Abstract:
Novel polyimide copolymers containing ether linkages were prepared by the reaction of an equimolar amount of dianhydride and a combination of diamines. The polyimide copolymers described herein possess the unique features of low moisture uptake, dimensional stability, good mechanical properties, and moderate glass transition temperatures. These materials have potential application as encapsulants and interlayer dielectrics.

Description:
ORIGIN OF INVENTION 
     The invention described herein was jointly made by an employee of the United States Government and a research associate with the National Research Council and may be manufactured and used by or for the Government for governmental purposes without payment of any royalties thereon or therefor. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field of the Invention 
     This invention relates generally to polyimide random copolymers. In particular it relates to polyimide copolymers containing ether linkages. Polyimide copolymers were synthesized using an equimolar amount of dianhydride and a combination of diamines. Additionally, copolymers of controlled molecular weight were synthesized. 
     2. Description of Related Art 
     Polyimides comprise a large family of heterocyclic polymers which were first prepared in the late 1950&#39;s. Since then a large volume of information has been generated concerning the synthesis, characterization, use, and physical and mechanical properties of these polymers. Polyimides have been extensively evaluated for aerospace and microelectronic applications because of their excellent thermal stability, low density, chemical resistance, and their good electrical and mechanical properties. 
     Polyimides are the first choice as material candidates for dielectrics because of their successful use in printed circuit boards, and planarization properties. Additional properties necessary for dielectric applications include: ease of processing, good adhesion, chemical resistance, low moisture uptake, thermal stability, low CTE, and cost effectiveness  Rothman, L. B. J. Electrochem. Soc. 1980, 127, 2216!. 
     Polyimides must possess high heat resistance because device processing may require sealing, packaging, die bonding, wirebonding, and soldering. Thermal stresses occur if there is a mismatch in the CTE of the polyimide and the substrate, resulting in peeling and cracking. A low CTE minimizes these effects. Additionally, a low dielectric constant is required to minimize propagation delay, interconnect capacitance, and crosstalk between lines; this allows circuits to be run at a lower power input  Monk, D. J.; Soane, D. S. In Polymers for Electronic and Photonic Applications; Wong, C. P., Ed.; Academic Press, Inc., 1993; p 119-165!. The dielectric constant of commercial polyimide film changes with absorbed moisture, reducing its overall performance, so a material with low moisture uptake is desirable. 
     Polyimides containing 1,2-bis(4-aminoanilino)cyclobutene-3,4-dione (SAPPD) were developed because of their low CTE. The first reports of a polyimide utilizing SAPPD appeared in 1993  St. Clair, T. L. U.S. Pat. No. 5,212,283, 1993 and Croall, C. I.; St. Clair, T. L.; Yumino, Y.; Mutoh, H.; and Ito, Y., &#34;Polyimides Containing Squaric Acid Derivatives&#34; presented at the Symposium on Recent Advances in Polyimides and Other High Performance Polymers, Reno/Sparks, Nev., Jan. 18-21, 1993!. The main focus of the patent was linear polyimides containing the cyclobutene-3,4-dione moiety. These polyimides exhibited glass transition temperatures greater than 500° C. and adhered tenaciously to glass. The main focus of the presentation was polyimides containing the cyclobutene-3,4-dione moiety for space applications. Several polyimides were synthesized and subsequently exposed to 1 MeV electrons. Mechanical properties were evaluated before and after electron radiation exposure. Polyimides prepared with 1,2-bis(4-aminoanilino)cyclobutene-3,4-dione (SAPPD) possess relatively low coefficients of thermal expansion but have somewhat lower thermal stability than commercial polyimide film. 
     Although these polyimides prepared with SAPPD have an extremely low CTE, their high glass transition temperatures are undesirable. Also, these polymers have further undesirable characteristics such as low thermal stability, outgasing problems and lack of flexibility. Traditionally, polyimides with low CTE&#39;s also have high glass transition temperatures. For the microelectronics industry, low CTE polymers are required which also possess good dimensional stability with low glass transition temperatures to allow for processing. Typically, low CTE polymers are either crystalline or rigid, and thus not processable. By the present invention, copolyimides were developed that have both low CTEs, as well as, low glass transition temperatures. These copolyimides contain ether linkages and may include the 1,2-bis(4-aminoanilino)cyclobutene-3,4-dione diamine. 
     SUMMARY OF THE INVENTION 
     A primary object of the invention is to provide new polyimide copolymers containing ether linkages which are useful as encapsulants and interlayer dielectrics. Another object is to provide new polyimide copolymers which have moderate glass transition temperatures, good mechanical properties, low moisture absorption, low coefficients of thermal expansion, and high thermal stability. 
