Thermally stable, highly fused quinoxaline composition and method of synthesis
United States Patent [1 1 Hedberg et al.  
 [ 1 Apr. 8, 1975 [75] Inventors: Frederick L. Hedberg, Dayton; Fred E. Arnold, Centcrville, both of Ohio [73] Assignee: The United States of America as represented by the Secretary of the Air Force, Washington, DC  
 [22] Filed: Apr. 17, 1974 [2]] Appl. No.: 461,689  
 [52] US. Cl...... 260/47 UA; 117/161 UN; 161/257; 260/30.4 N; 260/33.4 P; 260/33.8 UA;  
 [58] Field of Search C08g/15/00; 260/47 UA, 260/50. 65, 49. 47 R578 [56] References Cited UNITED STATES PATENTS 3,635,892 1/1972 Rabilloud et al. 260/47 CP 3.661.850 5/1972 Stille 260/50 3.792.017 2/1974 Arnold et a1 260/47 R 3,804,804 4/1974 Gerber et al 260/47 CP 3.804.807 4/1974 Duffy &#39;260/65 Primary Examiner-Lester L. Lee Attorney, Agent, or FirmHarry A. Herbert, Jr.; Cedric H. Kuhn [57] ABSTRACT Ouinoxaline polymers are synthesized by solution polycondensation of 2,2-bis(phenylethynyl)-5,5&#39;- diaminobenzidine with aromatic bis-benzils. Upon heating the polymers so prepared at a temperature ranging from about 240 to 300C., they undergo a thermal intramolecular cyclization reaction. thereby providing cured, thermally stable, highly fused quinoxaline compositions. Because of their solubility in aprotic solvents and the mechanism whereby they are cured. the quinoxaline polymer are particularly suitable for use in high temperature applications such as in the fabrication of plastic composites and protective coatings.  
 10 Claims, N0 Drawings THERMALLY STABLE. HIGHLY FUSED QUINOXALINE COMPOSITION AND METHOD OF SYNTHESIS RIGHTS OF THE GOVERNMENT The invention described herein may be manufactured and used by or for the Governmentof the United States for all governmental purposes without the payment of any royalty.  
 FIELD OF THE INVENTION This invention relates to quinoxaline polymers which after curing exhibit a high degree of thermal stability. In one aspect it relates to a method for synthesizing the quinoxaline polymers. In another aspect it relates to cured quinoxaline polymers which do not have a softening point below their decomposition temperature.  
 BACKGROUND OF THE INVENTION During the past several years. polymeric materials have been developed that possess heat resistance and strength properties found previously only in metals. In addition. the polymers are much lighter than metals. an important advantage where weight is a factor as in modern. high speed aerospace applications. By utilizing structure-property relationships. such as aromatic rings for thermal stability and aromatic-heterocyclic rings for adhesive adhesive and cohesive characteristics. it is possible to tailor polymer structures to provide desired end-use properties. such as strength. adhesiveness. elasticity. solvent-resistance. etc. While it may thus be possible to provide a suitable polymer system for a given application. the problem of processing the polymer into an end-use item must also be considered. The processing problem has probably been the most restric tive factor in limiting the use of high temperature resistant polymers.  
  To process a polymer into a composite structure. it is necessary to cause the polymer to flow in order to impregnate the reinforcing substrate and mold to the desired form. The lower the softening point (Tg) or the melting point (Tm) of a polymer. the easier it is to cause the polymer to flow. In general. a softening point of about 200C. or below is most desirable. While a composite fabricated with a polymer having a softening point of 200C. is suitable for use at 30C.. it will soften and lose its strength at temperatures approaching 200C. To render the composite suitable for use at temperatures greater than 200C. a method is required for subsequently raising the softening point of the polymer higher than the desired maximum use temperature. The  
 mer backbone.  
