3,699,075 United States Patent Office Patented Oct. 17, 1972 2 in preparing reinforced structures required the prepara 3,699,075 tion of polyimide prepolymers, i.e., the polyamide-acid SOLUBLE HGH MOLECULAR WEIGHT prepolymers which, when cured, results in the release of POLYMDE Hyman R. Lubowitz, Hawthorne, Calif., assignor to appreciable amounts of volatile material, due not only to TRW Inc., Redondo Beach, Calif. the evaporation of solvent, but also to the imidization re No Drawing. Filed Oct. 28, 1970, Ser. No. 84,935 action in the formation of the polymers. These cured Int, C. C08g 20/32 polymers, i.e., polyimides, are substantially insoluble in U.S. C. 260-49 3 Claims most organic solvents, and therefore could not be used for fabrication which required solutions for the preparations O of coatings, adhesives, films, fibers, reinforced composites, ABSTRACT OF THE DISCLOSUIRE and the like. This invention is directed to aromatic polycarboxylic Thus, most of the presently available polyimides are acids, the anhydrides and isomers thereof, and more par obtained from the lower-molecular weight precursors, i.e., ticularly, to the use of these acids and anhydrides for the prepolymers or amic-acids which are substantially soluble preparation of thermally stable, high-molecular weight 5 in most organic solvents. However, the prepolymers are polyamides. These polyamides are thermo-oxidatively highly susceptible to degradation due to oxidation, ther stable high-molecular weight polyimides which are com mal and hydrolytic conditions. Thus, it has been found paratively soluble in organic solvents. The polyimides are that many of these problems can be avoided by using a obtained by reacting approximately stoichiometric substantially soluble high-molecular weight polyimide in 20 accordance with this invention which contains thermally amounts of the polycarboxylic acid or the anhydrides stable phenoxyphenyl sulfone segments in the backbone of thereof with a polyfunctional compound selected from the polymer chain. These polyimides are particularly use the group consisting of polyamines, diisocyanates and ful in preparing solutions of polymers useful as sealants, combinations thereof. adhesives, fibers, films, coatings and composites for ad vanced aerospace structures, etc. By utilizing these novel acids or the anhydrides thereof, The invention described herein was made in perform it is possible to obtain polyimides which may be char ance of work under a NASA contract and is subject to the acterized as "stiff' polymers having aromatic and hetero provisions of Section 305 of the National Aeronautics and cyclic rings in the backbone. These "stiff’ polymeric ma Space Act of 1958, Public Law 85-568 (72 Statute 435: 30 terials were found to be stable under oxidative conditions 42 USC 2457). and to exhibit excellent dimensional stability at elevated This invention relates to thermally stable, high-molecu temperatures. While the polymeric materials of this in lar weight polyimides and to the use of novel acids and vention are cured by conventional means, the amount of the anhydrides for the preparation of thermally stable, volatile material released during the curing process is sub high-molecular weight polyimides. More specifically, this 35 stantially reduced due to the comparatively high-molecu invention relates to thermo-oxidatively stable high-mo lar weight of the monomers. Moreover, because the phen lecular weight polyimides which are highly soluble in yl groups of the acids are separated by thermally stable polar solvents. These polyimides may be used to provide linkages which also promote solubility, the fully cured tough, flexible, high-modulus resinous materials capable polyimide may be readily coated on various substrates, of being fabricated into high-performance films, fibers, 40 for example, with little difficulty. The preparation and coatings, reinforced composites, adhesives, etc. testing of glass-reinforced laminates, for example, ob The novel acids and anhydrides which are useful for tained from the polyimides of this invention showed the preparing the high-molecular weight polyimides may be polymers to have good wetting characteristics and pro characterized by the formula: duced products having a comparatively small percent of I. 45 voids as compared to the presently available materials. R. R. Thus, the acids and anhydrides thereof not only improve r t R. R. w the thermal and oxidative stability of the polyimides, but also provides a means for preparing otherwise difficult-to obtain useful objects. 