Nanoporous Polyurea from a Triisocyanate and Boric Acid: A Paradigm of a General Reaction Pathway for Isocyanates and Mineral Acids

Isocyanates react with carboxylic acids and yield amides. As reported herewith, however, transferring that reaction to a range of mineral acids (anhydrous H ₃ BO ₃ , H ₃ PO ₄ , H ₃ PO ₃ , H ₂ SeO ₃ , H ₆ TeO ₆ , H ₅ IO ₆ , and H ₃ AuO ₃ ) yields urea. The model system for this study was a triisocyanate, tris(4-isocyanatophenyl)methane (TIPM), reacting with boric acid in DMF at room temperature, yielding nanoporous polyurea networks that were dried with supercritical fluid CO ₂ to robust aerogels (referred to as BPUA-XX). BPUA-XX were structurally (CHN, solid-state ¹³ C NMR) and nanoscopically (SEM, SAXS, N ₂ -sorption) identical to the reaction product of the same triisocyanate (TIPM) and water (referred to as PUA-yy). Minute differences were detected in the primary particle radius (6.2-7.5 nm for BPUA-XX versus 7.0-9.0 nm for PUA-yy), the micropore size within primary particles (6.0-8.5 Å for BPUA-XX versus 8.0-10 Å for PUA-yy), and the solid-state ¹⁵ N NMR whereas PUA-yy showed some dangling -NH ₂ . All data together were consistent with exhaustive reaction in the BPUA-XX case, bringing polymeric strands closer together. Residual boron in BPUA-XX aerogels was quantified with prompt gamma neutron activation analysis (PGNAA). It was found very low (≥0.05% w/w) and was shown to be primarily from B ₂ O ₃ (by ¹¹ B NMR). Thus, any mechanism for systematic incorporation of boric acid in the polymeric chain, by analogy to carboxylic acids, was ruled out. (In fact, it is shown mathematically that boron-terminated star polyurea from TIPM should contain ≥3.3% w/w B, irrespective of size.) Retrospectively, it was fortuitous that this work was conducted with aerogels, and the model system used H ₃ BO ₃ , whereas the byproduct, B ₂ O ₃ , could be removed easily from the porous network, leaving behind pure polyurea. With other mineral acids, results could have been misleading, because the corresponding oxides are insoluble and remain within the polymer (via skeletal density determinations and EDS). On the positive side, the latter is a convenient method for in situ doping robust porous polymeric networks with oxide or pure metal nanoparticles (Au in the case of H ₃ AuO ₃ ) for possible applications in catalysis.