JPH0333722B2 - - Google Patents

Description

[Detailed description of the invention]

"Industrial Application Field" The present invention relates to a method for producing a copolymer comprising an N-aromatic maleimide, a vinyl aromatic compound and a vinyl cyanide compound and having excellent heat resistance and high temperature stability. "Prior Art" A copolymer consisting of a vinyl aromatic compound, especially styrene, and maleic anhydride (hereinafter referred to as SMA resin) is a styrene-acrylonitrile copolymer (hereinafter referred to as SA resin). , styrene-acrylonitrile-rubber elastic graft copolymer (hereinafter referred to as ABS resin), etc., and is known to have a high heat distortion temperature (good heat resistance). Therefore, a composition that is a blend of SMA resin, AS resin, and ABS resin provides a molded article with excellent heat resistance and good solvent resistance. However, its stability at high temperatures is poor; for example, when heated above 230°C, it causes discoloration, foaming, weight loss, crosslinking, etc. Therefore, when used as a normal injection molding material that is heated above 230°C, the commercial value is low. It has been difficult to obtain molded products with high . In order to improve the high-temperature stability of such compositions of SMA resin, AS resin, and ABS resin, attempts have been made to incorporate phenol-based, amine-based, or phosphorus-based antioxidants, etc. Practically satisfactory results have not been obtained. As described above, there has been a desire for a method for producing a copolymer that solves the drawback of poor high-temperature stability caused by conventional SMA resins. "Problems to be Solved by the Invention" In order to provide a method for producing a copolymer with excellent heat resistance and high temperature stability, the present inventor has conducted extensive research and found that the anhydrous maleate contained in SMA resin The present invention was achieved by discovering that the above-mentioned drawbacks can be overcome by imidizing acid residues using a specific method. "Means for Solving the Problems" The gist of the present invention is to produce a copolymer consisting of N-aromatic maleimide, a vinyl aromatic compound and a vinyl cyanide compound by a bulk-suspension polymerization method. A first step of copolymerizing maleic anhydride and a vinyl aromatic compound monomer in bulk to obtain a syrup containing a mixture of the copolymer and unreacted monomers; and a syrup obtained in the first step. A vinyl cyanide compound monomer is added and mixed into the mixture, and then the mixture of the syrup and the monomer is suspended in water to remove the maleic anhydride residues contained in the copolymer chain obtained in the first step. A second step of imidizing N-aromatic maleimide, a vinyl aromatic compound, and a vinyl cyanide compound with an aromatic amine. The present invention will be explained in detail below. The first step in the present invention is a step of copolymerizing maleic anhydride and a vinyl aromatic compound monomer in bulk to produce syrup in which the copolymer and unreacted monomers are mixed. A copolymer of maleic anhydride and vinyl aromatic compound monomer (hereinafter referred to as
Abbreviated as SMA copolymer. ) is a precursor to be imidized in a later step to produce a copolymer with excellent heat resistance and high temperature stability. The syrup in the present invention refers to a mixture of an SMA copolymer and an unreacted monomer dissolved therein.
Here, unreacted monomer means that most of the maleic anhydride was removed by the polymerization reaction in the first step.
