JPH07119278B2 - Method for producing copolyester - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a novel method for producing a copolyester. More specifically, the present invention has features such as a high elastic modulus, high strength and high elongation, and the formation of a thermotropic liquid crystal at the time of molding and excellent moldability. And a method for producing a copolyester suitable as a material for films, fibers, etc. with good reproducibility.

[Prior Art] In recent years, there has been an increasing demand for materials excellent in rigidity, strength, elongation, heat resistance, etc. regardless of whether they are fibers, films or molded products.

By the way, polyester is widely used in various fields including various molded articles in many fields. However, many polyesters are inferior in bending elastic modulus and bending strength, and thus have a drawback that they are not suitable for applications requiring high elastic modulus and high strength.

Therefore, the development of polyesters suitable for applications requiring high elastic modulus and high strength has been actively attempted, and liquid crystalline polyesters have recently attracted attention. This liquid crystalline polyester has attracted particular attention because it is made of polyethylene terephthalate and acetoxybenzoic acid and has a rigidity of 5 times or more, a strength of 4 times or more, and an impact strength of 25 times or more of polyethylene terephthalate. Due to the discovery of new thermal liquid crystal polymer compounds and their potential for high-performance resins [Journal of Polymer
Science Polymer Chemistry Edition "
Volume 14, Page 2043 (1976), Japanese Examined Patent Publication No. 56-18016. However, the thermal liquid crystal polymer compound has a drawback that it is extremely brittle, and its strength and elongation are not always sufficient.

Therefore, the inventors of the present invention have conducted extensive research to improve the strength and elongation of the thermal liquid crystal polymer compound, and previously made a copolyester having improved strength and elongation (JP-A-60-186527). ), And a new production method thereof (Japanese Patent Laid-Open No. 60-186525).

According to this new manufacturing method, the general formula (R ⁷ in the formula is a divalent aliphatic hydrocarbon group having 2 to 20 carbon atoms,
R ⁸ is a divalent aromatic hydrocarbon group. ) Is formed during the reaction and the unit is contained in the copolyester without lowering the elastic modulus,
It has become possible to obtain a copolyester having improved strength and elongation.

However, in this production method, since the unit component represented by the general formula (I) is formed during the reaction, the content of the unit in the copolyester is likely to change depending on the reaction conditions. there were.

[Problems to be Solved by the Invention] Under the circumstances, the present invention has a high elastic modulus,
In addition, the object of the present invention is to provide a method for producing a copolyester having excellent strength and elongation and having a good balance between elastic modulus and heat resistance with good reproducibility.

[Means for Solving the Problem] As a result of intensive research conducted by the present inventors to develop a method for producing the above-mentioned copolyester having excellent properties with good reproducibility, the oligo having a specific structure has been obtained. After reacting an ester or a polyester with an acyloxycarboxylic acid and a hydroxycarboxylic acid to form a copolymer oligomer, it was found that the objective can be achieved by carrying out the polymerization, and the present invention was completed based on this finding. Came to do.

That is, the present invention provides (A) general formula (R ¹ in the formula is a divalent alicyclic hydrocarbon group having 4 to 20 carbon atoms,
A divalent aliphatic hydrocarbon group having 1 to 40 carbon atoms or a divalent aromatic hydrocarbon group having 6 to 20 carbon atoms forming an aromatic ring, R ² is a divalent aliphatic group having 2 to 40 carbon atoms Hydrocarbon group, carbon number 4-20
Divalent alicyclic hydrocarbon group, divalent aromatic hydrocarbon group having 6 to 20 carbon atoms forming an aromatic ring, or divalent molecular weight 80 to
There are 8000 polyalkylene oxide residues. ) An oligoester or polyester comprising a repeating unit represented by (R ³ in the formula is a divalent aromatic hydrocarbon group having 6 to 20 carbon atoms forming an aromatic ring, and R ⁴ is an alkyl group having 1 to 8 carbon atoms or a phenyl group.) Carboxylic acid, and (C) general formula HOR ⁵ OR ⁶ COOH (IV) (wherein R ⁵ is a divalent aliphatic hydrocarbon group having 2 to 40 carbon atoms,
A divalent alicyclic hydrocarbon group having 4 to 20 carbon atoms, a divalent aromatic hydrocarbon group having 6 to 20 carbon atoms forming an aromatic ring, or a divalent polyalkylene oxide residue having a molecular weight of 80 to 8000 , R ⁶ is a divalent aromatic hydrocarbon group having 6 to 20 carbon atoms forming an aromatic ring. The present invention provides a method for producing a copolyester, which comprises reacting a compound represented by the formula (4) to form a copolymer oligomer and then polymerizing the oligomer.

