Exploring borinane-based multi-ammonium salts for epoxide (co)polymerization: insights into the structure-activity relationship

The reactivity of bifunctional borinane-based mono-ammonium salts has previously been demonstrated in various polymerization processes, including ring-opening polymerization (ROP) of epoxides and copolymerization with CO₂ or anhydrides. In this study, three bifunctional borinane-based multi-ammonium salts (N⁺/B) B, C and D (B₈N₃(C₆)Br₃; B₈N₃(C₂)Br₃; B₁₀N₄(C₂)Br₄) were synthesized with varying B/N ratios and linker lengths between two ammoniums, along with a monoammonium bifunctional salt, catalyst A, B₃NBr, used as a reference. The polymerization activities of these catalysts which were essentially used as initiators were evaluated in ROP of propylene oxide (PO), epichlorohydrin (ECH), and glycidyl azide (GA), and ROCOP of PO and ECH with CO₂. Specifically, this work focused on the ROP of ECH, which exhibited temperature-dependent reactivity. Lower temperatures favored chain propagation and resulted in well-controlled polymerization behavior, while higher temperatures favored chain transfer reactions to the monomer resulting in low molar mass polymers. Catalysts A and B demonstrated comparable reactivities across all polymerizations, indicating that catalyst B, with a higher B/N ratio and spatially arranged ammoniums, may be an ideal candidate. Overall, the general trend of catalytic activity was observed to be A ≅ B > C > D. This study provides valuable insights into the design and synthesis of forthcoming bifunctional N⁺/B catalysts.