Abstract:
A soluble aromatic random copolyimide and a process for preparing same are herein disclosed, wherein the copolyimide is prepared from (a) an aromatic diamine and two or more different aromatic dianhydrides or (b) an aromatic dianhydride and two or more different aromatic diamines; wherein at least one of the diamine or the dianhydride contains a sulfonyl group directly or indirectly linking the two amine moieties in said diamine or the two anhydride moieties in said dianhydride. The copolyimide is soluble at room temperature in polar organic solvents such as N-methyl-2-pyrollidone, N,N-dimethylacetamide, and dimethylformamide, and has a glass transition temperatures below its respective melting points. As such, the copolyimide is more easily solubilized and processed into working forms, while still retaining desirable characteristics such as heat resistance, and may be used in various applications for which aromatic polyimides are known to be useful.

Description:
FIELD OF INVENTION 
       [0001]    The present invention relates to a soluble aromatic random copolyimide and a process for preparing said copolyimide. 
       BACKGROUND OF THE INVENTION 
       [0002]    Aromatic polyimides are a class of polymers used in a variety of high performance and/or high temperature applications. A general discussion of the preparation, characterization and applications of these compounds is found in  Polyimides. Synthesis, Characterization and Applications , ed. K. L. Mittal, Plenum, N.Y., 1984. Polyimides are linear polymers synthesized by condensation polymerization of dianhydrides and diamines. In general, random polyimides can be synthesized from a single dianhydride and two or more diamines, or from a single diamine and two or more kinds of anhydrides. 
         [0003]    Aromatic polyimides have a number of useful physical properties, such as very high tensile strength, high tensile modulus, very high resistance to wear, as well as chemical, heat and radiation resistance, and excellent dimensional stability. Such polyimides have a low coefficient of friction and are able to bond strongly with metals. Aromatic polyimides are also able to form films while still retaining the desirable physical properties noted above. For all of these reasons, aromatic polyimides are used extensively in the preparation of high performance materials and composites. Such applications include films, matrix resins for composites, adhesives, and coatings on components or machine parts that are subject to chemical, thermal, electrical and/or mechanical stress. Some examples of the wide-ranging uses of aromatic polyimides include: components used in jet engines and aerospace engineering; insulation in electrical motors and wiring; automotive parts; advanced textiles and membranes; fire barriers; thermal and acoustic insulation; seals in pumps and valves; and coatings to protect metals and other sensitive materials from wear and corrosion. 
         [0004]    The useful properties of aromatic polyimides, such as solvent resistance, tensile strength and high temperature resistance, also render these compounds difficult to process into workable forms (e.g. films, coatings, sheets). Many aromatic polyimides are intractable due to high intermolecular forces, high polarity, and/or molecular stiffness and symmetry. This means that most polyimides are insoluble in commonly used organic solvents. Aromatic polyimides tend to be soluble only in halogenated organic solvents or upon heating in polar organic solvents with relatively high boiling points (typically greater than 100° C.), such as N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF) and N,N-dimethylacetamide (DMAc). Also, many of the totally aromatic polyimides have crystalline melting points are usually well above their thermal decomposition temperature, so that it is not possible to mold or reshape polyimides once formed. 
