Heterogeneous reaction between cellulose and alkyl ketene dimer under solvent-free conditions

Four native celluloses, i.e. hardwood bleached kraft pulp (HBKP), cotton linters, bacterial cellulose and microcrystalline cellulose, were heterogeneously reacted with alkyl ketene dimer (AKD) at 100 °C for 24 h in the presence of a small amount of 1-methylimidazole used as an esterification promoter under solvent-free conditions. AKD molecules were reacted with cellulose hydroxyl groups to form β-ketoester linkages by the above conditions. Each reaction product was separated into chloroform-soluble and -insoluble fractions, and weight ratios and degrees of substitution (DS) of the two fractions were measured. The weight ratios of the CHCl₃-insoluble fractions varied from 30 to 95 % of the products, and the DS values from 0.03 to 1.6, depending on the starting cellulose samples. X-ray diffraction and solid-state ¹³C-NMR analyses of the CHCl₃-insoluble fraction with DS 0.75 in the AKD-treated HBKP indicated that β-ketoester substituents were introduced to hydroxyl groups on the cellulose microfibril surfaces. This CHCl₃-insoluble fraction had thermoplastic properties, and was convertible to translucent or transparent films by hot pressing at 160–180 °C.