Borane Lewis pair-mediated ring-opening polymerization of epoxides: direct access to α-acyl azide/ω-hydroxyl heterotelechelic polyethers and latent isocyanate-derived polyurethane

Abstract

Heterotelechelic polyethers bearing two orthogonally addressable end groups are attractive precursors for modular macromolecular construction, yet their direct synthesis remains challenging when one terminus contains a highly reactive latent functionality such as an acyl azide. Herein, we report a borane Lewis pair-mediated strategy for the controlled ring-opening polymerization (ROP) of propylene oxide (PO) using 4-hydroxymethylbenzoyl azide as a bifunctional initiator precursor. In the presence of the multiborane Lewis acid POSS-B8 and phosphazene base ^tBuP₂, polymerization proceeded under bulk conditions at 25 °C to afford α-acyl azide/ω-hydroxyl-terminated poly(propylene oxide)s (PPOs) with targeted molecular weights from 0.5 to 10.0 kg mol⁻¹. The acyl azide end group remained intact throughout polymerization, enabling the direct preparation of well-defined heterotelechelic polyethers without post-polymerization end-group modification. Subsequent thermolysis triggered Curtius rearrangement to generate the corresponding α-isocyanate/ω-hydroxyl PPOs, which underwent self-polyaddition to yield polyurethane materials without the use of external diisocyanates. This approach integrates controlled epoxide polymerization with latent end-group activation and provides a concise route to functional PPOs and isocyanate-free-feed polyurethane synthesis.