Single-Step Methylation of Chitosan Using Dimethyl Carbonate as a Green Methylating Agent

Abstract

N,N,N-Trimethyl chitosan (TMC) is one chitosan derivative that, because of its improved solubility, has been studied for industrial and pharmaceutic applications. Conventional methods for the synthesis of TMC involve the use of highly toxic and harmful reagents, such as methyl iodide and dimethyl sulfate (DMS). Although the methylation of dimethylated chitosan to TMC by dimethyl carbonate (DMC, a green and benign methylating agent) was reported recently, it involved a formaldehyde-based procedure. In this paper we report the single-step synthesis of TMC from chitosan using DMC in an ionic liquid. The TMC synthesised was characterised by ¹H NMR spectroscopy and a functionally meaningful degree of quaternisation of 9% was demonstrated after a 12-h reaction time.

Keywords: N-methylation; dimethyl carbonate; green methylating agents; ionic liquids; trimethyl chitosan.

Figure 1

Structures of chitin and chitosan.

Scheme 1

Two-step method reported by Wu et al. for the synthesis of TMC using formaldehyde and DMC as methylating agents [26].

Figure 2

¹H NMR spectrum of TMC prepared using methyl iodide as the methylating agent recorded in D₂O at 25 °C.

Figure 3

¹H NMR spectrum of TMC prepared in [bmim]Cl using DMC as the methylating agent recorded in DCl/D₂O at 25 °C.

Figure 4

Calculated degrees of monomethylation (DM, ), dimethylation (DD, ) and quaternisation (DQ, ), and total degree of substitution () for TMC prepared using DMC as the methylating agent with varying reaction times (0–12 h).