Chemical Oxidative Polymerization vs. Grignard Metathesis Polymerization of Electron-Rich Thiophene-Based Polymers
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Conductive polymers are long chains of organic molecules that contain a conjugated structure consisting of alternating single and double bonds. Introduction of an electric field causes these polymers to change color, volume, conductivity, reactivity, and solubility. This ability has led to applications including sensors, energy conversion and storage, actuators, and display technologies. Conductive polymers can be functionalized with disease-specific biomolecules, producing an electrochemical response in the presence of genetic markers for diseases such as cancer and Parkinson’s Disease. Identifying inherited predisposition for diseases leads to early detection, which increases success of treatment.
We have synthesized soluble, electron-rich conductive polymers derived from thiophene. Two monomers were synthesized: 3,4-bis(hexyloxy)thiophene (BHOT, a symmetrical thiophene to help prevent specific defects), and 2,5-dibromo-3,4-bis(hexyloxy)thiophene (Br2BHOT). These monomers were prepared, purified using column chromatography, and characterized using nuclear magnetic resonance (NMR) spectroscopy. The monomers were polymerized along with 3-hexylthiophene (3HT, a monomer used as a baseline standard for comparison) using various polymerization techniques. Both Grignard metathesis (GriM) and chemical oxidative polymerization methods were used to produce conductive polymers. Gel permeation chromatography (GPC) was used to compare molecular weights and degrees of polymerization of polymers obtained from both methods. The best results were obtained for both polymers when using standard addition (adding oxidant to monomer), 4 eq. of FeCl3, and 24 hours for chemical oxidative polymerization.
While the applications of conductive polymers are promising, soluble polymers with high molecular weights are needed to enable their use in most potential applications. As chain length/molecular weight increase in conductive polymers, the polymers become more mechanically robust and more conductive, making them more useful in energy storage or biosensors.