POLYAMIC ACIDS AND METHODS TO CONVERT POLYAMIC ACIDS INTO POLYIMIDEBENZOXAZOLE FILMS

Abstract: This invention relates to polyamic acids and to methods for converting polyamic acid into polyimidebenzoxazole polymers. The polyamic acids are prepared by reacting a dianhydride with a diaminobenzoxazole under conditions sufficient to form polyamic acid with an inherent viscosity of greater than 2.0 dL/gram, as measured at 0.2 g/dL in N-methyl-pyrrolidinone at 25 DEG C. The polyamic acids are converted to polyimidebenzoxazole in the form of films, fiber and coatings using thermal and chemical processes.

Description: POLYAMIC ACIDS AND METHODS TO CONVERT POLYAMIC ACIDS INTO POLYIMIDEBENZOXAZOLE FILMS The present invention relates to polyamic acids ("PAA"), to a method for converting PAA into polyimidebenzoxazole ("PIBO") polymers, and to films and coatings of PIBO. In the preparation of polyimidebenzoxazole, a dianhydride and diaminobenzoxazole are first reacted to form a polyamic acid. The polyamic acid is then converted to PIBO by closing the amic acid linkages to form imide rings. Heretofore, only a limited number of dia ' nhydride/diaminobenzoxazole combinations have been disclosed for producing PIBO. While these limited number of PIBO polymers have been formed into fibers and films, the resulting fibers and films do not generally have sufficient physical and/or chemical properties (such as tensile modulus, tensile strength, elongation-at-break and coefficient of thermal expansion (CTE)) to make them generally useful in applications such as for the electronics industry. For example, U.S. Patent 4,087,409 to Preston "Ordered Heterocyclic Copolymers" broadly teaches reacting two symmetrical monomers to produce an essentially linear heterocyclic polymer having at least two different heterocyclic linkages. Polyamic acids can be prepared by selecting monomers from an otherwise long list of monomers (for example, 2,2'-p-phenylene bis(5-aminobenzoxazole) and pyromellitic dianhydride). Using the exemplified techniques, non-benzoxazole-containing polyamic acids are prepared at inherent viscosities of 1.13 dL/gram. Films made from these polymers and techniques are described in summary fashion, without specific processing techniques for preparing the films or physical properties disclosed. "Fibers from Ordered Benzheterocycle-lmide Copolymers," Appl. Poly. Sym., No. 9, pp. 145-158 (1969), describes the preparation of amic acid polymers by polycondensing specific aromatic dianhydrides and aromatic diaminobenzoxazoles. The polyamic acids exhibit inherent viscosities of 1.5 dL/gram and less. Without disclosing, in detail, the specific techniques, the polyamic acids are formed into PIBO fibers. These fibers have less than desirable physical properties, particularly tensile strengths. "New High-Temperature Polymers, VIII Ordered Benzoxazole- and Benzothiazole- -Imide Copolymers," J. Poly. Sci., Part A-1 (1969), 7(1), pp. 283-296, describes the preparation of ordered heterocycle copolymers prepared from aromatic dianhydrides reacted with aromatic diamines containing preformed benzoxazole or benzothiazole units. The ordered heterocyclic copolymers were prepared in their soluble polyamic acid form with inherent viscosities no greater than 1.78 dL/gram and fabricated to the desired film, fiber or metal coating. After fabrication, the polyamic acid is converted to PIBO such as by forming a film from the polyamic acid solution, and heating the film for 20 minutes at 100°C, 20 hours at 145 C C and then 300 G C for 1 hour. Films made from these polymers are described in summary fashion, using terms like "brittle" or "strong." "Azole Analogs of Polypyromellitimides," Vysokomol. Soed., Vol. (A) XIII, No. 11, 1971, pp. 2565-2570, discloses synthesis of azole-containing analogs of poly- -[N,N'-(p,p'-phenoxyphenylene)pyromellitimide]. Structure V in this paper is a PIBO prepared from 2,6-di(p,p'-aminophenoxyphenyl)benzo[1,2-d:5,4-d']bisoxazole and pyromellitic dianhydride. The inherent viscosity of the particular polyamic acid used to make this PIBO is 2.5 dL/gram. Similarly, "Correlations Between the Properties and Structures of some Polyhetero-arvlenes," Vysokomol. Soed., Vol. (A) XIV, No. 10, 1972, pp. 2174-2182, in comparing the mechanical properties of other polyimides with those of polyheteroarylenes of similar structure, shows a PIBO polymer in Structure II. The inherent viscosity of the particular polyamic acid used to make the described PIBO is 2.5 dL/gram or less. The physical/chemical properties of the film made from these polymers range from poor to moderate. U.S. Patent No. 4,866,873 to Mukai et al. also discloses an aromatic heterocyclic polyimide comprising substantially equimolar amounts of a specific aromatic, trans- -benzobisoxazole or trans-benzobisthiazole diamine such as 2,6-(4,4'-diaminodiphenyl)benzoI1,2-d:4,5-d']bisoxazole and a specific aromatic tetracarboxylic dianhydride such as pyromellitic dianhydride. The polyamic acids for these polyimides are prepared in an amide solvent and converted to PIBO. Inherent viscosities of suitable polyamic acids are described as being from 0.5 to 20 dL/g. Japanese Patent Application No. 2-41819 to Mitsubishi Kasei Corporation describes similar PIBO polymers from a polyamic acid made from specific aromatic tetracarboxylic dianhydrides such as pyromellitic dianhydride and specific aromatic, trans-benzobisoxazole or trans-benzobisthiazole diamines such as 2.6-(4,4'-diaminodiphenyl)benzo[1,2-d:4,5-d']bisoxazole. The polyamic acids have an inherent viscosity of at least 0.1 and preferably 1 or more dL/gram. Similar polyamic acids having an inherent viscosity ranging from 2.19 dL/gram to 3.07 dL/gram and PIBO polymers made therefrom are also described in "Novel Aromatic Heterocyclic Polyimide (PIBT) Having Ultra- -High Modulus of Elasticity" bySeiichi Nozawa, Kaqaku to Koqyo, 44(7), 1154 (1991). "Synthesis and Mechanical Properties of Novel Polyimide Containing Heterocycles" by Nozawa, Taytama, Kimura and Mukai presented at the 22nd International SAMPE Technical Conference (held November 6-8, 1990), teaches synthesis of one type of trans- -PIBT and trans-PIBO using a polyamic acid of 4,4'-diamino phenylenebenzobisthiazole and pyromellitic dianhydride with thermal and chemical ring closure techniques. The polyamic acid used to prepare the PIBO had an inherent viscosity of 3.09 to 3.79 dL/gram. The polymers prepared from the specific aromatic, trans-benzobisoxazole or trans-benzobisthiazole diamine and pyromellitic dianhydride are prepared as fibers. European Patent Publication No. 0355927 to Asahi discloses a polyimide precursor comprising a specific polyamic acid ester, polyamic amide or polyamic acid salt structure derived from a tetracarboxylic acid compound and a specific diamine compound exhibit low solution viscosity. The monomers used to make the polyimide precursors are generally rigid and symmetrical and each polymer repeat unit contains a pendant organic group having 1 to 20 carbon atoms. The polyamic acid precursors have reduced viscosities from 0.1 to 2.0 dL/gram. The polymers form a photopolymerizable composition with a photopolymeriza- tion initiator. The resulting PIBO polymer can be used to coat substrates for subsequent use in photolithography. Films produced from these cross-linked polymers exhibit relatively low tensile strengths and tensile modulus. Japanese Patent Application No. 41-42458 (Patent KOHO No. 45-8435) to Taoka Senryo Seizo K. . teaches the synthesis of PIBO from a polyamic acid with the polyamic acid being prepared from an aromatic diamine containing a single benzoxazole ring such as 5-amino-2-(4-aminophenyl)benzoxazole and an aromatic tetracarboxylic acid dianhydride using thermal ring closure techniques. In view of these deficiencies, it remains desirable to prepare PIBO polymers which have a more desired combination of chemical and physical properties than those previously reported. In addition, it remains desirable to convert these PIBO polymers into useful films and coatings. Accordingly, in one aspect, the present invention is a polyamic acid having an inherent viscosity of greater than 2.0 dL/g, as measured at 0.2 g/dL in N-methylpyrrolidinone at 25°C, which is the reaction product of a dianhydride of the formula: and a diaminobenzoxazole of any of the following formulae: Ar 11 — NH„ where Ar, Ar 1 , Ar 2 , Ar 3 , Ar 4 , Ar 5 , Ar 6 , Ar 7 , Ar 8 , Ar 9 , Ar 10 and Ar 1 ' can be any aromatic group or pyridine group; except that ! Ar does not equal where Y is a single covalent bond, -0-, -CH 2 -, -S-,-CO-, -S0 2 -, -C(CH 3 ) 2 - or -C(CF 3 ) 2 - and where X 0 and X 1 are hydrogen atoms, alkyl groups, alkoxy groups or halogen atoms or groups, when Ar 4 , Ar 5 , and Ar 6 are all ff " Y| ; 5 and except that Ar does not equal (Q) ♦ whenAr 1 is and Ar 2 and Ar 3 are both (QVo In another aspect, the present invention is a process for preparing a polyimidebenzoxazole from a solution of a polyamic acid (PA) by removing at least a portion of the solvent and exposing the polyamic acid to an imidization temperature from 160°C to 280°C to convert at least a portion of the polyamic acid to polyimidebenzoxazole. In a preferred embodiment, the polyamic acid/PIBO is further treated by exposing it to an additional temperature of from 250°C to 600°C for from 0.1 to 300 minutes. In a more preferred embodiment, this further heat-treatment comprises exposing the PA/PIBO to two or more different, successively higher temperatures. In a most preferred embodiment, the PA/PIBO is exposed to an annealing temperature from 250°C to 400°C for from 0.1 to 120 minutes and then optionally to a heat-treating temperature, higher than the annealing temperature, from 260°C to 600°C for from 0.1 to 120 minutes. ' n Y et another aspect, this invention is a method for preparing a polyimidebenzoxazole film which comprises the steps of: a) forming a liquid film of a polyamic acid solution; b) removing at least a portion of the solvent from the polyamic acid solution; and c) heating the polyamic acid at an imidization temperature from 160°C to 280°C for a period of from 5 to 90 minutes. In a preferred embodiment, the liquid film is contacted with a ring-closure agent prior to or while being heated to or at