Synthesis of High Molecular Weight Nylon 46 in Supercritical Carbon Dioxide

Nylon 46 with high molecular weight was successfully synthesized in supercritical carbon dioxide (sc-CO 2 ) by one-step three-stage polymerization using the salt prepared from 1,4-diaminobutane and 1,6-adipic acid as a monomer. The polymerization process was divided into three reaction stages: precondensation, postcondensation, and vacuum. To prepare high molecular weight nylon 46, the precondensation of the salts with pH over 7.23 was first carried out at 190 °C under 30 MPa for 2 h. Then CO 2 in the reactor was exhausted along with removing byproduct water, and the reactor was refilled with dry liquid CO 2 . Subsequently, the prepolymer formed was subjected to the postcondensation at 280 °C for 3 h under 15 MPa. Finally, the reactor was heated in vacuum at 270−280 °C for 30−45 min. The highest viscosity-average molecular weight of the nylon 46 sample synthesized from the above process is 48 200 g/mol. The chemical structure and thermal properties of the nylon 46 were characterized by Fourier transform infrared spectroscopy (FT-IR), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). The nylon 46 sample, as an example, has melting point of 310 °C, glass transition temperature of 67 °C, starting decomposition temperature of 400 °C, and maximum decomposition rate at 450 °C.