Efficient Synthesis of High Molecular Weight Semi-aromatic Polyamides with Biobased Furans over Metal-free Ionic Liquid

Abstract

Dimethyl furan-2,5-dicarboxylate (DMFDC) is a biobased monomer that can potentially serve as an alternative to terephthalic acid. It has been widely used in the production of biobased polyesters and polyamides. This study synthesized a series of PA4F with DMFDC and 1,4-butanediamine (BDA) through consecutive prepolymerization and melt polycondensation using ionic liquids (ILs) based on 1-butyl-3-methylimidazolium. The relationship between anions of ILs and the number-average molecular weight (Mn) of PA4F was systematically investigated, and the results show that [Bmim][H2PO4] has the best catalytic activity. PA4F with a Mn of 27 kg/mol catalyzed by it showed a high glass transition temperature of 151°C and elastic modulus of 3.7 GPa, which are the highest values reported to the best of our knowledge. NMR analysis and DFT calculations show that the strong interaction between [Bmim][H2PO4] and substrate facilitates the polymerization reaction. Furthermore, it was found that the pre-polymerization stage plays a crucial role in molecular weight growth. The hydrolysis of diester increases with the increase of the prepolymerization temperature and time, leading to a higher proportion of the furan dicarboxylic acid terminated segment in the oligomer, which greatly increases the possibility of decarboxylation and triamine formation in the polycondensation stage. Thus further limiting the molecular weight growth of PA4F. The impact of the pre-polymerization process was analyzed in detail by TG, DSC, ESI-MS, MALDI-TOF MS, and 2D NMR.