WO2013045613A1

WO2013045613A1 - Polyimid cross-linked by use of compound comprising at least two residues of oxazoline or at least two residues of dihydro-1,3-oxazine

Info

Publication number: WO2013045613A1
Application number: PCT/EP2012/069186
Authority: WO
Inventors: Jan-Erik Rosenberg; Daniel RÖME; David Persson
Original assignee: Nexam Chemical Ab
Priority date: 2011-09-30
Filing date: 2012-09-28
Publication date: 2013-04-04

Abstract

A composition comprising an aromatic oligo- or polyimide and a compound comprising at least two residues of oxazoline, or at least two residues of dihydro-1,3-oxazine, being cross-linkable is disclosed.

Description

POLYIMID CROSS-LINKED BY USE OF COMPOUND COMPRISING AT LEAST TWO RESIDUES OF OXAZOLINE OR AT LEAST TWO RESIDUES OF DIHYDRO-1 ,3-OXAZINE

Field of the invention

The present invention relates to a novel cross-linkable composition comprising an oligo- or polyimide. It also relates to an article produced by us of such a

composition, in which the oligo- or polyimide optionally has been cross-linked. Further, it relates to a method for obtaining a cross-linked oligo- or polyimide.

Background

Polymers has for long been used as replacement material for other materials, such as metals. They have the advantage of being light-weight materiel, which are relative easy to shape. However, polymers do typically have lower mechanical strength compared to metals and are less heat resistant.

The need for resistant polymers led to the development of aromatic polyimides. Polyimides are polymers comprising imide bonds. Aromatic polyimides are typically synthesized by condensation of aromatic carboxylic acid dianhydride monomers, such as pyromellitic dianhydride, 4,4'-oxydiphthalic anhydride, 2,2-bis-[4-(3,4- dicarboxyphenoxy)phenyl]-propane dianhydride, 3,3',4,4'-benzophenonetetracarboxylic acid dianhydride or 3,3',4,4'-tetracarboxybiphenyl dianhydride, with aromatic diamine monomers, such as 4,4'-oxydianiline, 1,4-diaminobenzene, 1,3-diaminobenzene, 1,3- bis-(4-aminophenoxy)benzene, 1 ,3-bis-(3-aminophenoxy)benzene, methyl enedianiline or 3,4'-oxydianiline. As an example, polyimides obtained via condensation of pyromellitic dianhydride and 4,4'-oxydianiline are among others sold as polyimide films under the trademarks Kapton®. They are materials which are lightweight and flexible, and which have good resistant to heat and chemicals.

Further, thermoset polyimides have inherent good properties, such as wear and friction properties, good electrical properties, radiation resistance, good cryogenic temperature stability and good flame retardant properties. Therefore, they are used in the electronics industry for flexible cables, as an insulating film on magnet wire, for medical tubing, as surface protecting layer and interlayer dielectrics of semiconductor chip. Polyimide materials are also used in high or low temperature exposed applications as structural parts were the good temperature properties is a prerequisite for the function.

The need to improve the processability, while keeping the mechanical properties, of polyimides for use in airplanes and aerospace applications led to the introduction of cross-linking technologies. As the polymer chains are cross-linked, they may be shorter whilst the mechanical properties are maintained or even improved. Shorter polymer chains have the advantage of being easier to process, as the viscosity of the polymer melt is lower. Examples of such cross-linking technologies include thermoset polyimides containing phenylethynyl-substituted aromatic species as reactive moieties.

As an example, polyimides end-capped with phenyl ethynyl phtalic anhydride (PEP A) have found use in the art. US 5,567,800 discloses phenylethynyl terminated imide oligomers (PETIs). Such oligomers may be prepared by first preparing amino terminated amic acid oligomers from dianhydride(s) and an excess of diamine(s) and subsequently end-cap the resulting amino terminated amic acid oligomers with phenylethynyl phtalic anhydride (PEP A). The amic acid oligomers are subsequently dehydrated to the corresponding imide oligomers. Upon heating, the triple bonds will react and cross-link the end-capped polyimid, thereby improving its heat resistance and mechanical strength. As disclosed by US 5,567,800 heating to at least 350°C is necessary to cure the PETI. As an alternative to PEP A, also ethynyl phtalic anhydride (EPA) has been used as cross-linker in polyimides (Hergenrother, P. M., "Acetylene- terminated Imide Oligomers and Polymers Therefrom", Polymer Preprints, Am. Chem. Soc, Vol. 21 (l), p. 81-83, 1980).

Although the mechanical as well as the thermal stability will be improved by use of such cross-linking system they suffer form other disadvantages limiting their use in certain applications. Especially, the polyimide, once cross-linked, will have a deep- red color, although being transparent. The typically red color resulting from the heat induced cross-linking of carbon-carbon triple bonds is an inherent feature of the system.

Colorless polyimide materials are deemed to be highly interesting for use in applications such as flexible electronics, e-paper, etc.

Thus, there is need within the art for alternative cross-linking systems for polyimides, which may be used to obtain cross-linked, transparent and essentially colorless polyimides. Summary

Consequently, the present invention seeks to mitigate, alleviate, eliminate or circumvent one or more of the above-identified deficiencies in the art and disadvantages singly or in any combination by providing composition comprising an aromatic oligo- or polyimide, comprising at least one pendant free aromatic hydroxyl (-OH), or at least one pendant free carboxy group (-C(O)OH or -C(0) and a compound comprising at least two residues of oxazoline, such as at least two

groups, or at least two

W 0

residues of dihydro- 1 ,3-oxazine, such as at least two groups. Such a

composition may be used to obtain an essentially colorless cross-linked aromatic oligo- or polyimide.

Another aspect of the invention relates to use of a compound comprising at least two residues of oxazoline, such as

groups, or at least two residues of

N ^O

dihydro- 1 ,3-oxazine, such as groups, for cross-linking an aromatic oligo- or polyimide, comprising at least one pendant free aromatic hydroxyl (-OH), or at least one pendant free carboxy group (-C(O)OH or -C(0)0^").

Another aspect of the invention relates to a method of cross-linking an oligo- or polyimide, comprising at least one pendant free aromatic hydroxyl (-OH) or group (-C(O)OH or -C(0)0^"). In such a method, a compound comprising at least two residues

of oxazoline, such as at least two groups, or at least two residues of dihydro-1 ,3-

W O

oxazine, such as at least two groups is added to the oligo- or polyimideto be cross-linked. The the resulting mixture is subsequently heated.

A further aspect of the invention relates to an article, such a film, comprising a cross-linked oligo- or polyimide obtainable by use of a composition comprising an aromatic oligo- or polyimide, comprising at least one pendant free aromatic hydroxyl (- OH), or at least one pendant free carboxy group (-C(O)OH or -C(0)0^"), and a compound comprising at least two residues of oxazoline, such as at least tw

groups, or at least two residues of dihydro-l,3-oxazine, such as at least two

groups.