  There are three major disadvantages to the crosslinking method of cure. One disadvantage results from the evolution of volatiles from any type of cure in which a condensation reaction is used. Because of the volatiles evolution. voids are formed by entrapped gases. effectively weakening the composite structure. A second disadvantage derives from the brittleness which is in herent in a three dimensional network. The third disadvantage lies in the fact that the softening point is raised only as high as the cure temperature because of freezing in&#34; of the reactive sites when the polymer softening point reaches the cure temperature. In&#39;other words. the polymer begins&#39;to soften as the use temperature ap&#39; proaches the cure temperature.  
  It is an object of this invention. therefore. to provide quinoxaline polymers which can be converted to thermally stable. highly fused quinoxaline compositions by non-volatile. intramolecular cyclization.  
  Another object of the invention is to provide a method for synthesizing quinoxaline polymers.  
  A further object of the invention is to provide a method of curing quinoxaline polymers that is not subject to disadvantages of crosslinking procedures.  
  Still another object of the invention is to provide quinoxaline polymers which are soluble in aprotic solvents.  
  A still further object of the invention is to provide a new monomer for use in the synthesis of quinoxaline polymers.  
  Other objects and advantages of the invention will become apparent to those skilled in the art upon consideration of the accompanying disclosure.  
 mers which consist essentially of repeating units having the following formula:  
 E1 Ea o era (a o 2%) Continued @ocfl ilo ($0,1 1), D-(OED);  
 There are generally at least two. preferably at least about room temperature to 100C. for a period of four. of the recurring units of the polymers. In general. the number of recurring units is such that the polymers have an inherent viscosity of about 0.10 to 1.0 dl/g in chloroform at C.  
  In one embodiment, the instant invention resides in a method for preparing the quinoxaline polymer by reacting 2.2&#39;-bis (phenylethynyl)-5.5-diaminobenzidine with an aromatic bis-benzil. The condensation reaction involved is illustrated by the following equation:  
 Polymer of Formula I about 1 to 96 hours. It is critical that the temperature does not exceed 100C. in order to ensure that the acetylenic groups do not react prematurely. Upon completion of the reaction, the product is conveniently recovered by adding the reaction mixture to an alcohol, such as methanol, thereby precipitating the polymer from solution. After recovery of the polymer, as by filtration or decantation, it is washed with an alcohol and finally air dried or dried under reduced pressure. ln order to purify the polymer further, the foregoing procedure may be repeated one or more times, i.e., dissolution of the dried polymer in a solvent, precipitation of the polymer by adding the solution to an alcohol, separation of the precipitated polymer, and drying of the separated polymer.  
  In the synthesis of the quinoxaline polymers, the monomers are generally employed in equimolar amounts. While a small excess of one of the monomers is not detrimental to the condensation reaction, a considerable excess of one of the reactants results in the production of lower molecular weight products.  
  The bis-dibenzil of Formula II] used in the polycondensation reaction are well known compounds that are described in the literature. Examples of such compounds include p-bis(phenylglyoxylyl)benzene,  
 m-bis(phenylglyoxylyl)benzene,  
 m-bis(p&#39;-ethylphenylglyoxylyl)benzene, m-bis(p-methoxyphenylglyoxylyl)benzene,  
 m-bis( p-phenoxyphenylglyoxylyl )benzene.  
 m-bis[p&#34;-(p&#34;-phenoxy)phenoxyphenylglyoxylyl]- benzene.  
 phenoxy]phenoxy)phenoxyphenylgloxylyl]benzene. m-bisIp-(p&#34;-cyano)phenoxyphenylglyoxylyl]benzene. m-bis(p&#39;-biphenylglyoxylyl)benzene, m-bis(p&#39;-hydroxyphenylglyoxylyl)benzene. p.p&#39;-bis(phenylglyoxylyl)diphenylmethane, p,p&#39;-bis( phenylglyoxylyl )benzophenone. p,p&#39;-bis(phenylglyoxylyl)diphenyl sulfide. p,p-bis(phenylglyoxylyl)diphenyl sulfone. p,p-bis(phenylglyoxylyl)diphenyl ether. p,p&#39;-bis(p&#34;-bromophenylglyoxylyl)diphenyl and p,p&#39;-bis(p&#34;-cyanophenylglyoxylyl)diphenyl ether.  
