JP7662575B2 - Benzoxazole Copolymer - Google Patents

Description

Application of Article 30, Paragraph 2 of the Patent Act (1) Japan Advanced Institute of Science and Technology, Graduate School of Advanced Science and Technology, Doctoral Dissertation Summary for Public Hearing, April 27, 2022

The present invention relates to a benzoxazole-based copolymer. More specifically, the present invention relates to a benzoxazole-based copolymer and a method for producing the same, as well as a film and a molding material containing the benzoxazole-based copolymer.

Polybenzoxazole is used as an engineering plastic due to its excellent heat resistance, and is used in fiber materials, etc. (see, for example, Non-Patent Document 1).

The benzoxazole skeleton of polybenzoxazole is produced by condensation of an o-aminophenol skeleton with a carboxylic acid, and therefore the use of 4-amino-3-hydroxybenzoic acid, which has the o-aminophenol skeleton, as a raw material monomer for polybenzoxazole has been investigated (see, for example, Non-Patent Document 2).

Recently, a method for producing 4-amino-3-benzoic acid from 4-aminobenzoic acid using a polypeptide having 4-aminobenzoic acid hydroxylation activity has been proposed (see, for example, Patent Documents 1 and 2). According to this method, 4-amino-3-hydroxybenzoic acid can be produced efficiently.

JP 2021-101626 A JP 2021-101627 A

Hirotaka Murase, SENI GAKKAISHI (Textiles and Industry), Vol. 66, No. 6, 2010 Lon J. Mathias et al. , Macromolecules, Vol. 18, No. 4, pp. 616-622 (1985)

The use of 4-amino-3-hydroxybenzoic acid as a raw material monomer for polybenzoxazole is being considered. However, the homopolymer of polybenzoxazole is brittle, so there is room for improvement in terms of stress characteristics, and there is also a need to improve heat resistance.

The present invention has been made in consideration of the above-mentioned conventional technology, and aims to provide a benzoxazole-based copolymer having excellent heat resistance and film formability and good mechanical properties, a method for producing the same, and a film and molding material containing the benzoxazole-based copolymer.

The present invention relates to
(1) Formula (I):

(In the formula, ^R1 and ^R2 each independently represent a hydrogen atom, an alkyl group, an alkoxy group, a hydroxyl group, a carboxyl group, an amino group, or a halogen atom.)
and a repeating unit represented by formula (II):

(In the formula, ^R3 and ^R4 each independently represent a hydrogen atom, an alkyl group, an alkoxy group, a hydroxyl group, a carboxyl group, an amino group, or a halogen atom.)
A benzoxazole-based copolymer having a repeating unit represented by
(2) A method for producing a benzoxazole-based copolymer according to (1) above, which comprises polymerizing a 4-amino-3-hydroxybenzoic acid compound with a 3,4-diaminobenzoic acid compound and then subjecting the resulting precursor of the benzoxazole-based copolymer to a ring-closing reaction;
(3) A film comprising the benzoxazole copolymer described in (1) above, and (4) a molding material comprising the benzoxazole copolymer described in (1) above.

The present invention provides a benzoxazole-based copolymer having excellent heat resistance and film-forming properties and good mechanical properties, a method for producing the same, and a film and molding material containing the benzoxazole-based copolymer.

1 is a graph showing the nuclear magnetic resonance ( ¹³ C-NMR) spectrum of the benzoxazole-based copolymer obtained in Example 1. 1 is a graph showing the Fourier transform infrared spectroscopy (FT-IR) spectrum of the benzoxazole-based copolymer obtained in Example 1. 2 is a graph showing the nuclear magnetic resonance ( ¹³ C-NMR) spectrum of the benzoxazole-based copolymer obtained in Example 2. 1 is a graph showing the Fourier transform infrared spectroscopy (FT-IR) spectrum of the benzoxazole-based copolymer obtained in Example 2. 1 is a graph showing the nuclear magnetic resonance ( ¹³ C-NMR) spectrum of the benzoxazole-based copolymer obtained in Example 3. 1 is a graph showing the Fourier transform infrared spectroscopy (FT-IR) spectrum of the benzoxazole-based copolymer obtained in Example 3. 1 is a graph showing the nuclear magnetic resonance ( ¹³ C-NMR) spectrum of the benzoxazole-based copolymer obtained in Example 4. 1 is a graph showing the Fourier transform infrared spectroscopy (FT-IR) spectrum of the benzoxazole-based copolymer obtained in Example 4. 1 is a graph showing the nuclear magnetic resonance ( ¹³ C-NMR) spectrum of the benzoxazole-based copolymer obtained in Example 5. 1 is a graph showing the Fourier transform infrared spectroscopy (FT-IR) spectrum of the benzoxazole-based copolymer obtained in Example 5. 1 is a graph showing the nuclear magnetic resonance ( ¹³ C-NMR) spectrum of the benzoxazole-based copolymer obtained in Example 6. 1 is a graph showing the Fourier transform infrared spectroscopy (FT-IR) spectrum of the benzoxazole-based copolymer obtained in Example 6. 1 is a graph showing the nuclear magnetic resonance ( ¹³ C-NMR) spectrum of the 2,6-polybenzoxazole obtained in Comparative Example 1. 1 is a graph showing the Fourier transform infrared spectroscopy (FT-IR) spectrum of the 2,6-polybenzoxazole obtained in Comparative Example 1.

