An Alternative Convenient Protocol for the Preparation of Regioregular Poly(3-hexylthiophene) via Thienylzincate Complex

Abstract

Regioregular head-to-tail poly(3-hexylthiophene), P3HT, has been found to be an unusual class of conducting polymer with good solubility, processibility, environmental stability, and high electrical conductivity. It has been used for a broad range of chemical devices such as photovoltaics, electrochromic devices, optical sensors, organic light-emitting diodes and field-effect transistors. Due to the significant role of this polymer in the field of functionalized material chemistry, many procedures have been developed and utilized for its production. To this aim, in general, two synthetic routes such as electrochemical methods and chemical methods have been developed. Chemical methods have extensively used for the synthesis of regioregular P3HT mainly because these methods generally provide high regioregularity of the polymer in excellent yields and can be readily carried out in large scale production. Among the chemical synthetic methods, organometallic reagents have been frequently employed as key intermediates. Among those, thienylzinc halides and thienylmagnesium halides are the most common reagents. In addition, a few other reports have exploited the Suzuki and Stille reagents. Although each protocol has provided the corresponding polymer with good qualities, the use of thienylzinc halide as an organometallic reagent has been proven to be the most reliable procedure for the preparation of regioregular P3HT. Rieke demonstrated the use of thienylzinc bromide intermediates prepared by the direct insertion of highly active zinc to 2,5-dibromo-3-alkylthiophenes. McCullough also reported the synthesis of the regioregular polythiophenes utilizing both thienylmagnesium halides and thienylzinc halides prepared by the Grignard metathesis followed by transmetallation with zinc halides. In our continuing studies on the preparation of P3HT, we next focused on the development of more practical synthetic procedures especially utilizing thienylzinc reagents. One of our studies was to use 2-bromo-3-hexyl-5-iodothiophene along with the dialkylzinc reagents. Recently, Higashihara also reported the use of 2-bromo-3-hexyl-5-iodothiophene along with a zincate complex of tBu4ZnLi2 for the synthesis of P3HT. Even though the use of 2-bromo-3-hexyl-5iodothiophene provides a satisfactory route, it still requires the regio-selective halogenation which is consisted of two separate steps. Thus, to alleviate this difficulty, the use of 2,5-dibromothiophene derivatives as a monomer would be highly recommended especially in large scale production. Herein, we report a more practical synthetic route for the preparation of thienylzinc reagents and its subsequent polymerization in the presence of a Ni-catalyst affording the high head-to-tail regioregular P3HT. The schematic diagram for the preparation of P3HT used in this study is described in Scheme 1. As depicted in Scheme 1, the monomer and trialkylzincate complex should be prepared prior to polymerization. 2,5Dibromo-3-hexylthiophene was prepared by the literature procedure; treatment of n-hexyl Grignard reagent with 3bromothiophene in the presence of Ni(dppe)Cl2, followed by bromination with NBS yielded 1. Simple distillation afforded the pure product (1) in good yield (Scheme 2). The trialkylzincate complex was also easily prepared as shown in Scheme 3. When zinc chloride was treated with 1.0