  The tetraamine monomer, which is characterized by having pendant phenylethynyl groups, is a new compound. The presence of these pendant groups on the quinoxaline polymer chain provides an internal crosslinking mechanism whereby the fusible polymer can be cured by internal cyclization to an infusible polymer. Furthermore, the pendant groups afford a mechanism whereby the softening point of the cured polymer is raised without any evolution of by-products. The preparation of the 2,2&#39;-bis(&#39;phenylethynyl)-5,5&#39;- diaminobenzidine involves five synthetic reactions are described hereinafter in Example I.  
  Curing of the quinoxaline polymers is accomplished by heating them in an inert atmosphere at a temperature ranging from about 240 to 300C. A heating period of from about 1 to 2 hours is usually sufficient to obtain a complete cure although longer times, e.g., up to 24 hours, can be used. In the curing operation, cyclization of the pendant phenylethynyl moieties occurs, resulting in a cured polymer having a dibenzoanthracene structure according to the following formula:  
 an Q9 where R, and R are as indicated above.  
  A more complete understanding of the invention can be obtained by referring to the following illustrative examples which are not intended, however, to be unduly limitative of the invention.  
 EXAMPLE I Preparation of 5,5 &#39;diaminobenzidine ether,  
 2,2 &#39;-bis( phenylethynyl u. N,N-Diacetyl-2,2-diiodobenzidineSixteen grams (.036 mol) of 2.2&#39;-diiodobenzidine was added rapidly. with stirring to 160 ml. of acetic anhydride. The reaction mixture turned almost clear after which a white precipitate formed. The white suspension was stirred and refluxed for 1.5 hours after which it was permitted to cool to 23C.  
  The product. N,N-diacetyl-2,-2&#39;-diiodobenzidine. was isolated by filtration from two runs carried out as described above. Yields of 97 and 100 percent were obtained and the product showed a melting point of 300-30lC.  
 Analysis Calcd. for C,,;H l N- ,O- (percent): C. 36.94: H. 2.71;  
  1.48.80; N, 5.38 Found (percent): C, 36.67. 37.00: H. 3.06, 3.22: l,  
  48.88: N. 5.01. b. N.N&#39;-Diacetyl-2,2&#39;-diiodo-5,5&#39;-dinitrobenzidine The suspension of N,N&#39;-diacetyl-2.2-diiodobenzidine in acetic anhydride. prepared as described above. was cooled in a cooling bath to l0C. Twenty ml. of concentrated nitric acid was added dropwise to the suspension at a rate such that the temperature did not rise above 0C. After the addition was completed, the bath temperature was raised to 10C. and allowed to remain at that temperature for 20 hours. Shortly after the temperature of the reaction mixture reached 10C., an orange solution formed, and about 30 minutes later a yellow precipate began to come outof solution. At the end of the reaction period, the precipitate was recovered by filtration, washed three times with 100 ml. of ether and dried by suction. The yellow solid (7.8g) was purified by warming in 300 ml. of tetrahydrofuran until dissolved and then reprecipitating with 2 lbs. of ether. After recovery by filtration, washing 2 times with 100 ml. of ether and drying in a vacuum oven at 600C. and 1 mm of Hg for 4 hours, the product, N,N&#39;-diacetyl- 2,2&#39;-diiodo-5,5&#39;dinitrobenzidine, weighed 6.4g. (29% yield) and had a melting point of 296-298C. Analysis Calcd. for C ,;H, l N,O, (percent: C. 31.50; H, 1.98;  
 l, 41.60; N. 9.19; molecular weight 610 Found (percent): C, 31.64, 31.24; H, 1.94, 1.85; l,  
 40.34, 40.10; molecular weight 610 (by mass spectrometry).  