the imidization temperature. In still another aspect, the present invention is a polyimidebenzoxazole film made using the described process. Using the method of the present invention, PIBO polymers having an excellent balance of chemical and physical properties can be prepared, including a good combination of strength, modulus and elongation-at-break. By selecting the monomer combination and specific conditions of preparation, it is also possible to modify and design these properties. The PIBO products can be prepared as rigid and stiff or flexible depending on the specific monomers selected. The synthesis techniques for making polyamic acid and processing techniques for converting the polyamic acid into PIBO results in polymers having good molecular weights (as evidenced by inherent viscosity) and films and coatings with excellent properties, including excellent tensile properties; dielectrical properties such as dielectric constant, dissipation factor, break down voltage and arc tracking, and dimensional stability properties. Among a number of other applications, the PIBO polymers of the present invention can be made into films and coatings in a multitude of electronics applications. In the present invention, polyamic acids (PAAs) are prepared by reacting a dianhydride monomer of the general formula: and a diaminobenzoxazole monomer of the following formula: where Ar, Ar', Ar*, Ar*, Ar 4 , Ar* , Ar 6 , Ar', Ar 8 , Ar 9 , Ar 1 ° and Ar 11 is an aromatic group, substituted aromatic group, or pyridine group. Ar does not equal when Ar*, Ar5, and Ar6 are all \ C \ . and f Ar does not equal fT ] , when Ar 1 is IT "" ) and Ar 2 and Ar 3 are both \0/ ~ ° \0 The preferred dianhydrides are: wherein Ar is: GH0H6: or where T is -O-, -S-, -S0 2 -, -CH 2 -, © ' ^ ) CH 3 0 CF 3 i « » -C- , -C- or -C- ι » CH 3 CF 3 The most preferred dianhydrides are pyromellitic dianhydride ("PMDA"): ,4'-oxydiphthalic anhydride ("ODPA"), ,2-bis-(3,4-dicarboxyphenyl)hexafluoropropane dianhydride ("6FDA"), ,3',4,4'-biphenyltetracarboxylic dianhydride ("BPDA"), ,3',4,4'-diphenylsulfonetetracarboxylic dianhydride (DSDA), and 3,3',4,4'-benzophenonetetracarboxylic dianhydride. Preferred diaminobenzoxazole monomers include: where Ar 1 and Ar 4 are: and where Ar 2 , Ar 3 , Ar 5 and Ar 6 are the same or different and are and T2 is -0-, -S0 2 -, -S-, CF 3 0 CH 3 II -C- -C- , -C- , and -CH 2 - . CF 3 CH 3 Preferred diaminobenzoxazole monomers include: where Ar 7 is: c , (Q oUUOό ) (§T -τgl where T 3 is selected from the same group as T 2 and where Ar8 is selected from the same group as Ar2, Ar 3 , Ar*> and Ar 6 . Preferred diaminobenzoxazole monomers include: where Ar 9 is: and T4 is : CF 3 1 -0-, -so 2 -, 0 -C- -S- , -CH2- , 1 -c- CF 3 and Ar 10 , Ar 11 are selected from the same group as Ar 7 . The most preferred diaminobenzoxazole monomers are 2,6-(4,4'-diaminodiphenyl)benzo[1,2-d:5,4-d']bisoxazole ("DABO"): 5-amino-2-(p-aminophenyl)benzoxazole ("pDAMBO"), 5-amino-2-(m-aminophenyl)benzoxazole ("mDAMBO"), 4,4'-diphenylether-2,2'-bis(5-aminobenzoxazole) {"OBA(DAMBO)."), 2,2'-p-phenylenebis(5-aminobenzoxazole) ("TA(DAMBO) 2 "), and and 2,2-bis(4-phenyl)hexafluoropropane-2,2'-bis(5-aminobenzoxazole) ( "6FA(DAMBO ) ."). The PIBO can be prepared by reacting one or more dia:nhy ~ ®drides with one or more diaminobenzoxazoles to form a polyamic acid and subsequently imidizing the reaction product. The polyamic acid and its properties will vary depending on the amount and type of dianhydride and diaminobenzoxazole, and, if employed, any additional monomers. In general, the dianhydride and diaminobenzoxazole are employed in amounts from 0.75 to 1.25, preferably from 0.8 to 1.2, more preferably from 0.9 to 1.1, equivalents of dianhydride per equivalent of diaminobenzoxazole. To prepare a high molecularweight polyamic acid, the dianhydride and diaminobenzoxazole are most preferably employed in equivalent amounts (that is, the reaction mixture contains equivalent number of amino and carboxylic anhydride groups). When non-stoichiometric amounts of the reactants are employed, the molecular weight is normally less than that achieved using stoichiometric amounts of reactants. It is not particularly critical how the aromatic dianhydride and aromatic diamine are contacted and, in general, they are dissolved in a suitable reaction medium and reacted to form a polyamic acid solution. In the practice of the present invention, the liquid reaction medium is a solvent for the monomers and the resulting polyamic acid and is sufficiently inert to the monomer(s) that it does not significantly and deleteriously affect polymerization. Typically, the solvent is a polar, aprotic liquid. While the polar, aprotic liquid most o advantageously employed as a liquid reaction medium is dependent on the specific dianhydride and diaminobenzoxazole employed, as well as the desired reaction product, preferred polar, aprotic solvents are generally amide solvents such as N,N-dimethylacetamide . N-methylpyrrolidinone, 1,3-dimethyl-5-imidazolidinone, N,N-dimethylformamide, 1,1,3,3-tetramethylurea, or N-cyclohexylpyrrolidinone. Mixtures of one or more polar aprotic 5 liquids may also be employed as the liquid reaction medium. To facilitate the formation of a solution of the monomers or the resulting polymers in the solvent, inorganic salts such as lithium chloride and/or calcium chloride, can be added to the reaction mixture. However, this practice is not normally preferred. It is acceptable to have small amounts of non-reactive, non- -solvents or polyamic acid (for example, toluene) in the polar/aprotic solvent. 0 The aromatic dianhydride and aromatic diamine are dissolved in a suitable reaction medium and reacted to form a polyamic acid solution. For example, the aromatic dianhydride and aromatic diamine can be added separately, either neat or in solution, to a solvent in which both the dianhydride and diamine are soluble and then reacted. Alternatively, a solution of either the aromatic dianhydride or aromatic diamine can be prepared and the 5 other reactant, either neat or dissolved in a solvent, can be added to the solution and then reacted in solution. Preferably, the monomers, either neat or as a solution, more preferably neat, are simultaneously added to the liquid reaction medium. The reaction of the dianhydride, diaminobenzoxazole, and other comonomers, if any, can be conducted at any temperature between the freezing point and the boiling point of the solvent, but within that constraint, is preferably conducted at a temperature from -40°C to 150°C, more preferably from -20°Cto 100°C, and most preferably from 0°C to 50°C. Due to the equilibrium between amic acid linkage, the monomers' reactive functionalities and the presence of water (generated from thermal imidization) and its hydrolysis of amic acid linkages, the polymerization, as well as subsequent storage, is most preferably conducted at a temperature below about 50°C. The monomers can be added to the liquid reaction medium in any amount which allows reaction but are typically limited by the solubility of the monomers and resulting polyamic acid. While the amount of the monomers most advantageously employed is dependent on a variety of factors, including the specific monomers and organic liquid employed and the resulting polymer, the monomers are preferably used in an amount such that their concentration in the liquid reaction medium is at least about 0.1 weight percent. In general, the monomers are employed in an amount from 0.5 to 50 percent based on the total weight of the monomers and the organic reaction liquid. In general, the monomers are preferably employed in an amount from 1 to 30 and more preferably in an amount from 2 to 20 weight percent based on the total weight of the monomers and organicreaction liquid employed. In preparing the polyamic acid, it is most preferred that the monomers, solvent, o and reactor vessel contain as little water as possible prior to reaction. In addition, conditions of extreme cleanliness are also most preferred. Preferably, there should be less than 5 mole percent water based on the total moles of dianhydride, diaminobenzoxazole and any optionally employed additional comonomers. If necessary, toluene, or other materials capable of forming an azeotrope with water, can be added to the solvent and later distilled off to 5 remove water. The polyamic acid is also preferably prepared and stored in both an oxygen-free and water-free atmosphere to enhance the retention of molecular weight. In addition, the monomers can be recrystallized and/or sublimed prior to use to reduce impurities. While not being bound by theory, the reaction of a diamine and dianhydride to form polyamic acid can be represented by: 0 wherein M is an aromatic-containing group such as depicted hereinbefore and it being understood that the amic acid linkages can exist as a variety of substituted isomers. In addition to the dianhydride and diamine, the polyamic acid may optionally be prepared using one or more other monomers, for example, a non-benzoxazole-containing diamine such as 1,4-phenylenediamine or 4,4'-oxydianiline or 3,4'-oxydianiline. 