The benzoxazole copolymer of the present invention has the formula (I):

(In the formula, ^R1 and ^R2 each independently represent a hydrogen atom, an alkyl group, an alkoxy group, a hydroxyl group, a carboxyl group, an amino group, or a halogen atom.)
and a repeating unit represented by formula (II):

(In the formula, ^R3 and ^R4 each independently represent a hydrogen atom, an alkyl group, an alkoxy group, a hydroxyl group, a carboxyl group, an amino group, or a halogen atom.)
The repeating unit has the following structure:

In the repeating unit represented by formula (I), R ¹ and R ² are each independently a hydrogen atom, an alkyl group, an alkoxy group, a hydroxyl group, a carboxyl group, an amino group, or a halogen atom. Suitable alkyl groups include, for example, alkyl groups having 1 to 4 carbon atoms, such as a methyl group, an ethyl group, a propyl group, and a butyl group. Suitable alkoxy groups include, for example, alkoxy groups having 1 to 4 carbon atoms, such as a methoxy group, an ethoxy group, a propoxy group, and a butoxy group. Suitable halogen atoms include a fluorine atom, a chlorine atom, a bromine atom, and an iodine atom. Among ^{R 1} and R ² , from the viewpoint of obtaining a benzoxazole-based copolymer having excellent heat resistance and film formability and good mechanical properties, a hydrogen atom, an alkyl group having 1 to 4 carbon atoms, and an alkoxy group having 1 to 4 carbon atoms are preferred, a hydrogen atom and an alkyl group having 1 to 4 carbon atoms are more preferred, and a hydrogen atom is even more preferred.

In the repeating unit represented by formula (II), R ³ and R ⁴ are each independently a hydrogen atom, an alkyl group, an alkoxy group, a hydroxyl group, a carboxyl group, an amino group, or a halogen atom. Suitable alkyl groups include, for example, alkyl groups having 1 to 4 carbon atoms, such as a methyl group, an ethyl group, a propyl group, and a butyl group. Suitable alkoxy groups include, for example, alkoxy groups having 1 to 4 carbon atoms, such as a methoxy group, an ethoxy group, a propoxy group, and a butoxy group. Suitable halogen atoms include a fluorine atom, a chlorine atom, a bromine atom, and an iodine atom. Among R ³ and R ⁴ , from the viewpoint of obtaining a benzoxazole-based copolymer having excellent heat resistance and film formability and good mechanical properties, a hydrogen atom, an alkyl group having 1 to 4 carbon atoms, and an alkoxy group having 1 to 4 carbon atoms are preferred, a hydrogen atom and an alkyl group having 1 to 4 carbon atoms are more preferred, and a hydrogen atom is even more preferred.

The molar ratio of the repeating units represented by formula (I) to the repeating units represented by formula (II) [repeating units represented by formula (I)/repeating units represented by formula (II)] is preferably 10/90 to 50/50, more preferably 15/85 to 45/55, and even more preferably 15/85 to 40/60, from the viewpoint of obtaining a benzoxazole-based copolymer having excellent heat resistance and film formability and good mechanical properties.

The viscosity average molecular weight of the benzoxazole-based copolymer of the present invention is not particularly limited, but from the viewpoint of obtaining a benzoxazole-based copolymer having excellent heat resistance and film formability and good mechanical properties, it is preferably 15,000 to 800,000 g/mol, more preferably 20,000 to 500,000 g/mol, and even more preferably 50,000 to 200,000 g/mol. The viscosity average molecular weight of the benzoxazole-based copolymer is a value measured based on the method described in the following examples.

The benzoxazole copolymer of the present invention may contain repeating units other than the repeating units represented by formula (I) and the repeating units represented by formula (II) within the scope of the present invention. The other repeating units include repeating units that are inevitably by-produced when a 4-amino-3-hydroxybenzoic acid compound and a 3,4-diaminobenzoic acid compound are polymerized, as well as repeating units based on a third monomer obtained by using a third monomer other than a 4-amino-3-hydroxybenzoic acid compound and a 3,4-diaminobenzoic acid compound. The third monomer can be used within the scope of the present invention.

The benzoxazole copolymer of the present invention can be obtained, for example, by polymerizing a 4-amino-3-hydroxybenzoic acid compound and a 3,4-diaminobenzoic acid compound, and then carrying out a ring-closing reaction of the resulting benzoxazole copolymer precursor.

4-amino-3-hydroxybenzoic acid compounds include, for example, compounds of formula (III):

(wherein R ¹ and R ² are the same as defined above, and R ⁵ represents an amino group or a group represented by the formula: --NH ₃ ⁺ X ^- (X represents a halogen atom).)
Examples of the 4-amino-3-hydroxybenzoic acid compound include those represented by the following formula: ^{R 1} and R ² are the same as above, but from the viewpoint of obtaining a benzoxazole-based copolymer having excellent heat resistance and film formability and good mechanical properties, a hydrogen atom, an alkyl group having 1 to 4 carbon atoms, and an alkoxy group having 1 to 4 carbon atoms are preferred, a hydrogen atom and an alkyl group having 1 to 4 carbon atoms are more preferred, and a hydrogen atom is even more preferred. X is a halogen atom. Examples of the halogen atom include a fluorine atom, a chlorine atom, a bromine atom, and an iodine atom. Of these halogen atoms, a chlorine atom is preferred.

In formula (III), the 4-amino-3-hydroxybenzoic acid compound in which R ¹ and R ² are each a hydrogen atom is 4-amino-3-hydroxybenzoic acid. 4-Amino-3-hydroxybenzoic acid can be efficiently prepared, for example, by the methods described in JP-A-2021-101626 and JP-A-2021-101627. In formula (III), the 4-amino-3-hydroxybenzoic acid compound in which X is a halogen atom can be easily prepared by halogenating 4-amino-3-hydroxybenzoic acid by a conventional method.

3,4-Diaminobenzoic acid compounds include, for example, compounds represented by formula (IV):

(wherein R ³ and R ⁴ are the same as defined above, and R ⁶ and R ⁷ each independently represent an amino group or a group represented by the formula: --NH ₃ ⁺ X ^- (X represents a halogen atom).)
Examples of the 3,4-diaminobenzoic acid compound represented by the formula: R ³ and R ⁴ are the same as above, but from the viewpoint of obtaining a benzoxazole-based copolymer having excellent heat resistance and film formability and good mechanical properties, a hydrogen atom, an alkyl group having 1 to 4 carbon atoms, and an alkoxy group having 1 to 4 carbon atoms are preferred, a hydrogen atom and an alkyl group having 1 to 4 carbon atoms are more preferred, and a hydrogen atom is even more preferred. In the group represented by the formula: -NH ₃ ⁺ X ^- , X is a halogen atom. Examples of the halogen atom include a fluorine atom, a chlorine atom, a bromine atom, and an iodine atom. Of these halogen atoms, a chlorine atom is preferred.

