Synthesis and Characterization of Two Regiochemically Defined Poly(dialkylbithiophenes): A Comparative Study

Abstract

The polymerization of 3,3’-dimethyl-2,2′-bithiophene and 3,3′-dihexyl-2,2′-bithiophene affords the stereoregular head-to-head versions of poly(3-methyl- and poly (3-hexylthiophene), respectively. The properties of the stereoregular polymers are compared to those of the conventional polymers obtained from polymerization of 3-methylthiophene and 3-hexylthiophene. Regiospecificity was determined via 500- MHz NMR (both 1H and 13C) in solution for the hexyl-substituted case and 300-MHz 13C CPMAS for all polymers. The regiospecific polymers have their maximum absorption at considerably shorter wavelengths than their nonregiospecific counterparts; thus, poly(3,3′-dimethyl-2,2′-bithiophene) absorbs at a 91 nm shorter wavelength (417 vs 508 nm) and poly(3,3′-dihexyl-2,2′-bithiophene) absorbs at a 110 nm shorter wavelength (398 vs 508 nm). This change in band positions is not due to a difference in molecular weight since the two types of polymer are essentially of equal Mw, with the stereoregular hexyl derivative having a slightly higher Mw. We conclude that the intrachain sulfur-alkyl steric repulsion must be dominant, forcing the backbone out of coplanarity and -conjugation. Interestingly, this pronounced steric effect on the electronic spectroscopy is not reflected in the conductivity. Finally, the stereoregular polymers are the most magnetic defect-free conjugated chains prepared to date, as determined via ESR spectroscopy. © 1990, American Chemical Society. All rights reserved.