| Nylon 46 has symmetrical molecular structure which is favorable to increase its crystallinity and melting point. Thus, nylon 46 exhibits excellent mechanical strength and good thermal resistance. Supercritical carbon dioxide (SC-CO2) is a green medium due to its inert, nontoxic, nonflammable and environmentally friendly properties. SC-CO2 can replaced the traditionan inorganic solvent in polymer synthesis which will pulleted the environment. In this paper, we try to find a route to prepare high molecular weight nylon 46 in SC-CO2.Nylon 46 was synthesized from the salt of 1,4-diaminobutane and 1,6-adipic acid in supercritical carbon dioxide (SC-CO2) by two-step polymerization. The process was divided to the precondensation and the post-condensation. To obtain high molecular weight nylon 46, nylon 46 salt with pH of 7.26 was used and the reaction conditions were investigated by orthogonal design. The optimum conditions are that the precondensation was firstly carried out at 190℃for 2 h, and then the post-condensation was performed at 280℃for 3 h, and the pressure of SC-CO2 in each step was 35 MPa. The sample obtained has the viscosity-average molecular weight of 25 900 g/mol. Further study showed that the molecular weight can be enhanced to 35 600 g/mol if the reaction system was evacuated for 1 h at the end of the post-condensation.The two step polymerization was developed to one-step three-stage polymerization. The polymerization process was divided into three reaction stages:precondensation, post-condensation and vacuum. To prepare high molecular weight nylon 46, the precondensation of the salts with pH of 7.26 was first carried out at 190℃under 30 MPa for 2 h. Then CO2 in the reactor was exhausted along with removing by-product water and the reactor was refilled with dry liquid CO2. Subsequently, the prepolymer formed was subjected to the post-condensation at 280℃for 3 h under 15 MPa. Finally, the reactor was heated in vacuum at 270-280℃for 45 min. The highest viscosity-average molecular weight of the nylon 46 sample synthesized from the above process is 48 200 g/mol.To remove by-product water and shift the reaction to the right, tetraethyl orthosilicate is used as dehydrant in the polycondensation system. Nylon 46 with high molecular weight of 53 400 g/mol was obtained and SiO2 with diameter less than 1 macrometers were also obtained.The chemical structure of the sample was characterized by fourier transform infrared spectroscopy (FT-IR) and the result reveals that nylon 46 has been successfully synthesized as it has all the usual bonds of polyamide. The thermal properties of nylon 46 were determined by differential scanning calorimeter (DSC) and thermogravimetric analysis (TGA). The nylon 46 sample has melting point of 310℃, glass transition temperature of 67℃, starting decomposition temperature of 400℃and maximum decomposition rate at 450℃.The kinetics of non-isothermal crystallization of nylon 46 was determined by DSC using the sample with molecular weight of 48 200 g/mol. Ozawa equation is liminted to describe the nonisothermal crystallization process of nylon 46 under the condition of this determination. Jeziorny equation showed that the nonisothermal crystallization process of nylon 46 is divided to two stages. Avrami exponent n for the primary stage and the secondly stage is 3-4 and about 1, respectively. The linear of the non-isothermal crystallization of nylon 46 described by Mo equation is fine and this results reveal that Mo method is an effective metnod. The cooling function (F(T)) of Mo equztion is range from 0.72-1.06 and the Mo exponent a is near 1.30.
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