Hereinafter, the present invention will be described in detail.

In the present invention, the polyester or oligoester used as the component (A) of the raw material has the general formula (In the formula, R ¹ and R ² have the same meanings as described above). When R ¹ or R ² in the general formula (II) is a divalent aromatic hydrocarbon group, the hydrogen atom of the aromatic ring of the aromatic hydrocarbon group is a halogen atom or an alkyl group having 1 to 4 carbon atoms. It may be substituted with a group or an alkoxy group.

The polyester or oligoester comprises a dicarboxylic acid represented by the general formula HOOC-R ¹ -COOH (V) (wherein R ¹ has the same meaning as described above), and a general formula HO-R ² -OH (( VI) (R ² in the formula has the same meaning as described above) and a diol represented by the following formula can be produced by condensation polymerization by a known method.

Examples of the dicarboxylic acid represented by the general formula (V) include terephthalic acid, methoxyterephthalic acid, ethoxyterephthalic acid, fluoroterephthalic acid, chloroterephthalic acid, methylterephthalic acid, isophthalic acid, phthalic acid, methoxyisophthalic acid, diphenylmethane- 4,4'-dicarboxylic acid, diphenylmethane-3,3'-dicarboxylic acid,
Diphenyl ether-4,4'-dicarboxylic acid, diphenyl-4,4'-dicarboxylic acid, naphthalene-2,6-dicarboxylic acid, naphthalene-1,5-dicarboxylic acid, naphthalene-
1,4-dicarboxylic acid, adipic acid, sebacic acid, azelaic acid, suberic acid, dodecanedicarboxylic acid, 3-methylazelaic acid, glutaric acid, succinic acid, cyclohexane-1,4-dicarboxylic acid, cyclohexane-1,3- Examples thereof include dicarboxylic acid and cyclopentane-1,3-dicarboxylic acid, and of these, terephthalic acid is preferable. These dicarboxylic acids may be used alone,
You may use in combination of 2 or more types.

On the other hand, as the diol represented by the general formula (VI),
For example, ethylene glycol, 1,3-propanediol,
1,2-propanediol, 1,3-butanediol, 1,4-
Butanediol, neopentyl glycol, 1,6 hexanediol, 1,12-dodecanediol, cyclohexane-1,4-diol, cyclohexane-1,3-diol, cyclohexane-1,2-diol, cyclopentane-1,3 −
Diol, diethylene glycol, polyethylene glycol, hydroquinone, resorcinol, bisphenol A, methylhydroquinone, chlorohydroquinone, 2,6-
Examples thereof include naphthalene diol, and among these, ethylene glycol and 1,4-butane diol are preferable. These diols may be used alone or in combination of two or more.

The component (A) of the raw material used in the method of the present invention may be any polyester or oligoester composed of the repeating unit represented by the general formula (II), and is not particularly limited, but polyethylene is available from the viewpoint of availability. Preference is given to terephthalates, polybutylene terephthalates and their oligomers, more preferably polyethylene terephthalates and their oligomers. In particular, polyethylene terephthalate having an ηinh of 0.2 or more is suitable.

The amount of the component (A) used is usually 5 with respect to all the raw material components, based on the repeating unit represented by the general formula (II).
˜70 mol%, preferably 5 to 50 mol%. If this amount is less than 5 mol%, the strength and elongation of the copolyester will be insufficient, and if it exceeds 70 mol%, the elastic modulus tends to decrease.

In the method of the present invention, as the component (B) of the raw material, a compound represented by the general formula (Wherein R ⁴ and R ³ have the same meaning as described above), an acyloxycarboxylic acid is used. In the general formula (III), R ³ may have a hydrogen atom of an aromatic ring substituted with a halogen atom, an alkyl group having 1 to 4 carbon atoms, an alkoxy group, or the like, or when R ⁴ is a phenyl group. , The hydrogen atom of the aromatic ring may be similarly substituted.

