Homogeneous Functionalization of Biopolymers Utilizing Ionic Liquids

Abstract

Recently there has been an increased interest in the utilization of natural polymers in order to move to a more sustainable economy. The natural polymers of interest are cellulose, a biopolymer consisting of repeating glucose units, and chitin, a biopolymer consisting of repeating N-acetylglucosamine units. Both polymers are currently used in a number of industrial areas, such as medical, textile, and food industries. Due to the strong inter- and intra- molecular interaction in the biopolymers, their dissolution has been limited, which has limited the degree to which they can be modified. Ionic liquids have opened the door to homogenous modification of biopolymers, which is leading to an increased degree of substitution and new modifications. Ionic liquids have been shown to dissolve various polymers, including cellulose and chitin.1-4 Their dissolution has opened up new avenues for the utilization of these biopolymers, including new composite materials and the homogenous modification mentioned previously. Heinze et al. have shown that using ionic liquids to dissolve cellulose and perform homogenous modification increases the degree of substitution (DS) for carboxymethylation and acylation reactions.5 , 6 An increased DS increases the effectiveness of the modification polymer. A modification of particular interest in our research is the addition of a chelating agent to the biopolymer, specifically the addition of the diethylaminoethyl functionality. Diethylaminoethyl-cellulose (DEAE-cellulose) is widely used as the stationary phase in ion exchange chromatography, where it functions as an anion exchanger. The addition of the DEAE branch to the cellulose backbone also increases its ability to chelate with metals in solution. The effectiveness of DEAE-cellulose to perform these functions is determined by its degree of substitution (DS). By increasing the DS, the effectiveness of the materials produced will be improved. Preliminary data, shown in figure 1, demonstrates in increase in the DS for the homogenous preparation of DEAE-cellulose in the ionic liquid, 1-butyl-3-methylimidazolium chloride ([C4mim][Cl]. There is an increase in the DS from 0.15 for the heterogeneous procedure to 0.35 for the homogenous procedure. The figure also shows the effect changes to the ionic liquid can have on the reaction. By changing the anion from chloride to acetate, there is a marked decrease in the DS of the cellulose. This research will explore the effects of various cations and anions on the homogenous modification of the biopolymers, cellulose and chitin. The DS for the modified biopolymers will be measured by titration and through FT-IR analysis. An investigation into the effectiveness of the modified polymers to remove various metals from aqueous solutions will also be discussed. (1) Swatloski, R. P.; Spear, S. K.; Holbrey, J. D.; Rogers, R. D. J Am Chem Soc 2002, 124, 4974. (2) Xie, H.; Zhang, S.; Li, S. Green Chem. 2006, 8, 630. (3) Qin, Y.; Lu, X.; Sun, N.; Rogers, R. D. Green Chem. 2010, 12, 968. (4) Wang, W.-T.; Zhu, J.; Wang, X.-L.; Huang, Y.; Wang, Y.-Z. J. Macromol. Sci., Part B: Phys. 2010, 49, 528. (5) Heinze, T.; Schwikal, K.; Barthel, S. Macromolecular Bioscience 2005, 5, 520. (6) Barthel, S.; Heinze, T. Green Chem. 2006, 8, 301. Figure 1