Meerwein Type Ion Pair Catalyzed Cationic Ring‐Opening Copolymerization of Cyclic Acetals and Cyclic Anhydrides

Ring‑opening copolymerization (ROCOP) of readily available monomers such as cyclic acetals and cyclic anhydrides offers an atom‑economical alternative strategy, yet efficient catalytic systems enabling precise control over chain sequence and molecular weight remain scarce. Herein, we report the use of the organic Meerwein‑type ion pair [Me₃O]⁺[B(C₆F₅)₄]^- as a highly active, metal‑free catalyst for the cationic ROCOP of cyclic acetals with cyclic anhydrides. This method affords structurally diverse poly(ether‑ester)s with tunable AB/ABB‑type linkages (A = anhydride and B = formaldehyde). By varying polymerization time, temperature, and monomer feed ratio, the content of AB linkages could be precisely regulated from 58 mol% to a fully alternating architecture, while M_n reached up to 70.4 kDa. The scope of the methodology was extended to 3 cyclic acetals and 6 cyclic anhydrides, yielding a library of 14 well‑defined copolymers. This work provides a robust and versatile catalytic approach to sequence‑controlled, high M_n poly(ether‑ester)s, significantly enriching the synthetic toolbox for sustainable polymer materials.