Since it is consumed as an SMA copolymer, it essentially consists only of vinyl aromatic compound monomers. In the present invention, the vinyl aromatic compound monomer is generally styrene, but examples include halogenated styrenes such as α-methylstyrene, t-butylstyrene, o-chlorostyrene, and p-chlorostyrene. These may be used alone or in a mixture of two or more. In the first step of the present invention, the SMA copolymer can be easily produced by the following two bulk copolymerization methods. A copolymerization method in which a vinyl aromatic compound monomer such as styrene is present in a bulk polymerization system, and a mixture of the vinyl aromatic compound monomer and maleic anhydride is continuously added to the polymerization system. A mixture of maleic anhydride and a vinyl aromatic compound monomer containing 5% by weight or less, preferably 1 to 3% by weight of maleic anhydride is present in the bulk polymerization system, and maleic anhydride is added to the polymerization system. A copolymerization method in which continuous addition is performed later. In addition, in any of the above copolymerization methods, the proportion of maleic anhydride in the monomer mixture of the vinyl aromatic compound monomer and maleic anhydride present in the polymerization system is 5% by weight. Hereinafter, it is preferable to carry out copolymerization while maintaining the amount preferably in the range of 1 to 3% by weight. In the case of the above method, the maleic anhydride and vinyl aromatic compound monomers are mixed at a concentration of 20 to 35% by weight based on 100 parts by weight of the vinyl aromatic compound monomer initially present in the polymerization system. It is preferable to add 70 to 170 parts by weight of the mixture consisting of the following. The amount of these mixtures added is determined by the content of the desired maleic anhydride component in the SMA copolymer to be produced, but since the solubility of maleic anhydride in vinyl aromatic compound monomers is not high, , it is not preferable to increase the concentration. In other words, when the concentration of maleic anhydride exceeds 35% by weight, maleic anhydride will precipitate from the solution of the mixture unless the mixture is kept at a temperature of 40°C or higher, and furthermore, if the mixture is kept at too high a temperature, maleic anhydride and vinyl aromatic compound monomers will be separated. This is undesirable because thermal polymerization with the polymer tends to occur, and both of these can cause clogging of the polymerization system charging piping. Furthermore, if the concentration of maleic anhydride in the monomer mixture is less than 20% by weight, it will be difficult to increase the content of the maleic anhydride component in the SMA copolymer, and the copolymer obtained by the method of the present invention will This is not preferred because the heat resistance of the polymer will be insufficiently improved. The above-mentioned method is preferably carried out by thermal polymerization, in which the polymerization system is heated to 100 to 130 DEG C., and proceeds without the use of an initiator. When the addition of the mixture of maleic anhydride and vinyl aromatic compound monomer is completed, the maleic anhydride in the polymerization system is rapidly consumed, and the contents of the polymerization system are the vinyl aromatic compound monomer and anhydride. Copolymerized with maleic acid
A syrup is obtained in which the SMA copolymer is dissolved in unreacted vinyl aromatic compound monomer. When using the above method, problems such as precipitation of maleic anhydride and clogging of the charging pipe due to polymerization do not occur, so the maleic anhydride component is contained in a high concentration.
Suitable for producing SMA copolymers. The concentration of maleic anhydride initially present in the polymerization system affects the polymerization rate and the content of maleic anhydride component in the SMA copolymer.
A suitable amount is 5% by weight or less, and more preferably 1 to 3% by weight. In the above method, 10 to 25 parts by weight of maleic anhydride is subsequently added continuously to 100 parts by weight of the mixture of maleic anhydride and vinyl aromatic compound monomer initially present in the polymerization system. is preferable. The amount of maleic anhydride added later is determined by the desired amount of maleic anhydride component in the SMA copolymer, but usually the content of maleic anhydride component in the SMA copolymer is 20 to 20%. Since it is selected from the range of 35% by weight, the above addition amount is appropriate. The polymerization reaction is preferably carried out by heating to 100 to 130°C for thermal polymerization. In addition, the rate of addition of maleic anhydride to the polymerization system is high at the beginning of polymerization when the viscosity of the polymerization system is low, and as the polymerization progresses and the viscosity of the system increases.
It is preferable to decrease it in proportion to the amount of SMA copolymer compression. In this way, heat removal becomes easy and the polymerization time can be further shortened. Note that a part of the vinyl aromatic compound monomer initially present in the polymerization system in the first step of the present invention may be replaced by acrylic acid alkyl ester, methacrylic acid ester such as methyl methacrylate, or ethyl methacrylate, acrylonitrile, It can be replaced with a monomer copolymerizable with maleic anhydride and vinyl aromatic compound monomers, such as methacrylonitrile. The syrup obtained in the first step of the present invention contains 35 to 60 unreacted vinyl aromatic compound monomers.
% by weight, and the remainder is an SMA copolymer containing 20 to 35% by weight of maleic anhydride component. In the second step of the present invention, a vinyl cyanide compound monomer is added to and mixed with the syrup obtained in the first step, and then the mixture with the syrup monomer is suspended in water. By copolymerizing the monomers contained in the syrup and imidizing the maleic anhydride residues contained in the copolymer chain obtained in the first step with an aromatic amine, N- A copolymer consisting of an aromatic maleimide, a vinyl aromatic compound and a vinyl cyanide compound is produced. In this second step, first, a vinyl cyanide compound monomer is added to the syrup obtained in the first step, and the mixture is stirred at a temperature of 70 to 130°C for 5 to 60 minutes.
