WO2024190545A1 - Method for producing polyester-imide copolymer - Google Patents

Abstract

The present invention relates to a method for producing a polyester-imide copolymer, the method comprising using a compound represented by general formula (2) and a compound represented by general formula (3) as polymerizing ingredients to form ester bonds by polycondensation, thereby obtaining a polymer represented by general formula (1). In general formulae (1) to (3), R¹ is a tetravalent organic, R² is a divalent organic group, R³ and R⁴ are each independently an arylene group, Z is an ester bond, and X¹, Y¹, X², and Y² are each independently a carboxy group, a hydroxy group, or an acetyloxy group. At least some of structure A in general formula (2) is an isoimide structure.

Description

Method for producing polyester-imide copolymer

The present invention relates to a method for producing a polyester-imide copolymer.

Polyimide film is widely used as a substrate material for printed wiring boards and the like due to its high heat resistance. In recent years, as electrical signals have become increasingly higher in frequency, polymer materials used in substrates and the like are required to have low transmission loss at high frequencies, particularly in the GHz range. Liquid crystal polyester is a low-dielectric material with a smaller relative dielectric constant and dielectric loss tangent than polyimide, so it is expected to reduce transmission loss at high frequencies (for example, Patent Document 1).

However, liquid crystal polyester has a problem in that its dielectric tangent has a large temperature dependency. This is presumably due to the low glass transition temperature of liquid crystal polyester. Patent Document 2 describes that a polyester-imide copolymer containing an imide structural unit and an aromatic ester structural unit has excellent heat resistance.

JP 2012-77117 A JP 2003-82101 A

The most common method for polymerizing polyester-imide copolymers is to use an imide compound (imide oligomer) as a raw material and carry out esterification through polycondensation. However, imide oligomers have a high melting point, so it is not easy to carry out a polymerization reaction using the melting method. This tendency is particularly noticeable when using a highly heat-resistant monomer to increase the heat resistance of the polymer.

The present invention relates to a method for producing a polyester-imide copolymer having a structure represented by the following general formula (1):

In the general formula (1), ^R1 is a tetravalent organic group, ^R2 is a divalent organic group, ^R3 and ^R4 are each independently an arylene group, and Z is an ester bond. m is an integer of 1 or more, and n is an integer of 0 or more.

The above polyester-imide copolymer is obtained by forming ester bonds through a polycondensation reaction using a compound represented by the following general formula (2) and a compound represented by the following general formula (3) as polymerization components.

In the general formulas (2) and (3), R ¹ to R ⁴ are the same as those in the general formula (1), and n is an integer of 0 or more. X ¹ , Y ¹ , X ² and Y ² are each independently a carboxy group or a hydroxy group. X ¹ , Y ¹ , X ² and Y ² may be a carboxy group or a hydroxy group that has been derivatized into a functional group suitable for an ester reaction. For example, X ¹ , Y ¹ , X ² and Y ² may be an acetyloxy group obtained by acylation of a hydroxy group with acetic anhydride.

Structure A in general formula (2) is (A1) or (A2) below, and at least a portion of structure A is an isoimide structure represented by (A2).

In the structure A in the general formula (2), the ratio of the isoimide structure to the total of the imide structure (A1) and the isoimide structure (A2) may be 10 mol % or more. The compound represented by the general formula (2) may have a melting point of 300°C or less.

In the general formula (3), R ⁴ is preferably a naphthalene skeleton or a benzene skeleton. The ratio of the compound in which R ⁴ has a naphthalene skeleton to the total amount of the compound represented by the general formula (3) may be 30 to 85 mol %.

In a polycondensation reaction in which a compound represented by general formula (2) and a compound represented by general formula (3) are used as monomer components, the amount of the compound represented by general formula (2) relative to the total amount of the compound represented by general formula (2) and the compound represented by general formula (3) may be 5 to 50 mol %. The polycondensation reaction may be carried out by a melt polymerization method. The temperature of the melt polymerization may be 350°C or lower.

The polyester-imide copolymer represented by general formula (1) has high heat resistance, excellent dielectric properties, and small orientation anisotropy when a film is produced. Since the compound represented by general formula (2) has an isoimide structure and a lower melting point than general imide oligomers, the present invention can provide the polyester-imide copolymer represented by general formula (1) by low-temperature melt polymerization.

The present invention relates to a polyester-imide copolymer having a repeating unit represented by the following general formula (1) and a method for producing the same.

^R1 is a tetravalent organic group, and ^R2 is a divalent organic group. ^R3 and ^R4 are each independently an aromatic group (arylene group), and Z is an ester bond (-CO-O- or -O-CO-). m is an integer of 1 or more, and n is an integer of 0 or more.

A polyester-imide copolymer having the structure of general formula (1) can be obtained by polycondensation of a compound (imide oligomer) represented by general formula (2) and a compound represented by general formula (3).

In general formula (2) and general formula (3), R ² , R ³ and R ⁴ are the same as those in general formula (1). X ¹ , Y ¹ , X ² and Y ² each independently represent a carboxy group or a hydroxy group.

Structure A in general formula (2) is (A1) or (A2) below, and at least a portion of structure A is an isoimide structure represented by (A2).

Below, we will explain the details of the compound of general formula (2) and the compound of general formula (3), and then explain the method for obtaining the polyester-imide copolymer of general formula (1).

[Compound of general formula (2)]
The compound represented by the general formula (2) is an imide-based compound (oligomer) in which at least a part of the compound has an isoimide structure. The imide-based oligomer represented by the general formula (2) is obtained by polymerizing a polyamic acid oligomer having an acid anhydride group at the end by reacting a tetracarboxylic dianhydride represented by the general formula (11) with a diamine represented by the general formula (12), reacting an aromatic amine having a carboxyl group or a hydroxyl group with the acid anhydride end of the polyamic acid to modify the end, and dehydrating and condensing the amic acid in the presence of a dehydrating and condensing agent.

<Tetracarboxylic acid dianhydride>
The tetracarboxylic dianhydride (hereinafter, sometimes simply referred to as "acid dianhydride") used as a monomer in the polymerization of polyamic acid is a compound represented by the above general formula (11). In general formula (11), R ^a is a tetravalent organic group, which is a tetracarboxylic acid residue obtained by removing four carboxy groups from a tetracarboxylic acid.