     Novel polyimide copolymers were prepared using a equimolar amount of dianhydride and a combination of diamines. Polyimide copolymers utilized either (1) hydroquinone diether dianhydride (HQDEA) and a combination of 4,4&#39;-oxydianiline (4,4&#39;-ODA) and 1,2-bis(4-aminoanilino)cyclobutene-3,4-dione (SAPPD); (2) oxydiphthalic dianhydride (ODPA) and a combination of 3,4&#39;-oxydianiline (3,4&#39;-ODA) and 1,2-bis(4-aminoanilino)cyclobutene-3,4-dione (SAPPD); (3) oxydiphthalic dianhyrdride (ODPA) and a combination of 3,4&#39;-oxydianiline (3,4&#39;-ODA) and p-phenylene diamine (p-PDA) or (4) benzophenone tetracarboxylic dianhydride (BTDA) and a combination of 4,4&#39;-oxydianiline (4,4&#39;-ODA) and p-phenylene diamine (p-PDA). These reactions were carried out in a polar solvent, such as N,N-dimethylacetamide (DMAc) or N-methylpyrollidone (NMP) at room temperature (RT) under nitrogen to give high molecular weight poly(amide acid) solutions. In some cases, the molecular weight was controlled by offsetting stoichiometry and endcapping. The poly(amide acid) solutions were cast as films and thermally cured to achieve cyclization to the polyimide. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preparation of dimensionally stable polyimide copolymers according to the present invention involves the reaction of two diamines in a solvent with an aromatic dianhydride according to the following ##STR1## Tables 1-4 summarize the polyimide copolymer compositions (i.e. 0-100 molar percent diamine), inherent viscosity data (η inh ), glass transition temperatures (T g ), weight loss profiles (TGA), and coefficients of thermal expansion (CTE) for the polyimide copolymers. Tables 5-8 summarize the tensile strengths, moduli, and elongations for the polyimide copolymers. Tables 9-12 summarize the moisture absorption data of selected copolymers. 
     
                                           TABLE 1__________________________________________________________________________Characterization Of HQDEA + 4,4&#39;-ODA:SAPPD Copolymers ##STR2##Ar ##STR3##            ##STR4##                               T.sub.g by                                    T.sub.g by                                         TGA,    CTE,molar %         molar %        η.sub.inh, dL/g                               DSC, °C.                                    TMA, °C.                                         10% wt. loss,                                                 ppm/°C.__________________________________________________________________________0               100            2.72 ND   ND   407     ND50              50             2.35 ND   277  470     35.560              40             1.45 254  274  474     37.070              30             1.03 271  258  496     39.975              25             1.42 266  256  503     38.990              10             1.18 251  244  516     42.6100             0              0.93 240  231  525     40.0__________________________________________________________________________ ND = not detected 
    
     
                                           TABLE 2__________________________________________________________________________Characterization Of ODPA + 3,4&#39;-ODA:SAPPD Copolymers ##STR5##Ar ##STR6##             ##STR7##                                 T.sub.g by                                      T.sub.g by                                            TGA,     CTE,molar %          molar %        η.sub.inh, dL/g                                 DSC, °C.                                      TMA, °C.                                            10% wt. Loss,                                                     ppm/°C.__________________________________________________________________________0                100            1.12  ND   ND    422      -2 to 650               50             1.14  ND   ND    456      30.875               25             1.19  ND   261   503      34.280               20             1.23  252  254   505      36.090               10             1.30  249  250   518      38.5100              0              1.34  238  237   511      39.5__________________________________________________________________________ ND = not detected 
    
     
                                           TABLE 3__________________________________________________________________________Characterization Of ODPA + 3,4&#39;-ODA:p-PDA Copolymers ##STR8##Ar ##STR9##            ##STR10##molar %         molar % η.sub.inh, dL/g                        T.sub.g by DSC, °C.                               T.sub.g by TMA, °C.                                       TGA, 10% wt. Loss,                                                  CTE,__________________________________________________________________________                                                  ppm/°C.0               100     0.88 ND     350     533        26.050              50      1.08 265    301     514        28.755              45      0.73 259    256     516        28.160              40      1.28 251    255     527        33.065              35      0.89 257    249     525        32.670              30      0.95 257    247     518        34.475              25      1.32 248    245     516        37.390              10      0.70 248    243     528        37.9100             0       1.34 238    237     511        39.5__________________________________________________________________________ ND = not detected 
    
     
                                           TABLE 4__________________________________________________________________________Characterization Of BTDA + 4,4&#39;-ODA:p-PDA Copolymers ##STR11##Ar ##STR12##            ##STR13##molar %         molar % η.sub.inh, dL/g                        T.sub.g by DSC, °C.                               T.sub.g by TMA, °C.                                       TGA, 10% wt. Loss,                                                  CTE,__________________________________________________________________________                                                  ppm/°C.0               100     1.21 ND     341     516        16.650              50      1.49 ND     357     501        24.875              25      1.38 292    355     512        31.990              10      1.21 277    276     519        32.9100             0       1.22 273    271     516        35.4__________________________________________________________________________ ND = not detected due to decomposition or outgassing of the polymer sampl 
    
     
