Homogeneous vinyl ester-based synthesis of different cellulose derivatives in 1-ethyl-3-methyl-imidazolium acetate

Abstract

A homogeneous acylation of cellulose with different vinyl esters in the biodegradable and less toxic ionic liquid 1-ethyl-3-methyl-imidazolium acetate ([EMIM]OAc) is described for the first time. The reaction proceeds in the absence of any additional catalyst and glucose- and cellulose-esters with chain lengths of C8 to C16 are accessible by using equimolar amounts of acyl donor. Cellulose esters with a degree of substitution (DS) in the range of 0.9-3.0 were synthesised successfully. Different reaction parameters like reaction time, temperature and amount of substrate were systematically changed and analysed by NMR, IR and HPLC-GPC. The highest DS was achieved at 80 °C and a reaction time of 2 hours. Taking into consideration the literature, the DS and degree of polymerisation (DP) of fatty acid chloride and vinyl ester-based synthesis routes were compared. Similar DS-values were obtained, but the DP was significantly reduced during the synthesis using fatty acid chlorides in [BMIM]Cl. As an undesirable side reaction, acetates from [EMIM]OAc are bound to the cellulose backbone. The quantity of bound acetate groups during vinyl ester-based synthesis rose with decreasing polarity of the substrates but overall proved to be much lower compared to the literature described anhydride or fatty acid chloride based synthesis routes in [EMIM]OAc. This novel process was extended by using further acyl donors like vinyl benzoate, pivalate and 2-ethylhexanoate to demonstrate the applicability of the vinyl ester-based cellulose modification in [EMIM]OAc. [EMIM]OAc was recycled with an efficiency of ∼90% and reused for subsequent syntheses.