Microwave-Assisted Carbamation and One-Pot Phosphorylation-Carbamation of Starch Using Molten Urea as a Reactive Solvent

This study presents a rapid and convenient approach for the carbamation of starch through the microwave irradiation of its mixture with an excess of urea. In this process, urea served not only as a dielectrically lossy material conducive to microwave heating but also as a solvent in its molten state and a source of isocyanic acid. The formation of starch carbamate was confirmed by the appearance of ν(C═O) and ν(C–N) vibrations in the FTIR spectrum, along with the detection of a carbamate carbonyl signal in the 13C NMR spectrum. The resultant derivative, with a degree of substitution of 0.71, exhibited exceptional cold-water solubility, resistance to retrogradation, and cold solubility in organic solvents such as DMSO, N,N-DMF, and DMAc. Additionally, this microwave-assisted technique could be modified to include other agents with urea. For example, introducing sodium dihydrogen orthophosphate to the starch-urea mixture led to simultaneous phosphorylation. Control experiments indicated that this concurrent phosphorylation-carbamation introduced phosphodiester linkages between the starch molecules in addition to carbamation, resulting in an absorbent material. This absorbent was capable of absorbing about 2200% of distilled water, even in its crude, unpurified form. This easy-to-synthesize absorbent, particularly in its crude form, holds immense promise in agriculture for providing combined nitrogen and phosphorus supplementation along with water retention.