5 A compound which has either a single functional group or a compound, which may have more than one functional group but only one of which reacts during the preparation of the polyamic acid (that is, is monof unctional in the terms of making amic acid linkages), may also optionally be employed in preparing the polyamic acid. The other group or groups, while not reactive under the conditions used to prepare polyamic acid, may be reactive under different conditions. In terms of polyamic acid preparation, these compounds act as chain terminators or "end-capping" agents. As chain terminators, the use of end-capping agents may reduce the molecular weight of the polyamic acid which might otherwise be obtained using the aromatic dianhydride and diaminobenzoxazole but the inherent viscosity of the polyamic acid should still be at least 2 dL/g. Useful end-capping agents include maleic anhydride, 2,3-dimethylmaleic anhydride, 3-(3-phenylethynlphenoxy) aniline, phenylethynlaniline, nadic anhydride, phthalic anhydride, aniline, propargylamine, vinyl o phthalic anhydride, ethynylaniline, and 4-aminobenzocyclobutene. These optionally employed monomers may be added to the reaction mixture prior to or during the reaction of the aromatic dianhydride and diaminobenzoxazole. In general, when employed, the monomers are employed in an amount from 0.01 to 50, preferably from 0.1 to 20, more preferably in an amount from 0.2 to 5, mole percent based on 5 the total moles of aromatic dianhydride and diaminobenzoxazole employed. The molecular weight of the polyamic acid can be expressed, albeit indirectly, in terms of its inherent viscosity. Inherent viscosity ("IV") is expressed as: •Qinh = In (ηrel c wherein η re | is the relative viscosity or t/t 0 where t is the flow time for the polyamic acid 0 solution and t 0 is the flow time for the pure solvent, and c is concentration of polyamic acid polymer in solution, given in grams per deciliter. The units for IV are deciliters per gram ("dL/g"). Measuring inherent viscosity of the polyamic acid solution is also a means of determining when the reaction is complete. For the purposes of this invention, reaction is presumed complete when the inherent viscosity does not increase between two measurements 5 taken at least twelve hours apart. The polyamic acids of the present invention exhibit an inherent viscosity of greater than 2.0 dL/gram, as measured at a solution concentration of 0.2 g/dL in N-methylpyrrolidinone at 25°C. At these relatively high inherent viscosities, the physical and chemical properties, such as one or more of tensile strength, tensile modulus, coefficient of thermal expansion and elongation-at-break, have been generally found to be excellent without the need for any further polymerization. More preferably, the polyamic acid exhibits an inherent viscosity of greater than about 2.5 dL/gram, most preferably greater than about 3.0 dL/gram, highly preferably greater than about 4.0 dL/gram, more highly preferably greater than about 5.0 dL/gram, and most highly preferably greater than about 6.0 dL/gram. Solutions of polyamic acid preferably have a viscosity of no more than about 500,000 centipoise. Following preparation, the polyamic acid, generally without subsequent isolation, is converted to polyimidebenzoxazole. While any method which converts the polyamic acid to the polyimidebenzoxazole can be employed in the practice of the present invention, the polyamic acid solutions are generally heated to a temperature sufficient to cause the closure of amic acid linkages; thereby converting the polyamic acid to polyimidebenzoxazole. The ring-closing reaction will form water and the total amounts of water generated increases as the reaction proceeds. While not bound by theory, the ring closure of amic acid linkages to form imide-linkages can theoretically be represented by the reaction scheme: - H O o Temperatures which can be employed while converting the polyamic acid to PIBO can vary from -10°C to 600°C. The reaction time required to convert the polyamic acid to the desired PIBO product ranges from as little as 5 minutes or less to as much as about 200 hours or more. Selection of specific temperatures and times are dependent upon a number of factors including the composition of the polyamic acid, the solvent, and concentration of polyamic acid in the solvent and the desired PIBO product. In the preferred method of the present invention, at least a portion of the liquid organic reaction medium (solvent) is removed from the polyamic acid solution and the polyamic acid then subjected to a temperature sufficient to convert the polyamic acid to PIBO. To remove the necessary amounts of the solvent from the polyamic acid solution, the solution can be contacted with a non-solvent for the polyamic acid such as water, methanol, acetone or other liquid; heated to a temperature to volatilize the solvent; or a combination of both. In general, sufficient amounts of the solvent are removed from the polyamic acid such that less than about 60 percent remains based on the weight of the solvent and the polyamic acid. Preferably, sufficient amounts of solvent are removed such that there is less than about 45, more preferably less than about 35, percent solvent remaining based on the weight of the solvent and polyamic acid. The polyamic acid typically binds some of the solvent until imidization; thereby making it difficult to remove sufficient solvent to reduce the solvent level by volatilization to below about 10 weight percent. In general, the polyamic acid solution is advantageously heated to remove at least a portion of the solvent and then subjected to a temperature sufficient to convert the polyamic acid to PIBO; at which time essentially all the solvent is removed. In the preferred method, the solvent is volatilized at a temperature of less than about 150°C to remove the desired amount of solvent and then heated to an imidization temperature of from 160°C to 240°C for a period sufficient to convert at least a portion of the polyamic acid to polyimidebenzoxazole. In general, the solvent is vaporized at a vaporization temperature from 50°C to 130°C, with the vaporization temperature(s) most advantageously employed within this range being dependent on a number of factors, particularly the vapor pressure/boiling point of the specific solvent employed. For example, a temperature of at least 50°C is preferred when the solvent is N.N-dimethylacetamide and a temperature of at least 80°C is preferred when the solvent is N-methylpyrrolidinone. This temperature may be achieved by steadily increasing the temperature to the desired value (known as "ramping") or by exposure to a constant heat o source set at the desired temperature. In many applications, the polyamic acid is kept at a vaporization temperature until it resembles more of a solid, shaped article than a polymer solution. The time the polyamic acid is heated at the vaporization temperature is dependent on a variety of factors including the solvent, percent solids, solution viscosity, exhaust rate, and the presence or absence of a coagulating fluid and chemical ring-closing agents, but, in 5 general, a minimum of 5 minutes is generally required with a maximum being less than about 2 hours. The polyamic acid is then heated to an imidization temperature from 160°C to 240°C to convert at least a portion of the polyamic acid into polyimidebenzoxazole. In general, the temperature of the polyamic acid is increased from the vaporization temperature to the 0 imidization temperature as quickly as possible. Preferably, the transition from an environment at the vaporization temperature to an environment at the conversion temperature should take less than about 30, preferably no more than about 10, and most preferably no more than about 5 minutes. The imidization temperature and time to which the polyamic acid is exposed is 5 dependent on a variety of factors, including the specific polyamic acid being processed, the desired PIBO properties, and the article being prepared. For example, in the preparation of a film coated on a support substrate, the substrate reduces the cracking, tearing, and breaking of the film during imidization which may occur if no support is employed. As such, the conversion temperature and method of imidization in preparing a supported film are not as critical as when an unsupported film is being prepared. In preparing an unsupported film, the conversion temperatu/e is more advantageously quickly achieved to minimize these defects. In general, an imidization temperature from 160°C to 280°C, more preferably from 185 C C to 240°C, and most preferably to 230°C is employed. The polyamic acid is kept at the imidization temperature(s) until the desired conversion is obtained. In general, this requires at least about 5 5 and no more than about 90, preferably from 10 to 80, more preferably from 15 to 75, minutes. The conversion of polyamic acid to PIBO is preferably conducted in an atmosphere of air or an inert gas that is preferably moisture free. While the polyamic acid is converted to PIBO at the imidization temperature, in the practice of the present invention, to further improve PIBO properties, the polyamic acid is preferably further treated by exposure to a temperature from about 250°C, preferably from about 300°C, to a temperature which is less than about 600°C and less than the glass transition temperature of the resulting polyimidebenzoxazole, for at least about 10 seconds. Preferably, the product is maintained at this elevated temperature for a sufficient period of time to ensure sufficiently complete conversion of the amic acid and/or isoimide linkages into imide linkages and until the desired PIBO properties are obtained. This further heat-treatment increases certain film properties such as tensile strength and/or elongation-at-break. If possible without o exceeding the glass transition temperature, by sequentially increasing these subsequent heat- -treating temperatures, the properties of the resulting PIBO, particularly the tensile strength and percent elongation-at-break, can unexpectedly be improved. For example, the polyamic acid/PIBO can be heated at a first lower temperature such as from 250°C to 350°C in air or an inert atmosphere, and then at a second temperature, higher than the first temperature, 5 preferably from 350°Cto 500°C, in air or an inert atmosphere, most preferably in a nitrogen atmosphere. In preparing PIBO articles, if the PIBO product is not soluble in the solvent for the polyamic acid, the polyamic acid solution is formed into the desired shape of the finished product such as a film, fiber or other article using conventional techniques, such as extrusion, 0 before substantial conversion of the polyamic acid to PIBO, that is, either before or during the removal of solvent. If the PIBO product is soluble in the solvent, it is not normally necessary and may not be desirable to pre-shape the polyamic acid solution. While the polyamic acid can be removed from solution and redissolved in another solvent, the polyamic acid is preferably processed directly from the organic liquid reaction solution (diluted, if necessary, with 5 additional amounts of the same or compatible solvent). Shaping the polyamic acid solution to form a film can be done by coating, casting, dipping, or spraying the polyamic acid solution onto an appropriate substrate and imidizing the polyamic acid either partially or completely. The film may be removed either before or after the imidization reaction. Films as thin as 1 micrometer, preferably having a thickness 0 from 1 to 2000 micrometers, can be made. Appropriate non-reactive substrates include materials such as glass, aluminum, stainless steel, silicon, copper, polyimide film and tetrafluoroethylene f luorocarbon polymers (marketed as Teflon τ ") to form a supported film. Often, a release agent such as Fluorogl.de"' or IMS Silicone Spray No. S316 is used to facilitate removal of the film from the substrate. 