         [0005]    In the past, workers have carried out the polymerization reaction in situ on a substrate (e.g. a machine component that is to be coated with the polyimide) or within a mold, in order to avoid the necessity for further processing. See for example, Meterko et al. in U.S. Pat. No. 5,171,828. By having to carry out the polymerization reaction in situ, this introduces additional problems to be overcome during the manufacturing process, such as having a substrate which is stable to the polymerization reaction conditions. Tamai et al. discloses a melt-processible polyimide in U.S. Pat. No. 5,268,446, in which the polyimide can be melted and formed into the required working shape. However, it would be advantageous to have a more tractable polyimide which is easily adaptable to a wide variety of manufacturing processes and conditions. 
         [0006]    Attempts to increase the ease of handling and processability of aromatic polyimides involve the modification of the polymer backbone through monomer selection. Bulky substituents on the polymer backbone to enhance the free volume of the main chain and disrupt the symmetry of the polymer improve solubility and lower the melt viscosity of the polymer. Modifications include the incorporation of the following types of groups into the polymer backbone: aliphatic groups, polar and non-polar pendent substituents, heteroatoms (i.e. non-carbon atoms) or heterocyclic groups. For example, in U.S. Pat. No. 4,9643,649, Wright et al. disclosed a copolyimide comprising an aromatic sulfone and aromatic fluoroaliphatic groups. 
         [0007]    Other methods to improve the handling ease of aromatic polyimides include the disruption of the symmetry of the polymer by either copolymerization with mixtures of two aromatic diamines or aromatic dianhydrides, or by forming a polymer from block segments of soluble oligomers which are themselves formed from flexible monomers. See for example, Bryant in U.S. Pat. No. 6,048,959. 
         [0008]    Accordingly, there is a need for aromatic polyimides which retain desirable characteristics, such as tensile strength, temperature resistance, and solvent resistance, while being easily processible once formed. Preferably, such polyimides should be soluble in readily available organic solvents without requiring heating for solubilization to occur. 
       SUMMARY OF THE INVENTION 
       [0009]    In accordance with a broad aspect of the present invention there is provided a soluble aromatic random copolyimide wherein said copolyimide is a reaction product of:
       (a) an aromatic sulfonyl diamine and two or more different aromatic dianhydrides;   (b) an aromatic dianhydride and two or more aromatic diamines, wherein at least one of the diamines contains a sulfonyl group directly or indirectly linking the two amine moieties of said diamine;   (c) an aromatic diamine and two or more different aromatic dianhydrides, wherein at least one of the dianhydrides contains a sulfonyl group directly or indirectly linking the two anhydride moieties of said dianhydride; or   (d) an aromatic sulfonyl dianhydride and two or more aromatic diamines.       
 