3,4-Diaminobenzoic acid represented by formula (IV) in which R ³ and R ⁴ are each a hydrogen atom is readily available commercially. 3,4-Diaminobenzoic acid compounds represented by formula (IV) in which X is a halogen atom can be readily prepared by halogenating 3,4-diaminobenzoic acid by a conventional method.

The molar ratio of the 4-amino-3-hydroxybenzoic acid compound to the 3,4-diaminobenzoic acid compound (4-amino-3-hydroxybenzoic acid compound/3,4-diaminobenzoic acid compound) is preferably 10/90 to 50/50, more preferably 15/85 to 45/55, and even more preferably 15/85 to 40/60, from the viewpoint of obtaining a benzoxazole-based copolymer having excellent heat resistance and film formability and good mechanical properties.

Note that compounds other than 4-amino-3-hydroxybenzoic acid compounds and 3,4-diaminobenzoic acid compounds may be used as long as they do not impair the objectives of the present invention. Examples of other compounds include aromatic diamine compounds such as phenylenediamine and xylylenediamine, but the present invention is not limited to these examples.

Methods for polymerizing a 4-amino-3-hydroxybenzoic acid compound and a 3,4-diaminobenzoic acid compound include, for example, bulk polymerization, solution polymerization, suspension polymerization, and emulsion polymerization, but the present invention is not limited to these examples. Among these polymerization methods, from the viewpoint of efficiently preparing a benzoxazole-based copolymer with a small amount of impurities, bulk polymerization and solution polymerization are preferred, and bulk polymerization is more preferred.

The polymerization reaction between the 4-amino-3-hydroxybenzoic acid compound and the 3,4-diaminobenzoic acid compound can be carried out in the presence or absence of an organic solvent.

The organic solvent is preferably an organic solvent capable of dissolving the 4-amino-3-hydroxybenzoic acid compound and the 3,4-diaminobenzoic acid compound at the polymerization reaction temperature and exhibiting poor solubility or insolubility in the benzoxazole copolymer. Examples of the organic solvent include aliphatic alcohols having 1 to 4 carbon atoms such as methanol, ethanol, propanol, and butanol, ketone compounds such as acetone and methyl ethyl ketone, ethyl acetate, tetrahydrofuran, dioxane, chloroform, dichloromethane, chlorobenzene, phenol, and cresol, but the present invention is not limited to these examples. These organic solvents may be used alone or in combination of two or more kinds. The amount of the organic solvent is not particularly limited as long as it is an amount that can efficiently polymerize the 4-amino-3-hydroxybenzoic acid compound and the 3,4-diaminobenzoic acid compound, but it is usually preferably about 3 to 20 times (mass) the total amount (mass) of the 4-amino-3-hydroxybenzoic acid compound and the 3,4-diaminobenzoic acid compound, and more preferably about 5 to 15 times (mass).

When a 4-amino-3-hydroxybenzoic acid compound and a 3,4-diaminobenzoic acid compound are polymerized, a suitable amount of a heat stabilizer can be used. Examples of heat stabilizers include phenylphosphonic acid (PPA), dimethyl phenylphosphonate, diethyl phenylphosphonate, and diphenylphosphonic acid, but the present invention is not limited to these examples.

The polymerization reaction temperature between the 4-amino-3-hydroxybenzoic acid compound and the 3,4-diaminobenzoic acid compound is not particularly limited, but is preferably about 120 to 220°C from the viewpoint of increasing the reaction efficiency. The polymerization reaction temperature may be constant or may be increased in stages.

The polymerization reaction time between the 4-amino-3-hydroxybenzoic acid compound and the 3,4-diaminobenzoic acid compound cannot be determined in general because it varies depending on the polymerization reaction temperature, but is usually about 10 to 40 hours. The atmosphere in which the 4-amino-3-hydroxybenzoic acid compound and the 3,4-diaminobenzoic acid compound are polymerized is preferably an inert gas such as nitrogen gas or argon gas, in order to avoid being affected by oxygen contained in the air.

By carrying out the above-mentioned polymerization reaction between the 4-amino-3-hydroxybenzoic acid compound and the 3,4-diaminobenzoic acid compound, a precursor of a benzoxazole-based copolymer is obtained. The fact that a precursor of a benzoxazole-based copolymer has been obtained can be confirmed, for example, by nuclear magnetic resonance spectroscopy, infrared spectroscopy, etc.

When the 4-amino-3-hydroxybenzoic acid compound and the 3,4-diaminobenzoic acid compound are polymerized, the ring-closing reaction of the precursor of the benzoxazole copolymer produced may proceed simultaneously. Therefore, the polymerization reaction of the 4-amino-3-hydroxybenzoic acid compound and the 3,4-diaminobenzoic acid compound and the ring-closing reaction of the precursor of the benzoxazole copolymer produced can be carried out continuously.

