Catalyst-free esterification of high amylose starch with maleic anhydride in 1-butyl-3-methylimidazolium chloride: The effect of amylose content on the degree of MA substitution
2020
Roy Goswami, Shrestha | Wang, Sen | Gnanasekar, Pitchaimari | Chauhan, Prashant | Yan, Ning
The limited reactivity of starch towards maleic anhydride (MA) affords maleate with a low degree of MA substitutions (CC and COOH groups). In this study, we investigated the relationship between the starch structure, controlled by its amylose (AM)/amylopectin (AP) ratio, and the DS of starch maleates using C₄[mim]Cl as the recyclable media, and catalyst. The results indicated that starches with varying AM/AP ratio produced maleates with comparable CC groups (DSNMR = 0.06–0.07). Following dissolution, the high amylose (DSₜᵢₜᵣₐₜᵢₒₙ = 1.17, yield = 69.2 %) and regular starches (DSₜᵢₜᵣₐₜᵢₒₙ = 1.17; yield = 59.3 %) produced high DSₜᵢₜᵣₐₜᵢₒₙ maleates (COOH groups) at MA/AGU ratio of 12:1 (80 °C, 10 min). Comparatively, DSₜᵢₜᵣₐₜᵢₒₙ value of waxy starch maleates (DSₜᵢₜᵣₐₜᵢₒₙ = 0.88, yield = 59.3 %) was lower than AM-based starches, possibly due to the crosslinking tendency of AP branches consisting of carboxylic end-groups. Interestingly, DSₜᵢₜᵣₐₜᵢₒₙ value for EHCS (1.17) ranged between its bulk (DSNMR: 0.06) and surface distribution of MA (DSSXPS 1.7); therefore, we considered it reliable for future reference.
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