Abstract
This review covers methods for modifying the structures of polysaccharides. The introduction of hydrophobic, acidic, basic, or other functionality into polysaccharide structures can alter the properties of materials based on these substances. The development of chemical methods to achieve this aim is an ongoing area of research that is expected to become more important as the emphasis on using renewable starting materials and sustainable processes increases in the future. The methods covered in this review include ester and ether formation using saccharide oxygen nucleophiles, including enzymatic reactions and aspects of regioselectivity; the introduction of heteroatomic nucleophiles into polysaccharide chains; the oxidation of polysaccharides, including oxidative glycol cleavage, chemical oxidation of primary alcohols to carboxylic acids, and enzymatic oxidation of primary alcohols to aldehydes; reactions of uronic-acid-based polysaccharides; nucleophilic reactions of the amines of chitosan; and the formation of unsaturated polysaccharide derivatives.
Highlights
With increasing oil prices and forecasts of a future lack of availability, renewable non-petrochemical-based alternatives to materials synthesis could become more important
The esterification of cellulose with various sulfonic acid chlorides, including the 2,4,6-trimethylbenzenesulfonyl group, under homogeneous conditions, was investigated in an attempt to improve the regioselectivity for substitution at O-6 [61], but in general, the products contained mixtures of 2- and 6-tosylation
In the early 1990s, Van Bekkum found that a homogeneous catalyst, TEMPO [i.e., (2,2,6,6-tetramethyl-piperidin1-yl) oxyl], could be used for the regioselective oxidation of the primary alcohols in polysaccharides to give the corresponding polyuronic acids with essentially complete conversion, (i.e., DSox ca 1.0) [125]
Summary
With increasing oil prices and forecasts of a future lack of availability, renewable non-petrochemical-based alternatives to materials synthesis could become more important. The simplest molecules consist of a monosaccharide repeating unit with hydroxyl groups as the only functional groups Examples of this type of structure include the very common polysaccharides, cellulose and amylose. Non-aqueous solvent mixtures that dissolve cellulose often consist of an organic liquid and an inorganic salt. Solvents for chitin include LiCl (5%)/DMA, LiCl/N-methyl-2-pyrrolidone, CaCl2/MeOH, and hexafluoroisopropyl alcohol Charged polysaccharides such as chitosan (which may be protonated on nitrogen) or polyuronates such as alginates (which can form carboxylate salts) will have very different solubility properties. Ionic liquids (room-temperature ionic liquids) are relatively new solvents that have found use in polysaccharide chemistry [7, 8] They can dissolve polysaccharides, including cellulose, hemicellulose, and wood, allowing derivatisation reactions to take place under homogeneous conditions. Volatile and distillable ionic liquids have been designed for polysaccharide derivatisation [10]
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