In this second step, an aromatic amine is added to the polymerization system, and the time of addition of this may be at the same time as the addition of the vinyl cyanide compound monomer,
It may be added after the polymerization mixture is suspended in water. Examples of the vinyl cyanide compound monomer in the present invention include acrylonitrile and methacrylonitrile. These may be one type or a mixture of two types. As the aromatic amine in the second step of the present invention, aniline is industrially easily available and is most suitable, but examples include halogenated aniline, nitroaniline, toluidine, α-naphthylamine, and phenylenediamine. These may be one type or a mixture of two or more types. In the second step, the amount of the vinyl cyanide compound monomer to be added and mixed is 10 to 25 parts by weight, assuming that the total amount of the vinyl aromatic compound monomer added to the polymerization system in the first step is 100 parts by weight. Appropriate. Also,
The amount of aromatic amine added is in molar ratio to the total amount of maleic anhydride added in the first step.
The amount is preferably 0.8 to 1.5 times. Adding aromatic amines, especially aniline,
It reacts with maleic anhydride residues contained in the SMA copolymer to produce a monoamide. Subsequently, the polymerization system is suspended in water. In this case, it is preferable to add a suspending agent and a polymerization initiator to the polymerization mixture. Examples of suspending agents include polyvinyl alcohol, polyacrylamide, barium sulfate, and the like. Examples of the polymerization initiator include those commonly used in suspension polymerization, such as azobisisobutyronitrile and benzoyl peroxide. The suspension polymerization reaction is carried out at a temperature of 60 to 120°C for 1 to 5 hours. Then a slightly higher temperature,
The temperature is raised to 120 to 190°C to carry out the N-aromatic maleimidation reaction. In this case, the higher the temperature is, the higher the imidization rate becomes. For the imidization reaction,
As described in JP 47-24024, when a basic substance such as triethylamine is used as a catalyst, the imidization rate can be improved at a relatively low temperature. The higher the amount of aromatic amine added, the better the imidization rate will be, but since the treatment of the excess amine after the reaction is a problem, the above range is appropriate. According to the method of the present invention, 60 mo% or more of the maleic anhydride residues contained in the SMA copolymer are imidized. The copolymer obtained by the method of the present invention can be used as it is as a raw material for producing molded articles. In addition, impact resistance can be improved by blending with other resins such as ABBC resin and AS resin. moreover,
Reinforcing agents such as glass fiber, carbon fiber, talc, calcium carbonate, fillers, lubricants of type and amount that do not impede the properties of the resin, mold release agents, coloring agents, ultraviolet absorbers, light stabilizers, stable heat resistance. Other additives such as flame retardants and flame retardants can be added. The copolymer and its resin composition obtained by the method of the present invention can be made into desired molded products by various processing methods such as injection molding, extrusion molding, and compression molding, and have excellent heat resistance and high temperature stability. It can be used in applications where high performance is required. "Effects of the Invention" The present invention has been described above, and has the following special and remarkable effects, and its industrial utility value is extremely large. (1) The copolymer obtained by the method of the present invention has excellent heat resistance. (2) The copolymer obtained by the method of the present invention has 270
Even if heated for 1 hour at a temperature of ℃, foaming, weight loss,
No discoloration or crosslinking occurs, and exhibits excellent high-temperature stability. (3) Copolymer and ABS obtained by the method of the present invention
The resin composition with resin etc. does not cause foaming, weight loss, crosslinking, etc. even when heated at a temperature of 270℃ for a period of time, and exhibits excellent high temperature stability, and the molded product does not become foamed during molding. Molded products with excellent appearance can be obtained. "Examples" Next, the present invention will be specifically explained based on Examples and Comparative Examples, but the present invention is not limited to the following examples unless it exceeds the gist thereof. Example 1 (1st step) 296 g of styrene was charged into a 2L autoclave equipped with a stirring blade, a raw material auxiliary additive addition device, and a nitrogen gas supply port, and while stirring and purging with nitrogen gas, the autoclave was heated from the outside to bring the internal temperature down. was heated to 100°C. A monomer mixture consisting of 280 g of styrene and 120 g of maleic anhydride was continuously added to this polymerization system at a constant rate of about 1.48 g/min over 4 hours and 30 minutes. After starting the continuous addition, the temperature of the polymerization system was increased from 100°C to 115°C in 15 minutes. Thereafter, while maintaining the temperature at 115° C. and continuously adding, bulk polymerization was continued to complete the first step. In the contents of the autoclave after completing the first step, the polymerization rate of the charged monomer mixture was 54% by weight, and the content of the maleic anhydride component of the SMA copolymer in the produced syrup was 30% by weight. (Second step) Next, add 104 g of acrylonitrile to the syrup obtained in the first step above and stir to bring the temperature of the system up.
