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

This study presents a rapid and convenient approach for\nthe carbamation\nof starch through the microwave irradiation of its mixture with an\nexcess of urea. In this process, urea served not only as a dielectrically\nlossy material conducive to microwave heating but also as a solvent\nin its molten state and a source of isocyanic acid. The formation\nof starch carbamate was confirmed by the appearance of ν(CO)\nand ν(C–N) vibrations in the FTIR spectrum, along with\nthe detection of a carbamate carbonyl signal in the <sup>13</sup>C\nNMR spectrum. The resultant derivative, with a degree of substitution\nof 0.71, exhibited exceptional cold-water solubility, resistance to\nretrogradation, and cold solubility in organic solvents such as DMSO, <i>N,N</i>-DMF, and DMAc. Additionally, this microwave-assisted\ntechnique could be modified to include other agents with urea. For\nexample, introducing sodium dihydrogen orthophosphate to the starch-urea\nmixture led to simultaneous phosphorylation. Control experiments indicated\nthat this concurrent phosphorylation-carbamation introduced phosphodiester\nlinkages between the starch molecules in addition to carbamation,\nresulting in an absorbent material. This absorbent was capable of\nabsorbing about 2200% of distilled water, even in its crude, unpurified\nform. This easy-to-synthesize absorbent, particularly in its crude\nform, holds immense promise in agriculture for providing combined\nnitrogen and phosphorus supplementation along with water retention.