Acid dianhydrides include 1,2,3,4-cyclobutane tetracarboxylic dianhydride, 1,2,3,4-cyclopentane tetracarboxylic dianhydride, 1,2,4,5-cyclohexane tetracarboxylic dianhydride, 1,2,3,4-tetramethyl-1,2,3,4-cyclobutane tetracarboxylic dianhydride, 4,4'-(ethyne-1,2-diyl)diphthalic anhydride, 4,4'-(hexafluoroisopropylidene)diphthalic anhydride, and naphthalene-1,4,5,8-tetracarboxylic dianhydride. Water, 3,4'-oxydiphthalic anhydride, 4,4'-oxydiphthalic anhydride, pyromellitic anhydride, 4,4'-sulfonyldiphthalic anhydride, 3,3',4,4'-benzophenonetetracarboxylic dianhydride, bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 2,3,3',4'-biphenyltetracarboxylic dianhydride, bicyclo[2.2.2]octane-2,3,5,6-tetracarboxylic dianhydride tetracarboxylic acid 2,3:5,6-dianhydride, 1,2,3,4-butanetetracarboxylic acid dianhydride, ethylenediaminetetraacetic acid dianhydride, 4,4'-(4,4'-isopropylidenediphenoxy)diphthalic anhydride, 4-(2,5-dioxotetrahydrofuran-3-yl)-tetralin-1,2-dicarboxylic acid anhydride, dicyclohexyl-3,4,3',4'-tetracarboxylic acid dianhydride, naphthalene-1,4,5,8-tetracarboxylic acid dianhydride, 2,3,6,7-naphthalene Examples of suitable dianhydrides include 9,9-bis(3,4-dicarboxyphenyl)fluorene dianhydride, norbornane-2-spiro-α-cyclopentanone-α'-spiro-2"-norbornane-5,5",6,6"-tetracarboxylic dianhydride, 5-(2,5-dioxotetrahydrofuryl)-3-methyl-3-cyclohexene-1,2-dicarboxylic anhydride, ethylene glycol bis(anhydrotrimellitate), and paraphenylene bis(trimellitate anhydride). Among these, pyromellitic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, and paraphenylene bis(trimellitate anhydride) are preferred.

<Diamine>
The diamine used as a monomer in the polymerization of polyamic acid is a compound represented by the above general formula (12). In general formula (12), ^Rb is a divalent organic group, and is a diamine residue obtained by removing two amino groups from a diamine.

The diamine used as a monomer for obtaining the general formula (2) is a C ₄ -C Aliphatic diamines with amino groups bonded to both ends of _{the 18} alkylenes, 1,5-diaminonaphthalene, 9,9-bis(4-aminophenyl)fluorene, 4,4'-methylenebis(2-methylcyclohexylamine), 1,4-bis[2-(4-aminophenyl)-2-propyl]benzene, m-xylylenediamine, p-xylylenediamine, 1,3-bis(aminomethyl)cyclohexane, 1,4-bis(aminomethyl)cyclohexane, bis(aminomethyl)norbornane, 1,2-cyclohexanediamine, 1,3-cyclohexanediamine, 1,4-cyclohexanediamine, 3-aminobenzylamine, 4-aminobenzylamine, bis(3-aminophenyl)sulfone, 3,3'-diamino-N-methyldipropylamine, 1,4-bis(3-aminopropoxy)butane, 1,4-bis(4-aminophenoxy)benzene, 1,3-bis(3-aminophenoxy)benzene benzene, 1,3-bis(4-aminophenoxy)benzene, 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane, 2,2-bis(4-aminophenyl)hexafluoropropane, 2,2-bis[4-(4-aminophenoxy)phenyl]propane, 4,4'-bis(4-aminophenoxy)biphenyl, bis[4-(3-aminophenoxy)phenyl]sulfone, bis[4-(4-aminophenoxy)phenyl] nyl]sulfone, 2,2'-bis(trifluoromethyl)benzidine, 1,1-bis(4-aminophenyl)cyclohexane, 9,9-bis(4-amino-3-methylphenyl)fluorene, 1,3-bis[2-(4-aminophenyl)-2-propyl]benzene, 1,4-bis(4-amino-2-trifluoromethylphenoxy)benzene, 4,4'-[naphthalene-2,7-diylbis(oxy)]dianiline, 4,4'-bis(3- (aminophenoxy)biphenyl, bis(4-aminophenyl)sulfone, 2,7-diaminofluorene, 4,4'-diaminobenzophenone, diethylene glycol bis(3-aminopropyl)ether, 3,4'-diaminodiphenyl ether, 3,3'-diaminodiphenylmethane, 3,4'-diaminodiphenylmethane, 4,4'-diaminodiphenylmethane, 1,6-diaminopyrene, 1,3-diaminopyrene, 2,5-dimethyl 1,4-phenylenediamine, 4,4''-diamino-p-terphenyl, 2,5-diaminotoluene, 4,4'-methylenebis(2-ethyl-6-methylaniline), 4,4'-methylenebis(2,6-diethylaniline), 4,4'-methylenebis(2,6-dimethylaniline), 4,4'-diaminodiphenyl ether, 3,3'-oxydianiline, 1,4-phenylenediamine, 1,3-phenylenediamine, o-tolidine , m-tolidine, 3,3',5,5'-tetramethylbenzidine, 2,4,6-trimethyl-1,3-phenylenediamine, 2,3,5,6-tetramethyl-1,4-phenylenediamine, 2-(trifluoromethyl)-1,4-phenylenediamine, 5-trifluoromethyl-1,3-phenylenediamine, 3,3'-dimethylnaphthidine, 4,4'-methylenebis(cyclohexylamine), 4,4'-diaminooctafluorobiphenyl Examples of the diamine include 4,4'-diamino-3,3'-dimethyldiphenylmethane, 4,4'-ethylenedianiline, 4,4'-bis(4-amino-2-trifluoromethylphenoxy)biphenyl, o-dianisidine, m-dianisidine, 4,4'-diamino-2,2'-dimethylbibenzyl, 3,3'-diaminobenzophenone, 1,3-bis(3-aminopropyl)tetramethyldisiloxane, and 4,4'-diaminobenzanilide. Among these, aliphatic diamines having an alkylene of _C4 to _C12 , 1,3-bis(3-aminophenoxy)benzene, 1,3-bis(4-aminophenoxy)benzene, and the like are preferred.