Further advantageous features of the invention are defined in the dependent claims. In addition, advantageous features of the invention are elaborated in

embodiments disclosed herein.

Detailed summary of preferred embodiments

Definitions:

In the context of the present application and invention, the following definitions apply:

As used herein, "alkyl" used alone or as a suffix or prefix, is intended to include both branched and straight chain saturated aliphatic hydrocarbon groups having from 1 to 12 carbon atoms or if a specified number of carbon atoms is provided then that specific number is intended. For example "CI -6 alkyl" denotes alkyl having 1, 2, 3, 4, 5 or 6 carbon atoms. When the specific number denoting the alkyl-group is the integer 0 (zero), a hydrogen-atom is intended as the substituent at the position of the alkyl-group. For example, "N(C0 alkyl)₂" is equivalent to "NH2" (amino).

As used herein, "alkylenyl" or "alkylene" refer to a bivalent radical that is derived from an alkane that has had two hydrogen atom removed from two distinct, or from the same, of its carbon atoms. The terms used alone or as a suffix or prefix, are intended to include branched and straight chain saturated aliphatic hydrocarbon groups, which may be cyclic or comprise a cycloalkylene group, having from 1 to 12 carbon atoms or if a specified number of carbon atoms is provided then that specific number is intended. For example "CI -6 alkylenyl" "CI -6 alkylene "denotes alkylenyl or alkylene having 1, 2, 3, 4, 5 or 6 carbon atoms. When the specific number denoting the alkylenyl or alkylene-group is the integer 0 (zero), a bond is intended to link the groups onto which the alkylenyl or alkylene-group is substituted. For example, "NH(C0

alkylene)NH₂" is equivalent to "NHNH₂" (hydrazino).

Examples of alkyl include, but are not limited to, methyl, ethyl, n-propyl, i- propyl, n-butyl, i-butyl, sec-butyl, t-butyl, pentyl, and hexyl. Examples of alkylene or alkylenyl include, but are not limited to, methylene, ethylene, propylene, butylenes and

As used herein, the term "aryl" refers to a ring structure, comprising at least one aromatic ring, made up of from 5 to 14 carbon atoms. Ring structures containing 5, 6, 7 and 8 carbon atoms would be single-ring aromatic groups, for example phenyl. Ring structures containing 8, 9, 10, 11, 12, 13, or 14 carbon atoms would be polycyclic, for example naphthyl. The aromatic ring may be substituted at one or more ring positions. The term "aryl" also includes polycyclic ring systems having two or more cyclic rings in which two or more carbons are common to two adjoining rings (the rings are "fused rings") wherein at least one of the rings is aromatic, for example, the other cyclic rings may be cycloalkyls, cycloalkenyls, cycloalkynyls, and/or aryls.

The terms ortho, meta and para apply to 1,2-, 1,3- and 1 ,4-disubstituted benzenes, respectively. For example, the names 1 ,2-dimethylbenzene and ortho- dimethylbenzene are synonymous.

As used herein, the term "arylene" and "arenediyl groups" refers to a bivalent radical that is derived from an aryl that has had a hydrogen atom removed from two of its ring carbon atoms. Examples of "arylene" include o-phenylene being equivalent to benzene- 1,2-diyl.

As used herein, the term "substitutable" refers to an atom to which a hydrogen may be covalently attached, and to which another substituent may be present instead of the hydrogen. A non-limiting example of substitutable atoms includes the carbon-atoms of pyridine. The nitrogen-atom of pyridine is not substitutable according to this definition. Embodiments

It has been found that an essentially colorless cross-linked aromatic oligo- or polyimide, may be obtained by heating a composition comprising an aromatic oligo- or polyimide, having at least one, such as at least 2, 5, 10 or even 15, aromatic pendant free hydroxyl group(s) (-OH) or at least one, such as at least 2, 5, 10 or even 15, pendant free carboxy group(s) (-C(O)OH or -C(0) a compound comprising at least two residues of oxazoline, such as at least two

groups, or at least two residues of dihydro- 1 ,3-oxazine, such as at least two

groups. Such a cross-linked oligo- or polyimide, in addition to being essentially colorless, will also typically be transparent. Thus, an embodiment relates to such a composition. Furthermore, the heat and chemical resistance will be improved by the cross-linking. Thus, such a cross-linked oligo- or polyimide may found use in applications in which there is a demand for color-less polymers displaying good chemical resistance and thermal stability. In addition, such a system may be cross-linked at a significantly lower temperature compared PEPA, such as at temperatures of 200 to 250°C

Typically, the aromatic group to which the pendant free hydroxyl or pendant free carboxy group is directly, or indirectly, bound will be part of the oligomer or polymer backbone. Although, and oligo- or polyimidesmay have free carboxy groups as end-groups, further reactive groups along the oligomer or polymer backbone has been found to be necessary for efficient cross-linking. Without being bounded be any theory, it is believed that cross-linking is achieved by having the oxazoline or dihydro-1,3- oxazine residues react with the pendant free aromatic hydroxyl groups, or with the pendant free carboxy groups. Contrary to cross-linking via ethynyl groups, such cross- linking as described herein results in an essentially colorless cross-linked material.

Further, the weight ratio between the oligo- or polyimide and the compound comprising at least two residues of oxazoline, or at least two residues of dihydro-1,3- oxazine in the composition may be 99.5 :0.5 to 4: 1 , such as 99: 1 to 9: 1. Furthermore, the molar ratio between the oligo- or polyimide and the compound comprising at least two residues of oxazoline, or at least two residues of dihydro -1,3 -oxazine in the composition may be 100: 1 to 1 :2, such 10:1 to 1 : 1.

According to an embodiment, the molar amount of the compound comprising at least two residues of oxazoline, or at least two residues of dihydro-l,3-oxazine, in the composition is selected in a manner such that number of residues of oxazoline or dihydro-l,3-oxazine is from 0.25 to 2, such 0.5 to 1.0, times the number of pendant free carboxy or aromatic hydroxyl groups of the oligo- or polyimide. Unreacted cross-linker, i.e. the compound comprising at least two residues of oxazoline, or at least two residues of dihydro- 1,3-oxazine, may act as plasticizer. Thus, the compound comprising at least two residues of oxazoline, or at least two residues of dihydro- 1,3 -oxazine, is preferably present in stoichiometric or sub- stoichiometric amount with respect to the molar amount of pendant free carboxy or aromatic hydroxyl of the oligo- or polyimide in the composition.

