Conventionally, polyimides, which exhibit excellent heat resistance, mechanical properties, electrical properties, etc.,  have found many applications in various fields as molding materials, composite materials, electrical and electronic materials, etc.  In particular, biphenyltetracarboxylic acid type polyimides, having a variety of characteristics that are different from those of pyromellitic acid type polyimides, are highly useful and thus become targets for many studies and utilization.  Of these, a polyimide having the repeating structure units indicated by chemical formulas (4) to (6) below: shows thermoplasticity, and therefore can provide molded products of diverse shapes by means of melt molding such as compression molding, extrusion, injection molding, and the like.  As a result, utility thereof is extremely high (see G. L. Wilkes et al.,  Macromolecules, 30, pp.  1012 (1997); S. Tamai et al.,  Polymer, 37 (16) pp.  3683 (1996); S. Tamai et al.,  Polymer, 39 (10) pp.  1945 (1998), etc.). 
Polyimide molded products are normally manufactured by sinter molding of non-thermoplastic polyimide powder or by melt molding of thermoplastic polyimide powder.  For this reason, many processes have been developed for manufacturing polyimide powder. 
For example, Japanese Patent Laid-Open Nos.  4-142332 and 2000-1545 have disclosed processes for preparing pyromellitic acid type polyimides, which are made to yield polyimides by thermally imidizing in aprotic polar solvents.  Pyromellitic acid type polyimides exhibit low solubilities in solvents, and thus form and precipitate as the reaction proceeds, leading to easy yield of polyimide powder. 
However, as indicated in T. Nakano, 2nd Intern.  Conf.  on IP, etc.,  biphenyltetracarboxylic acid type polyimides have high solubilities in solvents, and thus the aforementioned processes were difficult to utilize.  In other words, the processes caused problems that include no precipitation of generated biphenyltetracarboxylic acid type polyimides or solidification of the reaction system because the products contain a large amount of solvents and thus swell even though the polyimides were attempted to prepare by heating in aprotic polar solvents. 
For example, in G. L. Wilkes et al.,  Macromolecules, 30, pp.  1012 (1997), heating in N-methyl-2-pyrrolidone, an aprotic polar solvent, gives a polyimide having the repeating structure unit as indicated by chemical formula (4) above.  In this case, while the polymer forms and precipitates as the reaction proceeds, the concentration of the polymer is made to be about 10% by weight during the reaction.  This is due to the fact that setting of the concentration during polymerization to be equal to or higher than this concentration causes the reaction system to be in the form of clay, making it impossible to agitate because the precipitated polyimides contain a large amount of solvent and thus swell. 
Also, in Japanese Patent Laid-Open Nos.  2000-103854 and 9-302097, etc.,  heating in cresol for the imidization reaction yields a polyimide having the repeating structure unit as shown in chemical formula (4) or (6) above.  In this case, since the formed polymer is dissolved in the solvent, the polymer is made to precipitate using a large quantity of poor solvent after the reaction. 
As discussed above, preparation of a biphenyltetracarboxylic acid type polyimide by thermally imidizing in a solvent conventionally included reaction at a low concentration or dilution with a poor solvent subsequent to reaction at a high concentration, and so the volume efficiency was extremely poor and thus the productivity was extremely low.  That is, for example, the use of a reaction vessel of a capacity of 1 m3 provides less than 100 kg of an obtained polyimide, in accordance with the process of G. L. Wilkes et al.,  or provides less than 20 kg of a polyimide according to Japanese Patent Laid-Open No.  2000-103854. 
Furthermore, conventional processes, wherein precipitated polyimides contain large amounts of solvents, require strenuous efforts for solvent removal by means of drying or the like. 
In addition, Japanese Patent Laid-Open No.  6-220194 entitled “Polyimide Solution Compositions and Manufacturing Processes Thereof” describes a “process for manufacturing a polyimide wherein polycondensation is conducted in the presence of a phenolic compound in the process wherein a dianhydride is made to directly react with an aromatic diamine in an organic polar solvent to imidization reaction.” 
This invention is dealing with a solution of a polyimide containing aliphatic compounds that are soluble in an organic polar solvent and the object thereof is to suppress side reactions during the thermal imidization reaction in the solvent. 
On the other hand, the process of preparing a polyimide of the present invention is capable of obtaining a slurry reaction solution by precipitating a polyimide during the imidization reaction, and the object thereof is to simply obtain a polyimide only by direct filtration of the reaction solution. 
Therefore, the present invention is completely different from the aforementioned publications in their target and object. 
Moreover, in the aforementioned publications, a phenolic compound is utilized as a catalyst for suppressing a side reaction and the amount by weight thereof is also {fraction (1/10)} to ½ that of an organic polar solvent (9 to 33% by weight of the total amount of solvents), which is extremely small as compared with the present invention. 
In addition, the aforementioned Japanese Patent Laid-Open No.  6-220194 does not disclose at all a process of preparing a biphenyltetracarboxylic acid type polyimide with high productivity, as will be described in the present invention, in which a nitrogen-containing cyclic compound indicated by chemical formula (1), discussed later, of organic polar solvents is strongly associated with a phenol, resulting in dissolution of the polyimide precursor (polyamide acid) and undissolution of polyimide, and an equimolar composition has an extremely high boiling temperature.  For example, the publication indicates that synthesis of a polyimide, including as a solvent polar solvents of N-methyl-2-pyrrolidone and a phenol (mixing molar ratio of N-methyl-2-pyrrolidone to phenol is 65.5/34.5), is carried out at a reaction temperature of 180° C., however, it does not describe specificity concerning a boiling point, etc.  for the mixed solvent of N-methyl-2-pyrrolidone and a phenol, nor does it indicate that the mixed solvent thereof exhibits a high boiling temperature and does not mix with water.  Additionally, this publication does not disclose that the use of an equimolar composition having such specific properties as a solvent in a specified amount permits a biphenyltetracarboxylic acid type polyimide to precipitate during the imidization reaction. 
Therefore, a process of preparing a biphenyltetracarboxylic acid type polyimide with high productivity has been demanded. 
That is, the present invention has been made to solve problems associated with the prior art described above and an object of the present invention is to provide a process of preparing a polyimide by means of simple, easy, cost-effective steps without losing various properties (molding processability, sliding property, low water absorbability, electrical property, thermal oxidation stability, radiation resistance) of polyimides derived from diverse structures thereof.