         [0014]    In one embodiment, the copolyimide is prepared by reaction of an aromatic sulfonyl diamine and two or more different aromatic dianhydrides and has repeating units of formula (I): 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    wherein: 
         [0015]    A 1  and A 2  are trivalent aromatic radicals, A 1  and A 2  being same or different, each of A 1  and A 2  having 1 to 5 benzenoid-unsaturated rings of 6 carbon atoms wherein the two carbonyl groups bonded to each of A 1  and A 2  are directly bonded to adjacent carbon atoms in a benzene ring of each of A 1  and A 2 ; 
         [0016]    A 3  and A 4  are divalent aromatic radicals, A 3  and A 4  being same or different, each of A 3  and A 4  having 1 to 5 benzenoid-unsaturated rings of 6 carbon atoms, the nitrogen atom and the sulfonyl group bonded to each of A 3  and A 4  being directly bonded to different carbon atoms of a benzene ring in each of A 3  and A 4 ; 
         [0017]    A 5  is a tetravalent aromatic radical having 1 to 5 benzenoid-unsaturated rings of 6 carbon atoms wherein the four carbonyl groups bonded to A 5  are directly bonded to different carbon atoms in a benzene ring of A 5 , each pair of carbonyl groups being bonded to adjacent carbon atoms in a benzene ring of A 5 ; and 
         [0018]    Z is a divalent chemical group or bond, selected from the group consisting of a carbonyl group 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    an oxy group (—O—), a sulfonyl group (—SO 2 —) and a divalent C 1 -C 6  alkyl group optionally substituted with one or more aryl groups of formula (II), 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    wherein p is an integer selected from 0 to 6, q is an integer selected from 0 to 5, wherein q is the total number of substituents X, and X is independently selected from the group consisting of halogen and C 1 -C 6  branched or unbranched alkyl. 
         [0019]    In one preferred embodiment, Z is selected from the group consisting of: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0020]    In another preferred embodiment, the aromatic sulfonyl diamine is selected from the group consisting of 4,4′-diaminodiphenylsulfone (DDS), 3,3′-diaminodiphenylsulfone, 1,7′-diaminodinaphthylsulfone, 1,6′-diaminodinaphthylsulfone, bis[4-(4-aminophenoxy)phenyl]sulfone, bis[4-(3-aminophenoxy)phenyl]sulfone, bis(4-aminobenzophenone)sulfone, bis(3-aminobenzophenone)sulfone, and bis[2,2,-(4-aminophenyl-4-phenyl-4-phenyl)propane]sulfone. Preferably, each of said dianhydrides is selected from the group consisting of pyromellitic dianhydride (PMDA), 3,3′,4,4′-benzophenonetetracarboxylic dianhydride (BTDA), 2,3,3′,4′-benzophenonetetracarboxylic dianhydride, 2,2′,3,3′-benzophenonetetracarboxylic dianhydride, 4,4′,5,5′,6,6′-hexafluorobenzophenone-2,2′,3,3′-tetracarboxylic