                                           TABLE 5__________________________________________________________________________Mechanical Properties Of HQDEA + 4,4-ODA:SAPPDCopolymers ##STR14##Ar ##STR15##            ##STR16##molar %         molar %        Tensile strength, ksi                                   Tensile modulus,                                            Elong.,__________________________________________________________________________                                            %0               100            21.4     666.4    7.850              50             17.6     456.1    11.360              40             17.2     387.2    11.170              30             15.9     385.2    10.775              25             17.8     387.6    42.790              10             16.6     376.9    42.2100             0              23.8     370.2    93.1__________________________________________________________________________ 
    
     
                                           TABLE 6__________________________________________________________________________Mechanical Properties Of ODPA + 3,4&#39;-ODA:SAPPD Copolymers ##STR17##Ar ##STR18##            ##STR19##molar %         molar %        Tensile strength, ksi                                   Tensile modulus,                                            Elong.,__________________________________________________________________________                                            %0               100            25.9     873.3    6.450              50             18.8     601.4    4.275              25             18.5     494.4    9.680              20             20.7     463.2    10.690              10             18.6     439.0    63.4100             0              16.9     421.5    28.1__________________________________________________________________________ 
    
     
                                           TABLE 7__________________________________________________________________________Mechanical Properties Of ODPA + 3,4&#39;-ODA:p-PDA Copolymers ##STR20##Ar ##STR21##            ##STR22##molar %         molar % Tensile strength, ksi                            Tensile modulus, ksi                                     Elong., %__________________________________________________________________________0               100     30.0     832.3    14.750              50      21.5     534.4    6.555              45      21.6     531.1    21.860              40      21.1     505.6    27.665              35      19.3     418.2    51.670              30      19.2     456.1    9.275              25      18.9     371.9    47.890              10      17.9     363.9    33.1100             0       16.9     421.5    28.1__________________________________________________________________________ 
    
     
                                           TABLE 8__________________________________________________________________________Tensile Properties Of BTDA + 4,4&#39;-ODA:p-PDA Copolymers ##STR23##Ar ##STR24##            ##STR25##molar %         molar % Tensile strength, ksi                            Tensile modulus, ksi                                     Elong., %__________________________________________________________________________0               100     32.2     786.4    27.050              50      25.4     588.5    17.675              25      23.6     509.3    33.690              10      21.3     468.1    24.0100             0       22.3     426.6    49.5__________________________________________________________________________ 
    
     
                       TABLE 9______________________________________Moisture Uptake Of HQDEA + 4,4&#39;-ODA:SAPPD Copolymers           Moisture UptakePolyimide       (w/o)______________________________________HQDEA/SAPPD     2.3650:50           1.8870:30           1.4275:25           0.85HQDEA/4,4&#39;-ODA  0.83______________________________________ 
    
     
                       TABLE 10______________________________________Moisture Uptake of ODPA + 3,4&#39;-ODA:SAPPD Copolymers          Moisture UptakePolyimide      (w/o)______________________________________ODPA/SAPPD     3.0780:20          0.88ODPA/3,4&#39;-ODA  0.96______________________________________ 
    
     
                       TABLE 11______________________________________Moisture Uptake of ODPA + 3,4&#39;-ODA:p-ODA Copolymers          Moisture          UptakePolyimide      (w/o)______________________________________ODPA/3,4&#39;-ODA  0.9650:50          2.7455:45          1.4460:40          2.4265:35          1.5070:30          1.7275:25          1.0590:10          1.30ODPA/p-PDA     1.22______________________________________ 
    
     
                       TABLE 12______________________________________Moisture Uptake of BTDA + 4,4&#39;-ODA:p-PDA Copolymers          Moisture          UptakePolymer        (w/o)______________________________________BTDA/4,4&#39;-ODA  1.3450:50          2.4675:25          1.9190:10          1.35BTDA/p-PDA     2.12______________________________________ 
    
     Solubility of HQDEA+4,4&#39;-DDA:SAPPD and ODPA+3,4&#39;-ODA:SAPPD films was evaluated in NMP, DMAc, diglyme, DMF, and chloroform at intervals of 3 hr, 1 day, 3 days, and 5 days using a 1% solids concentration in a closed vial. Visual identification determined if the polyimides were soluble, partially soluble, or insoluble. Noted in the evaluations were discoloration of the solvent, swelling of the polymer, or other changes in the polymer film. Films in the HQDEA+4,4&#39;-ODA:SAPPD series were insoluble. Films in the ODPA+3,4&#39;-ODA:SAPPD series were insoluble. 