5 Alternatively, the PIBO can permanently coat these or other substrates, either as a single layer coating or as a coating applied in multiple layers. The polyamic acid solution can be applied at essentially any coating thickness and relatively thick coatings of up to 2000 micrometers can be prepared. While the desired thickness of the PIBO coating depends on the end-use application, it will generally vary from 0.1 to 100 micrometers. The coated substrates can be prepared using a variety of techniques well-known in the art for coating liquid solutions on a substrate and include spraying, or spin- or dip-coating the polyamic acid solution onto a clean substrate. To improve the adhesion of the PIBO to the substrate, an adhesion promoter such as 3-aminopropyltriethoxysilane can optionally be employed. The thickness at which the polyamic acid solution is applied as coating depends on the viscosity of the polyamic acid solution. In general, for any given polyamic acid, the coating thickness is reduced as the viscosity of the polyamic acid solution is reduced. The viscosity most preferably employed will depend on a variety of factors including the desired o thickness of the coating and the coating techniques. For example, using spin-coating techniques to prepare coatings of less than 100 micrometers, the viscosity of the polyamic acid solution (as measured by a Brookfield viscometer at 25°C) is preferably less than about 15,000 centipoise (cps), more preferably less than about 10,000 cps. Viscosity reduction can be achieved by simply diluting the polyamic acid solution to a desired viscosity. 5 One method for preparing multiple layered PIBO coatings involves applying a polyamic acid solution, volatilizing the solvent, applying another layer of polyamic acid solution and volatilizing the solvent, and repeating this process until such time as the desired thickness is obtained. The polyamic acid is then converted to PIBO. Alternatively, a multilayer coating can be prepared using a step-wise technique of repeated application and imidization o °f individual polyamic acid layers until the desired level of coating thickness has been applied. An optional adhesion promoter can be applied between layers. These types of thin PIBO coatings are useful for electrical insulating purposes and environmental resistance. While the reaction of the polyamic acid to form PIBO is effectively conducted by merely heating the polyamic acid, imidization can be conducted in the presence of a material 5 or combination of materials which facilitates or accelerates ring closing (for example, converts the amic acid linkages into imide linkages by catalyzing ring closure via dehydration). Of these materials, those whose by-products can be removed after or during imidization by vaporization and/or washing are most preferred. Materials which can be employed include dehydration agents such as acetic anhydride and other materials listed in column 5 of U.S. Patent 3,410,826, 0 which is incorporated by reference; with the preferred materials being acetic anhydride, propionic anhydride, ketene, and isobutyric dianhydride. While these materials can be employed alone, they are most preferably employed with an organic base, preferably those which do not form inorganic salts, which facilitate ring-closing activity such as picoline, pyridine, isoquinoline, triethylamine, lutidine or their mixtures, and other materials listed in 5 the previously incorporated reference in column 6 of U.S. Patent 3,410,826. These ring-closing agents (that is, the dehydrating agent and organic base) are generally mixed together before use, although they can be added separately to the polyamic acid solution. While the amount of ring-closing agent most advantageously employed is dependent on a variety of factors, including the specific ring-closing agent(s) employed, the desired reaction times and temperatures and the PIBO being prepared, the dehydration agent is preferably employed in an amount of more than about 10, more preferably more than about 100, and most preferably at least about 200, mole percent, preferably being used in an amount of less than about 500, more preferably less about 400, mole percent, said mole percent being based on the moles of dehydration agent per mole of amic acid linkages. The catalyst is employed in an amount of 1 to 200, preferably from 10 to 150, more preferably from 20 to 100, mole percent based on the theoretical maximum number of moles of amic acid linkages. The theoretical maximum number of polyamic acid linkages is easily determined by the moles of dianhydride employed. These ring-closing agents may by added to the polyamic acid solution before or during solvent removal or subsequent reaction. The temperature of the polyamic acid solution and ring-closing agents are advantageously adjusted to a temperature of from -20°C up to 140°C, more preferably from -20°C to 50°C, most preferably from -20°C to 15°C prior to mixing to minimize gelation or rapid build-up of viscosity of the polyamic acid solution. While the ring-closing agents, if any, can be added neat to the polyamic acid solution, they are preferably added as a solution, preferably in a solution of from 5 to 50 weight percent, in an organic liquid which is compatible with the polyamic acid solution. Preferably, the solution of the ring- -closing agents is prepared using the same solvent as employed in the polyamic acid solution. The polyamic acid can first be formed into a desired shape and the shaped article contacted with a solution of the ring-closing agents, generally for a time of at least about 30 seconds but less than about 30 minutes. In general, once the polyamic acid has been contacted with the ring-closing agent, conversion of amic acid linkages to imide linkages begins, with the rate of conversion being temperature dependent. In preparing PIBO articles, additives such as fillers, pigments, carbon black, conductive metal particles, abrasives and lubricating polymers are often advantageously employed. Conventional additives can be employed and the method of incorporating the additives is not critical. They can conveniently be added to the precursor solution prior to preparing the shaped article. The polyamic acid solution, alone or containing additives, may be applied by any of the usual techniques (doctoring, rolling, dipping, brushing, or spraying) onto a number of different substrates. If the PIBO polymer is prepared in solid form, the additives can be added to the solution prior to processing into a shaped article. For PIBO which remains soluble in the organic liquid reaction medium, it is generally preferred to add the ring-closing agents directly to the polyamic acid solution, allow the reaction to proceed at slightly elevated temperatures, for example, up to 50 C C, but most preferably room temperature (20°C to 25°C), followed by volatilization of the solvent and heat- -treatment. In general, when the resulting PIBO is soluble, ring-closing agents are preferably employed. The following examples are presented to illustrate the invention and should not be construed to limit its scope. In the examples, all parts and percentages are by weight unless 5 otherwise indicated. In these examples, inherent viscosities are measured by transferring an aliquot of the polyamic acid solution to a 25.0 mL volumetric flask and diluting with N-methylpyrrolidinone to achieve a solution concentration of 0.2 g/dL Solvent and solution flow times are measured at 25.0 C C using a Schott-Gerate CT 1450/AVS 310 viscometry apparatus 10 in a Ubbelohde viscometer tube with an inner diameter of approximately 0.63 mm. Tensile properties are measured according to ASTM D882. Adhesion is measured using a Cross-Hatch Tape Peel Adhesion Test. In such test, a coated wafer is scored using a surgical blade in a manner which produces 100 squares approximately 1 millimeter by 1 millimeter (mm). The coating is then attempted to be removed from the wafer using Scotch'" 15 810 tape and Scotch'" 600 tape. Four peels, two with each type tape, are conducted. The results are scored as number of squares pulled off by the tape per 100 squares. The lower the number, the better the adhesion of the film. The test is then repeated after the wafers are immersed in a 95°C water bath for one hour and cooled to room temperature. Examples 1-6 0 Int a 3-neck, 250-mL round-bottom flask equipped with agitation means and a Dean-Stark trap with a condenser was fed 28.9 mL of 1,3-dimethyl-2-imidazolidinone (DMI) and 14 mL of toluene. The flask was gently purged by nitrogen. Toluene was distilled off. Into the stirred, room-temperature solvent was added 3.042 g (8.8 mmol) of 2,6-(4,4'-diaminodiphenyl)benzo[1,2-d:5,4-d']bisoxazole (DABO) containing 0.96 weight 5 percent N-methylpyrrolidinone (NMP) and 1.919 g (8.80 mmol) of pyromellitic dianhydride (PMDA) with monomers rinsed down by 19 mL of dry N-methylpyrrolidinone. After about 16 hours, the resulting polyamic acid solution was diluted with 42.2 mL of a dry mixture of 60 volume percent DMI and 40 volume percent of NMP. Inherent viscosity (IV) was measured in NMP, at 25.0°C, and 0.2 g/dL The results of this testing are shown in Table 1. 0 Similar polyamic acid solutions (Examples 2-6) were made using essentially the same techniques with varying amounts of DABO and PMDA as summarized in Table 1 and the IV tested. The results of this testing are shown in Table 1. 5 Table 1 E " o Plβ DMI m T (mP DABO (mmol ) PMDA (mmol ) NMP (mL)* ^Jffl IY 1. 28.9 14 3.042 g (8.8) 1.919 g (8.8) 19 42.2 4.5 2. 51.4 25 5.383 g (15.62) 3.407 g (15.62) 33.7 ■ 74.9 3.3 3. 44.5 30 4.696 g (13.62) 2.971 g (13.62) 26.7 + 3.0 98.4 6.6 4. 