<Polymerization of polyamic acid>
According to a conventional method, polyamic acid is obtained by reacting an acid dianhydride with a diamine in an organic solvent. In this case, by making the number of moles of the acid dianhydride larger than the number of moles of the diamine, a polyamic acid having an acid anhydride group at both ends is obtained as shown in the following general formula (4). In general formula (4), R ^a is a tetracarboxylic acid residue which is a tetravalent organic group, and R ^b is a diamine residue which is a divalent organic group. k is an integer of 0 or more.

The amount of acid dianhydride in the polymerization of polyamic acid is preferably 1.1 to 3.0 times, more preferably 1.3 to 2.7 times, even more preferably 1.5 to 2.5 times, and may be 1.7 to 2.3 times or 1.8 to 2.2 times, in molar ratio relative to the amount of diamine. If the amount of acid dianhydride is small, polyamic acid having an acid anhydride at the end cannot be obtained. Also, if the amount of acid dianhydride is small (if the amount of acid dianhydride is equal to the amount of diamine), the molecular weight of the polyamic acid tends to be excessively large. If there is a large excess of acid dianhydride, the reaction efficiency tends to decrease.

The organic solvent used in the polymerization of polyamic acid is not particularly limited as long as it can dissolve polyamic acid, and for example, amide-based solvents such as N,N-dimethylformamide, N,N-dimethylacetamide, and N-methyl-2-pyrrolidone can be used. Among amide-based solvents, N,N-dimethylformamide and N-dimethylacetamide are particularly preferred. The solids concentration of polyamic acid is not particularly limited, and is, for example, about 5% to 35% by weight.

<Terminal Modification of Polyamic Acid>
A polyamic acid represented by general formula (5) having a carboxy group and/or a hydroxy group at its terminal is obtained by reacting an acid anhydride group at the terminal of a polyamic acid with an aromatic amino compound having a carboxy group or an aromatic amino compound having a hydroxy group as an end-capping compound (monoamine).

In the general formula (5), R ^a , R ^b and k are the same as those in the general formula (4). R ³ is arylene. ^{X 1} and Y ¹ each independently represent a carboxy group or a hydroxy group.

The aromatic amino compound having a carboxy group is a compound in which an amino group and a carboxy group are bonded to an aromatic ring, and is represented by the general formula H ₂ N-R ³ -COOH. Specific examples of the aromatic amino compound having a carboxy group include o-aminobenzoic acid, m-aminobenzoic acid, p-aminobenzoic acid, 3-amino-2-methylbenzoic acid, 3-amino-4-methylbenzoic acid, 2-amino-6-methylbenzoic acid, 4-amino-3-methylbenzoic acid, 2-amino-5-methylbenzoic acid, 2-amino-3-methylbenzoic acid, 4-amino-2-methylbenzoic acid, 5-amino-2-methylbenzoic acid, 6-amino-2-naphthoic acid, and 3-amino-2-naphthoic acid. Among these, m-aminobenzoic acid and p-aminobenzoic acid are preferred.

An aromatic amino compound having a hydroxy group is a compound in which an amino group and a carboxy group are bonded to an aromatic ring, and is represented by the general formula H ₂ N-R ³ -OH. Specific examples of aromatic amino compounds having a hydroxy group include o-aminophenol, m-aminophenol, p-aminophenol, 5-amino-1-naphthol, 5-amino-2-naphthol, 6-amino-1-naphthol, 3-amino-2-naphthol, and 8-amino-2-naphthol. Of these, m-aminophenol and p-aminophenol are preferred.

For the terminal modification of polyamic acid, only aromatic amino compounds having a carboxy group may be used, only aromatic amino compounds having a hydroxy group may be used, or both of them may be used. When only aromatic amino compounds having a carboxy group are used, the polyamic acid of general formula (5) has both X ¹ and Y ¹ as carboxy groups. When only aromatic amino compounds having a hydroxy group are used, the polyamic acid of general formula (5) has both X ¹ and Y ¹ as hydroxy groups. When aromatic amino compounds having a carboxy group and aromatic amino compounds having a hydroxy group are used, the polyamic acid of general formula (5) has both X ¹ and Y ¹ as carboxy groups, both X ¹ and Y ¹ as hydroxy groups, and a mixture of one of X ¹ and Y ¹ as carboxy groups and the other as hydroxy groups.

<Isoimidization>
The isoimide is generated by dehydrating and cyclizing the polyamic acid using a dehydrating condensation agent. The compound represented by the general formula (2) is obtained by dehydrating and cyclizing the terminal-modified polyamic acid represented by the general formula (5) using a dehydrating condensation agent.

^X1 and ^Y1 in general formula (2) are the same as those in general formula (5). ^R1 is a tetracarboxylic acid residue and is the same as ^Ra in general formula (5). ^R2 is a diamine residue and is the same as ^Rb in general formula (5). A in general formula (2) is an imide structure of the following (A1) or an isoimide structure of the following (A2).

Examples of dehydration condensation agents include carbodiimide compounds such as bis(2,6-diisopropylphenyl)carbodiimide, bis(trimethylsilyl)carbodiimide, N,N'-di-tert-butylcarbodiimide, N,N'-dicyclohexylcarbodiimide, N,N'-diisopropylcarbodiimide, and 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide; trifluoroacetic anhydride, and thionyl chloride. Among these, carbodiimide compounds are preferred, and N,N'-dicyclohexylcarbodiimide and N,N'-diisopropylcarbodiimide are particularly preferred.

The amount of dehydration condensation agent added is preferably 1.0 equivalent or more per equivalent of the amide acid (carboxy group) of the compound of general formula (5). If the amount is less than 1.0 equivalent, the isoimidization may be incomplete, and some of the polyamide acid may remain without ring closure. The reaction temperature for the dehydration condensation is preferably -10 to 30°C, and particularly preferably 0 to 10°C.

Generally, chemical imidization of polyamic acid is carried out in the presence of a catalyst such as a tertiary amine and a dehydrating agent such as acetic anhydride, and an imide structure is generated by dehydration and ring closure of the polyamic acid. Depending on the imidization conditions, a small amount of isoimide may be generated, but the majority of the imide structure is formed. On the other hand, when dehydration and ring closure is carried out in the presence of a dehydrating condensation agent such as a carbodiimide compound, a significant proportion of isoimide structures are generated in addition to the imide structure.