     The following specific examples are provided for purpose of illustration, and do not serve to limit the invention. 
     EXAMPLES 
     Example 1 
     Reaction of oxydiphthalic dianhydride (ODPA) with 3,4-oxydianiline (3,4&#39;-ODA) and p-phenylene diamine (p-PDA). 
     Into a 100 ml, three-neck round bottom flask equipped with mechanical stirrer, nitrogen gas inlet, and drying tube containing calcium carbonate was placed p-phenylene diamine (p-PDA) (0.3104 g, 0.00287 mol), 3,4&#39;-oxydianiline (3,4&#39;-ODA) (1.0673 g, 0.00533 mol), and DMAc (20 g). The solution was stirred until the diamines dissolved. To this solution was added oxydiphthalic dianhydride (ODPA) (2.5438 g, 0.0082 mol) and DMAc (16.1808 g). The final solution concentration was 9.78% solids. The solution was stirred at room temperature (RT) overnight under a nitrogen atmosphere. A 0.5% solution of poly(amide acid) in DMAc had an inherent viscosity of 0.89 dL/g measured at 25° C. Thin films were cast from the poly(amide acid) solution and thermally cured by heating for one hour at 100°, 200°, and 300° C. in a forced air oven. The thin film exhibited a DSC of 257° C. and 10% weight loss by TGA occurred at 525° C. The film exhibited a mean tensile strength of 19.3 ksi, and a modulus and elongation of 418 ksi and 51.6% respectively, at room temperature. The coefficient of thermal expansion was 32.6 ppm/°C. Moisture uptake by weight was 1.50%. 
     Example 2 
     By the same method and conditions as described in Example 1 of the present invention, 2.5438 g of oxydiphthalic dianhydride (ODPA) was added to 0.8210 g of 3,4&#39;-oxydianiline (3,4&#39;-ODA) and 0.4434 g of p-phenylene diamine (p-PDA) dissolved in 36.6577 g of dimethylacetamide (DMAC) to form a poly(amide acid) resin having an inherent viscosity of 1.08 dL/g. Resulting polyimide films of ODPA+50:50 3,4&#39;-ODA/p-PDA exhibited a T g  by DSC of 265° C. and 10% weight loss by TGA occurred at 514° C. The film exhibited a mean tensile strength of 21.5 ksi, and a modulus and elongation of 534 ksi and 6.5% respectively, at room temperature. The coefficient of thermal expansion was 28.7 ppm/°C. 
     Example 3 
     By the same method and conditions as described in Example 1 of the present invention, 2.5438 g of oxydiphthalic dianhydride (ODPA) was added to 1.2315 g of 3,4&#39;-oxydianiline (3,4&#39;-ODA) and 0.2217 g of p-phenylene diamine (p-PDA) dissolved in 45.0169 g of dimethylacetamide (DMAc) to form a poly(amide acid) resin having an inherent viscosity of 1.32 dL/g. Resulting polyimide films of ODPA+75:25 3,4&#39;-ODA/p-PDA exhibited a T g  by DSC of 248° C. and 10% weight loss by TGA occurred at 516° C. The film exhibited a mean tensile strength of 17.9 ksi, and a modulus and elongation of 364 ksi and 33.1% respectively, at room temperature. The coefficient of thermal expansion was 37.3 ppm/°C. 
     Example 4 
     By the same method and conditions as described in Example 1 of the present invention, 2.5438 g of oxydiphthalic dianhydride (ODPA) was added to 1.4778 g of 3,4&#39;-oxydianiline (3,4&#39;-ODA) and 0.0887 g of p-phenylene diamine (p-PDA) dissolved in 36.6489 g of dimethylacetamide (DMAc) to form a poly(amide acid) resin having an inherent viscosity of 0.70 dL/g. Resulting polyimide films of ODPA+90:10 3,4&#39;-ODA/p-PDA exhibited a T g  by DSC of 248° C. and 10% weight loss by TGA occurred at 528° C. The film exhibited a mean tensile strength of 18.9 ksi, and a modulus and elongation of 372 ksi and 47.8% respectively, at room temperature. The coefficient of thermal expansion was 37.9 ppm/°C. 
