Homogeneous synthesis of hydroxypropyl guar gum in an ionic liquid 1-butyl-3-methylimidazolium chloride

With potassium hydroxide (KOH) as catalyst, hydroxypropyl guar gum (HPG) was successfully prepared under homogenous conditions in an ionic liquid 1-butyl-3-methylimidazolium chloride (BmimCl). The value of degree of molar substitution (MS) of HPG, which was determined by means of 1H NMR, was easily controlled by varying the mass ratio of propylene oxide (PO) to guar gum (GG). The distribution of hydroxypropyl moieties among the seven hydroxy groups in HPGs was investigated by NMR and the results indicated that the homogeneous reaction preferentially substituted at the C3OH of the mannosyl residues in guar gum which was quite different from the heterogeneous reaction where substitution mostly occurred at C6OH. The average number of hydroxypropyl units per OH substitution of HPGs with MS value of 0.20, 0.37 and 0.60 was 1.82, 1.42 and 1.76, respectively.

► We reported a method of homogeneous synthesis of hydroxypropyl guar gum.
► The value of degree of molar substitution was determined by means of 1H NMR.
► The distribution of the hydroxypropyl moieties was investigated by 13C NMR.
► The reaction showed a preference at the C3OH of the mannosyl residues.