dianhydride, 3,3′,4,4′-diphenyl-tetracarboxylic dianhydride, 2,2′,3,3′-diphenyltetracarboxylic dianhydride, bis(3,4-dicarboxyphenyl)sulphone dianhydride, bis(2,5,6-trifluoro-3,4-dicarboxyphenyl)sulphone dianhydride, bis(3,4-dicarboxyphenyl)phenylphosphonate dianhydride, bis(3,4-dicarboxyphenyl)phenylphosphine oxide dianhydride, N,N-(3,4-dicarboxyphenyl)-N-methylamine dianhydride, bis(3,4-dicarboxyphenyl)diethylsilane dianhydride, 1,4,5,8-naphthalenetetracarboxylic dianhydride, 2,3,6,7-naphthalenetetracarboxylic dianhydride, 1,2,5,6-naphthalenetetracarboxylic dianhydride. 2,6-dichloronaphthalene-1,4,5,8-tetracarboxylic dianhydride, 2,7-dichloronaphthalene-1,4,5,8-tetracarboxylic dianhydride, 2,3,6,7-tetrachloronaphthalene-1,4,5,8-tetracarboxylic dianhydride, 1,4,5,8-tetrafluoronaphthalene-2,3,6,7-tetracarboxylic dianhydride, and phenanthrene-1,8,9,10-tetracarboxylic dianhydride. 
         [0021]    In yet another embodiment, there is provided a soluble aromatic random copolyimide having repeating units of formula (III): 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    The copolyimide of formula (III) can be prepared by reaction of 4,4′-diaminodiphenylsulfone (DDS) with pyromellitic dianhydride (PMDA) and 3,3′,4,4′-benzophenonetetracarboxylic dianhydride (BTDA). 
         [0022]    The copolyimide is soluble in an organic solvent with optional heating. The organic solvent is preferably a polar organic solvent. Preferred polar organic solvents are selected from the group consisting of m-cresol, N-methyl-2-pyrollidone (NMP), dimethyl sulfoxide (DMSO), N,N-dimethylacetamide (DMAc), and dimethylformamide (DMF). 
         [0023]    In another embodiment, the copolyimide has a glass transition temperature of about 310° C. to about 350° C. 
         [0024]    Another aspect of the present invention includes a composite material comprising the above-noted soluble aromatic random copolyimide. 
         [0025]    In another broad aspect of the present invention, there is provided a process for preparing a soluble aromatic random copolyimide, the process comprising:
       (i) forming a polyamic acid intermediate from (a) an aromatic sulfonyl diamine and two or more different aromatic dianhydrides; (b) an aromatic dianhydride and two or more aromatic diamines, wherein at least one of the diamines contains a sulfonyl group directly or indirectly linking the two amine moieties of said diamine; (c) an aromatic diamine and two or more different aromatic dianhydrides, wherein at least one of the dianhydrides contains a sulfonyl group directly or indirectly linking the two anhydride moieties of said dianhydride; or (d) an aromatic sulfonyl dianhydride and two or more aromatic diamines; and   (ii) thermally or chemically imidizing the polyamic acid intermediate to form said copolyimide.       
 
