The controlled polymerization of the 2-bromo-3-hexyl-S-iodothiophene (5) and 2-iodo-3-hexyl-5-bromothiophene (2), using Ni(dppp)Cl(2) as a catalyst and LiCl as an additive, was followed in real time by UV vis spectroscopy. It was shown that the propagation rate constant depends strongly on the ratio R = [LiCl]/[M]. For R = 4 the polymerization rate constant for the regular monomer 5 achieves a constant value 20 times higher than the one obtained without LiCl: k(p) = 44 +/- 5 mol(-1)Ls(-1). For the reverse monomer 2, the addition of LiCl (R = 4) leads to a polymerization reaction with a similar propagation rate constant. The slow initiation reaction of monomer 2, with and without LiCl, has been analyzed by (31)P NMR, UV vis spectroscopy, SEC. It was shown that the addition of LiCl increases the reactivity of the activated monomers under their Grignard forms but also modifies the active center by exchanging the bromine atom by a chlorine atom. This new process was extended to the synthesis of well-defined poly(dodecylthiophene) of high molecular weight.