Synthesis and physicochemical characterization of polyoxymethylene dimethyl ethers

Polyoxymethylene dimethyl ethers (H₃CO(CH₂O)_nCH₃, PODE_n or DMM_n, n ≥ 2) with unique physical and chemical properties are a potential additive for diesel fuels, which can effectively enhance the combustion efficiency and reduce the emission of pollutants. In this work, a series of pure PODE_n components (n=2-5) were synthesized from methylal and trioxymethylene and obtained with high purity through collaborative separation; their structure and properties were characterized by NMR, FT-IR, Raman, and DFT calculation and a detailed assignment of the expressions in the spectrogram to the various groups was performed. The density and viscosity of PODE_n were measured at 298.15-323.15 K. The results indicate that the density and viscosity of PODE_n decrease gradually with the increase of temperature. Meanwhile, with the increase in the number of -CH₂O-units (n) from 2 to 5, the density, viscosity, flash point, pour point, and the heat of fusion and solidification of PODE_n are all increased. These results are valuable for the practical synthesis and application of PODE_n.