Fundamental Studies on Preparation of Cellulose Oligomers with Narrow Molecular Weight Distribution

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A number of studies on the functionalization of cellulose have been made for a long time, however few on the functionalization of cellulose oligomers including cello-oligosaccharides. This is because these oligosaccharides are very expensive and hardly available. This research project concerns (i) the preparation of narrow-distributed cellulose oligomers with DP values ranging from 7 to 20 by using fractional precipitation and enzymatic degradation methods, and (ii) the functionality conversion of the reducing-end groups by taking advantage of their characteristic reactivity. The results are outlined below.1. Cellulose oligomers having a DP value of 10-20 and a polydispersity ratio (Mw/Mn) of 1.50 were prepared by homogeneous acid-hydrolysis of commercial cellulose triacetate (CTA) and the subsequent deacetylation, and used as a starting material. The extraction with hot DMSO (100。C) and the further fractional precipitation by hot DMSO/H_2O system afforded the relatively narrow-distributed cellulose oligomer (Mw/Mn=1.18) with a DP of 9.9 (yield 14%).2. The narrow-distributed cellulose oligomers (Mw/Mn=1.22-1.26) having a DP value of ca. 20 were obtained through enzymatic degradation with Cellulose T (Amano Pharmaceutical Co., Ltd.). It is worth noting that the degree of crystallinity of the degraded product became much higher than that of the starting cellulose, reflecting that the terminal non-crystalline region in the sample oligomers is preferentially degraded with celluose compared with the crystalline region.3. Cellulose tripropionate (CTP) was first depolymerized via acid-hydrolysis using HBr/CH_3COOH to obtain the end-brominated CTP oligomer. The subsequent glycosylation employing N-protected aminoalcohol as a glycosyl acceptor and the deprotection promoted with Pd-C/HCOONH_4 lead to the CTP oligomer with terminal primary amino function. ^1H-NMR and GPC analyzes revealed that the functionality of amino groups was close to unity.