This acyloxycarboxylic acid is obtained by converting an aromatic hydroxycarboxylic acid represented by the general formula HO-R ³ -COOH (VII) (wherein R ³ has the same meaning as described above) into the general formula R ⁴ COOH (VIII ) (Wherein R ⁴ has the same meaning as described above), or a reactive derivative such as an ester, an acid halide or an acid anhydride thereof.

Examples of the acyloxycarboxylic acid include p-acetoxybenzoic acid, 4-acetoxy-3-chlorobenzoic acid,
m-acetoxybenzoic acid, 4-acetoxy-3,5-dimethylbenzoic acid, 2-acetoxy-6-naphthoic acid, 1-
Acetoxy-5-naphthoic acid, 1-acetoxy-4-naphthoic acid, acetate of shringer acid, acetate of vanillic acid, 4-acetoxy-3-methylbenzoic acid and the like can be mentioned, and these may be used alone. Then
Two or more kinds may be used in combination, but it is preferable to use p-acetoxybenzoic acid alone, from the viewpoint of maintaining the melt anisotropy.

The amount of the acyloxycarboxylic acid used is usually selected in the range of 5 to 90 mol%, preferably 10 to 85 mol%, based on all the raw material components. If this amount is less than 5% by weight, the elastic modulus of the copolyester may be low, and if it exceeds 90 mol%, strength and elongation tend to be lowered, which is not preferable.

A major feature of the present invention is that, as one of the raw material components, (C)
A compound represented by the general formula HOR ⁵ OR ⁶ COOH (IV) (wherein R ⁵ and R ⁶ have the same meanings as described above) is used. By using this compound, strength and elongation can be improved without lowering the elastic modulus of the copolyester, and good reproducibility can be obtained in the reaction.

The divalent aromatic hydrocarbon group represented by R ⁶ in the general formula (IV) may have a hydrogen atom of an aromatic ring substituted with a halogen atom, an alkyl group having 1 to 4 carbon atoms, an alkoxy group, or the like, When R ⁵ is a divalent aromatic hydrocarbon group, the hydrogen atom of the aromatic ring may be substituted in the same manner as described above.

Examples of the compound represented by the general formula (IV) include 4-β-hydroxyethoxybenzoic acid, 3-β-hydroxyethoxybenzoic acid, 4-ω-hydroxybutoxybenzoic acid, 4-β-hydroxyethoxy-3-. Examples thereof include chlorobenzoic acid, 4-ω-hydroxybutoxy-3,5-dimethylbenzoic acid, 2-β-hydroxyethoxy-6-naphthoic acid, and 4- (4′-β-hydroxyethoxyphenyl) benzoic acid. Of these, 4-β-hydroxyethoxybenzoic acid is preferred. Moreover, these compounds may be used alone or in combination of two or more kinds.

The amount of the component (C) used in the method of the present invention is selected in the range of usually 1 to 30 mol%, preferably 2 to 25 mol%, and more preferably 3 to 20 mol% based on all the raw material components.
If this amount is less than 1 mol%, the strength and elongation of the copolyester may be low, and if it exceeds 30 mol%, the elastic modulus tends to decrease, such being undesirable.

In the method of the present invention, as the raw material components, (A),
In addition to the essential components of (B) and (C), if desired (D)
General formula (R ⁹ in the formula is a divalent aromatic hydrocarbon group having 6 to 20 carbon atoms forming an aromatic ring, and R ¹⁰ is an alkyl group having 1 to 8 carbon atoms or a phenyl group.) Group diacyl compound and / or (E) general formula (R ¹¹ in the formula is a divalent aromatic hydrocarbon group having 6 to 20 carbon atoms that forms an aromatic ring.) An aromatic dicarboxylic acid represented by the following formula can be used.

R ⁹ in the general formula (IX) and R ¹¹ in the general formula (X) are
The hydrogen atom of the aromatic ring may be substituted with a halogen atom, an alkyl group having 1 to 4 carbon atoms, an alkoxy group or the like,
When R ¹⁰ in general formula (IX) is a phenyl group, the hydrogen atom of its aromatic ring may be similarly substituted.

