Unprecedented Regioregular Poly(1,4‐arylene)s Prepared by Nickel(II)‐Catalyzed Cross‐Coupling Polymerization of 2,5‐Disubstituted Bromo(chloro)arylene

The unprecedented synthesis of regioregular head‐to‐tail‐type poly(1,4‐arylene)s bearing different substituents at the 2‐ and 5‐positions is described. They were prepared by the polymerization of 2,5‐disubstituted bromo(chloro)arylenes by selective halogen–metal exchange with a Grignard reagent and subsequent cross‐coupling polymerization with a nickel catalyst [NiCl2(dppp)]. Formation of the regioregular poly(1,4‐arylene)s were confirmed by NMR spectroscopy, and showed remarkable differences to those polymers having uncontrolled regioregularity. Polymerization of bromo(chloro)arylenes with a chiral alkoxy substituent also led to the regioregular head‐to‐tail‐type polyarylene, which demonstrated circular dichroism, thus suggesting formation of a structure with higher‐order regularity. One of the regulars: Polymerization of differently substituted 1,4‐bromo(chloro)arylenes afforded unprecedented head‐to‐tail (HT) regioregular poly(1,4‐arylene)s, whereas the regiocontrolled synthesis from the 1,4‐dibromoarylene was unsuccessful. A regioregular HT poly(1,4‐arylene) bearing a chiral substituent demonstrated a Cotton effect, as seen in the CD spectrum measured in a mixed solvent system, suggesting a helical structure.