Catalyst transfer polycondensation is the only method to prepare π-conjugated polymers in a chain-growth manner, yet several aspects that underlie this polymerization are not fully understood. Here, we investigate the nickel-catalyzed polymerization mechanisms of a series of thiophene monomers bearing different halogen functionalities (Cl, Br, I). We have discovered the significant role that halogens and magnesium salts play in this polymerization. More specifically, the catalyst resting state changes depending on the type of halogenated monomer. For chlorinated monomers a mixture of Ni(ii)-dithienyl and dissociated Ni(phosphine) complexes are the resting states, which results in uncontrolled polymerization. For brominated monomers, a Ni(ii)-dithienyl complex is the resting state, which leads to controlled polymerization. For iodinated monomers, a Ni(ii)-thienyl iodide complex is the resting state, and notable inhibition by magnesium salt by-products is observed. The catalyst resting state changes to a Ni(ii)-dithienyl complex when a turbo Grignard reagent (i-PrMgCl·LiCl) is used. These findings are used to guide the design of a new monomer, 2-bromo-3-(2-ethylhexyl)-5-iodotellurophene, which enables the first controlled polymerization of a tellurophene monomer containing a sterically encumbered 2-ethylhexyl side chain. These insights are crucial for deepening the mechanistic understanding of Kumada cross coupling reactions and the controlled synthesis of π-conjugated polymers.