The temperature was lowered to 95°C, a mixture of 108 g of aniline and 24 g of triethylamine was added, and the mixture was stirred at 95°C for 10 minutes. Add 0.3% polyvinyl alcohol suspending agent to this system.
g, 650 g of water containing 0.3 g of polyacrylic acid ester suspension agent and 3 g of sodium sulfate,
The syrup was suspended. To this, 0.4 g of azobisisobutyronitrile (hereinafter abbreviated as AIBN) was added to restart the polymerization, and after restarting the polymerization at a temperature of 80°C, it was held for 90 minutes, and then 155 The temperature was raised to ℃. After carrying out suspension polymerization and imidization reaction at this temperature for 120 minutes, the second step was completed and the polymerization system was cooled to room temperature. Subsequently, the obtained bead-like polymer was separated from the water with a cloth, washed with water, and dried. The yield of bead-like polymer was 840 g. The bead-like polymer was kneaded into pellets using a vented 1-inch extruder while devolatilizing. As a result of analysis, the composition of the pellets was determined to be 56 parts by weight of a copolymer in which 87 mol% of the maleic anhydride residues of the SMA copolymer with a maleic anhydride component content of 30% by weight were converted to N-phenylmaleimide; Acrylonitrile content
25% by weight styrene-acrylonitrile copolymer
It was a mixture with 44 parts by weight. (Evaluation of various physical properties) Test pieces for measuring physical properties were prepared from the pellets by injection molding, and tensile strength, Izot impact strength,
Vikato softening point measurements and gear oven tests were carried out. The results are shown in Table 1. Example 2 (First Step) The first step was carried out in the same 21 autoclave as described in Example 1 according to the same procedure as in the same example. (Second step) 104 g of acrylonitrile was added to the syrup obtained in the first step, and the mixture was stirred at 95° C. for 10 minutes. Next, 0.3g of polyvinyl alcohol suspension agent,
650 g of water containing 0.3 g of a polyacrylic acid ester suspending agent and 3 g of sodium sulfate were added to suspend the syrup and monomer mixture prepared in the above step in water. A mixture of 108g of aniline and 24g of triethylamine was added to this, and further AIBN0.4
g was added and held at a temperature of 80°C for 90 minutes, followed by raising the temperature to 155°C over 60 minutes. Suspension polymerization and imidization reaction were carried out at this temperature for 120 minutes to complete the second step, and the temperature of the polymerization system was cooled to room temperature. 850 g of bead-like polymer was obtained. continue,
Pelletization was performed in the same manner as in Example 1.
The composition of the pellets is that the maleic anhydride component content is 30
Weight% maleic anhydride residues in SMA copolymer 86
55 parts by weight of a copolymer with mol% of N-phenylmaleimide and 25 parts by weight of acrylonitrile content.
It was a mixture with 45 parts by weight of styrene-acrylonitrile copolymer. (Evaluation of various physical properties) Various physical properties of the obtained pellets were measured in the same manner as in Example 1. The results are shown in Table 1. Example 3 The first step was completed in the same manner as in Example 2, and then the second step was started and acrylonitrile was added.
A syrup containing 104 g was produced. Next, 0.3 g of a polyvinyl alcohol suspending agent and 0.3 g of a polyacrylic acid ester suspending agent were added to this system.
to suspend the syrup. 0.4 g of AIBN was added thereto, held at a temperature of 80°C for 90 minutes, and then suspended polymerized while raising the temperature to 155°C over 60 minutes. In this system,
A mixture of 108 g of aniline and 24 g of triethylamine was added, and imidization reaction and suspension polymerization were carried out at a temperature of 155° C. for 120 minutes to complete the second step, and the temperature of the polymerization system was cooled to room temperature. Subsequently, the obtained bead-like polymer was separated from the water with a cloth, washed with water, and dried. The yield of bead-like polymer was 850 g. The composition of the polymer was almost the same as that of the copolymer obtained in Example 2. Using a test piece prepared in the same manner as in Example 1, various physical properties were measured. The results are shown in Table 1. Comparative Example 1 A syrup containing 104 g of acrylonitrile was obtained in the same manner as in Example 2. Add 0.3% polyvinyl alcohol suspending agent to this system.
