Highly efficient ring‐opening polymerization of tetrahydrofuran by anhydrous ferric chloride

Poly(tetramethylene ether glycol) (PTMG), widely used as a precursor in the fabrication of commodity polymers, is typically produced byacid‐catalyzed polymerization of tetrahydrofuran (THF). Herein, we report a detailed investigation of the cationic ring‐opening polymerization of THF catalyzed by ferric chloride (FeCl 3 ), which can be considered as a strong Lewisacid. The polymerization reactions were performed in the presence of acetic anhydride at 20°C with FeCl 3 , which readily produced poly(tetramethylene ether glycol diester) (PTMG_DE). A maximum yield of 79.2% was obtained within 30 min using a FeCl 3 to acetic anhydride molar ratio of 5:4. The resulting polymers generally exhibited low molecular weights and a narrow polydispersity index and could be easily converted into PTMG using aqueous NaOH. In contrast with the FeCl 3 ‐acetic anhydride system, other iron‐based catalysts such as FeCl 2 and FeCl 3 ·6H 2 O did not show any noticeable activity in the polymerization of THF. The proposed mechanism involves initiation of the acetyl cation generated by FeCl 3 , propagation by nucleophilic addition of THF, and termination by the acetate anion, accounting for the high activity of the FeCl 3 catalyst for THF polymerization. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136 , 47999.