Without being bound by any theory, it is believed that the molar amount of residues of oxazoline or dihydro-l,3-oxazine will affect the ratio of inter vs. intra molecular cross-linking. The two types of cross-linking are believed to affect the properties of the oligo- or polyimide in various ways. Accordingly, the properties of the cross-linked oligo- or polyimide may be controlled by varying the relative molar amount of the compound comprising at least two residues of oxazoline, or at least two residues of dihydro-l,3-oxazine, in the composition.

According to an embodiment, the compound comprising at least two residues of oxazoline or dihydro-l,3-oxazine is a compound according to formula (I), or (II),

wherein

"A" is a direct bond, an arylene, such as phenylene, or a C 1-12 alkylene; Rl and R2, independently of each other, are selected from the group consisting of hydrogen and CI -5 alkyl;

"n" is an integer of 3 to 5;

"B" is, an arylene, such as phenylene, or a CI -12 alkylene.

Further, the compound comprising at least two residues of oxazoline or dihydro-l,3-oxazine may be a oli or polymer obtainable by co-polymerization of

2-vinyl-4,5-dihydrooxazole, i.e.

, or 2-(prop-l-en-2-yl)-4,5-dihydrooxazole, i.e.

and a co-monomer selected from the group consisting of ethene, propene, styrene, vinyl acetate, alkyl acrylate, such as butyl acrylate, alkyl methacrylate, such as methyl methacrylate, acrylonitrile, vinylchloride. As an example, polymers of the Epocross® series marketed by Nippon Shokubai, may be mentioned.

Preferably, the compound comprising at least two residues of oxazoline or dihydro-l,3-oxazine is selected from the group consisting of:

and polymers of the Epocross® series marketed by Nippon Shokubai.

Aromatic oligo- and polyimides, as well as aromatic oligo- and polyamic acid, having at least one pendant free aromatic hydroxyl (-OH) or carboxy group (-C(O)OH or -C(0)0^") are known within the art. As is well-known within the art, such oligo- and polymers may be obtained by co-polymerization of an aromatic dianhydride and an aromatic diamine, in the presence of an aromatic diamine and/or aromatic dianhydride having at least one free hydroxy or carboxy group. In obtaining the oligo- or polymer, a slight excess of diamine(s) or dianhydride(s) may be used.

The oligo- or polyimide to be cross-linked, may typically comprise residues of at least one aromatic dianhydride, and at least one aromatic diamine. Although, a corresponding oligo- or polyamic acid may be cross-linked by use of a compound comprising at least two residues of oxazoline or dihydro-l,3-oxazine , the cross-linked may not by completely dehydrated to the corresponding oligo- or polyimide, unless the oligo- or polyamic comprise residue(s) of at least one aromatic diamine and/or at least one aromatic dianhydride having at least one free hydroxy or carboxy group. Further, in order to be cross-linkable the oligo- or polyimide needs to comprise residue(s) of at least one aromatic diamine and/or at least one aromatic dianhydride having at least one hydroxy or carboxy group.

According to an embodiment, the aromatic dianhydride is pyromellitic dianhydride or a dianhydride according to the general formula (III),

wherein "G" represents a direct bond or a di-valent group selected from the group consisting of a carbonyl group, an amide group (-C(0)NH- or -NHC(O)-), an ester group (-C(O)O- or -OC(O)-), a methylene group, a sulfone group, a sulfide group, an ether group, an -C(0)-phenylene-C(0)- group, an isopropylidene group, a hexafluoroisopropylidene group, a 3-oxyphenoxy group, a 4-oxyphenoxy group, a 4'- oxy-4-biphenoxy group, and a 4-[l-(4-oxyphenyl)-l-methylethyl]phenoxy group; and wherein "G" may be connected to the 4- or 5-position and the 4'- or the 5 '-position, respectively, in the isobenzofuran-l,3-dione residues.

Symmetric aromatic dianhydrides as well asymmetric aromatic dianhydrides are equally possible.

Preferred examples of the aromatic dianhydrides comprise pyromellitic dianhydride, 4,4'-oxydiphthalic anhydride, 2,2-bis-[4-(3,4-dicarboxyphenoxy)phenyl]- propane dianhydride, 3,3',4,4'-benzophenonetetracarboxylic acid dianhydride, 3,3',4,4'- tetracarboxybiphenyl dianhydride, 4,4',5,5'-sulfonyldiphthalic anhydride, and 5,5'- (perfluoropropane-2,2-diyl)bis(isobenzofuran-l,3-dione).

According to an embodiment, the aromatic diamine is 1,4-diaminobenzene, 1,3-diaminobenzene, or a diamine according to the general formula (IV)

wherein the amino groups may be connected to any substitutable carbon atom in the benzene residues, i.e. to the 2-, 3- or 4-position, and the 2^', 3 ^', or 4^'-position, respectively; and "L" is a direct bond or a moiety selected from the group consisting of - 0-, -S-, -C(O)-, -C(CH₃)₂-, -C(CF₃)₂-, -CH₂-, -S0₂-, an amide group (-C(O)NH- or - NHC(O)-), an ester group (-C(O)O- or -OC(O)-), 3-oxyphenoxy group, 4-oxyphenoxy group, 4'-oxy-4-biphenoxy group, and 4-[l-(4-oxyphenyl)-l-methylethyl]phenoxy group.

Preferably, the amino groups are connected to the 3- or 4-position of the respective benzene residues. Symmetric di-amines, eg. 3,3 ^'- and 4,4^'-substited di- amines according to general formula (IV), as well as asymmetric di-amines, eg. 3,4^'-, or 4,3 ^'-substited di-amines according to general formula (IV), are equally possible.

As well known in the art, asymmetric aromatic diamines and dianhydrides may be used to prepare polyimides with a bent and rotationally hindered structure resulting in high Tg but also in improved processability and high melt fluidity along with and solubility of the resin in organic solvent.

Examples of preferred aromatic diamines comprise 4,4'-oxydianiline, 1,4- diaminobenzene, 1,3-diaminobenzene, l,3-bis-(4-aminophenoxy)benzene, l,3-bis-(3- aminophenoxy)benzene, methylenedianiline, or 3,4'-oxydianiline.

According to an embodiment, the aromatic dianhydride is pyromellitic dianhydride or 5,5'-(perfluoropropane-2,2-diyl)bis(isobenzofuran-l,3-dione) and the aromatic diamine is 4,4'-oxydianiline, 1 ,4-diaminobenzene, or 1,3-diaminobenzene.