43.1 28.8 4.540 g (13.19) 2.878 g (13.19) 25.6 + 2.9 88.3 5.7 5. 300 200 31.620 g (91.759) 20.014 g (91.759) 180 + 20 349.8 6.1 6. 21.6 10 2.266 g (6.572) 1.433 g (6.572) 14.2 31.5 3.4 I t-o I *When total volume of NMP is written x+y, it means that x was added to DMI and toluene at the start of the reaction with y being used to rinse down the monomers. Larger flasks are utilized as needed and different lots of DABO monomers have different NMP contents which can be back-calculated from the data in the table. Example 7 A 5.5 weight percent solution in NMP/DMI of the polyamic acid of Example 5 was cast onto a clean glass plate using a doctor blade with a 0.76 mm gap. The film was heated in an air-circulating oven at 80°C for 1.5 hours. The film was removed from the glass and placed in a solution of 35 percent acetic anhydride, 15 percent pyridine, and 50 percent toluene for 30 minutes, then washed twice with methanol. The film was secured in an aluminum frame and heated in a nitrogen atmosphere for 27 minutes at 30 C C, then heated 45 minutes to 300°C, maintained at 300°C for 1 hour, heated 30 minutes to reach 450°C, and maintained at 450°C for 1 hour. The sample was then cooled. The film sample exhibits a tensile strength of 35.6 Ksi (0.245 GPa); a tensile modulus of 1.26 Msi (8.69 GPa) and an elongation-at-break of about 9 percent. Example 8 To a dry, 3-neck, 250-mL round-bottom flask having a nitrogen atmosphere and equipped with agitation means and inlet and outlet adaptors was added 84.3 g of the polyamic acid solution of Example 5. About 12.1 mL of a dry mixture of 60 volume percent 1,3-dimethyl- imidazolidinone and 40 volume percent N-methylpyrrolidinone (DMI/NMP) was added to the flask and stirred overnight at room temperature. The flask was chilled by an ice-water bath and degassed by aspirator vacuum. The flask was brought back to atmospheric pressure with nitrogen and a pressure equalizing dropping funnel attached. A mixture of 11.7 mL of DMI/NMP, 3.19 mL of acetic anhydride (AA), and 0.44 mL β-picoline was added, using the dropping funnel, to the stirred, chilled polyamic acid solution over a 2-minute period. While still being chilled, the resulting mixture of polyamic acid and ring-closing agent was stirred for about 2 minutes and then degassed for about 6 minutes by aspirator vacuum while still chilled. The flask was brought to atmospheric pressure using nitrogen, closed and packed in ice. A 4.25 percent solution of the resulting polyamic acid in NMP/DMI was cast onto a clean glass plate with a doctor blade with a 0.030 inch (.762 mm) gap. The film was heated in an air-circulating oven at 80°C for 1.5 hours. The film was removed from the glass and placed in 10 percent NMP, 63 percent acetic anhydride and 27 percent pyridine for 30 minutes. The film was secured in an aluminum frame, air-dried for about 10 minutes and placed in an air- -circulating oven, heated for 20 minutes at 280 C C, heated to 340°C over a 20-minute period, and maintained at 340 C C for 1 hour. In a nitrogen atmosphere, the film was further heated for 27 minutes at 25°C, heated to 400°C over a 75-minute period and maintained at 400 C C for 2 hours. The film exhibited a tensile strength of 44.1 Ksi (0.304 GPa); a tensile modulus of 2.33 Msi (16.1 GPa); and an elongation-at-break of 3.6 percent. Example 9 Oxidized 3 inch (7.62 cm) silicon wafers having one-half of the wafer copper coated were plasma-cleaned with oxygen for 15 minutes and dump-rinsed with water 3 times. The wafers were then spun-dried in air. The wafers are treated on the spin-coater with either 1 percent acetic acid in deionized water (identified as HOAc in Table 2), 1 percent acetic acid in deionized water and an adhesion promoter (identified as HOAc and APS in Table 2), or were not surface treated (identified as None in Table 2). A 5 weight percent polyamic acid solution of Example 6 in DMI/NMP was spin-coated on the wafer using a spread cycle of 500 rpm for 38 seconds and a spin-cycle of 3000 rpm for 30 seconds in a spin-caster. The polyamic acid coated wafers were heated to 60°C for 15 minutes, heated at 225°C for 5 minutes, and at 300°C for 10 minutes. They were then cooled to room temperature overnight and their adhesion to the wafer evaluated. The results of this adhesion testing are reported in Table 2. Table 2 SCORE after SCORE Immersion Into a 3-neck, 250-mL round-bottom flask equipped with agitation means and a Dean-Stark trap with condenser was loaded 50.9 mL N,N-dimethylacetamide (DMAc), 20 mL toluene (T). The flask was gently purged by nitrogen via inlet and outlet adaptors. Toluene was distilled off. Into the stirred, room-temperature solvent was added 4.313 g (12.49 mmol) 2,6-(4,4'-diaminodiphenyl)benzo[1,2-d:5,4-d']bisoxazole (DABO) containing 0.89 weight percent NMP and 3.873 g (12.49 mmol) of 4,4'-oxydiphthalic anhydride (ODPA) with monomers rinsed down by 10.0 mL dry DMAc. After about 68 hours, the polyamic acid solution was diluted with dry 38.2 mL DMAc. The inherent viscosity (IV) of the resulting solution was measured and recorded in Table 3. Similar polyamic acid solutions (Examples 11-16) were made using essentially the same techniques with varying amounts of DABO and ODPA as summarized in Table 3. The inherent viscosity of each of the resulting polyamic acid solutions was measured and recorded in Table 3. I t l_π I Larger flasks were utilized as needed and different lots of DABO monomer have different NMP contents which can be back calculated from the data in the table. Example 17 In a similar manner to Example 9, three-inch oxidized wafers were prepared and a 10 percent solution of the polyamic acid of Example 10 in DMAc was spin-coated onto the wafers. The coated wafers were subjected to the same temperatures and tested for adhesion. The results of this adhesion testing are reported in Table 4. Table 4 SCORE after SCORE (initial) Immersion 0 5 A 6 inch (15.24 cm) x 6 inch (15.24 cm) piece of 0.005 inch (.127 mm) thick aluminum foil was taped to a glass plate. A 12.5 percent solution of the polyamic acid of Example 13 in N,N-dimethylacetamide (DMAc) was cast onto the aluminum foil using a doctor Q blade with a 0.010 inch (.254 mm) gap. The coated aluminum foil was then heated to 60°C in an air-circulating oven for 30 minutes. The coated aluminum foil was removed from the glass and secured in a brass frame with a 5.5 inch-square (13.97 cm) opening. The coated aluminum foil was then exposed to an environment heated for 30 minutes at 60°C, heated to 225°C for a 20-minute period, maintained at 225°Cfor 15 minutes, heated to 300 C C over a 20-minute 5 period, and maintained at 300°C for 60 minutes. The coated foil was removed from the oven, cooled and then, in a nitrogen atmosphere, heated for 27 minutes at 30°C, heated over 75 minutes to 350°C and maintained at 350°C for 2 hours. It was then cooled to room temperature. The resulting coating was bubble- -free and 12 to 16 μm thick. Q Example 19 A 0.050 inch (1.270 mm) titanium panel was cleaned with an abrasive pad, soap and water, rinsed in water, rinsed in acetone, rinsed in 1 ,1,1-trichloroethane and dried. The titanium panel was held in place on a glass plate by 32 mil Teflon ® fluorocarbon polymer shims. A 12.5 percent solution in DMAc of the polyamic acid of Example 14 was cast onto the titanium 5 panel using a doctor blade with a 0.030 inch (.762 mm) gap. The coated titanium panel was then placed in an air-circulating oven heated for 60 minutes at 60°C, heated for 30 minutes to 225°C, maintained at 225°C for 20 minutes, heated for 20 minutes to 300°C, and maintained for 60 minutes at 300°C. The titanium panel was removed from the oven and cooled. Using the tape peel test, the coating adhered well to the panel with no failures indicated. Other coated titanium panels were prepared in the same manner and, after the thermal treatment in air, they were heated in nitrogen for 27 minutes at 30°C, heated for 75 minutes to 400 C C, and maintained at 400°C for 2 hours. It was then cooled to room temperature. The adhesion of the coating was excellent on these samples as well. A piece of release-coated polyimide film (sold under the tradename Thermalimide*) was secured in an aluminum frame. Anothersample of a 12.5 percent solution of the polyamic acid of Example 14 in DMAc was cast onto the film using a Teflon'" fluorocarbon polymer doctor blade with a 0.015 inch (.381 mm) gap. The coated film was heated in an air-circulating oven for 60 minutes at60°C, for 20 minutes at 225°C, 20 minutes ramp to 300°C, and 60 minutes at 300°C. The PIBO ilm was peeled from the polyimide film and placed in a nitrogen atmosphere, heated for 27 minutes at 25°C, 75 minutes to 400 C C, and maintained at 400°C for 2 hours. It was then cooled to room temperature. The film exhibited a tensile strength of 39.0 Ksi (0.269 GPa), a tensile modulus 1.29 Msi (8.90 GPa), and an elongation-at-break of 9.7 percent. Yet another 12.5 percent solution of the polyamic acid of Example 14 in DMAc was cast onto a clean silylated glass plate with a doctor blade with a 0.015 inch (.381 mm) gap. The film was heated in an air-circulating oven at 60°C for 1 hour. The film was removed from the glass and placed in a mixture of 35 percent acetic anhydride, 15 percent pyridine and 50 percent toluene for 30 minutes, then washed twice with methanol. The film was secured in an aluminum frame and placed in a nitrogen atmosphere heated for 27 minutes at 30°C, for 45 minutes to 300°C, maintained at 300°C for 1 hour, heated for 30 minutes to 400°C, and maintained at 400°C for 1 hour. It was then cooled to room temperature. The film exhibited a tensile strength of 38.8 Ksi (0.267 GPa), a tensile modulus of 1.23 Msi (8.49 GPa), and an elongation-at-break of 18.4 percent. Example 20 A piece of heavy (5 mil) polished aluminum foil was secured in an aluminum frame. A 12.5 percent solution of the polyamic acid of Example 15 in DMAc was cast onto the foil using a Teflon'" fluorocarbon polymer doctor blade with a 0.010 inch (.254 mm) gap. The coated film was placed in an air-circulating oven, heated for 45 minutes at 60°C, for 25 minutes to 225°C, maintained at 225°C for 20 minutes, for 18 minutes to 300°C, and maintained at 300°C for 60 minutes. The film on the framed aluminum