Poly(3-hexylthiophene) was synthesized by chemical oxidation using FeCl3 in chloroform, dichloromethane, acetonitrile, nitromethane, nitrobenzene and mixtures of these solvents. Comparative reactions for 1 day at room temperature were carried out to investigate solvent effects. Polymer samples and the remaining reaction solutions were analyzed by MALDI-MS and LDI-MS to determine Mw and reaction completeness, respectively. Solubilities of FeCl3, FeCl2, and P3HT in each pure solvent were measured and correlated to the results. Open circuit potentials of FeCl3 and mixtures of FeCl3 and FeCl2 in pure solvents were measured and compared to the experimentally determined oxidation potential of 3-hexylthiophene. An optimized solvent system for reproducible synthesis of high M-w P3HT with quantitative yields is proposed based on MS analysis results. (C) 2017 Elsevier Ltd. All rights reserved.