         [0028]    In a preferred embodiment of the process, wherein the copolyimide is prepared from the combination (a) noted above, the aromatic sulfonyl diamine is reacted sequentially with two different aromatic dianhydrides to form a polyamic acid intermediate. Preferably, the sterically bulkier dianhydride of said two dianhydrides is reacted with said diamine first. 
         [0029]    The aromatic copolyimide disclosed herein has improved processability as compared to known aromatic polyimides, and is readily soluble in common organic solvents at room temperature, i.e. generally without heating, and its glass transition temperature (T g ) is lower than its respective melting temperature (T m ). Thus, the copolyimide of the invention may be easily handled by solubilizing in an organic solvent. The copolyimide of the invention also has enhanced film-forming and molding properties, as compared to known aromatic polymides, due to its lowered glass transition temperatures. The copolyimide of the invention also demonstrates good heat resistance, which is desirable for use in high performance materials. As such, the copolyimide of the invention retains the useful characteristics of the broader class of aromatic polyimides while being more easily processed than known aromatic polyimides. 
         [0030]    A further advantage of the present invention is providing a simple process for preparing copolyimides with greater solubility in common organic solvents. 
         [0031]    Other and further advantages and features of the invention will be apparent to those skilled in the art from the following detailed description of an embodiment thereof, taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0032]    The present invention will be further understood from the following detailed description of an embodiment of the invention, with reference to the drawings in which: 
           [0033]      FIG. 1(   a ) illustrates a general reaction scheme for the synthesis of a soluble random aromatic copolyimide from an aromatic sulfonyl diamine and two different aromatic dianhydrides, in accordance with a broad aspect of the present invention; 
           [0034]      FIG. 1(   b ) illustrates the reaction scheme for the synthesis of a soluble random aromatic copolyimide from diaminodiphenyl sulfone (DDS), and two different dianhydrides, 3,3′,4,4′-benzophenone tetracarboxylic dianhydride (BTDA), and pyromellitic dianhydride (PMDA), in accordance with a preferred embodiment of the present invention; 
           [0035]      FIG. 2  illustrates the Fourier transform infrared (FTIR) spectra of three soluble random aromatic copolyimides, CPI-1, CPI-2, and CPI-3, prepared in accordance with an embodiment of the present invention; 
           [0036]      FIG. 3  illustrates the  1 H nuclear magnetic resonance (NMR) spectrum for a copolyimide of the present invention, CPI-2, prepared according to Example 3; 
           [0037]      FIG. 4  illustrates the  13 C NMR spectrum for a copolyimide of the present invention, CPI-2; 
           [0038]      FIG. 5  illustrates the ultraviolet (UV) spectra of three copolyimides of the present invention, CPI-1, CPI-2 and CPI-3; 
           [0039]      FIG. 6  illustrates the differential scanning calorimetry (DSC) thermogram of a copolyimide of the present invention, CPI-2; and 
           [0040]      FIG. 7  illustrates the thermogravimetric analysis (TGA) curve of a copolyimide of the present invention, CPI-2. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0041]    In a preferred embodiment of the present invention, there is provided a soluble aromatic random copolyimide prepared from the polymerization reaction of an aromatic sulfonyl diamine and two or more different aromatic dianhydrides. 
         [0042]    The term “soluble”, as used herein means soluble in organic solvents, preferably polar organic solvents including m-cresol, and particularly polar aprotic solvents including N-methyl-2-pyrollidone (NMP), dimethyl sulfoxide (DMSO), N,N-dimethylacetamide (DMAc), and dimethylformamide (DMF). 
         [0043]    Preferably, the copolyimide of the invention is soluble in the organic solvent at room temperature, i.e. heating is optional and not required for solubilizing the copolymide. 
         [0044]    In a preferred embodiment, the aromatic copolyimide has the general formula (I), 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    wherein: 
         [0045]    A 1  and A 2  are trivalent aromatic radicals, A 1  and A 2  being same or different, each of A 1  and A 2  having 1 to 5 benzenoid-unsaturated rings of 6 carbon atoms wherein the two carbonyl groups bonded to each of A 1  and A 2  are directly bonded to adjacent carbon atoms in a benzene ring of each of A 1  and A 2 ; 
         [0046]    A 3  and A 4  are divalent aromatic radicals, A 3  and A 4  being same or different, each of A 3  and A 4  having 1 to 5 benzenoid-unsaturated rings of 6 carbon atoms, the nitrogen atom and the sulfonyl group bonded to each of A 3  and A 4  being directly bonded to different carbon atoms of a benzene ring in each of A 3  and A 4 ; 
         [0047]    A 5  is a tetravalent aromatic radical having 1 to 5 benzenoid-unsaturated rings of 6 carbon atoms wherein the four carbonyl groups bonded to A 5  are directly bonded to different carbon atoms in a benzene ring of A 5 , each pair of carbonyl groups being bonded to adjacent carbon atoms in a benzene ring of A 5 ; and 
         [0048]    Z is a divalent chemical group or bond, selected from the group consisting of a carbonyl group 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    an oxy group (—O—), a sulfonyl group (—SO 2 —) and a divalent C 1 -C 6  alkyl group optionally substituted with one or more aryl groups of formula (II), 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    wherein p is an integer selected from 0 to 6, q is an integer selected from 0 to 5, wherein q is the total number of substituents X, and X is independently selected from the group consisting of halogen and C 1 -C 6  branched or unbranched alkyl. 
         [0049]    Illustrative examples of Z that are preferred are: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0050]    In yet another preferred embodiment, Z is a carbonyl group 
         [0000]    
       