foil was removed from the oven and cooled. The aluminum foil was removed from the frame and the film on the foil was then placed in a nitrogen atmosphere, heated for 27 minutes at 30 C C, for 75 minutes to 400°C, and maintained at 400°C for 2 hours. The film was then cooled to room temperature. The excess aluminum foil around the film was trimmed off and the film on the aluminum was placed into a stirred 10 percent aqueous solution of NaOH until the film lifts from the aluminum (about 1 hour). The film was then thoroughly rinsed with deionized water, wiped free of surface residues with damp towels and allowed to air-dry. The film had a tensile strength of 33.8 Ksi (0.233 GPa), a tensile modulus of 1.17 Msi (8.07 GPa), and an elongation-at-break of 6.9 percent. Example 21 In a nitrogen atmosphere, 61.18 g of the polyamic acid solution of Example 16 was added to a dry, 3-neck, 250-mL round-bottom flask equipped with agitation means and o inlet and outlet adaptors. To the flask was added 7.2 mL of dry N,N-dimethylacetamide (DMAc). The resulting mixture was stirred overnight at room temperature. The flask was chilled for 10 minutes in an ice-water bath and then, while remaining in the ice bath, degassed by an aspirator vacuum. The flask was brought back to atmospheric pressure with nitrogen and a pressure equalizing dropping funnel attached. A ring-closing agent composed of 2.3 mL 5 acetic anhydride (AA) and 0.60 mL β-picoline in 8.7 mL of DMAc was added dropwise to the stirred, chilled polyamic acid solution over a 3-minute period. The resulting solution was stirred for 4 minutes and then degassed for about 13 minutes by aspirator vacuum while still being chilled. The flask was brought to atmospheric pressure by using nitrogen, closed and packed in ice. 0 A 9.5 percent solution of the polyamic acid in DMAc was cast onto a clean glass plate with a doctor blade with a 0.010 inch (.254 mm) gap. The film was heated in an air-circulating oven at 60°C for 30 minutes, then removed from the glass and secured in an aluminum frame. The framed film was placed in an air-circulating oven, heated for 15 minutes at 225°C, heated for 20 minutes to 300°C, and maintained at 300°C for 1 hour. 5 The film was further exposed to a nitrogen atmosphere, heated for 27 minutes at 25°C (N 2 purge cycle), 75 minutes to 400°C, and then maintained at 400°C for 2 hours. The film was then cooled to room temperature. The PIBO film was approximately 12 μm thick and exhibited a tensile strength of 51.62 Ksi (0.356 GPa), a tensile modulus of 1.87 Msi (12.9 GPa), and an elongation-at-break of 0 9.9 percent. Example 22 Into a 3-neck flask similar to that used in Example 1 , was added 60 mL of N,N-dimethylacetamide (DMAc) and 25 mL of toluene (T). The flask was gently purged by nitrogen via inlet and outlet adaptors. The toluene was distilled off. Into the room- 5 -temperature solvent was added 4.343 g (12.69 mmol) of 2,6-(4,4'-diaminodiphenyl)benzo[1,2-d:5,4-d']bisoxazole (DABO), 0.0035 mL (0.038 mmol) aniline, and 3.738 g (12.70 mmol) of 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA) with monomers rinsed down by 17.6 mL of dry DMAc. After about 68 hours, the polyamic acid solution was diluted with 21.0 mL of dry DMAc. After a reaction time of about 236 hours at room temperature, the inherent viscosity (IV) was measured as 2.76 dL/g (NMP, 25.0°C, 0.2 g/dL). An 8 percent solution of the resulting polyamic acid in DMAc was cast onto a clean glass plate with a doctor blade with a 0.015 inch (.381 mm) gap. The film was heated in an air-circulating oven at 50°C for 2 hours. The film was removed from the glass and placed in a mixture of 35 percent acetic anhydride, 15 percent pyridine, and 50 percent toluene for 30 minutes, then washed twice with methanol. The film was secured to a tetrafluoroethylene fluorocarbon polymer support and dried for approximately 18 hours under ambient conditions. The film was removed from the support and placed onto a piece of aluminum foil. The film was exposed in a nitrogen atmosphere to 25°Cfor 27 minutes (N 2 purge cycle), heated for 30 minutes to 100°C, maintained at 100°Cfor 1 hour, heated for 30 minutes to 200°C, maintained at 200°C for 1 hour, then heated for 30 minutes to 300°C, and maintained at 300°C for 1 hour. The film was then cooled. The resulting film was approximately 9 μm thick. The film was split into two pieces. The tensile properties of one piece were tested and the film found to exhibit a tensile strength of 27.4 Ksi (0.189 GPa), a tensile modulus of 1.24 Msi (8.56 GPa), and an elongation-at- -break of 4 percent. The other piece was exposed in a nitrogen atmosphere for 27 minutes at 25°C (N purge cycle), heated for 30 minutes to 350 C C, and maintained at 350°C for 2 hours. The film was then cooled. It exhibited a tensile strength of 34.75 Ksi (0.240 GPa), a tensile modulus of 1.29 Msi (8.90 GPa) and an elongation-at-break of 19.8 percent. Another 8 percent solution of the polyamic acid in DMAc was cast onto a clean glass plate with a doctor blade with a 0.020 inch (.508 mm) gap. The film was heated in an air-circulating oven at 60°C for 1.5 hours. It was then removed from the glass and secured in an aluminum frame and heated for 10 minutes at 245°C, for 35 minutes to 350°C, and maintained at 350°C for 30 minutes. It was then cooled to 100°C and removed from the oven. The f ilm,was then heated in a nitrogen atmosphere for 27 minutes at 25°C (N purge cycle), heated for 75 minutes to 400°C, and maintained at 400°C for 2 hours. It was then cooled to room temperature. The film exhibited a tensile strength of 41.97 Ksi (0.290 GPa), a tensile modulus of 1.61 Msi (11.1 GPa), and an elongation-at-break of 8.7 percent. Example 23 In a manner similar to that of Example 22, a polyamic acid solution was prepared using an initial solvent feed of 103.4 mL of 1,3-dimethyl-2-imidazolidinone (D I), 55.8 mL of N-methylpyrrolidinone (NMP), and 100 mL of toluene (T) and adding, after distilling off the toluene, 10.816 g (31.385 mmol) of 2,6-(4,4'-diaminodiphenyl)benzo[1,2-d:5,4-d']bisoxazole (DABO) containing 0.65 weight percent NMP, 1.110 g (5.543 mmol) of 4,4'-oxydianiline (ODA), and 8.055 g (36.93 mmol) of pyromellitic dianhydride (PMDA) with monomers rinsed down by 13.1 mL of dry N-methylpyrrolidinone (NMP). The viscous solution was diluted with a total of 83.1 mL of a dry mixture of 60 volume percent DMI and 40 volume percent NMP. After about 68 hours' reaction time at room temperature, the inherent viscosity of the reaction product was measured as 4.65 dL/g (NMP, 25.0°C, 0.2 g/dL). In a manner similar to that of Example 8, 57.60 g of the resulting polyamic acid solution was stirred overnight with 6.1 mL of a dry mixture of 60 volume percent DMI and 40 volume percent NMP. The flask was chilled by an ice-water bath and degassed by aspirator vacuum. After the flask was brought back to atmospheric pressure with nitrogen, a mixture of 9 mL of 60 percent DMI and 40 percent NMP, 2.78 mL of acetic anhydride, and 0.38 mL of 10 β-picoline was added dropwise to the stirred, chilled polyamic acid over a 2-minute period. The resulting mixture was stirred for 2 minutes and then degassed for about 6 minutes while still chilled. The flask wasbrought to atmospheric pressure using nitrogen. A 5.5 percent solution of the polyamic acid in NMP/DMI was formed into a film in a manner similar to Example 21 , except the solution was cast onto a clean, silylated glass plate 15 at 0.030 inch (.762 mm) and heated in the air-circulating oven for 1 hour at 80°C, and then in a nitrogen atmosphere for 27 minutes at 25°C (N 2 purge cycle), 45 minutes to 280°C, maintained at 280°C for 20 minutes, heated for 20 minutes to 350°C, maintained for 1 hour at 340°C, heated for 30 minutes at400°C, and maintained for 1 hour at 400°C; then cooled to room temperature. The film exhibited a tensile strength of 36.7 Ksi (0.0253 GPa), a tensile modulus of 20 2.01 Msi (13.9 GPa) and an elongation-at-break of 2.8 percent. The film exhibited a tensile strength of 36.7 Ksi (0.253 GPa). Example 24 In a manner similar to that of Example 22, a polyamic acid solution was prepared using an initial solvent feed of 234.7 mL of 1,3-dimethyl-2-imidazolidinone (DMI), 136.5 mL of 25 N-methylpyrrolidinone (NMP), and 100 mL of toluene (T). Into the stirred, room-temperature solvent, after distilling off the toluene, was added 22.209 g (64.57 mmol) of 2,6-(4,4'-diaminodiphenyl)benzo[1,2-d:5,4-d']bisoxazole (DABO) containing 0.45 percent NMP, 4.31 g (21.53 mmol) of 4,4'-oxydianiline (ODA), and 18.781 g (86.10 mmol) of pyromellitic dianhydride with monomers rinsed down by 20 mL of dry N-methylpyrrolidinone (NMP). The 30 viscous solution was diluted with a total of 355.5 mL of dry 60 percent DMI and 40 percent NMP. After about 116 hours' reaction time at room temperature, a reaction aliquot had its inherent viscosity (IV) measured. IV = 5.51 dL/g (NMP, 25.0°C, 0.2 g/dL). In a manner similar to that of Example 8, 41.87 g of the resulting polyamic acid solution was stirred overnight with 12.2 mL of a dry mixture of 60 volume percent DMI and 35 40 volume percent DMI/NMP. After the flask had been chilled in an ice-water bath, degassed and brought back to atmospheric pressure using nitrogen, a mixture composed of 5.8 mL of 60 percent DMI and 40 percent NMP, 1.67 mL of acetic anhydride, and 0.23 mL of β-picoline was added dropwise to the stirred, chilled polyamic acid over a 1 -minute period. The resulting mixture was stirred for 2 minutes and then degassed while still chilled. The flask was brought to atmospheric pressure using nitrogen. In a manner similar to Example 7, a 3.8 percent solution of the resulting polyamic acid in NMP/DMI was cast onto a clean, silylated glass plate with a doctor blade having a 0.03 inch (.762 mm) gap and formed as a film except that, after being washed with methanol, the resulting film was heated in an air-circulating oven for 1 hour at 300°C and then exposed to a nitrogen atmosphere, heated for 27 minutes at 25°C (N 2 purge cycle), for 75 minutes to 400°C, maintained at 400°C for 1 hour and subsequently cooled. The resulting film exhibited a tensile strength of 38.2 Ksi (0.264 GPa), a tensile modulus of 1.42 Msi (9.80 GPa), and an elongation-at- -break of 12.2 percent Example 