It is highly desirable to develop simple organocatalysts for the controlled ring-opening alternating copolymerization (ROAC) of epoxides and cyclic anhydrides, leading to high molecular weight polyesters. Hence, a phosphazenium salt, namely tri[tris (dimethylamino)phosphoranylidenamino]phosphonium chloride (P₄⁺Cl⁻), is developed as a catalyst for the ROAC of epoxides and cyclic anhydrides. Surprisingly, the combination of P₄⁺Cl⁻ with a protonic initiator, such as 1,4-benzenedimethanol (BDM) exhibited high efficiency in the copolymerization of propylene oxide (PO) and phthalic anhydride (PA). This led to the production of polyester with an exceptional high molecular weight (M_n) of up to 126 kDa, which represented a rare example of poly(PO-alt-PA) with M_n surpassing 100 kDa. Note that the core P atom is trivalent status and the tris[tris(dimethylamino)] phosphoranyl group will share one proton in the P₄⁺Cl⁻ salt. Once combined with protonic species, the P₄⁺Cl⁻ will not only serve as a proton acceptor but also as a hydrogen bonding donor for the cyclic anhydrides. Therefore, it was assumed that the P₄⁺ plus proton served dual role in mimic of base/urea pair to effectively catalyze ROAC, which was supported by density functional theory (DFT) calculations.