Controlled Ring-Opening Polymerization of Epoxides Catalyzed by Metal-Free Phosphazenium Salts (P<sub>5</sub><sup>+</sup>): Using Carboxylic Acid as an Initiator to Prepare Esterified Polyethers
Xiaoyu Liu, Chunhuan Jiang, Chuanli Ren, Zhibo Li
Abstract
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.