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 (P 5 + Cl -), combined with triethylborane (Et 3 B) 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 P 5 + cation and the electrostatic effects arising from loosely associated cation-anion pairs in P 5 + Cl -, the P 5 + Cl -/Et 3 B binary system exhibited excellent catalytic performance, achieving the highest 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 ( 1 H 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 P 5 + Cl -/Et 3 B catalytic system.