The ring-closing reaction of the precursor of the benzoxazole-based copolymer produced means a reaction in which the repeating units based on the 4-amino-3-hydroxybenzoic acid compound and the repeating units based on the 3,4-diaminobenzoic acid compound are each ring-closed. The ring-closing reaction of the precursor of the benzoxazole-based copolymer can be carried out by maintaining the temperature of the precursor of the benzoxazole-based copolymer at a temperature of 200°C or higher. The temperature of the benzoxazole-based copolymer when the ring-closing reaction of the precursor of the benzoxazole-based copolymer is carried out is preferably 200°C or higher, more preferably 210°C or higher, and even more preferably 220°C or higher, from the viewpoint of efficiently carrying out the ring-closing reaction. The upper limit of the temperature of the benzoxazole-based copolymer when the ring-closing reaction of the precursor of the benzoxazole-based copolymer is not particularly limited, but is preferably 300°C or lower, more preferably 280°C or lower, and even more preferably 260°C or lower, from the viewpoint of suppressing the decomposition of the benzoxazole-based copolymer produced and increasing energy efficiency. The temperature of the benzoxazole-based copolymer during the ring-closing reaction of the precursor of the benzoxazole-based copolymer may be constant or may be varied.

When carrying out the ring-closing reaction of the precursor of the benzoxazole-based copolymer, a suitable amount of a heat stabilizer can be used. Examples of the heat stabilizer include the same compounds as those described above.

The time required for the ring-closing reaction of the precursor of the benzoxazole-based copolymer cannot be determined in general because it varies depending on the heating temperature, but it is usually about 10 to 24 hours. The atmosphere in which the ring-closing reaction of the precursor of the benzoxazole-based copolymer is carried out is preferably an inert gas such as nitrogen gas or argon gas, in order to avoid the influence of oxygen contained in the air.

By carrying out the ring-closing reaction of the precursor of the benzoxazole-based copolymer as described above, the benzoxazole-based copolymer of the present invention can be obtained. The production of the benzoxazole-based copolymer can be confirmed, for example, by nuclear magnetic resonance spectroscopy, infrared spectroscopy, etc.

The benzoxazole copolymer obtained above may be used as it is, or may be purified by washing with water or neutralized to remove impurities contained in the benzoxazole copolymer. The benzoxazole copolymer obtained above may be recovered by a method such as filtration. The recovered benzoxazole copolymer may be dried, for example, by drying under reduced pressure.

The benzoxazole copolymer of the present invention has a repeating unit represented by formula (I) and a repeating unit represented by formula (II), and therefore has excellent heat resistance and film-forming properties, and has good mechanical properties.

The benzoxazole copolymer is usually a random copolymer having a repeating unit represented by formula (I) and a repeating unit represented by formula (II), but the repeating units represented by formula (I) and the repeating units represented by formula (II) may be present alternately or in a block form.

The benzoxazole copolymer may contain an appropriate amount of additives depending on the application, if necessary. Examples of additives include colorants such as pigments and dyes, UV absorbers, UV stabilizers, antioxidants, rust inhibitors, antibacterial agents, plasticizers, algae inhibitors, mildew inhibitors, flame retardants, and foaming agents, but the present invention is not limited to these examples. These additives may be used alone or in combination of two or more types. The amount of additive varies depending on the type of additive, so it cannot be determined in general, and it is preferable to determine the amount appropriately depending on the type of additive.

The benzoxazole-based copolymer of the present invention dissolves in various organic solvents, and can therefore be suitably used as a molding material for a molded body having a desired shape, a raw material for a film, etc. A molded body can be obtained, for example, by using a solution in which the benzoxazole-based copolymer is dissolved in an organic solvent as a molding material, and molding the molding material by a molding method such as blow molding, extrusion molding, or injection molding.

Examples of the organic solvent include trifluoroacetic acid, methylsulfonic acid, hexafluoroisopropanol, dimethylsulfoxide, dimethylacetamide, and dichloromethane, but the present invention is not limited to these examples. These organic solvents may be used alone or in combination of two or more kinds.

The amount of organic solvent relative to the benzoxazole copolymer of the present invention cannot be determined in general because it varies depending on the type of organic solvent, the desired viscosity of the organic solvent solution of the benzoxazole copolymer, etc. Usually, it is preferable to mix the benzoxazole copolymer and the organic solvent in a desired ratio so that the organic solvent solution of the benzoxazole copolymer has the desired viscosity at a specified temperature.

The film made of the benzoxazole-based copolymer of the present invention can be obtained by forming a film from an organic solvent solution of the benzoxazole-based copolymer by a method such as casting or flow casting. After the film is formed, the film may be dried to remove the organic solvent contained in the film. The thickness of the film formed cannot be determined in general because it varies depending on the application of the film, but is usually about 3 to 800 μm.

Next, the present invention will be described in more detail based on examples, but the present invention is not limited to these examples.

The structures and molecular weights of the polymers obtained in the following examples and comparative examples were determined based on the following method.

(1) Nuclear magnetic resonance ( ¹³ C-NMR) spectrum The nuclear magnetic resonance ( ¹³ C-NMR) spectrum of the polymer was measured using a nuclear magnetic resonance spectrometer (Bruker Avance III, 500 MHz, product name). 5 mg of the sample (polymer) was loaded into a zirconia rotor with a diameter of 7 mm and rotated at 8 kHz. The contact time and accumulation time were set to 2 seconds and 5 seconds, respectively, and the accumulation number was set to 15,000.

(2) Fourier transform infrared spectroscopy (FT-IR) spectrum Fourier transform infrared spectroscopy (FT-IR) was measured using a Perkin Elmer product name: Spectrum 100 (ATR method) as a measuring device, with the measurement wave number range set to 400 to 4000 cm ^-1 and the number of accumulations set to 16.

[Viscosity average molecular weight of polymer]
The viscosity average molecular weight of a polymer was determined based on the intrinsic viscosity of the polymer according to the following method.

A homogeneous solution was obtained by dissolving 100 mg of the polymer in 15 mL of 98% sulfuric acid. The relative viscosity of the obtained solution was measured at 25° C. using an Ubbelohde viscometer. The time (t ₀ ) for sulfuric acid to pass through a specific length of the viscometer and the time (t) for the polymer solution to pass through the same specific length were measured, and the relative viscosity (η _rel ) was calculated according to the following equation (1):
η _rel =t/t ₀ (1)
It was calculated based on the above.

