A green and highly efficient sulfur functionalization of starch

Abstract

In this study, sulfur functionalized starch has been successfully prepared, via an iso-thiouronium salt intermediate, from the reaction of thiourea with chloroacetylated starch. The later was synthesized by a chloroacetylation reaction. Thiolation via an iso-thiouronium ion intermediate is an innovative green approach to achieve thiol functionalization of starch with an innocuous low cost thiourea reagent. The thiol products are amenable for further derivatization and were modified to a sulfide and a sulphonium salt. The sulphonium structure was accomplished by exhaustive alkylation with iodoethane as an alkylating agent, which resulted in permanent charge generation on the starch backbone. The above-mentioned reactions were also carried out using microwave heating to achieve maximum yet uniform modification of starch. The extent of the derivatization processes was confirmed by iodine titration and precipitation titration and also by characterization of the synthesized samples by FTIR spectroscopy, ¹H-NMR, ¹³C-NMR, SEM-EDX, XRD, zeta potential measurements and elemental analysis (CHNS) to confirm the reaction output at every synthetic stage. Evidence that the degree of substitution of the thiol groups was ≥ 2 was obtained from titrimetry, SEM-EDX and elemental (CHNS) characterization results. Additionally, the generation of the permanent charge on the sulphonium structure was confirmed using zeta potential measurements.