Compounds having an isoimide structure have a lower melting point than imide compounds having only an imide structure, and are therefore more applicable to melt polycondensation, which will be described later. The melting point of the compound of general formula (2) is preferably 300°C or less, more preferably 280°C or less, and particularly preferably 250°C or less.

In the compound of general formula (2), the ratio of isoimide structures to the total of imide structures and isoimide structures is preferably 10% or more, more preferably 20% or more, even more preferably 25% or more, particularly preferably 30% or more, and may be 35% or more or 40% or more. The higher the ratio of isoimide structures, the lower the melting point tends to be. The ratio of isoimide structures may be 100%, or may be 80% or less, 70% or less, 60% or less, or 55% or less.

[Compound of general formula (3)]
In the general formula (3), ^R4 is arylene, and ^X2 and ^Y2 are each independently a carboxyl group or a hydroxyl group. As described below, ^X2 and ^Y2 may be derivatized to enhance ester formation.

From the viewpoints of the heat resistance of the polyester-imide copolymer and the cost and availability of raw materials, R ⁴ in the general formula (3) is preferably a naphthalene skeleton or a benzene skeleton.

When one of ^X2 and ^Y2 is a carboxy group and the other is a hydroxy group, the compound represented by the general formula (3) is an aromatic hydroxycarboxylic acid. By using an aromatic hydroxycarboxylic acid, an ester having an aromatic oxycarbonyl unit can be obtained.

Specific examples of suitable aromatic hydroxycarboxylic acids include p-hydroxybenzoic acid, m-hydroxybenzoic acid, o-hydroxybenzoic acid, 6-hydroxy-2-naphthoic acid, 5-hydroxy-2-naphthoic acid, 3-hydroxy-2-naphthoic acid, 4'-hydroxyphenyl-4-benzoic acid, 3'-hydroxyphenyl-4-benzoic acid, and 4'-hydroxyphenyl-3-benzoic acid. Of these aromatic hydroxycarboxylic acids, p-hydroxybenzoic acid and 6-hydroxy-2-naphthoic acid are preferred because they allow easy adjustment of the mechanical properties and melting point of the polymer.

When both ^X2 and ^Y2 are carboxy groups, the compound represented by the general formula (3) is an aromatic dicarboxylic acid. By using an aromatic dicarboxylic acid, an ester having an aromatic dicarboxy unit can be obtained.

Specific examples of suitable aromatic dicarboxylic acids include aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, 2,6-naphthalenedicarboxylic acid, 1,6-naphthalenedicarboxylic acid, 2,7-naphthalenedicarboxylic acid, 1,4-naphthalenedicarboxylic acid, and 4,4'-dicarboxybiphenyl. Of these aromatic dicarboxylic acids, terephthalic acid and 2,6-naphthalenedicarboxylic acid are preferred because they allow easy adjustment of the mechanical properties and melting point of the polymer.

When both ^X2 and ^Y2 are hydroxy groups, the compound represented by the general formula (3) is an aromatic diol acid. By using an aromatic diol, an ester having an aromatic dioxy unit is obtained.

Specific examples of suitable aromatic diols include hydroquinone, resorcinol, 2,6-dihydroxynaphthalene, 2,7-dihydroxynaphthalene, 1,6-dihydroxynaphthalene, 1,4-dihydroxynaphthalene, 4,4'-dihydroxybiphenyl, 3,3'-dihydroxybiphenyl, 3,4'-dihydroxybiphenyl, and 4,4'-dihydroxybiphenyl ether. From the viewpoint of polymer properties and reactivity during polymerization, hydroquinone, resorcinol, and 4,4'-dihydroxybiphenyl are preferred among these aromatic diols.

In preparing the polyester-imide copolymer, ester-forming derivatives of the above-mentioned aromatic hydroxycarboxylic acids, aromatic dicarboxylic acids, and aromatic diols may be used. Examples of ester-forming derivatives include ester derivatives and acid halides of carboxylic acids (carboxy groups), and acylated derivatives of hydroxy groups.

In preparing the polyester-imide copolymer, multiple types of compounds represented by general formula (3) may be used in combination. As the compound represented by general formula (3), multiple types of aromatic hydroxycarboxylic acids may be used, multiple types of aromatic dicarboxylic acids may be used, or multiple types of aromatic diols may be used. As the compound represented by general formula (3), two or more types may be used from among aromatic hydroxycarboxylic acids, aromatic dicarboxylic acids, and aromatic diols.

Among the compounds represented by the general formula (3) used in the preparation of the polyester-imide copolymer, the proportion (molar ratio) of the compound having a naphthalene skeleton (the compound in which R ⁴ has a naphthalene skeleton) is preferably 30 to 85%, more preferably 40 to 80%, even more preferably 45 to 75%, and may be 50 to 70% or 55 to 65%. When the proportion of the compound having a naphthalene skeleton is low, the heat resistance and dielectric properties of the resulting polymer tend to decrease. When the proportion of the compound having a naphthalene skeleton is excessively high, the melting point is high, so that the applicability to melt polymerization is poor, and polymerization may not proceed sufficiently.

<Ester Polymerization>
The polyester-imide copolymer represented by the general formula (1) is obtained by polycondensation of the compound represented by the general formula (2) (imide oligomer having an isoimide structure) and the compound represented by the general formula (3).

In the compound of general formula (2), at least a portion of structure A is an isoimide structure, but because the isoimide is isomerized to imide by heating during polycondensation, almost no isoimide structure remains in the resulting polymer.

Z in general formula (1) is an ester formed by condensation of ^X1 in general formula (2) with ^Y2 in general formula (3), and an ester formed by condensation of ^Y1 in general formula (2) with ^X2 in general formula (3). Depending on the combination of ^X1 , ^X2 , ^Y1 , and ^Y2 , the ester bond Z can be either -CO-O- or -O-CO-.

^R1 in formula (1) is the same as in formula (2) and is a tetracarboxylic acid residue, which is a tetravalent organic group. ^R3 and ^R4 in formula (1) are the same as ^R3 and ^R4 in formulas (2) and (3) and are arylene. ^R2 in formula (1) is a divalent organic group and is a diamine residue, which is the same as in formula (2), or is ^-R3 -Z- ^R3- .

