Isocyanates react with carboxylic
acids and yield amides. As reported
herewith, however, transferring that reaction to a range of mineral
acids (anhydrous H3BO3, H3PO4, H3PO3, H2SeO3, H6TeO6, H5IO6, and
H3AuO3) 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 CO2 to robust aerogels (referred to as BPUA-xx). BPUA-xx were structurally (CHN, solid-state 13C
NMR) and nanoscopically (SEM, SAXS, N2-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 15N NMR whereas PUA-yy showed some dangling −NH2. 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 B2O3 (by 11B 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 H3BO3, whereas the byproduct,
B2O3, 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 H3AuO3) for possible applications in catalysis.