  c. 2,2&#39;-Diiodo-5,5&#39;-dinitrobenzidine A 16g. amount (.025 mol) of N,N&#39;-diacetyl-2,2&#39;-diiodo-5,5&#39;- dinitrobenzidine was dissolved in ml. of concentrated sulfuric acid. To this solution there was added 260 ml. of water, the addition being made slowly so as to keep the product in solution. After addition was completed, the solution was heated at 100C. for 1 hour. After cooling, the solution was added to 2 liters of water, forming a bright orange precipitate. The precipitate was recovered by filtration, washed three times with 500 ml of water, and then dried by suction. The precipitate was taken up in 500 m] of water and passed through a column of alumina, eluting with tetrahydrofuran. The tetrahydrofuran was evaporated, leaving an orange solid which was dried in a vacuum oven at I 600C. and 1 mm of Hg for 4 hours. About 12.1g. (91% yield) of the product, 2,2 &#39;-diiodo-5 ,5 dinitrobenzidine, having a melting point of 346-348C. was obtained.  
 Analysis Calcd. for C H I N O (percent): C, 27.40; H, 1.53;  
 T, 48.25; N, 10.28; molecular weight 526.  
  7 Found (percent): C. 28.08, 28.72; H, 1.56, 1.47; l. 49.76; N, 10.28; molecular weight 526 (by mass spectrometry).  
  d. 2.2&#39;-Bis(phenylethynyl.)-5.5 dinitrobenzidine A solution of 2.2&#39;-diiodo-5,5&#39;-dinitrobenzidine 1 1.5g, .021 mol) in 1.200 ml. of dry pyridine was stirred at 23C. for minutes while dry nitrogen was bubbled through the solution. The nitrogen flow was continued while copper phenylacetylide (120g. .073 mol) was added and the reaction mixture was heated to reflux and refluxed for 48 hours. The pyridine was removed by distillation at 1 mm of Hg with a rotary evaporator. The residue was extracted three times with 200 ml. of tetrahydrofuran and filtered. The filtrate was passed through a column of alumina. eluting with tetrahydrofuran until thin layer chromatographic (TLC) analysis of the eluate showed only red colored spots. The desired product appeared as an orange spot on TLC analysis with an R, value higher than that ofsubsequent red spots. The tetrahydrofuran eluate was concentrated to dryness on a rotary evaporator and the residue was stirred and refluxed for 15 minutes with benzene. The  
 l c c continued for minutes and then the reaction mixture was cooled and extracted three times with 100 ml. of chloroform. The combined chloroform extracts were concentrated to dryness on a rotary evaporator. The residue was dissolved in methylene chloride (20 ml.) and filtered after which the filtrate was poured with stirring into 20 ml. of hexane. The resultant precipitate was filtered, washed with hexane. and dried at 23C. in a vacuum oven at 1 mm. of Hg for 24 hours to give .313g. (76% yield) of 2,2-bis(phenylethynyl)-5.5- diaminobenzidine which softened above 100C. to a viscous melt. Analysis Calcd. for C2NH2-3N4 (percent): C, 81.14; H, 5.34; N.  
  13.52; molecular weight 414. Found (percent): C, 80.79. 80.93; H, 5.06, 5.13; N, 13.25. 13.25; molecular weight 455. 456 (by vapor phase osmometry in benzene).  
 EXAMPLE 11 Synthesis of:  
 benzene extracts were filtered hot and allowed to cool to give 1.25g. of orange crystals of 2.2- bis(phenylethyny1)-5,5-dinitrobenzidine (melting point 252253C.). Concentration of the mother 1iquor to 400 ml. gave an additional .86g of material (melting point 252253C.) The mother liquor from this second crop was chromatographed on a column of alumina. After elution of acetylenic impurities with benzene. the orange band containing the desired product was eluted with methylene chloride. The eluate was concentrated to 100 ml and then diluted to 600 ml.  
 with hexane. The resultant precipitate was filtered and washed with hexane to give .92g. of 2,2-  
 bis(pheny1ethynyl)-5,5&#39;-dinitrobenzidine (melting point 252-253C.) for a total yield of 3.03g. (31% Analysis Calcd. for C H N O (percent): C, 70.88; H, 3.82;  
 N, 11.81; molecular weight 474.  
