Controlled Ring-Opening Polymerization of Epoxides Catalyzed by Metal-Free Phosphazenium Salts (P₅⁺): Using Carboxylic Acid as an Initiator to Prepare Esterified Polyethers

The applications of functional polyether materials include surfactants, lubricants, pharmaceuticals, and fundamental components in polyurethane products. The ring-opening polymerization (ROP) using carboxylic acid as an initiator offers enhanced pathways for synthesizing α,ω-heterobifunctional polyethers with cleavable or modifiable α-end junctions. The primary challenge lies in achieving a compatible balance between high efficiency and precise control over polymerization, as the ester groups generated during initiation steps may not tolerate highly basic catalysts. Herein, we present metal-free phosphazenium salts, namely, tetrakis [tris(dimethylamino) phosphoranylidenamino] phosphonium carboxylates (P 5 + RCOO – ), as catalysts for the carboxylic acid-initiated ROP of epoxides. Promoted by the appropriate basicity of P 5 + RCOO – and electrostatic effects of loose cation–anion pairs, the P 5 + RCOO – catalysts display high efficiency and controlled/living behavior even with a low catalyst loading. Moreover, the catalysts can be easily scaled up to the kilogram scale through simple procedures. Our strategy has successfully provided well-defined α-(carboxylic ester)-ω-hydroxy polyethers (PPO, PEO, and PBO) with controlled molecular weight, low dispersity, and high end-group fidelity. The effectiveness of the catalyst system is supported by kinetic investigations, 1 H NMR and size exclusion chromatography (SEC) spectra, and matrix-assisted laser desorption/ionization time-of-flight mass spectroscopy (MALDI-TOF-MS) analysis. This study provides a facile approach for synthesizing α,ω-heterobifunctional polyethers that have the potential for imparting degradability and amphiphilicity properties to polyether-based materials.