Electrooxidative Polymerization of 3-Hexylthiophene in a Flow Microreactor

Abstract

Poly(3-hexylthiophene) (P3HT) is used in a number of high-technology applications including OLEDs, OFETs and OPVs. Conventional synthesis method of P3HT is generally based on cross-coupling reaction.1,2 However, this synthetic method possesses several demerits such as multi-step process, long reaction time, difficulty of continuous large-scale synthesis, and use of toxic organometallic catalysts. To overcome these drawbacks, a simple, toxic catalyst-free, and continuous-large scale synthetic process should be developed. Electrooxidative polymerization of aromatic compounds is useful method for preparation of the corresponding p-conjugated polymers via oxidative coupling of aromatic compounds. In addition, this method can be operated under mild conditions and controlled by switching on and off and is regarded as green process. However, rapid oxidative coupling between monomers is generally occurred in the electrooxidative polymerization in a conventional batch-type reactor. As a result, increase in the molecular weight is occurred in the drastic manner, and hence insoluble polymers are usually deposited on the anode surface. Therefore, conventional electrooxidative polymerization in a batch-type reactor is unsuitable for the soluble polymer synthesis and molecular weight control. To overcome this, we envisioned electrochemical synthesis of monodisperse and soluble p-conjugated polymer such as P3HT using a flow microreactor. Figure 1 shows schematic representation of the flow microreactor assembly prepared in the present work. The flow microreactor employed in this work had a simple geometry with working and auxiliary electrodes directly facing each other with 80 μm distance. After connecting Teflon tubing to inlets and outlet, the reactor was sealed with epoxy resin. Bulk electrolysis was conducted with a constant current and solution flowing through the electrolysis cell. The flow rate was controlled by using syringe pump, and products are rapidly ejected from the reactor. This system has advantages such as precise control of resident time in the reactor and high efficiency of electrode reaction due to high surface-to-volume ratio.3 In this work, the electrochemical synthesis of P3HT was carried out by using a batch-type reactor and a flow microreactor under the same electrolysis condition. As shown in Table 1, the conversion of monomer and monodispersity were improved by using a flow microreactor. In addition, an insoluble polymer film could be observed on the anode surface after the electrolysis using a batch-type reactor. In contrast, no polymer deposition was confirmed on the anode surface after the electrolysis using a flow microreactor. This can be ascribed to high surface-to-volume ratio and short residence time in the flow microreactor. In the presentation, influences of experimental conditions on the conversion of monomer and monodispersity P3HT will be also discussed.