Thermally robust and highly active phosphazenium salt/Lewis acid catalyst for the ring-opening alternating copolymerization of epoxides and cyclic anhydrides

Abstract

Developing simple and highly efficient organocatalytic systems for the ring-opening alternating copolymerization (ROAC) of epoxides and cyclic anhydrides remains a significant challenge. Herein, a simple phosphazenium salt, specifically tetrakis[tris(dimethylamino)phosphoranylidenamino]phosphonium chloride (P₅⁺Cl⁻), combined with triethylborane (BEt₃) as a cocatalyst, has been demonstrated to be an efficient binary catalytic system for the ROAC of epoxides and cyclic anhydrides. Promoted by the bulky P₅⁺ cation and the electrostatic effects arising from loosely associated cation–anion pairs in P₅⁺Cl⁻, the P₅⁺Cl⁻/BEt₃ binary system exhibited excellent catalytic performance, achieving the maximum turnover frequency (TOF) of 9800 h⁻¹ at 180 °C in the ROAC of cyclohexene oxide (CHO) and phthalic anhydride (PA). Moreover, the ROAC proceeded in a controlled manner, which was supported by kinetic studies, nuclear magnetic resonance (¹H NMR) and gel permeation chromatography (GPC) spectra, and matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF MS) analysis. The methodology produced polyester with a high molecular weight (M_n) of up to 103 kDa, which offered a rare example of poly(PA-alt-CHO) exceeding 100 kDa. A series of well-defined polyesters were synthesized by the coupling of various epoxides (CHO, PO, BO, ECH, AGE and SO) with PA using the P₅⁺Cl⁻/BEt₃ catalytic system.