Examples of the component (D), that is, the aromatic diacyl compound represented by the general formula (IX) include, for example, hydroquinone, resorcinol, methylhydroquinone, t-butylhydroquinone, 2,5-di-t-butylhydroquinone, 2,6- The-t-
Butylhydroquinone, 2,4,5-trimethylresorcinol, 1,2,5-trimethylhydroquinone, 1,4-dihydroxynaphthalene, 1,5-dihydroxynaphthalene, 1,6-dihydroxynaphthalene, 2,6-dihydroxynaphthalene, 2 , 7-dihydroxynaphthalene, 2,2-bis (4-hydroxyphenyl) propane, 2- (3-hydroxyphenyl) -2- (4-hydroxyphenyl) propane,
Bis (4-hydroxyphenyl) methane, 1,1-bis (4-hydroxyphenyl) cyclohexane, 4,4′-
Dihydroxyphenyl, bis (4-hydroxyphenyl) ketone, bis (4-hydroxyphenyl) sulfide, bis (4-hydroxyphenyl) sulfone, bis (4-hydroxyphenyl) ether, (4-hydroxyphenyl) -4-hydroxybenzoate , 3,4'-
Examples thereof include diacyl compounds such as dihydroxyphenyl sulfone and 3,4′-dihydroxybenzophenone, but are not necessarily limited to diacetate.These aromatic diacyl compounds may be used alone. Good, or two or more kinds may be used in combination. Among the aromatic diacyl compounds,
Particularly preferred are hydroquinone, resorcinol, t-butylhydroquinone, 4,4'-dihydroxyphenyl, bis (4-hydroxyphenyl) sulfone and diacetate of 2,2-bis (4-hydroxyphenyl) propane.

The amount of these aromatic diacyl compounds used is selected in the range of preferably 40 mol% or less, more preferably 30 mol% or less, based on the total raw material components. If this amount exceeds 40 mol%, the strength and elongation of the copolyester tend to decrease, which is not preferable.

Examples of the component (E), that is, the aromatic dicarboxylic acid represented by the general formula (X) include terephthalic acid, isophthalic acid, naphthalene-2,6-dicarboxylic acid, naphthalene-1,5-dicarboxylic acid, diphenyl-4. 4,4'-dicarboxylic acid, methylterephthalic acid, methylisophthalic acid, diphenylether-4,4'-dicarboxylic acid, diphenylthioether-4,4'-dicarboxylic acid, 3,3'-diphenyldicarboxylic acid, diphenylsulfone-4, 4'-dicarboxylic acid, diphenylketone-4,4'-dicarboxylic acid, 2,2-diphenylpropane-4,4'-dicarboxylic acid, diphenyl terephthalate, diphenyl-3,3'-dicarboxylic acid and the like. However, it is not necessarily limited to these. Further, these aromatic dicarboxylic acids may be used alone or in combination of two or more. Among these aromatic dicarboxylic acids, terephthalic acid, isophthalic acid, diphenyl-3,3'-dicarboxylic acid and naphthalene-2,6-dicarboxylic acid are particularly preferable.

The amount of the aromatic dicarboxylic acid used is preferably 40 mol% or less, more preferably 30 mol%, based on all atomic ingredients.
It is selected within the following range. If this amount exceeds 40 mol%, the strength and elongation of the copolyester tend to decrease,
Not preferable.

In the method of the present invention, as the first step of the reaction, the acidolysis reaction is carried out by bringing the components (A), (B), (C) and optionally the components (D) and (E) used into contact with each other. , To form polyester fragments (oligomers). This reaction is usually carried out at a temperature in the range of 200 to 320 ° C., but it is desirable to carry out at a temperature in the range of 220 to 300 ° C., particularly 240 to 290 ° C. from the viewpoint of reaction efficiency and suppression of side reactions. The reaction time depends on the reaction temperature and cannot be determined unconditionally, but is usually 10 minutes to 5 minutes.
The time is preferably about 30 minutes to 3 hours.

In this reaction, substantially new (R ² and R ³ in the formula have the same meanings as described above), and the unit component represented by (R ⁵ and R ⁶ in the formula have the same meanings as described above) Since the amount of the unit component is defined, the unit represented by the general formula (XII) may be changed depending on the reaction conditions. Since the amounts of the components do not change, the desired copolyester can be obtained with good reproducibility. Further, the obtained copolyester contains the unit represented by the general formula (XII), whereby the strength and elongation are improved and the balance between elastic modulus and heat resistance is excellent.