     Example 5 
     By the same method and conditions as described in Example 1 of the present invention, 2.5438 g of oxydiphthalic dianhydride (ODPA) was added to 1.1494 g of 3,4&#39;-oxydianiline (3,4&#39;-ODA) and 0.2660 g of p-phenylene diamine (p-PDA) dissolved in 24.3098 g of dimethylacetamide (DMAc) to form a poly(amide acid) resin having an inherent viscosity of 0.95 dL/g. Resulting polyimide films of ODPA+70:30 3,4&#39;-ODA/p-PDA exhibited a T g  by DSC of 257° C and 10% weight loss by TGA occurred at 518° C. The film exhibited a mean tensile strength of 19.2 ksi, and a modulus and elongation of 456 ksi and 9.2% respectively, at RT. The coefficient of thermal expansion was 34.4 ppm/°C. 
     Example 6 
     By the same method and conditions as described in Example 1 of the present invention, 2.5438 g of oxydiphthalic dianhydride (ODPA) was added to 0.9031 g of 3,4&#39;-oxydianiline (3,4&#39;-ODA) and 0.3991 g of p-phenylene diamine (p-PDA) dissolved in 27.8800 9 of dimethylacetamide (DMAC) to form a poly(amide acid) resin having an inherent viscosity of 0.73 dL/g. Resulting polyimide films of ODPA+55:45 3,4&#39;-ODA/p-PDA exhibited a T g  by DSC of 259° C. and 10% weight loss by TGA occurred at 516° C. The film exhibited a mean tensile strength of 21.6 ksi, and a modulus and elongation of 531 ksi and 21.8% respectively, at RT. The coefficient of thermal expansion was 28.1 ppm/°C. 
     Example 7 
     By the same method and conditions as described in Example 1 of the present invention, 2.4818 g of oxydiphthalic dianhydride (ODPA) was added to 0.9612 g of 3,4&#39;-oxydianiline (3,4&#39;-ODA) and 0.3461 g of p-phenylene diamine (p-PDA) dissolved in 33.2751 g of dimethylacetamide (DMAc) to form a poly(amide acid) resin having an inherent viscosity of 1.28 dL/g. Resulting polyimide films of ODPA+60:40 3,4&#39;-ODA/p-PDA exhibited a T g  by DSC of 251° C. and 10% weight loss by TGA occurred at 527° C. The film exhibited a mean tensile strength of 21.1 ksi, and a modulus and elongation of 506 ksi and 27.6% respectively, at RT. The coefficient of thermal expansion was 33.0 ppm/°C. 
     Example 8 
     Reaction of benzophenone tetracarboxylic dianhydride (BTDA) with 4,4-oxydianiline (4,4&#39;-ODA) and p-phenylene diamine (p-PDA). 
     Into a 100 ml, three-neck round bottom flask equipped with mechanical stirrer, nitrogen gas inlet, and drying tube containing calcium carbonate was placed 4,4-oxydianiline (4,4&#39;-ODA) (1.0813 g, 0.0054 mol), p-phenylene diamine (p-PDA) (0.1947 g, 0.0018 mol), and DMAc (15 g). The solution was stirred until the diamines dissolved. To this solution was added benzophenone tetracarboxylic dianhydride (BTDA) (2.3201 g, 0.0072 mol) and DMAc (16.1393 g). The final solution concentration was 10.35% solids. The solution was stirred at RT overnight under a nitrogen atmosphere. A 0.5% solution of poly(amide acid) in DMAc had an inherent viscosity of 1.38 dL/g measured at 25° C. Thin films were cast from the poly(amide acid) solution and thermally cured by heating for one hour at 100°, 200°, and 300° C. in a forced air oven. The thin film exhibited a T g  by DSC of 277° C. and 10% weight loss by TGA occurred at 519° C. The film exhibited a mean tensile strength of 21.3 ksi, and a modulus and elongation of 468 ksi and 24.0% respectively, at RT. The coefficient of thermal expansion was 32.9 ppm/°C. Moisture uptake by weight was 1.38%. 
