We report the influence of the active Grignard monomer formation on the end groups and polydispersity of poly(3-hexylthiophene) (P3HT) for the catalyst transfer polymerization. The rate of the active Grignard monomer formation of 2,5-dibromo-3-hexylthiophene using t-BuMgCl was studied using H-1 NMR. Only in the absence of unreacted/excess t-BuMgCl, polymers with 100% H/Br end groups were formed. If the active Grignard monomer formation was incomplete and thus unreacted t-BuMgCl remained, the end groups depended on the polymerization time; the ratio of H/Br to H/H end groups decreased with increasing time. LiCl was shown to accelerate the active Grignard monomer formation but negatively affects the regioregularity to a small extent. It also increases the molecular weight of P3HT when used in combination with Ni(dppp)Cl-2 as a catalyst. Further, MeOH as a quenching reagent was identified to cause chain-chain coupling and hence an increase in the polydispersity. Thus, important parameters influencing the kinetics of the catalyst transfer polymerization were studied, and a series of P3HTs with 100% H/Br end groups and low polydispersities were synthesized by an optimized procedure. These findings are very relevant for extending this polymerization method to new monomers and for the realization of well-defined block copolymers.