The reaction in the first step is preferably performed in an inert gas atmosphere. In the reaction of the first step, fragmentation due to acidolysis occurs as a main reaction, but other condensation reaction also occurs and a low molecular compound such as acetic acid is eliminated. Since such desorbed substances are preferably distilled out of the system, the inert gas may be sealed and used, but it is advantageous to continuously introduce the inert gas into the system. As the inert gas, nitrogen, argon or the like can be used.

The polyester fragments disaggregated in this first step were separated in the next second step at 275 ° C. for 100 se.
Induced to high molecular weight polymer with melt viscosity at c ^-1 of 30 poise or more. The formation of the high-molecular weight substance is achieved by distilling out the low-molecular elimination product generated during the condensation to the outside of the system to shift the equilibrium.

Regarding the method for distilling off the desorbed low-molecular compound to the outside of the system, an inert gas may be introduced into the system and distilled along with this gas, or the system may be depressurized to distill off. However, it is preferable to reduce the pressure in terms of reaction time and the like. At this time, it is necessary to pay attention so that the necessary unreacted oligomer or monomer is not distilled out of the system, and for that purpose, it is desirable to gradually reduce the pressure. This depressurization is usually performed from atmospheric pressure to about 100 mmHg in about 20 minutes to 1 hour, from about 100 mmHg to 10 mmHg in about 20 minutes to 2 hours, and from about 10 mmHg to 1 mmHg in about 30 minutes to 2 hours. Is preferred. If the depressurization is performed faster than this, the amount of sublimate increases, and there is a possibility that an undesirable situation such as blockage of the distilling line may occur.

Then, the polymerization is carried out at a vacuum degree of 1 mmHg or less until the polymer reaches a desired melt viscosity. The polymerization time after starting the pressure reduction is usually about 1 to 20 hours.

The reaction temperature in this second step is preferably 240 to 350.
C., more preferably 270 to 320.degree. If this temperature is lower than 240 ° C, the reaction rate is too slow to be practical, and if it exceeds 350 ° C, a separation reaction easily occurs, which is not preferable.

In this reaction, a catalyst can be used if desired. Examples of the catalyst include Sb ₂ O ₃ , Zn (OAc) ₂ and Ge.
O ₂ , BuSnOOH, Bu ₂ SnO, Ti (OBu) ₄ , Sn (OAc) ₂ , Sn
(OAc) _{4 and the} like, and the amount used is usually 5 to 50,000 ppm, preferably 50 to 5000 ppm, relative to the polymer.

In the method of the present invention, the melt viscosity of the final product is usually
The value measured at 275 ° C. at 10 ² sec ⁻¹ is 30 poises or more, preferably 50 to 5000 poises, more preferably 100 to 2000 poises.

The copolyester obtained in this manner exhibits an optically anisotropic phase in the melt phase, and therefore has very good fluidity and therefore good moldability, and is generally used in extrusion molding, injection molding, compression molding and the like. Melt molding can be performed, and it can be easily processed into molded products, films, fibers and the like. Since it has a high fluidity, it is suitable for precision molded products.

Further, the copolyester obtained by the method of the present invention, when necessary, at the time of molding, for example, fibers such as glass fiber and carbon fiber, talc, mica, fillers such as calcium carbonate, core material, pigment, Fillers such as antioxidants, lubricants, stabilizers and flame retardants, additives, or other thermoplastic resins may be added to impart desired properties to the molded article.

Also, by blending with other polymers and alloying,
It is also possible to prepare compositions which combine the advantages of both other polymers and the copolyesters of the invention.

[Examples] Next, the present invention will be described in more detail with reference to Examples.
The invention is in no way limited by these examples.

The melt viscosity of the copolyester and the physical properties of the molded product were determined as follows. In addition, the molded pieces were prepared using a 0.1 oz injection molding machine manufactured by Nippon Steel Co., Ltd.

(1) Melt viscosity of copolyester Using a capillary rheometer (manufactured by Intesco),
The measurement was performed under the conditions of a temperature of 275 ° C., a shear rate (γ) of 100 sec ⁻¹ , and a cylinder nozzle length / diameter of 30.

(2) Tensile properties of molded products Tensile elastic modulus, tensile strength, and breaking elongation were measured using TENSILON / UTM-IIIL manufactured by Toyo Baldwin.

