Process for preparing aromatic polyimides, polyimides prepared thereby

A process for the preparation of a substantially cyclized polyimide of the formula:

• ##STR21## wherein Ar is a divalent aromatic oganic radical,
• Ar' is a tetravalent aromatic organic radical, the four carbonyl groups being attached directly to separate carbon atoms and each pair of carbonyl groups being attached to adjacent carbon atoms in the Ar' radical,
• A is NH.sub.2 --, ##STR22## where R" is defined hereinafter, A' is --R", --ArNH.sub.2, ##STR23## where R" is defined hereinafter, and n is 0 or a positive integer of at least one, soluble in a phenol which comprises:
• 1. reacting a reaction mixture of at least one tetracarboxylic acid dianhydride of the formula:
• ##STR24## wherein Ar' is as defined above, and at least one aromatic diamine of the formula:
• H.sub.2 N--Ar--NH.sub.2 (C)
• wherein Ar is as defined above, at a molar ratio of dianhydride to diamine in the range of m to m + 1:m to m + 1 where m is a positive integer of at least one and as high as n in formula (A), with the proviso that when the ratio of dianhydride to diamine is m + 1:m, the reaction mixture contains 0 or 2 moles of a monoamine of the formula R"NH.sub.2 wherein R" is a hydrocarbon radical of 1 to 12 carbon atoms which is saturated, has olefinic unsaturation or acetylenic unsaturation, and --CN, --CHO or --CH=NR" substituted derivatives thereof, ##STR25## where each R' is hydrogen or --CH.sub.3 in the presence of at least one organic azeotroping agent which:
• a. gives a water azeotrope having a boiling point less than 95.degree. C at atmospheric pressure,
• b. is non-reactive with the dianhydride and the diamine, and
• c. separates from water as a distinct phase, said reaction conducted at a temperature less than 140.degree. C and below the boiling point of said phenol and higher than the boiling point of said azeotroping agent with the vapor phase temperature being between that of the water azeotrope and no higher than 95.degree. C
• 2.