When ^X1 and ^Y1 in general formula (2) are both carboxy groups, and ^X2 and ^Y2 in general formula (3) are both hydroxy groups, the compound of general formula (2) and the compound of general formula (3) are alternately polymerized by polycondensation. Since the compounds of general formula (2) do not condense with each other, and the compounds of general formula (3) do not condense with each other, ^R2 in general formula (1) is a diamine residue, as in general formula (2), and m in general formula (1) is 1. The same applies when ^X1 and ^Y1 in general formula (2) are both hydroxy groups, and ^X2 and ^Y2 in general formula (3) are both carboxy groups.

When synthesizing the compound of general formula (2), if an aromatic amino compound having a carboxy group and an aromatic amino compound having a hydroxy group are used in combination as aromatic amino compounds for terminal modification of polyamic acid, ^X1 and ^Y1 in general formula (2) contain both a hydroxy group and a carboxy group. In this case, during polycondensation, the compounds of general formula (2) are condensed with each other to form an ester bond Z, and ^R2 in general formula (1) has a structure of ^-R3 -Z- ^R3- .

When an aromatic hydroxycarboxylic acid is used as the compound of general formula (3) or when an aromatic dicarboxylic acid and an aromatic diol are used in combination, ^X2 and ^Y2 in general formula (3) contain both a hydroxy group and a carboxy group. In this case, the compounds of general formula (3) are condensed with each other during polycondensation to generate an ester bond Z, so that general formula (1) contains a structural unit (polyester unit) in which m is 2 or more.

The ratio of the compound of general formula (2) to the compound of general formula (3) is not particularly limited, but from the viewpoint of polymerizability, it is preferable that the total amount of carboxy groups and the total amount of hydroxy groups in ^X1 and ^Y1 of general formula (2) and ^X2 and ^Y2 of general formula (3) as a whole are approximately equimolar. The ratio of the total amount of carboxy groups to the total amount of hydroxy groups is preferably 0.7 to 1.3, more preferably 0.8 to 1.2, even more preferably 0.9 to 1.1, and may be 0.95 to 1.05, 0.98 to 1.02, or 0.99 to 1.01. The amount of carboxy groups here includes those in which the carboxy groups have been converted into ester-forming derivatives (e.g., esters and acid halides), and the amount of hydroxy groups includes those in which the hydroxy groups have been converted into ester-forming derivatives (e.g., acylated products).

From the viewpoint of increasing the heat resistance of the polymer while lowering its dielectric constant, the amount of the compound of general formula (2) relative to the total of the compound of general formula (2) and the compound of general formula (3) is preferably 5 to 50 mol %, more preferably 10 to 40 mol %. If the proportion of the compound of general formula (2) is small and the proportion of the imide structure in the polymer is low, the heat resistance may be insufficient. If the proportion of the compound of general formula (2) is excessively large and the proportion of the imide structure in the polymer is high, the relative dielectric constant and dielectric tangent of the polymer tend to be high.

The molar amount of the compound of general formula (2) can be calculated based on the amount (mass) of the compound of general formula (2) charged and the theoretical molecular weight M calculated from the following formula (I) based on the type and amount of the acid dianhydride and diamine used in the polymerization of the polyamic acid.
M=(m _P −2×m _w )×{N _A /(N _A −N _B )} …(I)

m _P : Molecular weight per unit of polyamic acid m _w : Molecular weight of water (= Molecular weight per unit of polyamic acid (= 18.02)
N _A : Total moles of acid dianhydride N _B : Total moles of diamine

The molecular weight _mP per unit of polyamic acid is calculated based on the following formula:

m _a : molecular weight of acid dianhydride a N _a : moles of acid dianhydride a N _A : total moles of acid dianhydride m _b : molecular weight of diamine b N _b : moles of diamine b N _B : total moles of diamines m _p : molecular weight of terminal blocking compound (monoamine) p N _b : moles of terminal blocking compound p N _B : total moles of terminal blocking compounds

In the polycondensation of the compound of general formula (2) and the compound of general formula (3), the hydroxyl groups and carboxyl groups of X ¹ , Y ¹ , X ² and Y ² can be esterified by condensation as they are. However, from the viewpoint of polymerization efficiency, it is preferable to derivatize the hydroxyl groups and/or carboxyl groups into functional groups having high ester-forming properties and then polycondense the resulting mixture.

In the polymerization of aromatic polyesters, it is preferable to acylate the hydroxyl groups and then perform polycondensation by deesterification. The acylation is preferably carried out by reacting the hydroxyl groups of the compound of general formula (2) or (3) with a fatty acid anhydride. Specific examples of fatty acid anhydrides include acetic anhydride and propionic anhydride. From the standpoint of cost and ease of handling, acetic anhydride is particularly preferable. The amount of fatty acid anhydride used is preferably 1.0 to 1.5 equivalents per equivalent of hydroxyl groups, and more preferably 1.05 to 1.3 equivalents.

The compounds of general formula (2) and general formula (3) are mixed, and derivatized as necessary, such as by acylation. The monomer mixture is then heated, and the by-product of polycondensation (for example, when the hydroxyl group is acylated with a fatty acid anhydride, the by-product is a fatty acid) is distilled off to obtain a polyester-imide copolymer represented by general formula (1). The polycondensation is preferably carried out in the presence of a catalyst. Examples of catalysts include metal compounds such as magnesium acetate, stannous acetate, tetrabutyl titanate, lead acetate, sodium acetate, potassium acetate, and antimony trioxide, and nitrogen-containing heterocyclic compounds such as 1-methylimidazole. The amount of catalyst used is generally 0.1 molar parts or less per 100 molar parts of the total monomers.

Polycondensation can be carried out by any of solution polymerization, melt polymerization and solid-phase polymerization, but melt polymerization is preferred because it can be applied to monomers with low solubility in solvents and has high reaction efficiency. When polymerization is carried out only by melt polycondensation, the temperature of melt polycondensation is preferably 150°C to 350°C, more preferably 250°C to 320°C. When polymerization is carried out in two stages by carrying out solid-phase polymerization described later after melt polycondensation, the temperature of melt polycondensation is preferably 120°C to 320°C, more preferably 200°C to 300°C. The time of the polycondensation reaction is not particularly limited as long as a polymer having the desired melting point and molecular weight is obtained. The reaction time of polycondensation may be, for example, 30 minutes to 5 hours.