Next, the specific viscosity (η _sp ) is calculated using equation (2):
η _sp =(t−t ₀ )/t ₀ =η _rel −1 (2)
Then, the intrinsic viscosity [η _int (unit: dL/g)] was calculated based on the Solomon-Ciuta formula (3):

(wherein c represents the concentration of the polymer in the solution).
For reference, R. Pamies, JG Hernandez Cifre, M. del Carmen Lopez Martinez and J. Garcia de la Torre, Colloid Polym. Sci., 2008, 286, 1223-1231 is listed as a reference for Solomon-Ciuta's formula (3).

Next, the viscosity average molecular weight [ _Mw (unit: g/mol)] is calculated according to the Mark-Houwink formula (4):
η _int = KM _w ^a (4)
(In the formula, K is 1.94×10 ⁻⁴ and a is 0.791.)
For reference, the following references for Mark-Houwink's formula (4) are listed: J. Lobato, P. Canizares, MA Rodrigo, JJ Linares and JA Aguilar, J. Memb. Sci., 2007, 306, 47-55.

Polymer Properties
(1) Heat Resistance of Polymers Using a thermogravimetric analyzer (Seiko Instruments Inc., product number: SSC/5200SII), thermogravimetric analysis of polymers was performed in a nitrogen gas atmosphere at a temperature range of 25 to 800°C at a heating rate of 10°C/min. Based on the results of the thermogravimetric analysis, the 5% weight loss temperature (T _d5 ) and the 10% weight loss temperature (T _d10 ) were determined. The 5% weight loss temperature means the temperature at which the mass of the sample (polymer) is reduced by 5%, and the 10% weight loss temperature means the temperature at which the mass of the sample (polymer) is reduced by 10%.

(2) Breaking stress, elongation at break, Young's modulus and toughness of film Using a film having a width of 7 mm, a length of 40 mm and a thickness of 15 μm, the maximum strength (breaking stress) until the film breaks was measured by pulling it at room temperature at a crosshead speed of 0.4 mm/sec with a tensile tester (manufactured by INSTRON, product number: 3365-L5). The elongation at break of the film is the value obtained by measuring the tensile strength of the film, determining the length of the film stretched when the film breaks, dividing the stretched length by the original length of the film, and multiplying the result by 100. The Young's modulus of the film was determined from the initial slope of the stress-strain curve. The toughness of the film was determined as the integrated area of the region surrounded by the stress-strain curve.

[Polymer Solubility]
5 mg of polymer was added to 2 mL of each solvent, and left to stand at room temperature (about 20° C.). Whether the polymer was dissolved in the solvent was examined 1 hour after the start of the addition of the polymer. If the polymer was not dissolved in the solvent even after 1 hour from the start of the addition of the polymer, it was examined whether it was dissolved by irradiating with ultrasonic waves or by heating to 60° C.

The evaluation criteria for solubility are as follows:
[Evaluation Criteria]
+: Polymer completely dissolved ±: Polymer partially dissolved -: Polymer insoluble

Preparation Example 1 (Preparation of 4-amino-3-hydroxybenzoic acid hydrochloride)
6.0 g (39.2 mmol) of 4-amino-3-hydroxybenzoic acid was dispersed in 30 mL of methanol to obtain a dispersion. 12 N hydrochloric acid was added dropwise to the dispersion obtained above under stirring until 4-amino-3-hydroxybenzoic acid was completely dissolved. After the color of the dispersion changed from pink to dark red, stirring was continued for another 4 hours at room temperature to obtain a yellow dispersion of 4-amino-3-hydroxybenzoic acid hydrochloride. The solvent was removed from the dispersion obtained above by evaporation to obtain 5.4 g of 4-amino-3-hydroxybenzoic acid hydrochloride (yield: 94%).

Preparation Example 2 (Preparation of 3,4-diaminobenzoic acid hydrochloride)
3,4-Diaminobenzoic acid hydrochloride was obtained in the same manner as in Preparation Example 1, except that 9.0 g (40.0 mg) of 3,4-diaminobenzoic acid hydrochloride was used instead of 4-amino-3-hydroxybenzoic acid.

Example 1 (Preparation of 2,6-benzoxazole copolymer)

25 g of polyphosphoric acid was placed in a three-necked round-bottom flask equipped with a magnetic stirrer, and the contents of the flask were heated at 100°C for 1 hour under a nitrogen gas atmosphere to remove traces of moisture. Subsequently, 226.8 mg (1.20 mmol) of 4-amino-3-hydroxybenzoic acid and 1,080.0 mg (4.80 mmol) of 3,4-diaminobenzoic acid hydrochloride were added to the flask, and the contents of the flask were continuously stirred for 1 hour to dissolve each compound.

The contents of the flask were then heated at 160°C for 4 hours, then at 180°C for 4 hours, then at 200°C for 4 hours, and finally at 220°C for 12 hours, to successively carry out the polymerization reaction of 4-amino-3-hydroxybenzoic acid with 3,4-diaminobenzoic acid hydrochloride and the ring-closing reaction of the resulting precursor of the 2,6-benzoxazole copolymer, and the color of the contents changed from red to dark brown.

The solution obtained above was dispersed in water, and the resulting dispersion was stirred for 12 hours. The polyphosphoric acid was then removed from the dispersion, and the dispersion was filtered to obtain a brown solid.

The solid obtained above was dried under reduced pressure, and then pulverized to obtain a powder. The obtained powder was suspended in deionized water, and 1M aqueous sodium hydroxide solution was slowly added to the obtained suspension, and the suspension was stirred for 1 hour until the pH of the suspension reached 7.0.

The solid was collected from the suspension by filtration and dried under reduced pressure to obtain 621.0 mg (5.4 mmol) of brown 2,6-benzoxazole copolymer (hereinafter referred to as PBOBI-A) powder (yield: 89%).

The nuclear magnetic resonance ( ¹³ C-NMR) spectrum of the PBOBI-A obtained above is shown in FIG. 1, and the Fourier transform infrared spectroscopy (FT-IR) spectrum is shown in FIG.

