Regioselective Grignard Metathesis Reaction of 2,5-Dibromo-3-(6′-hexylpyridine-2′-yl)thiophene and Kumada Coupling Polymerization

2,5-Dibromo-3-(6'-hexylpyridine-2'-yl)thiophene (DBPyTh) was synthesized by the Suzuki coupling reaction between two aromatic compounds followed by the bromination. The Grignard metathesis reaction of DBPyTh with isopropylmagnesium chloride proceeded in 85% conversion and the regioselective halogen-metal exchange at the 2-position was confirmed. Namely, 5-bromo-2-chloromagnesio-3-(6'-hexylpyridine-2'-yl)thiophene and 2-bromo-5-chloromagnesio-3-(6'-hexylpyridine-2'-yl)thiophene were generated in 90:10 molar ratio. Subsequently, the Kumada coupling polymerization was carried out using 1,3-bis(diphenylphosphinopropane)nickel(II) dichloride to obtain poly(3-(6'-hexylpyridine-2'-yl)thiophene) (PolyPyTh). The polymer molecular weight could be roughly controlled by the catalyst concentration and the molecular weight distribution ranged from 1.25 to 1.80. The gas chromatograph analysis indicated that 5-bromo-2-chloromagnesio-3-(6'-hexylpyridine-2'-yl)thiophene was preferentially polymerized in 90% conversion and the percentage of the head-to-tail content (regioregularity) was calculated to be 96%. The matrix-assisted laser desorption/ionization time-of-fright mass spectrum indicated that both polymer chain ends were substituted with the hydrogen atom. The absorption maxima of polymer in CHCl3 and thin film were observed at 447 and 457 nm, respectively, which were blue-shifted compared with poly(3-(4'-octylphenyl)thiophene). From the CV measurement of the polymer thin film, highest occupied molecular orbital (HOMO) (-5.31 eV) and lowest unoccupied molecular orbital (LUMO) (-3.76 eV) energy levels were calculated from the oxidation and reduction onset potentials, respectively, and the electrochemical band gap energy was determined to be 1.62 eV. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 4013-4020, 2011