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 alpha,omega-heterobifunctional polyethers with cleavable or modifiable alpha-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 (P5+RCOO-), as catalysts for the carboxylic acid-initiated ROP of epoxides. Promoted by the appropriate basicity of P5+RCOO- and electrostatic effects of loose cation-anion pairs, the P5+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 alpha-(carboxylic ester)-omega-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, H-1 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 alpha,omega-heterobifunctional polyethers that have the potential for imparting degradability and amphiphilicity properties to polyether-based materials.