                 
         
             
             
         
       
     
         [0051]    Without being limited to the following list, illustrative examples of aromatic sulfonyl diamines that are preferred for use in the methods disclosed herein are 4,4′-diaminodiphenylsulfone, 3,3′-diaminodiphenylsulfone, 1,7′-diaminodinaphthylsulfone, 1,6′-diaminodinaphthylsulfone, bis[4-(4-aminophenoxy)phenyl]sulfone, bis[4-(3-aminophenoxy)phenyl]sulfone, bis(4-aminobenzophenone)sulfone, bis(3-aminobenzophenone)sulfone, bis[2,2,-(4-aminophenyl-4-phenyl-4-phenyl)propane]sulfone and obvious chemical equivalents thereof. 
         [0052]    Without being limited to the following, illustrative examples of aromatic dianhydrides which are suitable for use in preparing the polyimide according to the methods disclosed herein are: pyromellitic dianhydride (PMDA), 3,3′,4,4′-benzophenonetetracarboxylic dianhydride (BTDA), 2,3,3′,4′-benzophenonetetracarboxylic dianhydride, 2,2′,3,3′-benzophenonetetracarboxylic dianhydride, 4,4′,5,5′,6,6′-hexafluorobenzophenone-2,2′,3,3′-tetracarboxylic dianhydride, 3,3′,4,4′-diphenyl-tetracarboxylic dianhydride, 2,2′,3,3′-diphenyltetracarboxylic dianhydride, bis(3,4-dicarboxyphenyl)sulphone dianhydride, bis(2,5,6-trifluoro-3,4-dicarboxyphenyl)sulphone dianhydride, bis(3,4-dicarboxyphenyl)phenylphosphonate dianhydride, bis(3,4-dicarboxyphenyl)phenylphosphine oxide dianhydride, N,N-(3,4-dicarboxyphenyl)-N-methylamine dianhydride, bis(3,4-dicarboxyphenyl)diethylsilane dianhydride, 1,4,5,8-naphthalenetetracarboxylic dianhydride, 2,3,6,7-naphthalenetetracarboxylic dianhydride, 1,2,5,6-naphthalenetetracarboxylic dianhydride. 2,6-dichloronaphthalene-1,4,5,8-tetracarboxylic dianhydride, 2,7-dichloronaphthalene-1,4,5,8-tetracarboxylic dianhydride, 2,3,6,7-tetrachloronaphthalene-1,4,5,8-tetracarboxylic dianhydride, 1,4,5,8-tetrafluoronaphthalene-2,3,6,7-tetracarboxylic dianhydride, and phenanthrene-1,8,9,10-tetracarboxylic dianhydride and obvious chemical equivalents thereof. 
         [0053]    Referring to  FIG. 1(   a ), in the first step of the polymerization reaction, the aromatic sulfonyl diamine (1) is reacted sequentially with two different aromatic dianhydrides (2, 3) to form a polyamic acid intermediate (4). The dianhydrides (2, 3) shown in  FIG. 1(   a ) may be added in any order, however preferably, the sterically bulkier dianhydride (2) of the two different anhydrides is added first in order to control the rate of reaction in the first stage of polymerization. Next, the cyclization of the polyamic acid (4) to the corresponding polyimide (5) (where “n” is a positive integer greater than or equal to 1) is carried out by heating the polyamic acid to temperatures between about 100° C. and 300° C., or by heating with a dehydrating agent by itself or in combination with a tertiary amine (e.g. triethylamine or pyridine). 
         [0054]    Preferred solvents for the polymerization reaction are polar aprotic solvents such as N,N-dimethylacetamide (as shown in  FIG. 1(   a )), N,N-diethylacetamide, N,N-dimethylformamide and N-methyl-2-pyrrolidone and obvious chemical equivalents thereof. After completion of the polymerization reaction, the solvent can be removed by filtration and drying in vacuo. 
         [0055]    Examples of preferred dehydrating agents are acetic anhydride, propionic anhydride and dicyclohexylcarbodiimide, optionally with a tertiary amine and obvious chemical equivalents thereof. A preferred dehydrating agent is a mixture of acetic anhydride and triethylamine or pyridine. In yet another preferred embodiment, the cyclisation was done by heating the polyamic acid between 100° C. and 130° C. and using a mixture of acetic anhydride and pyridine. 
         [0056]    The copolyimides of the invention may be used in alone or in the preparation of composite materials. The copolyimides and composite materials comprising the copolyimides can be used in high performance materials and other applications for which aromatic polyimides are known to be useful. For example, the copolyimides can be compounded with silicon dioxide or other silicon materials to form silicon composites. Due to their enhanced processability, the copolyimides of the invention can be readily formed into laminates and films. 