     Example 9 
     By the same method and conditions as described in Example 8 of the present invention, 2.3201 g of benzophenone tetracarboxylic dianhydride (BTDA) was added to 0.7209 g of 4,4&#39;-oxydianiline (4,4&#39;-ODA) and 0.3893 g of p-phenylene diamine (p-PDA) dissolved in 29.2856 g of dimethylacetamide (DMAc) to form a poly(amide acid) resin having an inherent viscosity of 1.49 dL/g. Resulting polyimide films of BTDA+50:50 4,4&#39;-ODA/p-PDA exhibited a T g  by TMA of 357° C. and 10% weight loss by TGA occurred at 501° C. The film exhibited a mean tensile strength of 25.4 ksi, and a modulus and elongation of 589 ksi and 17.6% respectively, at RT. The coefficient of thermal expansion was 24.8 ppm/°C. 
     Example 10 
     By the same method and conditions as described in Example 8 of the present invention, 2.3201 g of benzophenone tetracarboxylic dianhydride (BTDA) was added to 1.2976 g of 4,4&#39;-oxydianiline (4,4&#39;-ODA) and 0.0779 g of p-phenylene diamine (p-PDA) dissolved in 31.2252 g of dimethylacetamide (DMAC) to form a poly(amide acid) resin having an inherent viscosity of 1.21 dL/g. Resulting polyimide films of BTDA+90:10 4,4&#39;-ODA/p-PDA exhibited a T g  by TMA of 276° C. and 10% weight loss by TGA occurred at 519° C. The film exhibited a mean tensile strength of 21.3 ksi, and a modulus and elongation of 468 ksi and 24.0% respectively, at RT. The coefficient of thermal expansion was 32.9 ppm/°C. 
     Example 11 
     Reaction of hydroquinone diether dianhydride (HQDEA) with 1,2-bis(4-aminoanilino)cyclobutene-3,4-dione (SAPPD) and 4,4-oxydianiline (4,4&#39;-ODA). 
     Into a 100 ml, three-neck round bottom flask equipped with mechanical stirrer, nitrogen gas inlet, and drying tube containing calcium carbonate was placed 1,2-bis(4-aminoanilino)cyclobutene-3,4-dione (SAPPD) (0.7505 g, 0.00255 mol), 4,4&#39;-oxydianiline (4,4&#39;-ODA) (1.1914 g, 0.00595 mol), and DMAc (10 g). The solution was stirred until the diamines dissolved. To this solution was added hydroquinone diether dianhydride (3.4197 g, 0.0085 mol) and DMAc (40.7709 g). The final solution concentration was 9.55% solids. The solution was stirred at room temperature overnight under a nitrogen atmosphere. A 0.5% solution of poly(amide acid) in DMAc at 25° C. had an inherent viscosity of 1.03 dL/g. Thin films were cast from the poly(amide acid) solution and thermally cured by heating for one hour at 100°, 200°, and 300° C. in a forced air oven. The thin film exhibited a T g  of 271 ° C. by DSC and 10% weight loss by TGA occurred at 496° C. The film exhibited a mean tensile strength of 15.9 ksi, and a modulus and elongation of 386 ksi and 10.7% respectively, at room temperature. The coefficient of thermal expansion was 39.9 ppm/°C. Moisture uptake by weight was 1.42%. 
     Example 12 
     By the same method and conditions described in Example 11 of the present invention, 3.2588 g of hydroquinone diether dianhydride (HQDEA) was added to 0.8110 g of 4,4&#39;-oxydianiline (4,4&#39;-ODA) and 1.1920 g of 1,2-bis(4-aminoanilino)cyclobutene-3,4-dione (SAPPD) dissolved in 65.7458 g of dimethylacetamide (DMAc) to form a poly(amide acid) resin having an inherent viscosity of 2.35 dL/g. Resulting polyimide films of HQDEA+50:50 4,4&#39;-ODA/SAPPD exhibited a T g  by TMA of 277° C. and 10% weight loss by TGA occurred at 470° C. The film exhibited a mean tensile strength of 17.6 ksi, and a modulus and elongation of 456 ksi and 11.3% respectively, at RT. The coefficient of thermal expansion was 35.5 ppm/°C. Moisture uptake by weight was 1.88%. 
     Example 13 
     By the same method and conditions described in Example 11 of the present invention, 2.0116 g of hydroquinone diether dianhydride (HQDEA) was added to 0.7509 g of 4,4&#39;-oxydianiline (4,4&#39;-ODA) and 0.3679 g of 1,2-bis(4-aminoanilino)cyclobutene-3,4-dione (SAPPD) dissolved in 25.6263 g of dimethylacetamide (DMAc) to form a poly(amide acid) resin having an inherent viscosity of 1.42 dL/g. Resulting polyimide films of HQDEA+75:25 4,4&#39;-ODA/SAPPD exhibited a T g  of 266° C. by DSC and 10% weight loss by TGA occurred at 503° C. The film exhibited a mean tensile strength of 17.8 ksi, and a modulus and elongation of 388 ksi and 42.7% respectively, at RT. The coefficient of thermal expansion was 38.9 ppm/°C. Moisture uptake by weight was 0.85%. 
