Synthesis of regiospecific poly(3-substituted-thiophenes) using Ni-diimine-initiated cross-coupling polymerization

Introduction Polythiophenes (PTs) continue to be one of the most versatile and extensively studied conjugated polymer systems due to their exceptional spectroscopic and electronic properties, and the ease of synthesis of PTs derivatives. Practical and potential applications of PT derivatives include rechargeable battery, electrochromic devices (ECDs) (e.g., poly(alkylenedioxythiophene) [PXDOT]), chemical and optical sensors, light-emitting diodes (LEDs), field-effect transistors (FETs). Structurally regioregular PTs can be obtained by several methods that utilize a metalinitiated cross-coupling reaction that proceeds by a chain-growth mechanism. The choice of the metal and the ligands used in polymerization is crucial to achieving a high degree of regioregularity in the product PT. After comparing Ni vs. Pd with different ligands (e.g., monodentate (PPh3) or bidentate (Ph2PCH2CH2PPh2; dppe), it has been observed that cross-coupling selectivity was a function of the steric environment of the metal, where the most sterically hindered Ni(dppe)Cl2 produced regioregular PT and the least hindered Pd(Ph3)4 produced regiorandom PT. Here we report synthesis of regioregular poly(3-alkyl/alkoxysubstituted-thiophenes) via the Grignard Metathesis (GRIM) method employing a Ni-!-diimine catalyst [(ArN=C(An)-C(An)=NAr)NiBr2 with Ar = 2-t-BuC6H4)] (see Figure 1, 2) that has a great tolerance to different functional groups and is mainly used for living polymerization of !-olefines. Utilizing this catalyst leads to regioregular polymers in good yield with narrow polydispersities (PDIs) (e.g., PDI=1.25 for poly(3-hexylthiophene) (1a)) with similar electrical and spectroscopic properties previously reported for PT synthesized by the well-established methods. Furthermore, the synthesis of the catalyst and employing the GRIM method provide a quick, easy and cost effective procedure for the synthesis of regiospecific PT derivatives on an industrial level.