Pure, Transparent‐Melting Starch Esters: Synthesis and Characterization

Abstract

Long chain starch esters were prepared by a new method using molten imidazole as solvent for the biopolymer. The advantage is the simplicity of the reaction mixture. Imidazole is acting not only as solvent, but also as reagent and base. The reaction succeeds via the imidazolide, which is formed in situ with an acid chloride. It yields highly pure derivatives, as could be shown by NMR spectroscopy and elemental analysis. No hints for desoxychloro substituents or other impurities could be found. The high quality of the products prepared is responsible for the occurrence of colorless melts. Although DSC measurements show a variety of thermal transitions, the formation of melts in the range of 40 to 255 °C could be observed with a hot stage microscope. The melting behavior can be adjusted by the type of ester moiety and the amount of ester functions introduced. In case of starch palmitates completely transparent melts are obtained within two distinct DS regions namely around 1.5 and 2.2 to 3.0. Upon cooling the melts form homogeneous films on different supports including glass. They show good adhesion and should therefore be a suitable basic material for the preparation of composites like laminated glass.

Recent Advances in the Preparation and Characterization of Intermediately to Highly Esterified and Etherified Starches: A Review

“…Thermal behavior of a highly substituted starch follows the typical rheological solid at a “glass→rubber→melt”, then it degrades as temperature is increased . Depending on the nature of the substituents, degree of substitution, and presence of a plasticizer, the temperatures of these thermal transitions vary.…”

“…The chain length at which melting of associated side chain ceases has not been established. However, C 10 and C 12 esters, which are considered rather short fatty acid chains have been reported to exhibit a distinguishable side‐chain melting transition . Frequently, highly substituted starch esters of fatty acids have apparent “hot‐gel” properties but it is indeed the interplay of melted side‐chain and increased mobility of starch molecules …”

Catalytic Transesterification of Starch with Plant Oils: A Sustainable and Efficient Route to Fatty Acid Starch Esters

“…[11] The functional groups presenti nt riglycerides offer many possibilities for chemical modification,s uch as hydrolysis, transesterification, hydrogenation, metathesis, and many others. [12] Moreover,f attya cid-derived compounds such as fatty-acidc hlorides [13] vinyl esters, [14] or fatty-acid imidazolides [15] were employed for the chemical functionalization of starch, leading to different degrees of substitutions (DSs). Commonly,t he film-forming capacity of modified starches,e special-The transesterification of maize starch with olive oilo rh igh oleic sunflower oil was studied under homogeneous conditions in the presence of 1,5,7-triazabicyclo [4.4.0]dec-5-ene (TBD) as catalyst.…”

Engineered Polysaccharides: α‐1,3‐Glucan Acetates Showing Upper Critical Solution Temperature in Organic Solvents

“…These polymers are soluble in chloroform (no signals in the peak range of interest) and do not interact by means of hydrogen bonds. An intriguing observation was that perpropionylation of samples with DS Ac > 2.2 could not be achieved, which is in complete contrast to other polysaccharide ester systems like dextran, cellulose, and starch . It is assumed that the full accessibility of the hydroxyl groups is not given by a conformation of the glucan acetate in solution.…”