Example 1 26.9 g (0.14) of polyethylene terephthalate (IV = 0.60)
Mol) with 100.8 g (0.65 mol) of p-acetoxybenzoic acid and 5.1 g (0.028 mol) of 4-β-hydroxyethoxybenzoic acid in a glass polymerization tube equipped with a stirring blade, a nitrogen inlet, and a pressure reducing port. After charging, evacuating and purging with nitrogen three times, the polymerization tube was immersed in an oil bath at 120 ° C. and dried under vacuum for 1 hour. After the completion of drying, purging with nitrogen was performed, and the temperature of the oil bath was raised to 275 ° C. in a nitrogen atmosphere.

When this mixture is stirred at 275 ° C. under a nitrogen atmosphere, acetic acid gradually begins to distill from the polymerization tube, and after 1 hour, most of the acetic acid distills to form a polyester fragment having a low melt viscosity. Next, stirring was continued for 5 hours at 275 ° C. and a vacuum degree of 0.3 mmHg to obtain a white opaque copolyester having a melt viscosity of 350 poise.

Using a 0.1 oz molding machine, this polyester was molded at 275 ° C to prepare a molded piece, and its physical properties were examined.The tensile elastic modulus was 68,000 kg / cm ² , the tensile strength was 1230 kg / cm ² , and the elongation at break was
It was 4.0%.

Comparative Example 1 The procedure of Example 1 was repeated except that 4-β-hydroxyethoxybenzoic acid was not used.

The melt viscosity of the obtained copolyester was 870 poise, and a molded piece obtained by molding this polyester in the same manner as in Example 1 had a tensile elastic modulus of 35,000 kg / cm ² , a tensile strength of 590 kg / cm ² , The breaking elongation was 2.3%.

Example 2 In Example 1, as a raw material mixture charged in a glass polymerization tube, polyethylene terephthalate 0.12 mol p-acetoxybenzoic acid 0.20 mol 4-β-hydroxyethoxybenzoic acid 0.06 mol hydroquinone diacetate 0.10 mol terephthalic acid 0.10 mol When the reaction was carried out in the same manner as in Example 1 except that was used, a copolyester having a melt viscosity of 520 poise was obtained.

Molded pieces obtained by molding this polyester in the same manner as in Example 1 had a tensile elastic modulus of 77,000 kg / cm ² and a tensile strength of 1,78.
The elongation at break was 0 kg / cm ² and 4.8%.

[Effects of the Invention] According to the method of the present invention, liquid crystallinity is exhibited during melt molding, and since it has extremely high fluidity, it is excellent in moldability,
Moreover, a tenacious copolyester having a high elastic modulus and excellent strength and elongation can be easily produced.

The copolyester may be alloyed or may be blended with a reinforcing agent such as glass fiber or carbon fiber so as to have more excellent mechanical properties and heat resistance. For example, a molding material, a film, It is preferably used as a material such as fibers, particularly as a precision molding material.

Claims (1)

[Claims] 1. A general formula (A) (R ¹ in the formula is a divalent alicyclic hydrocarbon group having 4 to 20 carbon atoms,
A divalent aliphatic hydrocarbon group having 1 to 40 carbon atoms or a divalent aromatic hydrocarbon group having 6 to 20 carbon atoms forming an aromatic ring, R ² is a divalent aliphatic group having 2 to 40 carbon atoms Hydrocarbon group, carbon number 4-20
Divalent alicyclic hydrocarbon group, divalent aromatic hydrocarbon group having 6 to 20 carbon atoms forming an aromatic ring, or divalent molecular weight 80 to
There are 8000 polyalkylene oxide residues. ) An oligoester or polyester comprising a repeating unit represented by (R ³ in the formula is a divalent aromatic hydrocarbon group having 6 to 20 carbon atoms forming an aromatic ring, and R ⁴ is an alkyl group having 1 to 8 carbon atoms or a phenyl group.) Carboxylic acid and (C) general formula HOR ⁵ OR ⁶ COOH (wherein R ⁵ is a divalent aliphatic hydrocarbon group having 2 to 40 carbon atoms,
A divalent alicyclic hydrocarbon group having 4 to 20 carbon atoms, a divalent aromatic hydrocarbon group having 6 to 20 carbon atoms forming an aromatic ring, or a divalent polyalkylene oxide residue having a molecular weight of 80 to 8000 , R ⁶ is a divalent aromatic hydrocarbon group having 6 to 20 carbon atoms forming an aromatic ring. ) A compound represented by is reacted to form a copolymerized oligomer, and then polymerization is carried out, to produce a copolymerized polyester.