Rapid flow-based synthesis of poly(3-hexylthiophene) using 2-methyltetrahydrofuran as a bio-derived reaction solvent

We report the synthesis of poly(3-hexylthiophene) (P3HT) by Grignard metathesis (GRIM) polymerization using the bio-derived ‘green’ solvent 2-methyltetrahydrofuran (2-MeTHF). Using a standard flask-based reaction, the molecular weight distribution, regioregularity and product yield were found to be similar to those obtained under equivalent conditions using tetrahydrofuran (THF) as a reaction solvent. The synthesis was subsequently adapted to a novel “tube-in-shell” droplet-based flow reactor, using a newly developed high-solubility catalyst derived from nickel(II) bromide ethylene glycol dimethyl ether complex (Ni(dme)Br2) and 1,3-bis(diphenylphosphino)propane (dppp). Use of the new catalyst together with an increased reaction temperature of 65 °C (enabled by the higher boiling point of 2-MeTHF) resulted in an approximate four-fold increase in reaction rate compared to a standard THF-based synthesis at 55 °C, with full conversion reached within one minute. The purified flow-synthesized polymer had an Mw of 46 kg mol−1, a low PDI of 1.4, and a regioregularity of 93%, indicating the suitability of flow-based GRIM polymerization in 2-MeTHF for the high-throughput synthesis of high quality P3HT.