The syrup was suspended by adding 650 g of water containing g, 0.3 g of a polyacrylic acid ester suspending agent, and 3 g of sodium sulfate. Add 0.4g of AIBN to this,
Hold at a temperature of 80℃ for 90 minutes, followed by 60 minutes
After the temperature was raised to 155°C and suspension polymerization was carried out at this temperature for 120 minutes, the temperature of the polymerization system was immediately cooled to room temperature. 750 g of bead-like polymer was obtained. The beaded polymer was pelletized as in Example 1. The composition of the obtained pellets was a mixture of 52 parts by weight of SMA copolymer containing 30% by weight of maleic anhydride component and 48 parts by weight of styrene-acrylonitrile copolymer containing 25% by weight of acrylonitrile. Regarding this pellet, various physical properties were measured in the same manner as in Example 1. The results are shown in Table 1. Comparative Example 2 The physical properties of a commercially available styrene/acrylonitrile copolymer (SMA-C, manufactured by Mitsubishi Monsanto Chemical Co., Ltd.) were evaluated, and the results are shown in Table 1.

[Table] Example 4 (First step) 576 g of styrene and 15.4 g of maleic anhydride were placed in a 2-liter autoclave, and the temperature was raised to 115° C. while purging with nitrogen while stirring. To this system, 104.6 g of maleic anhydride kept at 60° C. was continuously added according to the addition rate shown in Table 2, and bulk polymerization was carried out at a temperature of 115° C. for 186 minutes to complete the first step. At the end of the first step, the polymerization rate was 54% by weight, and the maleic anhydride component content of the SMA copolymer in the resulting syrup was 30% by weight.

[Table] (Second step) Add 104 g of acrylonitrile to the syrup obtained in the first step, stir, and lower the temperature of the system to 95°C.
Further, a mixture of 108 g of aniline and 24 g of triethylamine was added, and the mixture was stirred at a temperature of 95° C. for 10 minutes. Next, 0.3g of polyvinyl alcohol suspension agent,
A syrup containing acrylonitrile, aniline, etc. was suspended by adding 650 g of water containing 0.3 g of a polyacrylic acid ester suspending agent and 3 g of sodium sulfate. Add 0.4g of AIBN to this and heat to 80℃.
temperature for 90 minutes, followed by 60 minutes at 155°C.
After the second step was completed, the temperature of the polymerization system was cooled to room temperature. Separate the resulting bead-like polymer from the water with a cloth,
Washed with water and dried. The yield of bead-like polymer is
It was 840g. The bead-like polymer was kneaded in a vented 1-inch extruder while devolatilizing to form pellets. As a result of analysis, the composition of the pellet was found to be 56 polymerized parts of a copolymer in which 87 mol% of the maleic anhydride residues of the SMA copolymer with a maleic anhydride component content of 30% by weight were converted to N-phenylmaleimide, It was a mixture with 44 parts by weight of a styrene/acrylonitrile copolymer containing 25% by weight of acrylonitrile. (Evaluation of various physical properties) A test piece was prepared from the above pellet using a 1-ounce injection molding machine, and various physical properties were evaluated. The results are shown in Table 3. Example 5 (First step) A syrup containing an SMA copolymer was produced in the same manner as in Example 4. (Second Step) 104 g of acrylonitrile was added to the above syrup, and the mixture was stirred for 10 minutes while keeping the temperature of the system at 95°C. Add 0.3% polyvinyl alcohol suspending agent to this system.