 Found (percent) C, 70.63, 70.48; H, 3.66, 3.73; N,  
 11.80. 11.84; molecular weight 474 (by mass spectrometry.  
 e. 2,2&#39;-Bis(phenylethynyl)-5,5-diaminobenzidine solution of 2,2&#39;-bis(phenylethynyl)-5,5-  
 dinitrobenzidine (.474g., .00100 mol) in dioxane (50 ml., freshly distilled from FeSO, (3.65g., .024 mol),  
 was added to a boiling solution of Fe SQ, (3.65g., .024  
 mol) in water (50 ml.) which was stirred under nitrogen. After 5 minutes, ammonium hydroxide (20 ml.,  
 30%) was added slowly followed by 20 m1. more of dioxane. The resultant dark suspension was stirred and refluxed for 30 minutes at which point additional ammonium hydroxide (20 ml.) was added. Refluxing was Poly [2,2-(p-phenyleneoxy-p-phenylene)-6.6&#39;-bis(3- phenyl-7-phenylethynylquinoxaline A mixture of 2,2&#39;-bis(pheny1thyny1)-5,5&#39;- diaminobenzidine (.3 1 3g, .000755 mol) and p,p&#39;- bis(phenylgly0xylyl)diphenyl ether (.328g., .000755 mol) was stirred under a flow of dry nitrogen for 5 minutes. Thereafter. m-cresol (50 ml.) was added. The reaction mixture was stirred at 23C. for 24 hours and at 90C. for 18 hours. o-Phenylenediamine (.50g.) was added to end-cap and stirring was continued for 24 hours at 23C. The polymer was then precipitated by adding the reaction mixture to 500 m1. of methanol and purified by successive repricipitations from chloroform solution with methanol, ether and ethyl acatate. After drying at 60C. and 1 mm of Hg pressure for 24 hours, the polyl2.2-phenyleneoxy-p-phenylene-6,6&#39;-bis(3- phenyl-7-phenylethynylquinoxaline)] showed an inherent viscosity of 0.30 dl/g in chloroform at 30C. The polymer was soluble in methylene chloride, chloroform. tetrahydrofuran, dioxane, and m-cresol. Analysis Calcd. for C ,;H N 0 (percent): C, 86.57; H, 4.15;  
  N, 7.21. Found (percent): C, 85.05, 84,98; H, 3.84, 3.89; N,  
 EXAMPLE 111 A solution of the polymer of Example 11 was poured into a flat bottomed pan. The pan was heated at a temperature sufficient to evaporate the solvent. A flexible film having good physical properties was thereafter recovered from the pan.  
 {gig  
 EXAMPLE iv A sample of the polymer of Example II was subjected to differential thermal analysis (AT 20C/min). According to the analysis, there was a strong exotherm at 250C. upon heating from 23C. to 325C, An infrared spectrum of the polymer showed an intense acetylene band at 2,220 cm which diminished in a spectrum of a polymer sample which had been heated to 280C. under nitrogen. The acetylene band disappeared completely in a spectrum of a polymer sample which had been heated to 320C. under nitrogen No softening point was obtained for the polymer of Example II by either thermomechanical analysis or torsion braid analysis.  
 In the heating operations during which the polymer was cured, there was no evolution of by-products.  
 EXAMPLE V Synthesis of:  
 Poly phenyoxyphenyl]-7-phenyl-ethynylquinoxaline)1 A solution of I &#39;2,2-bis(phenylethynyl)-5,5&#39;- diaminobenzidine (.255 000615 mol)&#39;in m-cresol (50 ml.) was stirred at 23C. under dry nitrogen while adding m-bis(p-phenoxyphenylglyoxylyl) benzene (.324g., .000615 mol.). The reaction mixture was stirred for 96 hours at 23C. after which the polymer was precipitated by adding the reaction mixture to 1 liter of methanol. The precipitate was washed with methanol, reprecipitated twice with methanol from chloroform solution, and dried at 60CJand 1 mm of Hg pressure for 24 hours. The poly [2,2-(mphenylene)-6,6&#39;-bis(3-[p-phenoxyphenyl]-7-phenylethynylquinoxaline)] showed an inherent viscosity of .13 dl/g in chloroform at- 30C. Analysis Calcd. for C I-I ,;N O (percent): C, 85.69; H, 4.18;  
  N, 6.45 Found (percent): C, 85.12: H, 4.50; N, 5.83.  