According to an embodiment, the aromatic diamine having at least one hydroxy or carboxy group is a compound selected from the group consisting of:

or a compound according to the general formula V, Va, Vb, Vbl, Vc, or Vd:

(Vb) (Vbl) (COOH)_p2

I

H₂N Ar, E-i Ar₂ E₂ Ar₃ NH₂

(COOH)_p1 (COOH)_{p3 ( Vc )}

(OH)_q2

H₂N-Ar₁-E_rAr₂-E₂-Ar₃ NH₂

(OH)_q1 (OH)_{q3 ( Vd )} wherein

"E" represents a direct bond or a di-valent group selected from the group consisting of a carbonyl group, an amide group (-C(O)NH- or -NHC(O)-), an ester group (-C(O)O- or -OC(O)-), a methylene group, a sulfone group, a sulfide group, an ether group, an -C(0)-phenylene-C(0)- group, an isopropylidene group, a

hexafluoroisopropylidene group, a 3-oxyphenoxy group, a 4-oxyphenoxy group, a 4'- oxy-4-biphenoxy group, a 4-[l-(4-oxyphenyl)-l-methylethyl]phenoxy group, and a 9,9- tetramethyl-9H-fluorene-3,6-diyl group; and "E" is connected to the 3- or 4-position and the 3 ^'- or the 4 ^'-position, respectively, in the terminal aniline residues; the amino groups being positioned in the 1- and 1 ^'-position, respectively;

"Arl", "Ar2", and "Ar3", independently of each other, respectively is an arylene, such as an arylene selected from the group consisting of benzene, naphthalene, anthracene, and phenanthrene residues;

"El" and "E2", independently of each other, respectively represents a direct bond or a di-valent group selected from the group consisting of a carbonyl group, an amide group (-C(O)NH- or -NHC(O)-), an ester group (-C(O)O- or -OC(O)-), a methylene group, a sulfone group, a sulfide group, an ether group, an -C(0)-phenylene- C(O)- group, an isopropylidene group, a hexafluoroisopropylidene group, a 3- oxyphenoxy group, a 4-oxyphenoxy group, a 4'-oxy-4-biphenoxy group, and a 4-[l-(4- oxyphenyl)-l-methylethyl]phenoxy group; and "El" and "E2", respectively, is connected to any substitutable atom of "Arl", "Ar2", and "Ar3", respectively;

pi, p2, and p3 are integers of 0 (zero) to 2, wherein at least one of pi, p2, and p3, is 1 or 2;

ql, q2, and q3 are integers of 0 (zero) to 2, wherein at least one of ql, q2, and q3, is 1 or 2.

tl and t2 are integers of 0 (zero) to 4, wherein the sum of tl and t2 at least is 1, such as 1 or 2. In one embodiment, tl and t2 are both 1. In such an embodiment, the phenolic groups of formula (Va), and the carboxy groups of formula (Vbl), respectively, typically are positioned orto with respect to the amino groups.

Similarly, the aromatic dianhydride having at least one hydroxy or carboxy group, is, according to an embodiment, a compound selected from the group consisting of:

or an aromatic dianhydride according to the general formula (VI) or (Via)

(°^H)p3 (via) wherein "Arl", "Ar2", and "Ar3", independently of each other, respectively is an arylene, such as an arylene selected from the group consisting of benzene, naphtene, antracene, and phenanthrene residues;

"El" and "E2", independently of each other, respectively represents a direct bond or a di-valent group selected from the group consisting of a carbonyl group, an amide group (-C(O)NH- or -NHC(O)-), an ester group (-C(O)O- or -OC(O)-), a methylene group, a sulfone group, a sulfide group, an ether group, an -C(0)-phenylene- C(O)- group, an isopropylidene group, a hexafluoroisopropylidene group, a 3- oxyphenoxy group, a 4-oxyphenoxy group, a 4'-oxy-4-biphenoxy group, and a 4-[l-(4- oxyphenyl)- 1 -methylethyljphenoxy group; and "El" and "E2", respectively is connected to any substitutable atom of "Arl", "Ar2", and "Ar3", respectively;

pi, p2, and p3 are integers of 0 (zero) to 2, wherein at least one of pi, p2, and p3, is 1 or 2.

An example of aromatic diamine having at least one hydroxy or carboxy group is di-aminobenzoic acid. Further examples of common types of aromatic diamines having at least one hydroxy or carboxy group are compounds according to formula (Va) or (Vbl), wherein "E" is selected from the group consisting of a direct bond and a divalent group selected from the group consisting of a carbonyl group, a sulfone group, a sulfide group, an isopropylidene group, a hexafluoroisopropylidene group, and a 9,9- tetramethyl-9H-fluorene-3,6-diyl group. Typically, both tl and t2 are 1 in such examples. Further, the phenolic groups of formula (Va), and the carboxy groups of formula (Vbl), respectively, typically are positioned orto with respect to the amino

diamines having at least one hydroxy or carboxy group.

Such oligo- or polymers to be cross-linked may have a number average molecular weight of about 1,000 to 20,000, such as from about 2,500 to 10,000. The number average molecular weight, as well as the weight average molecular weight, may be determined with gel permeation chromatography (GPC) or size exclusion chromatography (SEC), by use a of a combination multi-angle light scattering (MALS) detection and refractive index (RI) detection.

An oligomer to be cross-linked may have a weight average molecular weight of about 1,000 to 10,000, such as from about 2,500 to 7,5000. Further, a polymer to be cross-linked may have a weight average molecular weight of about 1 ,000 to 200,000, such as from about 25,000 to 100,000.

As an example, oligo- or polymers to be cross-linked and having low molecular weight, e.g. comprising less than 20 di-amine residues, may comprise, such as consist of:

- at least one but less than twenty residues of said aromatic diamine;

- at least one but less than twenty residues of said aromatic dianhydride; and

- at least one, such as at least 2, 3, 4, or 5, but less than twenty residues of said aromatic diamine having at least one hydroxy or carboxy group; and/or

- at least one, such as at least 2, 3, 4, or 5, but less than twenty residues of said aromatic dianhydride having at least one hydroxy or carboxy group.

As a further example, oligo- or polymers to be cross-linked and having intermediate molecular weight, e.g. comprising 20 or more diamine residues, but less than 200 diamine residues, may comprise, such as consist of:

- at least 20 but less than 200 residues of said aromatic diamine;

- at least 20 but less than 200 residues of said aromatic dianhydride; and

- at least 1, such as at least 2, 5, 10, or 20, but less than 200, such as less than 100, or 50, residues of said aromatic diamine having at least one hydroxy or carboxy group; and/or

- at least 1 such as at least 2, 5, 10, or 20, but less than 200, such as less than 100, or 50, residues of said aromatic dianhydride having at least one hydroxy or carboxy group.

