Regulatory Mechanism of Ionic Liquids on the Structure and Molecular Weight of the Melamine-Formaldehyde Prepolymer: Experimental and Theoretical Insights

Melamine-formaldehyde (MF) is widely used in catalysis and flame-retardant materials. While ionic liquids (ILs) can regulate the molecular weight of melamine-formaldehyde prepolymer (pre-MF), their underlying mechanism remains unclear. In this work, the influence of ILs on pre-MF synthesis was investigated by experimental research studies and theoretical calculations. Pre-MF was synthesized in an aqueous solution of five different ILs, and structural characterization revealed that the anions of ILs predominantly govern the molecular weight distribution of pre-MF. Furthermore, the stability of pre-MF solution and the formaldehyde concentration during synthesis were strongly correlated with the nature of the anions of ILs. Theoretical calculations further indicated that strong hydrogen bond (H-bond) interactions exist between the ILs and pre-MF, with binding strengths following the order: 1-ethyl-3-methylimidazolium dimethyl phosphate > 1-ethyl-3-methylimidazolium diethyl phosphate ≫ 1-ethyl-3-methylimidazolium chloride > 1-allyl-3-methylimidazolium chloride > 1-butyl-3-methylimidazolium chloride. These findings collectively demonstrate that the regulatory effects of ILs on pre-MF synthesis are governed by the strength of H-bond interactions between IL anions and the amino groups and hydroxymethyl of pre-MF. Ultimately, the synthesized pre-MF was subsequently processed into MF fibers, which exhibited a tensile strength of 158 MPa, an elongation at break of 8.5%, and a limiting oxygen index of 37.5%. This work provides key insights for designing ILs-based systems for the controlled synthesis of pre-MF with tailored molecular weight characteristics.