Since imide compounds have a high melting point and are hardly melted in the above temperature range, it is necessary to heat them to a high temperature of about 350°C or higher in order to obtain a polyester-imide copolymer by melt polymerization. On the other hand, since at least a part of structure A of the compound represented by general formula (2) is an isoimide structure, it has a lower melting point than general imide oligomers, can be melt polymerized even at a low temperature of 350°C or lower, and has excellent reactivity. As mentioned above, isoimide isomerizes to imide at high temperatures, so even when a compound having an isoimide structure is used, the ester of general formula (1) obtained by melt polymerization contains almost no isoimide structure, and almost all of structure A of general formula (2) is imidized.

If necessary, the polymer obtained by melt polymerization may be heated in a solidified state (solid phase) to carry out polycondensation (solid phase polymerization) for the purpose of further increasing the molecular weight, etc.

<Physical Properties of Polyester-Imide Copolymer>
The melting point of the polyester-imide copolymer of general formula (1) obtained by the above method is not particularly limited, but from the viewpoint of heat resistance, it is preferably 250° C. or higher, and more preferably 280° C. or higher. By using a compound having an isoimide structure represented by general formula (2), it becomes possible to obtain a polyester-imide copolymer having such a high melting point by melt polymerization. From the viewpoint of processability into films, etc., the melting point of the polyester-imide copolymer of general formula (1) is preferably 400° C. or lower, and more preferably 350° C. or lower.

The glass transition temperature of the polyester-imide copolymer of general formula (1) is not particularly limited, but from the viewpoint of heat resistance and electrical properties at high temperatures (particularly suppression of increases in the dielectric constant and dielectric tangent), it is preferably 130°C or higher, more preferably 140°C or higher, and even more preferably 150°C or higher.

The glass transition temperature and melting point of a polyester-imide copolymer are temperatures determined from the inflection point and crystalline melting peak in a DSC chart measured with a differential scanning calorimeter (DSC) at a heating rate of 10°C/min. Specifically, the sample is heated from room temperature at 10°C/min, and after an endothermic peak is observed, the sample is heated to a temperature 20-50°C higher than the endothermic peak temperature and held for 10 minutes, after which the sample is cooled to room temperature at a cooling rate of 20°C/min. After that, in the DSC chart when the temperature is raised at 10°C/min, the intersection point between the baseline and the tangent at the inflection point is taken as the glass transition temperature, and the temperature at which the endothermic peak reaches its peak is taken as the melting point.

The melting point and glass transition temperature of the polyester-imide copolymer are influenced by the types of tetracarboxylic acid residue R ¹ (R ^a ), diamine residue R ² (R ^b ), and aromatic groups (arylene groups) R ³ and R ⁴ , and the ratio of imide structures derived from the compound of general formula (2). As described above, the higher the ratio of imide structures, the higher the heat resistance tends to be. In addition, when the aromatic group R ⁴ of the compound of general formula (3) has a naphthalene skeleton (when the ratio of compounds having a naphthalene skeleton among the compounds represented by general formula (3) is high), the heat resistance tends to be high. In addition, when R ⁴ has a naphthalene skeleton, the polymer is likely to exhibit liquid crystallinity, and the relative dielectric constant and dielectric loss tangent tend to be low accordingly.

<Applications of polyester-imide copolymer>
The polyester-imide copolymer may be processed into a molded product such as a film when put into practical use. Processing methods include press molding, injection molding, extrusion molding, blow molding, solution casting, etc. The polyester-imide copolymer has excellent low dielectric properties in the high frequency band, and therefore can be suitably used as a substrate film material for flexible printed wiring boards with low transmission loss.

The film made from the polyester-imide copolymer preferably has a dielectric constant Dk of 3.50 or less at a frequency of 40 GHz, and a dielectric loss tangent Df of 0.0035 or less at a frequency of 40 GHz. It is more preferable that the dielectric constant Dk is 3.20 or less, and the dielectric loss tangent Df is 0.0030 or less. As mentioned above, the lower the ratio of imide structures in the polymer, the lower the dielectric constant and dielectric loss tangent tend to be.

The linear expansion coefficient in the in-plane directions (X and Y directions) of a film made from the above polyester-imide copolymer is preferably 70 ppm/K or less, more preferably 60 ppm/K or less. The linear expansion coefficient in the thickness direction (Z direction) of the film is preferably 130 ppm/K or less, more preferably 110 ppm/K or less. The linear expansion coefficient α is the slope of the dimensional change rate versus temperature in the range of 25°C to 150°C by thermomechanical analysis (TMA), and is expressed by the following formula:

T ₁ : Starting point temperature (=25℃=298K)
T ₂ : End point temperature (=150℃=423K)
L ₁ : sample length at temperature T ₁ L ₂ : sample length at temperature T ₂

The film made of the polyester-imide copolymer has smaller anisotropy and a smaller difference between the linear expansion coefficient _αXY in the in-plane direction (XY) and the linear expansion coefficient _αZ in the thickness direction (Z) than a film made of a liquid crystal polyester. The ratio of the linear expansion coefficients in the thickness direction and the in-plane direction of the film, _αZ / _αXY , is preferably 4 or less, more preferably 3 or less, and may be 2.5 or less or 2 or less. When the ratio _αZ / _αXY is small, when used in an electronic circuit board or the like, the connection reliability in the thickness direction is improved and warping of the film when a metal layer or the like is laminated tends to be reduced.

The polyester obtained by polycondensing only the compound represented by the above general formula (3) is prone to packing of the aromatic group R ⁴ and generally exhibits liquid crystallinity. Liquid crystal polyester is prone to packing of aromatic groups between molecules, and when molded into a film, the molecular chains of the polymer are prone to be oriented in-plane, so the anisotropy of the film is large and the linear expansion coefficient α _Z in the thickness direction tends to be large. The polyester-imide copolymer obtained by copolymerization of the compound represented by general formula (3) and the imide-based compound (oligomer) represented by general formula (2) may have liquid crystallinity, but compared to liquid crystal polyester, the packing of aromatic groups is less and the liquid crystallinity is low, so the in-plane orientation of the polymer chains is suppressed and the anisotropy tends to be low.

As mentioned above, by using a compound of general formula (2) having an isoimide structure as an imide oligomer, melt polymerization at low temperatures becomes possible. Therefore, according to the present invention, it is possible to provide a polymer and a film that have high heat resistance, excellent dielectric properties, and suppressed orientation anisotropy by low-temperature solution polymerization.