As an additional example, oligo- or polymers to be cross-linked and having high molecular weight, e.g. comprising at least 200 diamine residues, may comprise, such as consist of:

- at least 200 residues of said aromatic diamine;

- at least 200 residues of said aromatic dianhydride; and

- at least 1, such as at least 5, 25, or 50, residue(s) of said aromatic diamine having at least one hydroxy or carboxy group; and/or

- at least 1, such as at least 5, 25, or 50, residue(s) of said aromatic dianhydride having at least one hydroxy or carboxy group. As well known within the art, the preparation of aromatic oligo- and polyimides, or the intermediate aromatic oligo- and polyamic acid, are preferably performed in, but not limited to, aprotic solvents, such as dimethylacetamide, dimethylformaide or N-Methylpyrrolidone. Further examples of solvents and mixtures of solvents used in the preparation of aromatic oligo- and polyimides are cresol, cresol/toluene, N-Methylpyrrolidone/orto-dichlorobenzene, benzoic acid, and nitrobenzene. Such solvents may evidently be used to obtain aromatic oligo- and polyimides to be cross-linked.

Even further examples of solvents include:

Phenol solvents, such as phenol, o-chlorophenol, m-chlorophenol, p- chlorophenol, o-cresol, m-cresol, p-cresol, 2,3-xylenol, 2,4-xylenol, 2,5- xylenol, 2,6-xylenol, 3,4-xylenol, and 3,5-xylenol;

- Aprotic amide solvents, such as Ν,Ν-dimethylformamide, N,N- dimethylacetamide, N,N-diethylacetamide, N-methyl-2-pyrrolidone, 1,3- dimethyl-2-imidazolidine, N-methylcaprolactam, and

hexamethylphosphorotriamide;

Ether solvents, such as 1,2-dimethoxyethane, bis(2-methoxy ethyl) ether, 1 ,2-bis(2-methoxyethoxy)ethane, tetrahydrofuran, bis[2-(2- methoxyethoxy)ethyl] ether, 1 ,4-dioxane, and diphenyl ether;

- Amine solvents, such as pyridine, quinoline, isoquinoline, alpha -picoline, beta -picoline, gamma -picoline, isophorone, piperidine, 2,4-lutidine, 2,6- lutidine, trimethylamine, triethylamine, tripropylamine, and tributylamine; as well as

Other solvents, such as dimethyl sulfoxide, dimethyl sulfone, sulphorane, diphenyl sulfone, tetramethylurea, anisole, and water.

Further, alkanols, such as methanol or ethanol, may be used as solvent in obtaining oligo- and polyimides

Typically, aromatic oligo- and polyimides, or the intermediate aromatic oligo- and polyamic acids, are prepared at a dry weight of the monomers corresponding to about 10 to 40 wt%.

In the preparation of aromatic oligo- and polyimides, or the intermediate aromatic oligo- and polyamic acid, the monomers are mixed at ambient or at slightly elevated temperature, typically from about 25 °C to 50°C, to obtain an aromatic oligo- or a polyamic acid as intermediate. Then, the aromatic oligo- or polyamic acid intermediate is imidized at a much higher temperature, such as about 180°C, by dehydration eliminating water.

The dehydration, may also be chemical driven, such as by addition if acetic anhydride, whereby by the imidization may be performed at lower temperature, such as room temperature, i.e. about 20 to 25 °C, to about 100 to 150°C.

By heating the aromatic oligo- or polyimide having at least one pendant free hydroxyl (-OH) or carboxy group (-C(O)OH or -C(0)0^") in the presence of the compound comprising at least two residues of oxazoline or dihydro-l,3-oxazine, the oxazoline or dihydro-l,3-oxazine residues may be caused to react with the pendant free hydroxyl (-OH) or carboxy group (-C(O)OH or -C(0)0^") of the oligo- or polyimide. If each of the at least two the oxazoline or dihydro-l,3-oxazine residues reacts with distinct oligo- or polyimide, cross-linking will be achieved. If the oligo- or polyimide has more than the one pendant free hydroxyl (-OH) or carboxy group (-C(O)OH or - C(0)0^"), more dense cross-linking may be obtained.

According to an embodiment, the aromatic oligo- or polyimide to be cross- linked has least two, such as at least 5, 10 or 15, pendant free hydroxyl (-OH) and/or carboxy groups (-C(O)OH or -C(0)0^").

A further embodiment relates to a method of cross-linking an aromatic oligo- or polyimide, comprising at least one pendant free aromatic hydroxyl (-OH) or carboxy group (-C(O)OH or -C(0)0^"). Such oligomers and polymers have been disclosed herein. The cross-linking method comprises adding an compound comprising at least two residues of oxazoline or dihydro-l,3-oxazine, as disclosed herein, to the aromatic oligo- or polyimide. Subsequently to the addition the resulting mixture is heated.

Similarly, an embodiment relates to use of a compound comprising at least two

residues of oxazoline, such as or at least two residues of dihydro-1,3-

Nr O

J

oxazine, such as I \^ groups, for cross-linking an aromatic oligo- or polyimide, comprising at least one pendant free aromatic hydroxyl (-OH), or at least one pendant free carboxy group (-C(O)OH or -C(0)0^"). Examples of compounds comprising at least two residues of oxazoline, or at least two residues of dihydro-l,3-oxazine, have been disclosed herein. Further, also aromatic oligo- or polyimides, comprising at least one pendant free aromatic hydroxyl (-OH), or at least one pendant free carboxy group (-C(O)OH or -C(0)0^") have been disclosed herein. According to one embodiment, the compositions disclosed herein are used to produce polyimide films. As known in the art, polyimide films may be obtained by various methods. One method for obtaining polyimide films, which also may be used to obtain cross-linkable polyimide films, comprising the compositions disclosed herein, is to pour such a composition dissolved in NMP onto a glass plate. Subsequently, a thin film may be obtained by using a BYK filmknife. The resulting thin film may then be baked in a vacuum oven at elevated temperature (eg. 90°C) and reduced pressure (e.g. 50 mbar) to obtain a film. Thereafter the film may be cured at higher temperatures. Typically the film is cured under inert atmosphere, such as under nitrogen atmosphere. The curing may be performed at temperatures of 150 to 300°C, typically temperatures of 200 to 250°C. As an example a curing cycle may be curing at 200°C for 0.5-1 hour followed by curing at 250°C for 0.5-1 hour. The resulting polyimide films may be display improved thermal stability, improved chemical resistance, altered stiffness, and/or altered dimension stability, etc. The thickness of the produced films may vary. As an example the films may have a thickness of 150 to 200μιη.

Another embodiment relates to a polyimide film comprising or consisting of a compositions disclosed herein. Preferably the polyimide film has been cross-linked by heating the film.

According to one embodiment, the compositions disclosed herein are used as coatings. As known in the art, surfaces may be coated with a composition comprising a polyimide by various methods. Once the surface has been coated, the coating may be heated to initiate cross-linking. Cross-linked polyimides coatings may be display improved thermal stability, improved chemical resistance, and/or scratch resistance, etc.

Another embodiment relates to a coating comprising or consisting of a compositions disclosed herein. Preferably the coating has been cross-linked by heating it.

Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The preferred specific embodiments described herein are, therefore, to be construed as merely illustrative and not limitative of the remainder of the description in any way whatsoever. Further, although the present invention has been described above with reference to specific embodiments, it is not intended to be limited to the specific form set forth herein. Rather, the invention is limited only by the accompanying claims and, other embodiments than the specific above are equally possible within the scope of these appended claims, e.g. different than those described above. In the claims, the term "comprises/comprising" does not exclude the presence of other elements or steps. Additionally, although individual features may be included in different claims, these may possibly advantageously be combined, and the inclusion in different claims does not imply that a combination of features is not feasible and/or advantageous.

In addition, singular references do not exclude a plurality. The terms "a", "an", "first", "second" etc do not preclude a plurality.

Experimental

The following examples are mere examples and should by no mean be interpreted to limit the scope of the invention. Rather, the invention is limited only by the accompanying claims.

Methods for film preparation & analysis

Film preparation

A commercial available carboxy functionalized polyimide resin with at least one pendant free carboxy group (resin A) or commercial available a hydroxy

functionalized polyimide resin having at least one aromatic hydroxyl group (resin B) was diluted with NMP (N-Methyl-2-pyrrolidone), typically 50% by weight and heated to 80 to 100°C in a glass tube under stirring. After one hour, the cross-linker (PBO, i.e. Phenylene-l,3-Bis(2-Oxazoline), PBOX, i.e. phenylene-l,4-bis(5,6-dihydro-4H-l,3- oxazines), or DOX (4,4',5,5'-tetrahydro-2,2'-bioxazole; also known as 2-(4,5- dihydrooxazol-2-yl)-4,5-dihydrooxazole)) dissolved in NMP (typically 15% NMP of the resin weight) was added to the resin and the resulting mixture was stirred for additional one hour.

The thus obtained solution was poured onto a glass plate (22x15x0,45 cm) and a thin film was made using a BYK filmknife (setting: 20 μιη), which film was baked in a vacuum oven at 90°C and 50 mbar for 17-34 hours. Thereafter the film was cured in an oven over nitrogen atmosphere at 200°C for 0.5-1 hour and 250°C for 0.5-1 hour. The obtained cured films, were clear transparent films, having essentially the same color as the reference. The cured films were analyzed by various methods indicated below. Analysis methods

Thermal decomposition

Instrument: TA instruments TGA Q50

Method: Temperature ramp 25-550°C (20°C/min)

N₂ flow: 60 mL/min

Table 1 - Thermal stability

Base resin Crosslinker Value

Type Amount (wt.%) TD5 Onset

A none 0 353 290

A PBO 5 377 300

A PBO 2 360 299

A PBOX 2 389 291 As can been be seen from table 1, the thermal stability of the base resin could be increased by employing a cross-linker comprising least two residues of oxazoline.

CTE & Tg determination (dimensional stability and softening point)

Instrument: TA instruments TMA Q400

Method: Temperature ramp 40-300°C (5°C/min)

Probe: Expansion

Preload force: 0.02 N

Applied force: 0.02 N

Table 2 - dimensional stability and softening point

Base resin Crosslinker Value

Type Amount (wt%) CTE value Tg (°C)

A none 0 28.2 255

A PBO 5 38.7 235

A PBO 2 14.8 261

B none 0 11.4 252

B PBO 2 28.7 259 Flexural modulus (stiffness)

Instrument: TA instruments TMA Q400

Method: Stress ramp 1.00 MPa / min to 20 MPa, isothermal at 50°C

Probe: Flexural

Geometry: Rectangular

Preload force: 0.02 N

Table 3 - stiffness

Base resin Crosslinker Modulus (GPa)

Type Amount (wt.%) Value

A None (ref.) 0 9.2

A PBO 5 7.5

A PBO 2 15.7 As can been be seen from table 3, the stiffness the base resin could be increased by employing a cross-linker comprising least two residues of oxazoline.

Chemical resistance

Two polyimide films comprising resin A was produced one with and one without (PBO). The film without PBO had a thickness of 175 μιη and the one with PBO had a thickness of 150 μιη. A small piece of the resulting films polyimide film (25 - 60 mg) was placed in a glass tube and 5-10 mL of solvent (Hypochlorite 15% and NMP) was added. The films were left in the solvent for 24 hours and were then dried on the surface with a cleanex paper. The thermostability of the films after the chemical treatment was analyzed by TGA.

Table 4 - Chemical resistance

Base resin Crosslinker Chemical treatment Value

Amount

Type Solvent Time (h) TD5 Onset

(wt%)

A none - none 0 353 290

Hypochlorite

A none - 20 316 279

15%

Hypochlorite

A PBO 5 20 327 323

15%

A none - NMP 20 326 275

A PBO 5 NMP 20 341 283 As can been be seen from table 4, the chemical resistance of the base resin could be increased by employing a cross-linker comprising least two residues of oxazoline.

Claims

1. A composition comprising an aromatic oligo- or polyimide, comprising at least one pendant free aromatic hydroxyl (-OH), or at least one pendant free carboxy group (-C(O)OH or -C(0)0^~) and a compound comprising at least two residues of oxazoline, such as at least tw groups, or at least two residues of dihydro-1,3-

oxazine, such as at least two

groups.

2. The composition according to claim 1, wherein said compound comprising at least two residues of oxazoline or dihydro- 1,3 -oxazine is a compound according to formula (I), or (II),

wherein

"A" is a direct bond, an arylene, such as phenylene, or a C 1-12 alkylene;

Rl and R2, independently of each other, are selected from the group consisting of hydrogen and CI -5 alkyl;

"n" is an integer of 3 to 5;

"B" is, an arylene, such as phenylene, or a CI -12 alkylene;

or wherein said compound comprising at least two residues of oxazoline or dihydro-l,3-oxazine is a compound obtainable by co-polymerization of 2-vinyl-4,5-

dihydrooxazole, i.e.

and a co-monomer selected from the group consisting of ethene, propene, styrene, vinyl acetate, alkyl acrylate, such as butyl acrylate, alkyl methacrylate, such as methyl methacrylate, acrylonitrile, vinylchloride.

3. The composition according to any one of the preceding claims, wherein said compound comprising at least two residues of oxazoline or dihydro-l,3-oxazine is selected from the group consisting of:

and polymers of the Epocross® series marketed by Nippon Shokubai.

4. The composition according to any one of the preceding claims, wherein the weight ratio between said oligo- or polyimide and said compound comprising at least two residues of oxazoline or dihydro-l,3-oxazine is 99.5:0.5 to 4: 1, such as 99:1 to 9:1.