     Example 14 
     By the same method and conditions described in Example 11 of the present invention, 3.5002 g of hydroquinone diether dianhydride (HQDEA) was added to 1.5679 g of 4,4&#39;-oxydianiline (4,4&#39;-ODA) and 0.2561 g of 1,2-bis(4-aminoanilino)cyclobutene-3,4-dione (SAPPD) dissolved in 53.5329 g of dimethylacetamide (DMAc) to form a poly(amide acid) resin having an inherent viscosity of 1.18 dL/g. Resulting polyimide films of HQDEA+90:10 4,4&#39;-ODA/SAPPD exhibited a T g  by DSC of 251° C. and 10% weight loss by TGA occurred at 516° C. The film exhibited a mean tensile strength of 16.6 ksi, and a modulus and elongation of 377 ksi and 42.2% respectively, at RT. The coefficient of thermal expansion was 42.6 ppm/°C. 
     Example 15 
     By the same method and conditions described in Example 11 of the present invention, 3.4197 g of hydroquinone diether dianhydride (HQDEA) was added to 1.0212 g of 4,4&#39;-oxydianiline (4,4&#39;-ODA) and 1.007 g of 1,2-bis(4-aminoanilino)cyclobutene-3,4-dione (SAPPD) dissolved in 71.6206 g of dimethylacetamide (DMAc) to form a poly(amide acid) resin having an inherent viscosity of 1.45 dL/g. Resulting polyimide films of HQDEA+60:40 4,4&#39;-ODA/SAPPD exhibited a T g  by DSC of 254° C. and 10% weight loss by TGA occurred at 474° C. The film exhibited a mean tensile strength of 17.2 ksi, and a modulus and elongation of 387 ksi and 11.1% respectively, at RT. The coefficient of thermal expansion was 37.0 ppm/°C. 
     Example 16 
     By the same method and conditions described in Example 11 of the present invention, 3.0576 g of hydroquinone diether dianhydride (HQDEA) was added to 2.2386 g of 1,2-bis(4-aminoanilino)cyclobutene-3,4-dione (SAPPD) dissolved in 78.0760 g of dimethylacetamide (DMAc) to form a poly(amide acid) resin having an inherent viscosity of 2.72 dL/g. Resulting polyimide films of HQDEA+SAPPD did not exhibit a T g  by DSC and 10% weight loss by TGA occurred at 407° C. The film exhibited a mean tensile strength of 21.4 ksi, and a modulus and elongation of 666 ksi and 7.8% respectively, at room temperature. The coefficient of thermal expansion was not detected. Moisture uptake by weight was 2.36%. 
     Example 17 
     Reaction of oxydiphthalic dianhydride (ODPA) with 1,2-bis(4-aminoanilino)cyclobutene-3,4-dione (SAPPD) and 3,4-oxydianiline (3,4&#39;-ODA). 
     Into a 100 ml, three-neck round bottom flask equipped with mechanical stirrer, nitrogen gas inlet, and drying tube containing calcium carbonate was placed 1,2-bis(4-aminoanilino)cyclobutene-3,4-dione (SAPPD) (0.5651 g, 0.00192 mol), 3,4&#39;-oxydianiline (3,4&#39;-ODA) (1.5379 g, 0.00768 mol), and DMAc (20 g). The solution was stirred until the diamines dissolved. To this solution was added oxydiphthalic anhydride (ODPA) (2.9781 g, 0.0096 mol) and DMAc (33.5457 g). The final solution concentration was 9.49% solids. The solution was stirred at RT overnight under a nitrogen atmosphere. A 0.5% solution of poly(amide acid) in DMAc had an inherent viscosity of 1.23 dL/g measured at 25° C. Thin films were cast from the poly(amide acid) solution and thermally cured by heating for one hour at 100°, 200°, and 300° C. in a forced air oven. The thin film exhibited a T g  by DSC of 252° C. and 10% weight loss by TGA occurred at 505° C. The film exhibited a mean tensile strength of 20.7 ksi, and a modulus and elongation of 463 ksi and 10.6% respectively, at room temperature. The coefficient of thermal expansion was 36.0 ppm/°C. Moisture uptake by weight was 0.88%. 
