<p>The need for sustainable alternatives to petroleum-based polymers has driven the development of advanced catalysts for polyester synthesis. Here we present a series of covalently tethered borane–oxyanion organocatalysts for the ring-opening copolymerization of epoxides and cyclic anhydrides. These catalysts achieve outstanding efficiency, with turnover frequencies up to 13,500 h⁻¹ and high molecular weights (<i>M</i>_n) up to 174.0 kDa of the resultant polymers. Mechanistic studies reveal that intramolecular cooperation between borane and propagating species accelerates the rate-limiting epoxide ring-opening step, resulting in nearly equivalent energy barriers for epoxide and anhydride ring opening. Notably, the covalent tethering strategy not only enhances performance but also imparts remarkable air stability, addressing key limitations of conventional borane-based catalysts. Furthermore, our catalysts exhibit broad substrate scope and high thermal stability, facilitating the production of metal-free polyesters with tailored characteristics. This work establishes a sustainable and robust platform for polyester synthesis, with promising applications in biomaterials and packaging.</p><p></p>