5. The composition according to any one of the preceding claims, wherein the molar ratio between said oligo- or polyimide and said compound comprising at least two residues of oxazoline or dihydro-l,3-oxazine is 100: 1 to 1 :2, such as 10:1 to 1 :1.

6. The composition according to any one of the preceding claims, wherein the molar amount of the compound comprising at least two residues of oxazoline, or at least two residues of dihydro-l,3-oxazine, in the composition is selected in a manner such that number of residues of oxazoline or dihydro-l,3-oxazine is from 0.25 to 2, such as 0.5 to 1.0, times the number of pendant free carboxy or aromatic hydroxyl groups of the oligo- or polyimide in the composition.

7. The composition according to any one of the preceding claims, wherein said composition further comprises an additional polymer, a solvent, a polycarboxylic acid, such as tri-carboxylic acid, and/or a polyphenol, such as a tri-phenol.

8. The composition according to any one of the preceding claims, wherein said aromatic oligo- or polyimide is obtainable by co-polymerization of:

- an aromatic dianhydride selected from the group consisting pyromellitic dianhydride and an aromatic dianhydride according to the general formula (III),

wherein "G" represents a direct bond or a di-valent group selected from the group consisting of a carbonyl group, an amide group (-C(0)NH- or -NHC(O)-), an ester group (-C(O)O- or -OC(O)-), a methylene group, a sulfone group, a sulfide group, an ether group, an -C(0)-phenylene-C(0)- group, an isopropylidene group, a

hexafluoroisopropylidene group, a 3-oxyphenoxy group, a 4-oxyphenoxy group, a 4'- oxy-4-biphenoxy group, and a 4-[l-(4-oxyphenyl)-l-methylethyl]phenoxy group; and "G" is connected to the 4- or 5-position and the 4^'- or the 5 ^'-position, respectively, in the isobenzofuran- 1 ,3-dione residues;

- an aromatic diamine selected from the group consisting of 1 ,4- diaminobenzene, 1,3-diaminobenzene, and an aromatic diamine according to the general formula (IV)

wherein

the amino groups may be connected to any substitutable carbon atom in the benzene residues; and

"L" is a direct bond or a moiety selected from the group consisting of -0-, -S-, -C(O)-, -C(CH₃)₂-, -C(CF₃)₂-, -CH₂-, -S0₂-, an amide group (-C(0)NH- or -NHC(O)-), an ester group (-C(O)O- or -OC(O)-), 3-oxyphenoxy group, 4-oxyphenoxy group, 4'- oxy-4-biphenoxy group, and 4-[l-(4-oxyphenyl)-l-methylethyl]phenoxy group; and

- an aromatic diamine having at least one hydroxy or carboxy group, wherein said aromatic diamine is a compound selected from the group consisting of:

eral formula

V, Va, Vb, Vbl , Vc, or Vd:

(COOH)_p2

H₂N A^- ET Ar₂ E₂ Ar₃ NH₂

(COOH)_p1 (C00H)_P3 _(VC)

(OH)_q2

H₂N Ar_r Ei Ar₂ E₂ Ar₃ NH₂

(OH)_q1 (OH)_P3 _{( VD )} wherein

hexafluoroisopropylidene group, a 3-oxyphenoxy group, a 4-oxyphenoxy group, a 4'- oxy-4-biphenoxy group, a 4-[l-(4-oxyphenyl)-l-methylethyl]phenoxy group, and a 9,9- tetramethyl-9H-fluorene-3,6-diyl group; and "E" is connected to the 3- or 4-position and the 3 ^'- or the 3 ^'-position, respectively, in the terminal aniline residues; the amino groups being positioned in the 1- and 1 ^'-position, respectively;

"El" and "E2", independently of each other, respectively represents a direct bond or a di-valent group selected from the group consisting of a carbonyl group, an amide group (-C(O)NH- or -NHC(O)-), an ester group (-C(O)O- or -OC(O)-), a methylene group, a sulfone group, a sulfide group, an ether group, an -C(0)-phenylene- C(O)- group, an isopropylidene group, a hexafluoroisopropylidene group, a 3- oxyphenoxy group, a 4-oxyphenoxy group, a 4'-oxy-4-biphenoxy group, and a 4-[l-(4- oxyphenyl)-l-methylethyl]phenoxy group; and "El" and "E2", respectively is connected to any substitutable atom of "Arl", "Ar2", and "Ar3", respectively;

ql, q2, and q3 are integers of 0 (zero) to 2, wherein at least one of ql, q2, and q3, is 1 or 2; and

tl and t2 are integers of 0 (zero) to 4, wherein the sum of tl and t2 at least is 1; and/or

- an aromatic dianhydride having at least one hydroxy or carboxy group, wherein said aromatic dianhydride is a compound selected from the group consisting of:

O , and O or an aromatic dianhydride according to the general formula (VI) or (Via)

wherein

"Arl", "Ar2", and "Ar3", independently of each other, respectively is an arylene, such as an arylene selected from the group consisting of benzene, naphtene, antracene, and phenanthrene residues;

"El" and "E2", independently of each other, respectively represents a direct bond or a di-valent group selected from the group consisting of a carbonyl group, an amide group (-C(O)NH- or -NHC(O)-), an ester group (-C(O)O- or -OC(O)-), a methylene group, a sulfone group, a sulfide group, an ether group, an -C(0)-phenylene- C(O)- group, an isopropylidene group, a hexafluoroisopropylidene group, a 3- oxyphenoxy group, a 4-oxyphenoxy group, a 4'-oxy-4-biphenoxy group, and a 4-[l-(4- oxyphenyl)-l-methylethyl]phenoxy group; and "El" and "E2" respectively is connected to any substitutable atom of "Arl", "Ar2", and "Ar3", respectively;

9. The composition according to claim 8, wherein

- said aromatic dianhydride is selected from the group consisting of

pyromellitic dianhydride, 4,4'-oxydiphthalic anhydride, 2,2-bis-[4-(3,4- dicarboxyphenoxy)phenyl]-propane dianhydride, 3,3',4,4'-benzophenonetetracarboxylic acid dianhydride, 3,3',4,4'-tetracarboxybiphenyl dianhydride, 4,4',5,5'-sulfonyldiphthalic anhydride, and 5,5'-(perfluoropropane-2,2-diyl)bis(isobenzofuran-l,3-dione); and

- said aromatic di-amine is selected from the group consisting of 4,4'- oxydianiline, 1,4-diaminobenzene, 1,3-diaminobenzene, l,3-bis-(4- aminophenoxy)benzene, l,3-bis-(3-aminophenoxy)benzene, methylenedianiline, and 3,4'-oxydianiline.