25 A polyamic acid solution was prepared in a manner identical to Example 24. A 4 percent solution of the polyamic acid in DMI/NMP was cast on a clean glass plate with a doctor blade with 0.020 inch (.508 mm) gap. The polymer was coagulated from solution by immersing the coated glass plate in methanol for 10 minutes. The resulting film was collected and imidized by immersing it for 5 minutes in a mixture of 700 mL acetic anhydride, 300 mL pyridine, and 1000 mL toluene. The film was washed in methanol for 10 minutes. The film was then framed onto an aluminum frame and air-dried for 45 minutes at ambient conditions. The air-dried film was placed in an air-circulating oven, heated for 50 minutes at 225°C, for 30 minutes to 300°C, and maintained at 300°C for 1.5 hours. The film was removed from the oven and cooled to room temperature. The film was then exposed, in a nitrogen atmosphere, heated for 27 minutes at 30 C C, 75 minutes to 400°C, maintained at 400°C for 2 hours, and then cooled. The resulting film had a tensile strength of 41.5 Ksi (0.286 GPa), a tensile modulus of 1.47 Msi (10.1 GPa), and an elongation-at-break of 11.3 percent. Example 26 A polyamic acid solution was prepared in a manner similar to Example 1 except that the initial solvent was 135 mL N,N-dimethylacetamide (DMAc) and 50 mL toluene (T). After the toluene was distilled off, 9.737 g (43.23 mmol) of 5-amino-2-(p-aminophenyl)- benzoxazole (pDAMBO) and 9.429 g (43.23 mmol) of pyromellitic dianhydride (PMDA) rinsed down by 15 mL dry DMAc was added. A total of 34.1 mL of DMAc was used to dilute the viscous solution. After about 44 hours, an aliquot of the polyamic acid solution was found to have an inherent viscosity (IV) of 4.31 dL/g (NMP, 25.0°C, 0.2 g/dL). In a manner similar to Example 8, 54.15 g of the resulting polyamic acid solution was stirred overnight with 7.2 mL of dry DMAc at room temperature. After the flask was chilled and degassed, brought back to atmospheric pressure with nitrogen, a mixture of 7.2 mL (DMAc), 2.49 mL acetic anhydride, and 0.66 mL β-picoline was added dropwise to the stirred, chilled polyamic acid over a 1-minute period. The resulting mixture was stirred for about 3.5 minutes and then degassed for about 8 minutes. The flask was brought to atmospheric pressure using nitrogen. In a manner similar to Example 8, a portion of the resulting mixture was cast as a 0.030 inch (.762 mm) film which was placed in an air-circulating oven heated at 60°C for 90 minutes. The film was removed from the glass and secured in an aluminum frame, then heated for 27 minutes at 225°C, 18 minutes to 300°C, and maintained at 300°C for 90 minutes. The film was then exposed, in a nitrogen oven, to a temperature of 300°C for 27 minutes then heated to 400°C over 75 minutes and maintained at 400 C C for 2 hours. The resulting film exhibited tensile strength of 57.8 Ksi (0.399 GPa), a tensile modulus of 1.61 (11.1 GPa), and an elongation-at-break of 21.1 percent. Example 27 A polyamic acid solution was prepared in a manner similar to Example 1 except that the initial solvent was 78.8 mL 1,3-dimethyl-2-imidazolidinone (DMI), 37.5 mL N-methylpyrrolidinone (NMP) and 50 mL toluene (T). After the toluene was distilled off, 9.907 g (43.98 mmol) of 5-amino-2-(p-aminophenyl)benzoxazole (pDAMBO) and 9.593 g (43.98 mmol) of pyromellitic dianhydride (PMDA) rinsed down with 15 mL of dry N MP was added to the solvent at room temperature. A total of 100 mL of a dry mixture of 60 percent DMI and 40 percent NMP was used to dilute the viscous solution. After about 44 hours, the inherent viscosity of the polyamic acid was 3.53 dL/g (NMP, 25.0°C, 0.2 g/dL). A 7.5 weight percent solution of polyamic acid solution was cast as a 0.02 inch (.508 mm) film. The film layer was coagulated in a mixture of 700 mL acetic anhydride, 300 mL pyridine, and 1000 mL toluene. The coagulated film was then washed in methanol for 15 minutes. The film was framed and placed in an air-circulating oven, heated for 50 minutes at 225°C, then for 20 minutes to 300°C, and maintained for 60 minutes at 300°C. It was then cooled to room temperature and heated, in a nitrogen atmosphere, at 400°C for 2 hours. The resulting film exhibited a tensile strength of 47.6 Ksi, (0.328 GPa), tensile modulus of 1.23 Msi, (8.49 GPa) and an elongation-at-break of 25.9 percent. Example 28 In a manner similar to Example 1, a polyamic acid solution was prepared using an initial feed of 61.5 mL N,N-dimethylacetamide (DMAc) and 20 mL toluene, with the polyamic acid made using 4.974 g (22.08 mmol) of 5-amino-2-(p-aminophenyl)benzoxazole (pDAMBO) and 6.850 g (22.08 mmol) of 4,4'-oxydiphthalic anhydride (ODPA) rinsed with 10.0 mL dry DMAc. After 68 hours, the resulting polyamic acid was diluted with N-methylpyrrolidinone (NMP) and had an inherent viscosity of 2.67 dL/g (0.203 g/dL, 25.0°C). In a manner similar to Example 8, 78.8 g of the polyamic acid was chilled and degassed. After the flask was returned to atmospheric pressure using nitrogen, 16.7 mL DMAc, 4.37 mL acetic anhydride, and 1.16 mL β-picoline were added dropwise to the stirred, chilled PA for 2.25 minutes. The resulting mixture was then degassed for 15.0 minutes and returned to atmospheric pressure using nitrogen. A 12.5 percent solution of the resulting polyamic acid in DMAc was cast as a 0.015 inch (.381 mm) film. The film was heated in an air-circulating oven at 60°C for 45 minutes, removed from the glass and secured in an aluminum frame. It was then placed in an air-circulating oven, heated for 20 minutes at 225°C, for 20 minutes to 300°C, maintained for 1 hour at 300°C, then cooled to room temperature; and, in a nitrogen atmosphere, heated for 27 minutes at 30°C (N 2 purge cycle), 75 minutes to 400°C, maintained at 400°C for 2 hours, and then cooled to room temperature. The film's tensile strength was 44.11 Ksi (0.304 GPa), its tensile modulus was 1.04 Msi (7.18 GPa), and its elongation-at-break 22.6 percent. Examples 29-46 Following the procedure of Example 28, except using the amounts of solvent and reactants set forth in Table 5, a series of films were prepared. The tensile properties of these films were tested and reported in Table 5. Table 5 I ω *•• I I ω 1 - Initial solvent was recited in abbreviated form with NMP being N-methylpyrrolidinone, T being toluene and DMAc being dimethylacetamide. The first number was the amount, in mL, used as the reaction medium and the second, the amount to rinse the reaction product. 2 - Reactants were recited in abbreviated form with mDAMBO: 5-amino-2-(m-aminophenyl)benzoxazole; ODPA: 4,4'oxydiphthalic dianhydride; pDAMBO: 5-amino-2-(p- aminophenyl)benzoxazole; PMDA - pyromellitic dianhydride; 6FDA: 2,2-bis-(3,4-dicarboxyphenyl)hexa- fluoropropane dianhydride; DSDA: 3,3' ,4,4'-diphenyl- sulfonetetracarboxylic dianhydride; DABO: 2,6-(4,4'- diaminodiphenyl)benzo[1,2-d:5,4-d' jbisoxazole; BPDA: 3,3' ,4,4'-biphenyltetracarboxylic dianhydride; TA(DAMBO>2: 2,2'-p-phenylenebis(5-aminobenzoxazole) ; 6FA(DAMB0) 2 : 2,2-bis(4-phenyl)hexafluoropropane-2,2'- bis(5-aminobenzoxazole). 3 - Catalyst mixture was recited in abbreviated form with DMAc: N,N-dimethylacetamide; AA: acetic anhydride: B-P: β-picoline; NMP: N-methylpyrrolidinone. 4 - Heat cycle was recited as the heating in air or nitrogen with the cycle being recited as time in minutes at a specific temperature in C C; with a notation 20/60/300, or similar notation, meaning the sample was exposed for 20 minutes to an oven being heated to 300°C and then maintained at 300°C for 60 minutes. Air heating was conducted first and nitrogen subsequently when both air and nitrogen heating was conducted. 5 - Tensile properties were reported as tensile strength (T.S.) in Ksi (GPa), the tensile modulus (T.M.) in Msi (GPa), and Elongation-at-break (Elong.) in percent. Example 47 In a manner similar to Example 1, 1.158 g (5.143 mmol) of 5-amino-2-(m-aminophenyl)benzoxazole and 2.285 g (5.143 mmol) of 2,2-bis(3,4-dicarboxyphenyl)hexaf luoropropane dianhydride was added to an initial feed of 5 15.7 mL of N-methylpyrrolidinone (NMP) and 10 mL toluene. After 44 hours' reaction, the resulting polyamic acid had an inherent viscosity of 2.20 dL/g (0.202 g/dL, 25.0°C). The flask was chilled with an ice-water bath and a solution of 3.2 mL NMP, 1.17 mL acetic anhydride, and 0.30 mL β-picoline was added dropwise to the stirred, chilled flask over 5 minutes. The reaction mixture was allowed to warm to room temperature and was stirred 68 hours at room 10 temperature. The resulting solution was cast as a 0.5 mm film on a glass plate. The film was heated in an air-circulating oven at 100°C for 1.0 hour, removed from the glass and secured in an aluminum frame. It was then placed in an air-circulating oven, heated at 225°C for 20 minutes, heated for 20 minutes to 300 C C, maintained at 300 C C for 1.0 hour, and then cooled 15 to room temperature. The resulting film exhibited a tensile strength of 18.3 Ksi (0.126 GPa), a tensile modulus of 482.9 Msi (3.33 GPa), and an elongation-at-break of 22.2 percent. Example 48 In a manner similar to Example 1, 1.533 g (6.807 mmol) of 20 5-amino-2-(p-aminophenyl)benzoxazole (pDAMBO) and 3.024 g (6.807 mmol) of 2,2-bis(3,4-dicarboxyphenyl)hexaf luoropropane dianhydride was added to an initial feed of 25 mL of N-methylpyrrolidinone (NMP) and 10 mL toluene. After 68 hours, the resulting polyamic acid had an inherent viscosity of 3.27 dL/g (0.198 g/dL, 25.0 C C). The flask was chilled with an ice-water bath and a solution of 6.1 mL NMP, 1.55 mL acetic anhydride, and 0.4 mL 25 β-picoline was added dropwise to the stirred, chilled PA over 5 minutes. The reaction mixture was allowed to warm to room temperature and was stirred 68 hours at room temperature. The resulting solution was cast as a 0.5 mm film on a glass plate. The film was heated in an air-circulating oven at 100°Cfor 1.0 hour, removed from the glass and secured in an aluminum frame. It was then placed in an air-circulating oven and heated at 225°C for 30 20 minutes, heated for 20 minutes to 300°C, maintained at 300°C for 1 hour, and then cooled to room temperature. 35 The resulting film exhibited a tensile strength of 20.3 Ksi (0.140 GPa), a tensile modulus of 575.6 Msi (3.97 GPa), and an elongation-at-break of 30.9 percent.

