Poly(3-octadecylthiophene)s (P3OTs) were prepared via Grignard metathesis polymerization of 2,5-dibromo-3-octadecylthiophene in the presence of palladium and nickel catalysts. The effect of catalyst structure and reaction temperature on the regioregularity of P3OTs was investigated. Nickel catalysts provided P3OTs with higher regioregularity, while palladium catalysts gave lower regioregularity. Surprisingly, the regioregularity of P3OTs increased when the polymerization was conducted at higher temperature. The catalyst and temperature dependence of the regioregularity is consistent with two competing mechanisms. Polymerizations at higher temperature with nickel catalysts occur primarily via chain-growth reactions, while polymerizations at lower temperature with palladium catalysts have competing step-growth and chain growth reactions. P3OTs with higher regioregularity have longer wavelength visible absorptions, while P3OTs with lower regioregularity have shorter wavelength absorptions. (C) 2004 Wiley Periodicals, Inc.