Example 2 (Preparation of 2,6-benzoxazole-based copolymer)
A brown powder of 2,6-benzoxazole-based copolymer (hereinafter referred to as PBOBI-B) was obtained in the same manner as in Example 1, except that the amount of 4-amino-3-hydroxybenzoic acid was changed to 113.4 mg (0.6 mmol) and the amount of 4-amino-3-hydroxybenzoic acid was changed to 1215 mg (5.4 mol).

The nuclear magnetic resonance ( ¹³ C-NMR) spectrum of the PBOBI-B obtained above is shown in FIG. 3, and the Fourier transform infrared spectroscopy (FT-IR) spectrum is shown in FIG.

Example 3 (Preparation of 2,6-benzoxazole-based copolymer)
A brown benzoxazole-based copolymer (hereinafter referred to as PBOBI-C) powder was obtained in the same manner as in Example 1, except that the amount of 4-amino-3-hydroxybenzoic acid was changed to 340.2 mg (1.8 mmol) and the amount of 3,4-diaminobenzoic acid hydrochloride was changed to 945 mg (4.2 mmol).

The nuclear magnetic resonance ( ¹³ C-NMR) spectrum of the PBOBI-C obtained above is shown in FIG. 5, and the Fourier transform infrared spectroscopy (FT-IR) spectrum is shown in FIG.

Example 4 (Preparation of 2,6-benzoxazole-based copolymer)
A brown benzoxazole-based copolymer (hereinafter referred to as PBOBI-D) powder was obtained in the same manner as in Example 1, except that the amount of 4-amino-3-hydroxybenzoic acid was changed to 453.6 mg (2.4 mmol) and the amount of 3,4-diaminobenzoic acid hydrochloride was changed to 810 mg (3.6 mmol).

The nuclear magnetic resonance ( ¹³ C-NMR) spectrum of the PBOBI-D obtained above is shown in FIG. 7, and the Fourier transform infrared spectroscopy (FT-IR) spectrum is shown in FIG.

Example 5 (Preparation of 2,6-benzoxazole-based copolymer)
A brown powder of 2,6-benzoxazole-based copolymer (hereinafter referred to as PBOBI-E) was obtained in the same manner as in Example 1, except that the amount of 4-amino-3-hydroxybenzoic acid was changed to 567.0 mg (3.0 mmol) and the amount of 3,4-diaminobenzoic acid hydrochloride was changed to 675 mg (3.0 mmol).

The nuclear magnetic resonance ( ¹³ C-NMR) spectrum of the PBOBI-E obtained above is shown in FIG. 9, and the Fourier transform infrared spectroscopy (FT-IR) spectrum is shown in FIG.

Example 6 (Preparation of 2,6-benzoxazole-based copolymer)
A brown powder of 2,6-benzoxazole-based copolymer (hereinafter referred to as PBOBI-F) was obtained in the same manner as in Example 1, except that the amount of 4-amino-3-hydroxybenzoic acid was changed to 680.4 mg (3.6 mmol) and the amount of 3,4-diaminobenzoic acid hydrochloride was changed to 540 mg (2.4 mmol).

The nuclear magnetic resonance ( ¹³ C-NMR) spectrum of the PBOBI-F obtained above is shown in FIG. 11, and the Fourier transform infrared spectroscopy (FT-IR) spectrum is shown in FIG.

Comparative Example 1 (Preparation of 2,6-polybenzoxazole)

25 g of polyphosphoric acid was placed in a three-necked round-bottom flask equipped with a magnetic stirrer, and the contents of the flask were heated at 100°C for 1 hour under a nitrogen gas atmosphere to remove traces of moisture. Subsequently, 1134 mg (6 mmol) of 4-amino-3-hydroxybenzoic acid hydrochloride was added to the flask, and the contents of the flask were continuously stirred for 1 hour to dissolve each compound.

The contents of the flask were then heated at 160°C for 4 hours, then 180°C for 4 hours, then 200°C for 4 hours, followed by 220°C for 12 hours.

The contents of the flask heated as described above were dispersed in water, and the resulting dispersion was stirred for 12 hours. The polyphosphoric acid was then removed from the dispersion, and the dispersion was filtered to obtain a solid.

The solid obtained above was dried under reduced pressure, and then pulverized to obtain a powder. The obtained powder was suspended in deionized water, and 1M aqueous sodium hydroxide solution was slowly added to the obtained suspension, and the suspension was stirred for 1 hour until the pH of the suspension reached 7.0.

The solid was collected from the suspension by filtration and dried under reduced pressure to obtain 2,6-polybenzoxazole (hereinafter referred to as PBO-A).

The nuclear magnetic resonance ( ¹³ C-NMR) spectrum of the PBO-A obtained above is shown in FIG. 13, and the Fourier transform infrared spectroscopy (FT-IR) spectrum is shown in FIG.

Comparative Example 2 (Preparation of 2,5-polybenzoxazole)

25 g of polyphosphoric acid was placed in a three-necked round-bottom flask equipped with a magnetic stirrer, and the contents of the flask were heated at 100°C for 1 hour under a nitrogen gas atmosphere to remove traces of moisture. Subsequently, 1134 mg (6 mmol) of 3-amino-4-hydroxybenzoic acid hydrochloride was added to the flask, and the contents of the flask were continuously stirred for 1 hour to dissolve each compound.

The contents of the flask were then heated at 160°C for 4 hours, then 180°C for 4 hours, then 200°C for 4 hours, followed by 220°C for 12 hours.

The contents of the flask heated as described above were dispersed in water, and the resulting dispersion was stirred for 12 hours. The polyphosphoric acid was then removed from the dispersion, and the dispersion was filtered to obtain a solid.

The solid obtained above was dried under reduced pressure, and then pulverized to obtain a powder. The obtained powder was suspended in deionized water, and 1M aqueous sodium hydroxide solution was slowly added to the obtained suspension, and the suspension was stirred for 1 hour until the pH of the suspension reached 7.0.