The thickness of the film made of polyester-imide copolymer is not particularly limited and may be determined appropriately depending on the application. For example, when manufacturing a metal-clad laminate for use in the manufacture of printed wiring boards, the thickness of the film is preferably 5 to 200 μm, more preferably 10 to 100 μm.

Metal-clad laminates can be produced by known methods, such as thermocompression bonding of a film and a metal foil, or casting a polymer melt on a metal foil to form a film on the metal foil. Metal-clad laminates may be produced by laminating metal foil on only one main surface of a film, or may be produced by laminating metal foil on both main surfaces of a film. Examples of metal foil include copper, copper alloys, nickel, nickel alloys (e.g., alloy 42), aluminum, aluminum alloys, and stainless steel. From the viewpoints of electrical conductivity, workability, and bonding strength with the film, copper foil is particularly preferred as the metal foil. Functional layers such as an anti-rust layer, a heat-resistant layer, and an adhesive layer may be provided on the surface of the metal foil as necessary.

　A printed wiring board is manufactured by etching the metal foil of a metal-clad laminate to form wiring. A printed wiring board in which metal wiring is provided on a film made of the above-mentioned polyester-imide copolymer has a high transmission speed and low transmission loss, making it suitable for use as a circuit board for high-frequency applications.

The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples.

<Production Example 1>
In a separable flask equipped with a stirring blade, 1,8-diaminooctane (0.041 mol) was placed as a diamine and dissolved in 160 g of N,N-dimethylformamide (DMF). 3,3',4,4'-biphenyltetracarboxylic dianhydride (0.082 mol) was added as an acid dianhydride, and the mixture was stirred at room temperature for 3 hours under a nitrogen atmosphere. A solution of aminophenol (0.041 mol) and aminobenzoic acid (0.041 mol) dissolved in 40 g of DMF was added to the reaction solution, and the mixture was stirred at room temperature for 3 hours. Diisopropylcarbodiimide (0.196 mol) was added as a dehydration condensation agent in an ice bath, and the mixture was stirred for 2 hours. The reaction solution was added to a large amount of isopropanol to precipitate the reactant, and the precipitated powder was collected. The mixture was repeatedly washed until no impurities were present, and then dried under reduced pressure at 100°C for 12 hours to obtain an imide oligomer 1 containing an isoimide group. The ratio of isoimide to the total of imide and isoimide (isoimidization rate) calculated from the intensity ratio of the peak near 1780 cm ⁻¹ (peak derived from imide) to the peak near 1800 cm ⁻¹ (peak derived from isoimide) in the infrared spectrum was 39%.

<Production Example 2>
Imide oligomer 2 having an isoimide group was synthesized in the same manner as in Production Example 1, except that 1,3-bis(aminophenoxybenzene) was used as the diamine, paraphenylene bis(trimellitate anhydride) was used as the acid dianhydride, and the reaction solvent was changed to N-methyl-2-pyrrolidone (NMP). The conversion rate of isoimide in imide oligomer 2 was 50%.

<Production Example 3>
Except for using paraphenylene bis(trimellitate anhydride) as the acid dianhydride and changing the reaction solvent to NMP, the same procedure as in Production Example 1 was carried out to obtain a compound 3 having an isoimide group. The ratio of isoimide in the imide oligomer 3 was 46%.

<Production Example 4>
1,8-diaminooctane (0.041 mol) was placed in a separable flask equipped with a stirring blade and dissolved in 160 g of DMF. 3,3',4,4'-biphenyltetracarboxylic dianhydride (0.082 mol) was added thereto and stirred at room temperature for 3 hours under a nitrogen atmosphere. Pyridine (0.082 mol) and acetic anhydride (0.107 mol) were added to this solution and the mixture was stirred at 120°C for 2 hours to carry out imidization. The reaction solution was added to a large amount of isopropanol to precipitate the reactant, and the precipitated powder was collected. The mixture was repeatedly washed until no impurities were present, and then dried under reduced pressure at 150°C for 12 hours to obtain imide oligomer 4.

<Examples 1 to 7>
A reaction vessel equipped with a stirrer with a torque meter, a distillation tube, and a reflux condenser was charged with each of the materials shown in Table 1, and further added with 1.1 equivalents of acetic anhydride per equivalent of hydroxyl groups in all of the charged compounds, and a catalytic amount of 1-methylimidazole. The amounts of the imide oligomer and aromatic hydroxycarboxylic acid (4-hydroxybenzoic acid and 6-hydroxy-2-naphthoic acid) in Table 1 are expressed in mmol. The amount of the imide oligomer is a calculated value based on the average molecular weight calculated from the above formula (I).

The inside of the reaction vessel was thoroughly replaced with nitrogen gas, and the vessel was heated to 145°C while stirring and held for 30 minutes, after which it was quickly heated to 185°C and held for 30 minutes. The by-product acetic acid was then distilled off while the temperature was raised to 300°C over 30 minutes, and held for 2 hours and 30 minutes. The pressure inside the reaction vessel was then reduced to 7.5 torr over 10 minutes and held for 1 hour. Stirring was continued while maintaining the vacuum, and when the torque reached a specified value, stirring was stopped and the reaction was terminated. The resulting solid matter was removed and allowed to cool to room temperature, then pulverized in a grinder, washed with methanol, and dried. The vessel was then held for 2 hours at 320°C in a nitrogen gas atmosphere to allow the polymerization reaction to proceed in the solid phase, and the resulting solid matter was pulverized again in a grinder to obtain a polyester-imide copolymer powder.

<Comparative Examples 1 and 2>
Without using an imide oligomer, the aromatic hydroxycarboxylic acids shown in Table 1 were used as raw materials, and acylation with acetic anhydride and polymerization were carried out in the same manner as in Examples 1 to 7 to obtain powders of polymers (aromatic polyesters).

<Comparative Example 3>
Polymerization was attempted in the same manner as in Examples 1 to 7 using the imide oligomer 4 prepared in Production Example 4 as the imide oligomer. However, even when the temperature was raised to 300° C., the imide oligomer did not melt, and melt polymerization could not be performed.

[evaluation]
The resins of Examples 1 to 7 and Comparative Examples 1 and 2 were evaluated for glass transition temperature, linear expansion coefficient, dielectric constant, and dielectric loss tangent according to the following methods.

