High-Pressure Polycondensation of Salt Monomers

Fig. 6. Time dependence of inherent viscosity of polyimide P-12PM formed by the high-pressure polycondensation of salt monomer 12PMA under 220 MPa at 320 °C...

Then the reactivity of the tetracarboxylic acid-based salt monomers was compared with that of the salts consisting of tetracarboxylic acid half diesters. The P-XPM series polyimides were prepared by the high-pressure polycondensation of salt monomers XPME derived from the aliphatic diamines and pyromellitic acid half diethyl ester (see Eq. 5, Ar=PM and R=ethyl) [20], in addition to the polymers obtained from the pyromellitic acid-based salt monomer XPMA [24] already shown in Table 1. The polycondensation of salts XPME proceeded readily under high pressure of 250 MPa at 240 °C for 15 h, even with the elimination of ethanol and water as the by-products in the closed reaction vessel, and this afforded the polyimides with inherent viscosities up to 1.6 dL/g. Therefore, the reactivity of salt monomers XPME was found to be almost comparable to that of the parent salt XPMA, and the properties of the resultant P-XPM series polyimides from XPME were the same as those obtained from salts XPMA. 

In practice, the high-pressure polycondensation of the salt monomers producing polyimides was carried out by using a piston-cylinder type hot-pressing apparatus with the use of a Teflon capsule as a reaction vessel (see Eq. 5) [34]. 

The results of the high-pressure polycondensations of the aliphatic-aromatic salt monomers XPMA, XBPA, XTPE, XOPA, and XBTA under 200-600 MPa at 220-320°C for 3-30 h leading to the formation of a series of aliphatic polyimides (see Eq. 5) are summarized in Table 1 [22,24-26]. 

These salt monomers have been prepared as white crystalline solids by dissolving an equimolar amount of each individual diamine and tetracarboxylic acid half diester in hot ethanol (or methanol), and subsequently cooling the resultant solution. The author has been found that imidization (polycondensation) of salt run to polylmide for 10 min at 250 °C. The high-pressure polycondensation of the salt monomer has been applied. The pressure affect on the temperature and reaction time that directly afforded high molecular-weight polyimide. This method is useful for the s)mthesis of the polyimides having well defined structures, compared with the other S3mthetic methods [4]. 

Quite recently, we have investigated systematically the high-pressure polycondensation leading to the formation of a variety of polyimides, and polyben-zoxazoles as well [19,33]. Here we applied the salt monomer method to the high-pressure synthesis of aliphatic polyimides. 

Contrary to the high-pressure polycondensation, when the polycondensation of the salt monomers was conducted in a molten state under atmospheric or reduced pressure for the preparation of the polyimides having Tm below 300°C, this often led to the formation of crosslinked aliphatic polyimides that were insoluble even in concentrated sulfuric acid. Therefore, the high-pressure polycondensation process provides a simple and effective method for the synthesis of the linear polyimides with well-defined structures that caused high crystallinity, compared with the other synthetic methods. 

An aqueous solution of salt monomer 9PME was mixed with tetramethoxysi-lane, yielding a gel, which was then vacuum-dried forming a precursor powder composed of silica gel and the salt monomer. This was subjected to high-pressure polycondensation under 235 MPa at 230 °C for 5 h, producing polyimide-silica hybrid molding. By varying the ratio of tetramethoxysilane to the salt... 

In a typical experiment, a monomer or its salt (2 g) in a polar high boiling solvent (1-2 ml) was irradiated under nitrogen atmosphere. The microwave assisted polycondensation proceeded rapidly and was completed within 5 min (inherent viscosity around 0.5 dL/g) for the polyamide synthesis [68] and within 2 min for the polyimides (inherent viscosity above 0.5 dL/g) [69]. The rate of polycondensation of the salt monomers under various conditions decreased in the following series the microwave induced polycondensation>solid-state thermal polymeriza-tion>high-pressure thermal polycondensation [71]. 

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