         [0057]    Further details of preferred embodiment of the invention are illustrated in the following Examples which are understood to be non-limiting with respect to the appended claims. 
       EXAMPLE 1 
       [0058]    The following series of reactions are as provided in the reaction scheme of  FIG. 1(   b ). Abbreviations used in  FIG. 1(   b ) are as defined below. 
         [0059]    In a 50 mL three necked round bottom flask equipped with dry nitrogen inlet, outlet and a condenser was added 0.4966 g (2 mmol) 4,4′-diaminodiphenyl sulfone (DDS) and 3 mL N,N-dimethylacetamide (DMAc). The diamine, DDS, was dissolved in the solvent, DMAc, and 0.1288 g (0.4 mmol) 3,3′,4,4′-benzophenone tetracarboxylic dianhydride (BTDA) was added and stirred. The reaction mixture was heated to between 60° C. and 80° C. for about 4 h. After the mentioned time, 0.3496 g (1.6 mmol) pyromellitic dianhydride (PMDA) and 2 mL DMAc was added and the reaction was continued for about 4 h. The molar ratio of BTDA:DDS:PMDA was 0.2:1:0.8. In  FIG. 1(   b ), “n” is a positive integer greater than or equal to 1. 
         [0060]    The mixture was stirred at room temperature overnight, after which the temperature of the reaction mixture was raised to about 100 to 130° C. About 0.5 mL pyridine (Py) and about 1 mL acetic anhydride (Ac 2 O) was added to the reaction mixture and stirred for about 2 to 3 h. The copolyimide precipitated out slowly. The precipitated polymer was washed with acetone and with water and filtered. The polymer was dried under vacuum at about 120° C. for 8 h. The copolyimide was found to have a glass transition temperature (T g ) of about 335° C. 
       EXAMPLE 2 
       [0061]    The procedure for Example 1 was followed to prepare the following copolyimide, entitled “CPI-1”. 0.2577 g (0.8 mmol) 3,3′,4,4′-benzophenone tetracarboxylic dianhydride (BTDA) and 0.2614 g (1.2 mmol) pyromellitic dianhydride (PMDA) was used for the synthesis of the copolyimide CPI-1. The molar ratio of BTDA:DDS:PMDA was 0.4:1:0.6. The resultant copolyimide CPI-1 was found to have a glass transition temperature (T g ) of about 330° C. 
       EXAMPLE 3 
       [0062]    The procedure for Example 1 was followed to prepare the following copolyimide, entitled “CPI-2”. 0 3222 g (1.0 mmol) 3,3′,4,4′-benzophenone tetracarboxylic dianhydride (BTDA) and 0.2181 g (1.0 mmol) pyromellitic anhydride (PMDA) was used for the synthesis of the copolyimide CPI-2. The molar ratio of BTDA:DDS:PMDA was 0.5:1:0.5. The copolyimide CPI-2 was found to have a glass transition temperature (T g ) of about 325° C. 
       EXAMPLE 4 
       [0063]    The procedure for Example 1 was followed to prepare the following copolyimide, entitled “CPI-3”. 0.3864 g (1.2 mmol) 3,3′,4,4′-benzophenone tetracarboxylic dianhydride (BTDA) and 0.1745 g (0.8 mmol) pyromellitic dianhydride (PMDA) was used for the synthesis of the copolyimide CPI-3. The molar ratio of BTDA:DDS:PMDA was 0.6:1:0.4. The copolyimide CPI-3 was found to have a glass transition temperature (T g ) of about 325° C. 
       EXAMPLE 5 
       [0064]    The procedure for Example 1 was followed to prepare the following copolyimide. 0.5155 g (1.6 mmol) 3,3′,4,4′-benzophenone tetracarboxylic dianhydride (BTDA) and 0.0872 g (0.4 mmol) pyromellitic anhydride (PMDA) was used for the synthesis of the copolymer. The molar ratio of BTDA:DDS:PMDA was 0.8:1:0.2. The copolyimide was found to have a glass transition temperature (T g ) of about 310° C. 
       EXAMPLE 6 
       [0065]    The copolyimides of Examples 2, 3 and 4, i.e. CPI-1, CPI-2 and CPI-3, respectively, were tested for their solubility in a number of common organic solvents: N-methyl-2-pyrollidone (NMP), dimethyl sulfoxide (DMSO), m-cresol, N,N-dimethylacetamide (DMAc), and dimethylformamide (DMF). The solubility results are summarized in Table 1 below. 
         [0000]    
       
