A Convenient and Highly Efficient Strategy for Esterification of Poly (γ-Glutamic Acid) with Alkyl Halides at Room Temperature

The presented work discusses the highly efficient esterification of poly (γ-glutamic acid) (γ-PGA) with alkyl halides at room temperature. The esterification reaction was completed within 3 h, and the prepared γ-PGA esters were obtained with excellent yields (98.6%) when 1,1,3,3-tetramethylguanidine (TMG) was used as a promoter. The influence of the amount of TMG, solvent, reaction conditions, and alkyl halides on the esterification reaction was examined. It was found that polar aprotic solvents, such as <i>N</i>-Methylpyrrolidone (NMP) and 1,3-Dimethyl-2-imidazolidinone (DMI), were favorable for the esterification. Non-polar or weakly polar solvents (i.e., dichloroethane, acetonitrile) were not favorable for the esterification. Water as a solvent had a negative effect on esterification. The reactivity of bromine halogenated compounds was higher than that of chlorine halogenated compounds but lower than that of iodine halogenated compounds. The structures of the prepared γ-PGA ester were confirmed by ¹H NMR and FT-IR spectroscopy. Thermal stability and hydrophobic properties of the resulting product were tested. The results showed that the prepared γ-PGA propyl ester had high thermal stability (up to 267 °C) and showed good hydrophobicity (contact angle 118.7°).