The solid was collected from the suspension by filtration and dried under reduced pressure to obtain 2,5-polybenzoxazole (hereinafter referred to as PBO-B).

It was confirmed by ¹³ C-NMR spectrum and infrared absorption spectrum that PBO-B was obtained in Comparative Example 2.

Comparative Example 3 (Preparation of benzoxazole copolymer)

25 g of polyphosphoric acid was placed in a three-necked round-bottom flask equipped with a magnetic stirrer, and the contents of the flask were heated at 100°C for 1 hour under a nitrogen gas atmosphere to remove traces of moisture. Subsequently, 567.0 mg (3.0 mmol) of 4-amino-3-hydroxybenzoic acid hydrochloride and 567.0 mg (3.0 mmol) of 3-amino-4-hydroxybenzoic acid hydrochloride were added to the flask, and the contents of the flask were continuously stirred for 1 hour to dissolve each compound.

The contents of the flask were then heated at 160°C for 4 hours, then at 180°C for 4 hours, then at 200°C for 4 hours, and then at 220°C for 12 hours, thereby successively carrying out the polymerization reaction of 4-amino-3-hydroxybenzoic acid with 3-amino-4-hydroxybenzoic acid hydrochloride and the ring-closing reaction of the resulting precursor of 2,6-polybenzoxazole.

The contents of the flask heated as described above were dispersed in water, and the resulting dispersion was stirred for 12 hours. The polyphosphoric acid was then removed from the dispersion, and the dispersion was filtered to obtain a solid.

The solid obtained above was dried under reduced pressure, and then pulverized to obtain a powder. The obtained powder was suspended in deionized water, and 1M aqueous sodium hydroxide solution was slowly added to the obtained suspension, and the suspension was stirred for 1 hour until the pH of the suspension reached 7.0.

The solid was collected from the suspension by filtration and dried under reduced pressure to obtain 2,6-polybenzoxazole (hereinafter referred to as PBOBO).

It was confirmed by ¹³ C-NMR spectrum and infrared absorption spectrum that PBOBO was obtained in Comparative Example 3.

Comparative Example 4 (Preparation of 2,5-benzoxazole copolymer)

25 g of polyphosphoric acid was placed in a three-necked round-bottom flask equipped with a magnetic stirrer, and the contents of the flask were heated at 100°C for 1 hour under a nitrogen gas atmosphere to remove traces of moisture. Subsequently, 226.8 mg (1.20 mmol) of 3-amino-4-hydroxybenzoic acid and 1,080.0 mg (4.80 mmol) of 3,4-diaminobenzoic acid hydrochloride were added to the flask, and the contents of the flask were continuously stirred for 1 hour to dissolve each compound.

The contents of the flask were then heated at 160°C for 4 hours, then at 180°C for 4 hours, then at 200°C for 4 hours, and finally at 220°C for 12 hours, to successively carry out the polymerization reaction of 3-amino-4-hydroxybenzoic acid with 3,4-diaminobenzoic acid hydrochloride and the ring-closing reaction of the resulting precursor of the 2,5-benzoxazole copolymer, and the color of the contents changed from red to dark brown.

The solution obtained above was dispersed in water, and the resulting dispersion was stirred for 12 hours. The polyphosphoric acid was then removed from the dispersion, and the dispersion was filtered to obtain a brown solid.

The solid obtained above was dried under reduced pressure, and then pulverized to obtain a powder. The obtained powder was suspended in deionized water, and 1M aqueous sodium hydroxide solution was slowly added to the obtained suspension, and the suspension was stirred for 1 hour until the pH of the suspension reached 7.0.

The solid was collected from the suspension by filtration and dried under reduced pressure to obtain 621.0 mg (5.4 mmol) of brown 2,5-benzoxazole-based copolymer (hereinafter referred to as PBOBI-G) powder (yield: 89%).

It was confirmed by ¹³ C-NMR spectrum and infrared absorption spectrum that PBOBI-G was obtained in Comparative Example 4.

Comparative Example 5 (Preparation of 2,5-benzoxazole-based copolymer)
In Comparative Example 4, the amount of 3-amino-4-hydroxybenzoic acid was changed to 453.6 mg (2.4 mmol), and the amount of 3,4-diaminobenzoic acid hydrochloride was changed to 810 mg (3.6 mmol), but the same procedure as in Comparative Example 4 was repeated to obtain a brown 2,5-benzoxazole-based copolymer (hereinafter, referred to as PBOBI-H) powder.

The nuclear magnetic resonance ^{( 13} C-NMR) spectrum and infrared absorption spectrum of the thus obtained PBOBI-H were both similar to those of PBOBI-G obtained in Comparative Example 4.

Comparative Example 6 (Preparation of 2,5-benzoxazole-based copolymer)
In Comparative Example 4, the amount of 3-amino-4-hydroxybenzoic acid was changed to 945.0 mg (5.0 mmol), and the amount of 3,4-diaminobenzoic acid hydrochloride was changed to 1125 mg (5.0 mmol), but the same procedure as in Comparative Example 4 was repeated to obtain a brown 2,5-benzoxazole-based copolymer (hereinafter referred to as PBOBI-I) powder.

The nuclear magnetic resonance ^{( 13} C-NMR) spectrum and infrared absorption spectrum of the thus obtained PBOBI-I were both similar to those of PBOBI-G obtained in Comparative Example 4.

Comparative Example 7 (Preparation of 2,5-Benzoxazole-Based Copolymer J)
A brown powder of 2,5-benzoxazole-based copolymer (hereinafter referred to as PBOBI-J) was obtained in the same manner as in Comparative Example 4, except that the amount of 3-amino-4-hydroxybenzoic acid was changed to 680.4 mg (3.6 mmol) and the amount of 3,4-diaminobenzoic acid hydrochloride was changed to 540 mg (2.4 mmol).