         
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Copol- 
                   
                   
                   
                 m- 
                   
                   
               
               
                 ymer 
                 BTDA:DDS:PMDA 
                 NMP 
                 DMSO 
                 cresol 
                 DMAc 
                 DMF 
               
               
                   
               
             
             
               
                 CPI-1 
                 0.4:1:0.6 
                 + 
                 + 
                 + 
                 + 
                 ± 
               
               
                 CPI-2 
                 0.5:1:0.5 
                 + 
                 + 
                 + 
                 + 
                 + 
               
               
                 CPI-3 
                 0.6:1:0.4 
                 + 
                 + 
                 + 
                 + 
                 + 
               
               
                   
               
               
                 + soluble at room temperature 
               
               
                 ± soluble on heating 
               
             
          
         
       
     
         [0066]    As can be seen in Table 1, CPI-2 and CPI-3 were soluble in NMP, DMSO, m-cresol, DMAC and DMF at room temperature. CPI-1 was soluble in NMP, DMSO, m-cresol and DMAc at room temperature, but required heating to dissolve in DMF. 
         [0067]    The weight average molecular weight (M w ), the number average molecular weight (M n ), and the polydispersity index (M w /M n ) were determined for each of copolyimides CPI-1, CPI-2, and CPI-3, and are summarized in Table 2 below. 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Copolyimide 
                 BTDA:DDS:PMDA 
                 M n  (g/mol) 
                 M w  (g/mol) 
                 M w /M n   
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 CPI-1 
                 0.4:1:0.6 
                 191466 
                 1.421 × 10 6   
                 7.42 
               
               
                   
                   
                 5096 
                 7061 
                 1.39 
               
               
                 CPI-2 
                 0.5:1:0.5 
                 170881 
                 1.531 × 10 6   
                 8.96 
               
               
                   
                   
                 5316 
                 7161 
                 1.35 
               
               
                 CPI-3 
                 0.6:1:0.4 
                 785604 
                 3.814 × 10 6   
                 4.85 
               
               
                   
                   
                 5488 
                 7987 
                 1.46 
               
               
                   
               
             
          
         
       
     
         [0068]    The FTIR spectrum of each of CPI-1, CPI-2 and CPI-3 is provided in  FIG. 2 . The spectrum for each of CPI-1, CPI-2 and CPI-3 showed peaks at 1780 and 1720 cm −1 , corresponding to the asymmetrical and symmetrical C═O imide stretching vibration. The peak at 1360 cm −1  corresponds to C—N stretching vibrations of the imide ring within the copolyimide. 
         [0069]    The UV spectrum of each of CPI-1, CPI-2 and CPI-3 is provided in  FIG. 3 . The spectrum for each of CPI-1, CPI-2 and CPI-3 showed a peak between 250 to 325 nm, which is due to the carbonyl group and aromatic rings within the copolyimide. 
         [0070]    CPI-2 was further analyzed by  1 H NMR,  13 C NMR, differential scanning calorimetry (DSC) and thermogravimetric anaylsis (TGA). 
         [0071]    The  1 H NMR spectrum of CPI-2 is shown in  FIG. 3 . Referring to  FIG. 3 , there are peaks observed between 7.6 and 8.4 ppm that are due to the aromatic protons. The peak at 8.4 ppm is due to the protons of PMDA in CPI-2. 
         [0072]    The  13 C NMR spectrum of CPI-2 is shown in  FIG. 4 . Referring to  FIG. 4 , there is a peak observed at 191.5 ppm, which is due to the carbonyl carbon within CPI-2. The remaining peaks in the  13 C NMR spectrum of CPI-2 correspond to the carbon atoms of the aromatic rings within CPI-2. 
         [0073]    The DSC curve for CPI-2 is shown in  FIG. 6 . As shown in the DSC curve for CPI-2, the T g  is around 325° C. 
         [0074]    The TGA curve for CPI-2 is shown in  FIG. 7 . Significant degradation does not occur in CPI-2 until temperatures above around 500° C. As such, CPI-2 shows very good heat resistance, which is a desirable characteristic and very useful in many high performance applications. 
         [0075]    Numerous modifications, variations, adaptations may be made to the particular embodiments of the invention described above without departing form the scope of the invention, which is defined in the following claims.