Head-to-Tail Regioregular Polythiazole Prepared via Kumada-Coupling Polycondensation

Head-to-tail regioregular poly(4-alkylthiazoles) (C-pTz, C-pTz, alkyl = n-C₉H₁₉, n-C₁₃H₂₇) have been synthesized and spectroscopically and electrochemically characterized. The PTzs were obtained by transmetalation of 2-chloro-5-bromo-4-alkylthiazoles with PrMgCl, followed by Kumada-coupling polycondensation. The polymers are largely insoluble in organic solvents but dissolve readily in the presence of, e.g., trifluoro acetic acid (TFA). Analyses of soluble trace fractions of the polymers gave number average molecular weights (M_n) of 1.9-2.4 kDa (C-pTz, PDI ≈ 1.1-1.3) and 2.9-3.0 kDa (C-pTz, PDI ≈ 1.1-1.2), as determined by gel permeation chromatography (GPC) relative to polystyrene standards, while the molecular weight of the bulk material is presumed to be considerably higher. Comparison of the ¹H NMR spectra of the PTzs with a quaterthiazole model compound (4Tz) and head-to-head-tail-to-tail regioregular polybithiazole (PBTz) confirmed the head-tail structure and the high degree of regioregularity. The optical and electrochemical band gaps of C-PTz were found to be similar to those of poly(3-hexylthiophene) (P3HT), while the frontier orbital levels are stabilized by 0.3-0.5 eV relative to those of the polythiophene. The synthesis of PTz via selective transmetalation of the precursor at the sterically hindered 5-position renders this synthesis a rare polycondensation of a reversed monomer. The implications for the polymerization mechanism are also discussed.