4 R5 R5 RA 50 Accordingly, it is an object of this invention to provide wherein R1, R2, R3, R4 and R5 are either the same or polyimides obtained from aromatic polycarboxylic acids different, and are selected from the class consisting of hy and the anhydrides thereof which are oxidative and drogen atoms, carboxylic-acid groups and the anhydrides thermally stable at elevated temperatures. of said carboxylic-acid groups. For purposes of this in It is another object of this invention to provide poly vention, however, it is important that at least two of the 55 imides which are thermo-oxidatively stable and are com adjacent R groups in each phenyl group are acid groups paratively soluble in organic polar solvents. to give a tetracarboxylic-acid monomer or the dianhydride It is still a further object of this invention to provide thereof, e.g., bis(3,4-dicarboxyphenoxyphenyl) sulfone di soluble high-molecular weight polyimides which can be anhydride. converted to product forms having thermo-oxidative Generally, thermally stable resins are capable of being 60 stability, high mechanical properties and a long shelf-life fabricated into various products including laminates, ad in the varnish form. hesives, coatings, films, molded objects and the like and It is still a further object of this invention to provide are particularly useful in advance space-vehicles and high stable high-molecular weight polyimides which are soluble performance aircraft. Thus, thermally stable aromatic and in polar organic solvents, and therefore can be easily heterocylic materials have been investigated in recent 65 fabricated to various product forms. These and other years with considerable progress in attempts to obtain objects of the invention will become apparent from a thermally stable high-molecular weight polymers. How further and more detailed description of the invention. ever, a number of the presently available synthetic poly More specifically, this invention relates to the prepara mers have one or more deficiencies which limits their use tion of polyimides which are obtained by reacting approxi particularly in the fabrication of reinforced structures or mately stoichiometric amounts of at least one polyfunc the like. Heretofore, for example, the use of polyimides tional compound selected from the group consisting of di 3,699,075 3 4 amines, diisocyanates and combinations thereof with an about 145 C. in an atmosphere of nitrogen. The solution acid, the anhydride, and the isomers thereof having the was poured into ice water with stirring. The white solid formula: formed was washed with cold distilled water and filtered. ... R. R. The white solid was then dried in a vacuum at 90° C. and O R.1 R.: 5 then recrystallized twice in propanol. The product was R o–K X- i-K X-O- R foundC. The to product have a wasmelting analyzed point andof aboutfound 128.5° to have to the 130.0 fol R. R. 5 R4 lowing analysis: wherein R. R. R. R. and R5 are either the same or Theoretical calculations (C28H26OS)-Theory (per different and are selected from the class consisting of hy- 9 &R E. ygen, 5.72. Found (percent): drogen atoms, carboxylic-acid groups, the anhydrides and arbon, 72.93; hydrogen, 5.51. isomers of said acid groups. It is essential, however, in EXAMPLE II preparing the polyimides of this invention that on each phenyl group, and at least two of the adjacent R groups, A composition bis(3,4-dicarboxyphenoxyphenyl) sul i.e., k,R,R,R, and Rs are carboxylic acid groups or 15 fone was prepared by dissolying 45.9 parts by weight of the dianhydride thereof. the previously prepared bis(3,4-dimethylphenoxyphenyl) The acids and the anhydrides thereof may be prepared sulfone in a solution containing 500 ml. of pyridine and by various methods. For purposes of illustration, approxi- 100 ml. of water. The solution was heated until refluxed mately stoichiometric amounts of an alkali-metal, C1-Cs and then 63.2 parts by weight of potassium permanganate mono- or polyalkyl substituted phenoxide may be reacted 20 was added as was necessary to maintain the reflux tem with bis(chlorophenyl) sulfone in an organic solvent as perature. After the addition was completed, the suspen illustrated. sion was refluxed for two hours, filtered hot through a III. CH O C - > onCoNa -- c-(Cl )--(D-a-C - Cs C on-K D-o-( >{-->-enO fris The intermediate bis(3,4-dialkyl phenoxyphenyl) sulfone 35 diatomaceous earth filter bed, washed with hot pyridine, is heated in the presence of oxygen or a chemical oxidizing and then evaporated under a vacuum. The residue was agent to obtain the corresponding bis(dicarboxyphenoxy- dissolved in 500 ml. of 1.8 normal sodium hydroxide solu phenyl) sulfone from which the dianhydrides are derived tion. The filtrate from the basic solution was again as illustrated. oxidized and filtered using 63.2 parts by weight of potas W. O O Oe- - O Ho-i-KX-0-{ X--K X-0-{D-g-on -A O w O dO o^ O 1.No () It is obvious that the chemical structure of the final sium permanganate. The excess permanganate was de product will depend upon the particular reactants used stroyed with ethanol. The filtrate from this oxidation was initially in obtaining the intermediate sulfones. Thus, for cooled to 10 C. and acidified to a pH of 1 with 6 normal example, the 2,3-dialkylphenoxide or the 3,4-dialkylphen- hydrochloric acid.