10. The composition according to claim 8 or 9, wherein said aromatic diamine having at least one hydroxy or carboxy group is selected from the group consisting of:

- di-aminobenzoic acid;

- compounds according to formula (Va) or (Vbl) of 8, wherein

"E" is selected from the group consisting of a direct bond and a di-valent group selected from the group consisting of a carbonyl group, a sulfone group, a sulfide group, an isopropylidene group, a hexafluoroisopropylidene group, and a 9,9-tetramethyl-9H- fluorene-3,6-diyl group;

tl and t2 are 1; and

the phenolic groups of formula (Va), and the carboxy groups of formula (Vbl), respectively, are positioned orto with respect to the amino groups.

11. The composition according to any of the claims 1 to 7, wherein said aromatic oligo- or polyimide comprises: - at least one residue of an aromatic dianhydride selected from the group consisting pyromellitic dianhydride and an aromatic dianhydride according to the general formula (III),

wherein "G" represents a direct bond or a di-valent group selected from the group consisting of a carbonyl group, an amide group (-C(0)NH- or -NHC(O)-), an ester group (-C(O)O- or -OC(O)-), a methylene group, a sulfone group, a sulfide group, an ether group, an -C(0)-phenylene-C(0)- group, an isopropylidene group, a hexafluoroisopropylidene group, a 3-oxyphenoxy group, a 4-oxyphenoxy group, a 4'- oxy-4-biphenoxy group, and a 4-[l-(4-oxyphenyl)-l-methylethyl]phenoxy group; and "G" is connected to the 4- or 5-position and the 4^'- or the 5 ^'-position, respectively, in the isobenzofuran-l,3-dione residues;

- at least one residue of an aromatic diamine selected from the group consisting of 1 ,4-diaminobenzene, 1,3-diaminobenzene, and an aromatic diamine according to the general formula (IV)

wherein

"L" is a direct bond or a moiety selected from the group consisting of -0-, -S-, -C(O)-, -C(CH₃)₂-, -C(CF₃)₂-, -CH₂-, -S0₂-, an amide group (-C(0)NH- or -NHC(O)-), an ester group (-C(O)O- or -OC(O)-), 3-oxyphenoxy group, 4-oxyphenoxy group, 4'- oxy-4-biphenoxy group, and 4-[l-(4-oxyphenyl)-l-methylethyl]phenoxy group; and - at least one residue of an aromatic diamine having at least one hydroxy or carboxy group, wherein said aromatic diamine is a compound selected from the group consisting of:

_{or an} aromatic diamine according to the general formula

V, Va, Vb, Vbl , Vc, or Vd:

(COOH)_p2

Ai^ ET Ar₂ E₂ Ar₃ NH₂

(COOH)_p1 (COOH)p3 _{( Vc )}

(OH)_q2

Art ET Ar₂ E₂ Ar₃ NH₂

(OH)_{p 1} (OH)_{q3 ( Vd )} wherein

"E" represents a direct bond or a di-valent group selected from the group consisting of a carbonyl group, an amide group (-C(0)NH- or -NHC(O)-), an ester group (-C(O)O- or -OC(O)-), a methylene group, a sulfone group, a sulfide group, an ether group, an -C(0)-phenylene-C(0)- group, an isopropylidene group, a

- at least one residue of an aromatic dianhydride having at least one hydroxy or carboxy group, wherein said aromatic dianhydride is a compound selected from the group consisting of:

O , and O or an aromatic dianhydride according to the general formula VI or (Via)

wherein

12. The composition according to claim 11, wherein

- said aromatic dianhydride is selected from the group consisting of

13. The composition according to claim 11 or 12, wherein said aromatic diamine having at least one hydroxy or carboxy group is selected from the group consisting of:

- compounds according to formula (Va) or (Vbl) of 11, wherein "E" is selected from the group consisting of a direct bond and a di-valent group selected from the group consisting of a carbonyl group, a sulfone group, a sulfide group, an isopropylidene group, a hexafluoroisopropylidene group, and a 9,9-tetramethyl-9H- fluorene-3,6-diyl group;

tl and t2 are 1; and

14. The composition according to any one of the preceding claims, wherein said oligo- or polyimide has a number average molecular weight of about 1 ,000 to 20,000.

15. The composition according to any one of the preceding claims, wherein said oligo- or polyimide has a weight average molecular weight of about 1,000 to 200,000.

16. The composition according to any one of the claims 1 to 15, wherein said oligo- or polyimide comprises:

- at least one but less than twenty residues of said aromatic diamine;

- at least one but less than twenty residues of said aromatic dianhydride; and

- at least one but less than twenty residues of said aromatic diamine having at least one hydroxy or carboxy group; and/or

- at least one but less than twenty residues of said aromatic dianhydride having at least one hydroxy or carboxy group.

17. The composition according to any one of the claims 1 to 15, wherein said oligo- or polyimide comprises:

- at least 20 but less than 200 residues of said aromatic diamine;

- at least 20 but less than 200 residues of said aromatic dianhydride; and

- at least 1 but less than 200 residues of said aromatic diamine having at least one hydroxy or carboxy group; and/or

- at least 1 but less than 200 residues of said aromatic dianhydride having at least one hydroxy or carboxy group.

18. The composition according to any one of the claims 1 to 15, wherein said oligo- or polyimide comprises:

- at least 200 residues of said aromatic diamine;

- at least 200 residues of said aromatic dianhydride; and

- at least 1 residue of said aromatic diamine having at least one hydroxy or carboxy group; and/or

- at least 1 residue of said aromatic dianhydride having at least one hydroxy or carboxy group.

19. Use of a compound comprising at least two residues of oxazoline, such as

, or at least two residues of dihydro- 1 ,3-oxazine, such as cross- linking an aromatic oligo- or polyimide comprising at least one pendant free aromatic hydroxyl (-OH), or at least one pendant free carboxy group (-C(O)OH or -C(0)0^").

20. The use according to claim 19, wherein said compound comprising at least two residues of oxazoline or dihydro-l,3-oxazine, is a compound according to claim 2 or 3.

21. A method of cross-linking an oligo- or polyimide, comprising at least one pendant free hydroxyl (-OH) or carboxy group (-C(O)OH or -C(0)0^"), comprising the step of:

-adding a compound comprising at least two residues of oxazoline, such as at least two

groups, or at least two residues of dihydro- 1, 3 -oxazine, such as at least

N 0

two groups, to said oligo- or polyimide; and

- heating the resulting mixture.

22. The method according to claim 21, wherein said compound comprising at least two residues of oxazoline, such as at least two roups, or at least two

residues of dihydro- 1,3 -oxazine, such as at least two

groups is a compound according to claim 2 or 3.

23. The method according to claim 21 or 22, wherein said oligo- or polyimide is as an oligo- or polyimide according to any one of the claims 8 to 18.

24. An article, such a film, comprising a cross-linked oligo- or polyimide obtainable by use of the composition according to any one of the claims 1 to 15.