The nuclear magnetic resonance ^{( 13} C-NMR) spectrum and infrared absorption spectrum of the thus obtained PBOBI-J were both similar to those of PBOBI-G obtained in Comparative Example 4.

[Molecular weight of each polymer]
The data regarding the molecular weight of each polymer obtained above is shown in Table 1 below.

[Film Preparation]
Each of the polymers obtained above was taken in an amount of 100 mg, and each polymer was dispersed in a mixed solvent of 3 mL of trifluoroacetic acid and 0.19 mL of methanesulfonic acid. The dispersion obtained above was stirred at 60° C. for 24 hours to dissolve the polymer and obtain a dark brown solution. The solution obtained above was taken with a dropper, uniformly applied onto a silicon plate, and dried at room temperature for 12 hours to evaporate the solvent, forming a film.

Next, the film obtained above was immersed in water at 25°C for 12 hours to remove the methanesulfonic acid contained in the film. The film was then removed from the water, sandwiched between two silicon plates, and dried under reduced pressure for 12 hours to obtain a film with a thickness of approximately 30 μm.

When films were prepared in the same manner as above using PBOBI-A to PBOBI-J as the benzoxazole copolymers, it was found that the durability of the film decreased as the content of 4-amino-3-hydroxybenzoic acid in the benzoxazole copolymer increased.

It was confirmed that when the content of 4-amino-3-hydroxybenzoic acid in the benzoxazole-based copolymer is 20 mol % or less, the film becomes flexible. It was also confirmed that when the content of 4-amino-3-hydroxybenzoic acid in the benzoxazole-based copolymer is 50 mol % or more, the film becomes brittle. From these findings, it was confirmed that the molar ratio of the repeating unit represented by formula (I) to the repeating unit represented by formula (II) [repeating unit represented by formula (I)/repeating unit represented by formula (II)] is preferably 20/80 to 50/50.

On the other hand, when a film was produced in the same manner as above using 100 mg of the PBO-A obtained in Comparative Example 1, the film was broken into powder when an attempt was made to peel it off the silicon plate, indicating that the film was very brittle and that the mechanical properties of the film could not be measured.

Furthermore, when films were produced in the same manner as above using 100 mg of PBO-B obtained in Comparative Example 2 or 100 mg of PBOBO obtained in Comparative Example 3, the films obtained were easily destroyed when attempting to peel them off from the silicon plate, and the mechanical properties of these films could not be measured.

[Heat resistance of polymer]
The 5% weight loss temperatures and 10% weight loss temperatures were used as indicators of heat resistance. The measurement results of the 5% weight loss temperatures and 10% weight loss temperatures of PBOBI-A to PBOBI-J, PBO-A, PBO-B and PBOBO are shown in Table 2.

The results shown in Table 2 show that the 5% weight loss temperatures and 10% weight loss temperatures of PBO-A to PBO-F obtained in each Example are superior to those of PBOBI-G to PBOBI-J obtained in each Comparative Example.

[Film breaking stress, elongation at break, Young's modulus and toughness]
The mechanical properties of PBOBI-A, PBOBI-B, PBOBI-D, PBO-A, PBO-B, PBOBO, PBOBI-G and PBOBI-I were examined in terms of film breaking stress, elongation at break, Young's modulus and toughness, and the results are shown in Table 3.

The results shown in Table 3 show that the benzoxazole copolymers obtained in each Example have significantly better breaking stress, elongation at break, rigidity (Young's modulus), and toughness than the PBO-A, PBO-B, and PBOBO obtained in each Comparative Example, and that the breaking stress, elongation at break, rigidity, and toughness improve as the content of the repeating unit represented by formula (II) in the benzoxazole copolymer increases.

[Polymer Solubility]
The solubility of PBOBI-A to PBOBI-J, PBO-A, PBO-B and PBOBO obtained in each Example was examined, and the results are shown in Table 4.

The results shown in Table 4 show that the benzoxazole copolymers obtained in each Example are all easily soluble in various solvents. This shows that a solution in which the benzoxazole copolymer obtained in each Example is dissolved in a solvent can be used as a molding material, and that the solution can be made into a film by a method such as casting to produce a film.

The benzoxazole-based copolymer of the present invention is excellent in heat resistance and film formability and has good mechanical properties, and is therefore expected to be used as molding materials for electronic parts, electronic information materials, aerospace materials, etc., which require heat resistance, molding materials for various films, enamels for electric wires used in high-performance motors, etc. In addition, the benzoxazole-based copolymer of the present invention is not only excellent in heat resistance and film formability but also has good mechanical properties, and is therefore expected to be used in various applications, such as materials for disposable diapers and cosmetic sheets such as lotion wipes.

Claims (5)

Formula (I):

(In the formula, ^R1 and ^R2 each independently represent a hydrogen atom, an alkyl group, an alkoxy group, a hydroxyl group, a carboxyl group, an amino group, or a halogen atom.)
and a repeating unit represented by formula (II):

(In the formula, ^R3 and ^R4 each independently represent a hydrogen atom, an alkyl group, an alkoxy group, a hydroxyl group, a carboxyl group, an amino group, or a halogen atom.)
A benzoxazole-based copolymer having a repeating unit represented by the formula: The benzoxazole copolymer according to claim 1, wherein the molar ratio of the repeating unit represented by formula (I) to the repeating unit represented by formula (II) [repeating unit represented by formula (I)/repeating unit represented by formula (II)] is 10/90 to 50/50. The method for producing a benzoxazole-based copolymer according to claim 1 or 2, which comprises polymerizing 4-amino-3-hydroxybenzoic acid and 3,4-diaminobenzoic acid, and then carrying out a ring-closing reaction of the resulting precursor of the benzoxazole-based copolymer. A film containing the benzoxazole copolymer according to claim 1 or 2. A molding material comprising the benzoxazole copolymer according to claim 1 or 2.

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