In‐Situ End‐Group Functionalization of Regioregular Poly(3‐alkylthiophene) Using the Grignard Metathesis Polymerization Method

Due to their excellent electrical properties, regioregular polythiophenes are rapidly being developed into new commercial products with applications ranging from transistors to hole-transport layers in organic light-emitting diodes (OLEDs) to new specialty plastics. [1±3] As regioregular poly-thiophenes grow in importance, so does the development of inexpensive and convenient methods for the synthesis of these polymers. While a great deal of work has been reported for the modification and variation of the side chains of regioregu-lar, head-to-tail coupled polythiophenes (HT-PTs), [4] less attention has been given to the nature and control of the end groups of such polymers. [5,6] Procedures published to date to functionalize end groups of conjugated polymers are limited in scope and number. [5,6] End-group functionalization of HT-PTs would lead to a number of new uses for these polymers including end-group-driven self-assembly onto surfaces and into conducting polymer networks, and their use as building blocks for the synthesis of block copolymers. [7] Previously, our group reported a multistep synthesis of HT-PTs bearing amino and hydroxyl groups. [5] The hydroxylated PTs were further derivatized and used to synthesize, e.g., HT-PT±polystyrene diblock and triblock copolymers. [7] These copolymers self-assemble into nanowires, resulting in surprisingly high conductivities, even with relatively low percentages of HT-PT in the copolymer. While these materials have very interesting properties, their seven-step synthesis limits their ease of preparation and usefulness. To fully realize the potential of these materials, new convenient synthetic methods must be developed that are simple and highly reproducible. Herein, we report a new protocol for controlling the end-group composition of the polymer. This method is used for the simple synthesis of HT-PTs bearing a wide variety of functional end groups, including critical functionalities that will allow for the synthesis of conducting block copolymers in only three steps. The main objective of this work was to find a method to systematically functionalize the end groups of HT-PT using the simple Grignard metathesis polymerization (GRIM) [3] method developed by our group. Typical polymerization of 2,5-di-bromo-3-hexylthiophene 1, using the GRIM method, yields a polymer that has primarily one end-group composition, namely H/Br. Selective conversion of the Br terminated polymer to yield a functionalized end-capped PT can be accomplished by a post-polymerization functionalization reaction. Since the polymerization of 1 follows a chain-growth mechanism , [8] we believed that the simple addition of a Grignard reagent would terminate the polymer growth and give mono-capped HT-PTs. Indeed, we have found …