Claims: WHAT IS CLAIMED IS: 1. A polyamic acid having an inherent viscosity of greater than 2.0 dL/g, as measured at 0.2 g/dL in N-methylpyrrolidinone at 25°C, which comprises the reaction product, in polymerized form, of a dianhydride of the formula: 0 and a diaminobenzoxazole of the following formula: where Ar, Ar 1 , Ar*, Ar 3 , Ar 4 , Ar 5 , Ar 6 , Ar?, Ar β , Ar9, Ar 10 and Ar 11 can be any aromatic group or pyridine group, except that: where Y is a single covalent bond, -0-, -CH 2 -, -S-,-CO-, -S0 2 -, -C(CH 3 ) 2 - or -(CF 3 ) 2 - and where X and X 1 are hydrogen atoms, alkyl groups, alkoxy groups or halogen atoms or groups, when Ar* , Ar5 , and Ar6 are all f Y) . and except that Ar does not equal f j » when Ar 1 is [ ] and Ar 2 and Ar 3 are " 2. The polyamic acid of Claim 1 in which the polyamic acid has an inherent viscosity of greater than 3 dL/g, as measured at 0.2 g/dL in N-methylpyrrolidinone at 25°C. 3. The polyamic acid of Claim 1 in which the polyamic acid has an inherent viscosity of greater than 5 dL/g, as measured at 0.2 g/dL in N-methylpyrrolidinone at 25°C. 4. The polyamic acid of Claim 1 in which the polyamic acid has an inherent viscosity of greater than 6 dL/g, as measured at 0.2 g/dL in N-methylpyrrolidinone at 25 C C. 5. The polyamic acid of Claim 1 in which the dianhydride is selected from the group 4,4'-oxydiphthalic anhydride, pyromellitic dianhydride, 2,2-bis-(3,4- -dicarboxyphenyl)hexaf luoropropane dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 3,3' . 4,4'-diphenylsulfonetetracarboxylic dianhydride and 3,3'4,4'- -benzophenonetetracarboxylic dianhydride. 6. The polyamic acid of Claim 1 in which the diaminobenzoxazole is selected from the group 2, 6-(4,4'-diaminodiphenyl)benzo[1,2-d:5,4-d']bisoxazole, 5-amino-2- -(p-aminophenyl)benzoxazole, 5-amino-2-(m-aminophenyl)benzoxazole, 4,4'-diphenylether- -2,2'-bis(5-aminobenzoxazole), 2,2'-p-phenylenebis(5-aminobenzoxazole), and 2,2-bis(4- -phenyl)hexafluoropropane-2,2'-bis(5-aminobenzoxazole). 7. A solution of the polyamic acid of Claim 1 in an organic liquid, the polyamic acid solution having a bulk solution viscosity of, at most, 500,000 centipoise. 8. A process for preparing the polyamic acid of Claim 1 by dissolving the dianhydride and diaminobenzoxazole in an organic liquid reaction medium and maintaining the resulting mixture at a temperature from -40°C to 150°C. 9. The process of Claim 8 wherein the temperature is maintained from 0°C to 50°C. 10. The process of Claim 8 wherein the reaction medium further comprises a non- -benzoxazole-containing diamine. 11. The process of Claim 10 wherein the non-benzoxazole-containing diamine is 1 ,4-phenylenediamine or 4,4'-oxydianiline or 3,4'-oxydianiline. 12. The process of Claim 8 wherein the reaction mixture further comprises a chain terminator. 13. The process of Claim 12 wherein the chain terminator is maleic anhydride, 2,3- -dimethylmaleic anhydride, nadic anhydride, phthalic anhydride, aniline or 4- -aminobenzocyclobutene. 14. A method for preparing a polyimidebenzoxazole from a solution of the polyamic acid of Claim 1 and an organic liquid solvent, the method comprising removing at least a portion of the solvent and then exposing the polyamic acid to an imidization temperature from 160 C C to 240°C to convert at least a portion of the polyamic acid to polyimidebenzoxazole. 15. A method for preparing polyimidebenzoxazole from a solution of the polyamic acid of Claim 1 and an organic liquid solvent, the method comprising heating the polyamic acid solution to a volatilization temperature of from 50°C to 130°C to remove at least a portion of the solvent and subsequently to an imidization temperature from 160°C to 280 C C. 16. The method of Claim 15 wherein the polyamic acid solution is heated atthe volatilization temperature for a time from 5 minutes to 90 minutes and at the imidization temperature for a time from 5 minutes to 120 minutes. 17. The method of Claim 16 further comprising heating the imidization product to a temperature from 250 C C to 600°C and below the glass transition temperature of the polyimidebenzoxazole. 18. A method for preparing a polyimidebenzoxazole which comprises preparing a solution of a polyamic acid having an inherent viscosity of greater than 2.0 dL/g, as measured at 0.2 g/dL in N-methylpyrrolidinone at 25°C in an organic liquid; the polyamic acid being the reaction product, in polymerized form, of a dianhydride of the formula: and a diaminobenzoxazole of any of the following formulae:where Ar, A , Ar 2 , Ar 3 , Ar 4 , Ar 5 , Ar 6 , Ar 7 , Ar 8 , Ar 9 , Ar 10 and Ar 11 can be any aromatic group or pyridine group; removing at least a portion of the solvent; sequentially exposing the polyamic acid to an imidization temperature from 160°C to 240°C to convert at least a portion of the polyamic acid to polyimidebenzoxazole; to an annealing temperature from 250°C to 325°C and then to a heat- -treating temperature, higher than the annealing temperature, of from 250°C to 600°C for from 0.1 to 120 minutes; said annealing and heat-treating temperatures being below the glass transition temperature of the polyimidebenzoxazole. 19. The method of Claim 18 wherein the organic liquid is partially removed by coagulating the polyamic acid. 20. A method for preparing a coating of a polyimidebenzoxazole by applying a solution of the polyamic acid of Claim 1 in an organic liquid to a substrate, removing at least a portion of the organic liquid from polyamic acid, and exposing the coated article to a temperature from 160°C to 240°C. 21. A method for preparing a polyimidebenzoxazole film from a solution of a polyamic acid in an organic liquid by forming a liquid film of the polyamic acid solution, removing at least a portion of the organic liquid from the polyamic acid solution; heating the polyamic acid at an imidization temperature from 160°C to 240°C to convert at least a portion of the polyamic acid to polyimidebenzoxazole; and subsequently to a temperature from 250°C o to 600°C, but below the glass transition temperature of the polyimidebenzoxazole. 22. The method of Claim 21 in which imidization is conducted in the presence of a material or combination of materials which facilitate or accelerate ring closing. 23. The method of Claim 22 in which the ring-closing agents are the combination of a dehydration agent and an organic base which facilitates their ring-closing activity. 5 24. The method of Claim 21 in which the organic liquid is at least partially removed by coagulating the polyamic acid from solution. 25. A polyamic acid which is the reaction product, in polymerized form, of a) (i) 2,2-bis-(3,4-dicarboxyphenyl)hexafluoropropane dianhydride and (ii) 5-amino-2-(m-aminophenyl)benzoxazole; 5-amino-2-(p-amino- 0 phenyl)benzoxazole, 2,2-bis(4-phenyl)hexafluoropropane-2,2'-bis(5- -aminobenzoxazole), or 4,4'-diphenylether-2,2'-bis(5- -aminobenzoxazole); b) (i) 3,3',4,4'-diphenylsulfonetetracarboxylic dianhydride and (ii) 5- -amino-2-(m-aminophenyl)- benzoxazole, or 2,2-bis(4- 5 -phenyl)hexafluoropropane-2,2'-bis(5-aminobenzoxazole); or c) 4,4'-oxydiphthalic anhydride and 2,2-bis(4-phenyl)hexafluoropropane- -2,2'-bis(5-aminobenzoxazole). 26. A polyimidebenzoxazole film of, in polymerized form, pyromellitic dianhydride and 5-amino-2-(p-aminophenyl)benzoxazole, 0 2,6-(4,4'-diaminodiphenyl)benzo[1,2-d:5,4-d']bisoxazole, or 2,2'-p-phenylene-bis(5- -aminobenzoxazole), which film has a tensile modulus of at least about 6 GPa. 27. A polyimidebenzoxazole film of, in polymerized form, 2,2-bis-(3,4- -dicarboxyphenyl)- hexafluoropropane dianhydride and 5-amino-2-(m- -aminophenyl)benzoxazole or 5-amino-2-(p-aminophenyl)benzoxazole. 5 28. A polyimidebenzoxazole film of, in polymerized form, 3,3',4,4'-biphenyltetra- carboxylic dianhydride and either 5-amino-2-(p-aminophenyl)- benzoxazole or 2,6-(4,4'- -diaminodiphenyl)benzo[1,2-d: 5,4-d']bisoxazole, which film has a tensile modulus of at least about 3 GPa. 29. A polyimidebenzoxazole film of, in polymerized form, 4,4'-oxydiphthalic dianhydride and 2,6-(4,4'-diaminodiphenyl)benzo[1,2-d:5,4-d']bisoxazole.

Inventors: HARRIS WILLIAM J, HWANG WEN-FANG, HARRIS, WILLIAM, J., HWANG, WEN-FANG
Applicants: DOW CHEMICAL CO, THE DOW CHEMICAL COMPANY
Application year: 1995
Main classification: C08G73
IPC codes: B05D7, B29C41, B29K79, B29L7, C08G73, C08J5