     Example 18 
     By the same method and conditions as described in Example 17 of the present invention, 2.9781 g of oxydiphthalic dianhydride (ODPA) was added to 0.9612 g of 3,4&#39;-oxydianiline (3,4&#39;-ODA) and 1.4127 g of 1,2-bis(4-aminoanilino)cyclobutene-3,4-dione (SAPPD) dissolved in 47.3337 g of dimethylacetamide (DMAc) to form a poly(amide acid) resin having an inherent viscosity of 1.14 dL/g. Resulting polyimide films of the ODPA+50:50 3,4&#39;-ODA/SAPPD did not exhibit a T g  by DSC and 10% weight loss by TGA occurred at 456° C. The film exhibited a mean tensile strength of 18.8 ksi, and a modulus and elongation of 601 ksi and 4.2% respectively, at room temperature. The coefficient of thermal expansion was 30.8 ppm/°C. 
     Example 19 
     By the same method and conditions as described in Example 17 of the present invention, 2.9781 g of oxydiphthalic dianhydride (ODPA) was added to 1.4417 g of 3,4&#39;-oxydianiline (3,4&#39;-ODA) and 0.7064 9 of 1,2-bis(4-aminoanilino)cyclobutene-3,4-dione (SAPPD) dissolved in 55.2011 g of dimethylacetamide (DMAc) to form a poly(amide acid) resin having an inherent viscosity of 1.19 dL/g. Resulting polyimide films of ODPA+75:25 3,4&#39;-ODA/SAPPD exhibited a T g  by TMA of 261° C. and 10% weight loss by TGA occurred at 503° C. The film exhibited a mean tensile strength of 18.5 ksi, and a modulus and elongation of 494 ksi and 9.6% respectively, at room temperature. The coefficient of thermal expansion was 34.2 ppm/°C. 
     Example 20 
     By the same method and conditions as described in Example 17 of the present invention, 2.9781 g of oxydiphthalic dianhydride (ODPA) was added to 1.7301 g of 3,4&#39;-oxydianiline (3,4&#39;-ODA) and 0.2825 g of 1,2-bis(4-aminoanilino)cyclobutene-3,4-dione (SAPPD) dissolved in 38.7055 g of dimethylacetamide (DMAc) to form a poly(amide acid) resin having an inherent viscosity of 1.30 dL/g. Resulting polyimide films of the ODPA+90:10 3,4&#39;-ODA/SAPPD exhibited a T g  by DSC of 249° C. and 10% weight loss occurred at 518° C. The film exhibited a mean tensile strength of 18.6 ksi, and a modulus and elongation of 439 ksi and 63.4% respectively, at room temperature. The coefficient of thermal expansion was 38.5 ppm/°C. 
     Example 21 
     Into a 500 ml, three-neck round bottom flask equipped with mechanical stirrer, nitrogen gas inlet, and drying tube containing calcium carbonate was placed 1,2-bis(4-aminoanilino)cyclobutene-3,4-dione (SAPPD) (5.8863 g, 0.0200 mol), 3,4&#39;-oxydianiline (3,4&#39;-ODA) (16.0194 g, 0.08 mol), and N-methylpyrollidinone (NMP) (150 g). The solution was stirred until the diamines dissolved. To this solution was added oxydiphthalic anhydride (ODPA) (31.0223 g, 0.1000 mol) and NMP (141.75 g). The final solution concentration was 15.35% solids. The solution was stirred at RT overnight under a nitrogen atmosphere. A 0.5% solution of poly(amide acid) in NMP had an inherent viscosity of 1.03 dL/g measured at 25° C. 
     Example 22 
     Controlled molecular weight polyimide copolymers by stoichiometric imbalance and endcapping with phthalic anhydride. 
     Into a 100 ml, three-neck round bottom flask equipped with mechanical stirrer, nitrogen gas inlet, and drying tube containing calcium carbonate was placed 1,2-bis(4-aminoanilino)cyclobutene-3,4-dione (SAPPD) (0.5886 g, 0.0020 mol), 3,4&#39;-oxydianiline (3,4&#39;-ODA) (1.6019 g, 0.0080 mol), phthalic anhydride (0.0592 g, 0.0004 mol), and N-methylpyrollidinone (NMP) (20 g). The solution was stirred until the diamines dissolved. To this solution was added oxydiphthalic anhydride (ODPA) (3.0402 g, 0.0098 mol) and NMP (13.7865 g). The final solution concentration was 13.40% solids. The solution was stirred at RT overnight under a nitrogen atmosphere. A 0.5% solution of poly(amide acid) in NMP had an inherent viscosity of 0.6 dL/g measured at 25° C.