<Glass transition temperature>
The glass transition temperature of the resin was measured by a differential scanning calorimeter ("Q1000" manufactured by TA Instruments). 5 to 8 mg of the resin was placed in an aluminum sample pan and covered with a lid. The resin was completely melted by heating from room temperature to 350 to 400°C at a heating rate of 10°C/min under a nitrogen gas flow, and then cooled to 0°C at a heating rate of 20°C/min. The temperature was then raised at a heating rate of 10°C/min, and the intersection of the baseline and the tangent at the inflection point (the point where the upward convex curve changes to a downward convex curve) in the DSC chart at this time was taken as the glass transition temperature.

<Coefficient of Linear Expansion (CTE)>
(Film Preparation)
Using a tabletop mini press (manufactured by Toyo Seiki Co., Ltd.), the resin was placed in a 300 μm thick brass spacer and hot pressed to produce a film with a thickness of about 300 μm. A film with a thickness of about 1 mm was produced in the same manner using a 1 mm thick brass spacer.

(In-plane linear expansion coefficient)
The thermal expansion coefficient in the in-plane direction was measured by a tensile load method using a thermomechanical analyzer ("TMA4000SA" manufactured by NESZSCH). A film with a thickness of 300 μm cut into a size of 5 mm x 10 mm was used as a sample, and the temperature was raised to 80 to 150 ° C. at a temperature increase rate of 10 ° C. / min under a nitrogen gas flow, and then the temperature was lowered to 30 ° C. at a temperature decrease rate of 20 ° C. / min. Thereafter, the temperature was increased at a temperature increase rate of 10 ° C. / min, and the linear expansion coefficient α _XY was calculated from the sample length L ₁ at 25 ° C. and the sample length L ₂ at 150 ° C. in the TMA chart at this time.

(Coefficient of linear expansion in the thickness direction)
The thermal expansion coefficient in the thickness direction was measured by a compression load method using a thermomechanical analyzer (Rigaku's "TMA8310"). A 1 mm thick film cut into a size of 5 mm x 5 mm was used as a sample, and the temperature was raised to 80 to 150°C at a heating rate of 10°C/min under a nitrogen gas flow, and then cooled to 25°C at a heating rate of 8°C/min. The temperature was then raised at a heating rate of 10°C/min, and the linear expansion coefficient _αZ was calculated from the sample length L ₁ at 25°C and the sample length L ₂ at 150°C in the TMA chart at this time.

<Dielectric constant and dielectric loss tangent>
A sample having a size of 35 mm x 50 mm was cut out from a film having a thickness of 300 μm prepared in the same manner as above, and allowed to stand for 24 hours under an environment of a temperature of 23° C. and a relative humidity of 50%. Then, the relative dielectric constant and the dielectric loss tangent were measured at a frequency of 40 GHz using a network analyzer ("N5222B" manufactured by KEYSIGHT) and a split cylinder resonator (manufactured by KEYSIGHT).

Table 1 shows the types and amounts of raw materials used in the preparation of the polymers of the Examples and Comparative Examples, as well as the evaluation results of the polymers and films.

In Comparative Example 3, in which imide oligomer 4 without an isoimide structure was used, no polymer was obtained. However, by using the imide oligomer of general formula (2) with an isoimide structure, the melting point of the compound is lowered, and it can be seen that a polyester-imide copolymer can be easily obtained by melt polymerization.

The polyester-imide copolymers of Examples 1 to 7 obtained by melt polymerization of the imide oligomer of general formula (2) and the compound of general formula (3) (aromatic hydroxycarboxylic acid) had both good dielectric properties and high heat resistance. A comparison of Examples 1 to 4 and Examples 6 and 7 shows that the higher the ratio of the compound represented by general formula (2) during polymerization and the larger the ratio of the imide structure in the polymer, the higher the heat resistance and the lower the anisotropy of the linear expansion coefficient tend to be.

The polymers (aromatic polyesters) of Comparative Examples 1 and 2 obtained by melt polymerization of aromatic hydroxycarboxylic acid had good dielectric properties similar to those of Examples 1 to 7, but had lower glass transition temperatures and inferior heat resistance compared to the polyester-imide copolymers of Examples 1 to 7. In addition, the film produced using the polyester of Comparative Example 1 showed a large value of α _Z /α _XY , which is an index showing the anisotropy of the linear expansion coefficient.

Claims (8)

A method for producing a polyester-imide copolymer having a structure represented by the following general formula (1),

A method for producing a polyester-imide copolymer, comprising forming an ester bond by a polycondensation reaction using a compound represented by the following general formula (2) and a compound represented by the following general formula (3) as polymerization components:

In general formulas (1) to (3), R ¹ is a tetravalent organic group, R ² is a divalent organic group, R ³ and R ⁴ each independently represent an arylene group, Z is an ester bond, m is an integer of 1 or more, n is an integer of 0 or more, X ¹ , Y ¹ , X ² and Y ² each independently represent a carboxy group, a hydroxy group or an acetyloxy group,
The structure A in the general formula (2) is the following (A1) or (A2):

At least a part of A in the general formula (2) is an isoimide structure represented by (A2). The method for producing a polyester-imide copolymer according to claim 1, wherein the compound represented by the general formula (2) has a melting point of 300°C or less. 3. The method for producing a polyester-imide copolymer according to claim 1, wherein R ⁴ in the general formula (3) contains at least one selected from the group consisting of a naphthalene skeleton and a benzene skeleton. 3. The method for producing a polyester-imide copolymer according to claim 1, wherein the ratio of the compound in which R ⁴ has a naphthalene skeleton is 30 to 85 mol % of the total amount of the compound represented by the general formula (3). The method for producing a polyester-imide copolymer according to claim 1 or 2, wherein the ratio of the isoimide structure to the total of the imide structure of (A1) and the isoimide structure of (A2) in the structure A in the general formula (2) is 10 mol % or more. The method for producing a polyester-imide copolymer according to claim 1 or 2, wherein in the polycondensation reaction, the amount of the compound represented by general formula (2) relative to the total amount of the compound represented by general formula (2) and the compound represented by general formula (3) is 5 to 50 mol %. The method for producing a polyester-imide copolymer according to claim 1 or 2, in which the polycondensation reaction is carried out by a melt polymerization method. The method for producing a polyester-imide copolymer according to claim 7, wherein the temperature of the melt polymerization is 350° C. or lower.