strong mechanistic influence on the polymerization.