 EXAMPLE VI A sample of the polymer of Example V was subjected to differential thermal analysis (AT 20C./min.). According to the analysis, there was a strong exotherm at 246C. upon heating from 23C. to 325C. Upon cooling to room temperature and recyclingto 400C, no exotherm was observed.  
  Thermomechanical analysis (AT 20C./min;&#39;expansion probe, 5g. load) showed a strong penetration indicative of glass transition (softening print) at 215C. and a weak penetration at 378C. upon &#39;heating from 23C. to 450C. A sample of the polymer which had been heated undennitrogen to 320C. showed only a weak penetratioii&#39;at 370C. upon heating from 23C. to 450C. in the thermomechanical analyzer.  
  An infrared spectrunrof the polymer of Example V showed an intense acetylene band at 2.210 cm&#34;. The  
 band diminished considerably in a spectrum of a poly- I 215C. Cyclization of the pendant phenylethynyl moieties to a dibenzoanthracene structure according to Formula IV, in which R is phenoxyphenyl and R is phenylene, occurred at 246C. with concurrent solidification. The cured polymer subsequently showed a new softening point at about 370C. The cured polymer of Example V had a structure according to Formula IV, in which R is p-phenoxyphenyl and R is m-phe nylene.  
  The data in Example IV show that the&#39;polymer of Example II in curing underwent an intramolecular cyclization of its pendant phenylethynyl groups at 260C. This occurred in the solid state since there was no prior softening of the polymer. The cured polymer had a structure according to Formula IV, in which R is phenyl and R is phenyleneoxyphenylene.  
  Quinoxaline polymers of this invention are unique in that the polymers can be cured to a polymer which can be used at temperatures above their cure temperature. Thus, the polymer of Example V can be molded into a laminate at about 215C. (The 215C. softening temperature would be decreased when molding under pressure.) The laminate is then cured at a temperature of about 246C., and the resultant cured laminate is useable at temperatures up to 370C. Quinoxaline polymers having the structure of the polymer of Example V are, therefore, the preferred polymers of this invention. Since the outstanding properties possessed by the polymer of Example V are believed to be attributable to its pendant phenoxyphenyl groups in combination with its pendant phenylethynyl groups, quinoxaline polymers are also preferred that have repeating units according to Formula I in which R is p-phenoxyphenyl.  
  As will be evident to those skilled in the art, modifications of the present invention can be made in view of the foregoing disclosure without departing from the spirit and scope of the invention.  
  1 l 12 We claim: 1. A quinoxaline polymer consisting essentially of repeating units having the following formula:  
  N I N l I W K in wherein R is m l 4 @f z s a K D&#39; 2 one (ane wherein R&#39; is@ Z g&#39;ji owe owe (as- 21a or me.  
 2. The polymer of claim 1 in which R is 5. The polymer of claim 1 in which R is 0 and R is and R is 3. The polymer of claim 1 in which R is i 6, The polymer of claim 1 in which R is and R is i 4. The polymer of claim 1 in which R is 0 7. A method for synthesizing a quinoxaline polymer ,5 which comprises reacting substantially equimolar and R s I amounts of 2,2-bis (phenylethynyl)-5,5&#39;-  
 . diaminobenzidine and an aromatic bis-benzil having D the following formula:  
 R E3 @1 1 e &#39;t @W @MEDWKQA 0 or H, and wherein is @IJ out) lson raw @5024? at a temperature ranging from about room temperature to 100C. for a period of about 1 to. 96 hours.  
  8. A thermally stable quinoxaline composition consisting essentially of repeating units having the following formula:  
  O I whereln N R I H [o R N O (a, m @ocn &#34;@-o@t @f f@)fl EN I I 0 or on and&#39;wherein R&#39; is apote, othe one pla e 0? 9. The quinoxuline composition of claim 8 in which 10. The qiiinoxaline composition of claim 8 in which R is R is &#34;r.- O T9 v5 k o and R is i A I lhd R is