Silylcellulose from silylation/desilylation of cellullose in ammonia

Abstract

Silylation of cellulose with hexamethyldisilazane (HMDS) in liquid ammonia is described emphasizing mechanistic features. Conditions for complete as well as for stoichiometric partial silylation have been established which proved that the ratio of silylating agent and hydrogen bonding capability of the solvent are crucial for complete silylation. The heterogeneous reaction follows first order kinetics in the presence of saccharine as catalyst indicating that the accessability of hydroxy groups is not the rate determining factor. Stoichiometric desilylation of trimethylsilyl cellulose is also possible in tetrahydrofurane containing ammonia as hydrogen bond donor. Trimethylsilyl celluloses have a solubility window in THF with a DS from 1.6 to 2.6 which is narrower for higher molecular weight of the cellulose. Trimethylsilyl celluloses with DS above 2.7 show thermotropic behaviour.