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Nanoporous Polyurea from Isocyanates Reacting with Mineral Acids

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*Abstract

While it is known that isocyanates react with carboxylic acid and yield amides, it is reported herewith that reaction of isocyanates with a range of anhydrous mineral acids, (H3BO3, H3PO4, H3PO3, H2SeO3, H6TeO6 and H5IO6) 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 to form nanoporous polyurea networks that were dried with supercritical fluid CO2 towards robust aerogels (referred to as BPUA-xx), which were structurally (CHN, solid-state 13C NMR) and nanoscopically (SEM, SAXS, N2-sorption) identical to the reaction product of the same isocyanate and water (referred to in this paper 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 of PUA-yy showing dangling –NH2. All data together were considered 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. It was found to be very low (≤0.05 % w/w) and was shown to be primarily B2O3 (11B NMR). Thus, any systematic mechanism for incorporation of boric acid by analogy to the reaction with carboxylic acids was ruled out. (In fact it is shown theoretically that boron-terminated star polyurea from a TIPM core should contain ≥3.3% w/w boron irrespective of size.) It was fortuitous that this work was focused on open-pore polymers (aerogels) and that the model system was based on H3BO3, in which the byproduct, B2O3, could be removed easily to leave behind pure polyurea. Had we focused on another mineral acid, results could have been misleading since the corresponding oxides are insoluble and remain within the polymer (via skeletal density determinations and EDS). In retrospect, however, the latter may prove to be a convenient method for in situ doping robust porous polymeric networks with oxide nanoparticles for possible applications in catalysis.

*Principal Investigator

Name: Nicholas Leventis, Associate Professor

Department:

Name: Chariklia Sotiriou-Leventis

Department:

Name: Malik Adnan Saeed, Grduate Student

Department:

Country/Region

USA