The syrup and monomer mixture prepared in the above step was suspended by adding 650 g of water containing g, 0.3 g of a polyacrylic acid ester suspending agent and 3 g of sodium sulfate. To this, add a mixture of 108 g of aniline and 24 g of triethylamine, and further add 0.4 g of AIBN.
was added and kept at a temperature of 80 °C for 90 min, followed by
The temperature was raised to 155°C over 60 minutes, and suspension polymerization and imidization reaction were carried out at this temperature for 120 minutes to complete the second step, and then the temperature of the polymerization system was cooled to room temperature. The yield of bead-shaped polymers separated from water, washed with water, and dried in the same manner as in Example 4 was 850
It was hot at g. Pelletization was carried out in the same manner as in Example 4. The composition of the pellets is 55 parts by weight of a copolymer in which 86 mol% of the maleic anhydride residues of a SMA copolymer with a maleic anhydride component content of 30% by weight are N-phenylmaleimide, and an acrylonitrile content of 25% by weight. It was a mixture with 45 parts by weight of styrene-acrylonitrile copolymer. (Evaluation of various physical properties) Various physical properties of the obtained pellets were measured in the same manner as in Example 4. The results are shown in Table 3. Example 6 A syrup consisting of an SMA copolymer containing 104 g of acrylonitrile was obtained in the same manner as in Example 5. Add to this 0.3g of polyvinyl alcohol suspending agent,
The above syrup was suspended by adding 650 g of water containing 0.3 g of a polyacrylic acid ester suspending agent and 3 g of sodium sulfate. Add 0.4g of AIBN to this,
Hold at a temperature of 80℃ for 90 minutes, followed by 60 minutes
Suspension polymerization was carried out while raising the temperature to 155°C. In this system,
A mixture of 108 g of aniline and 24 g of triethylamine was added, and imidization reaction and suspension polymerization were carried out at a temperature of 155° C. for 120 minutes, after which the temperature of the polymerization system was immediately cooled to room temperature. The yield of the bead-like polymer obtained by separating it from water, washing it with water, and drying it in the same manner as in Example 4 was 850 g. The bead-shaped polymer was pelletized in the same manner as in Example 4. The composition of the pellets was almost the same as that of the copolymer obtained in Example 5. Table 3 shows the results of measuring various physical properties of the pellets obtained. Comparative Example 3 Acrylonitrile was prepared in exactly the same manner as in Example 2.
A syrup containing 104 g was obtained. Next, 0.3g of polyvinyl alcohol suspension agent,
The syrup was suspended in water by adding 650 g of water containing 0.3 g of a polyacrylate suspending agent and 3 g of sodium sulfate. 0.4 g of AIBN was added thereto, held at a temperature of 80°C for 90 minutes, then heated to 155°C over 60 minutes, and suspension polymerization was carried out at this temperature for 120 minutes. Immediately after the polymerization was completed, the temperature of the polymerization system was cooled to room temperature. The yield of the bead-shaped polymer obtained by post-treatment in the same manner as in Example 4 was 750 g. The above beaded polymer was pelletized as in Example 4. The composition of the pellets is
It was a mixture of 52 parts by weight of an SMA copolymer having a maleic anhydride component content of 30% by weight and 48 parts by weight of a styrene-acrylonitrile copolymer having a 25% by weight acrylonitrile content. Various physical properties of the obtained pellets were measured. The results are shown in Table 3.

【table】

[Table] From Tables 1 and 3, the following becomes clear. (1) The copolymers produced by the method of the present invention have a high Vikato softening point and excellent heat resistance (see Examples 1 to 6). On the other hand, copolymers that deviate from the method of the present invention have low Vikato softening points and poor heat resistance (see Comparative Example 2). (2) The copolymer produced by the method of the present invention has 270
Even when heated for 1 hour at a temperature of .degree. C., no foaming or weight loss was observed, indicating excellent high-temperature stability (see Examples 1 to 6). On the other hand, copolymers that deviate from the method of the present invention have poor high temperature stability, and foaming and weight loss are observed (see Comparative Examples 1 and 3).

Claims (1)

[Scope of Claims] 1. In producing a copolymer consisting of N-aromatic maleimide, a vinyl aromatic compound, and a vinyl cyanide compound by a bulk-suspension polymerization method, maleic anhydride and vinyl aromatic compound monomers and A first step of copolymerizing in bulk to obtain a syrup containing a mixture of the copolymer and unreacted monomers; Adding and mixing a vinyl cyanide compound monomer to the syrup obtained in the first step, Next, the mixture of the syrup and monomers is suspended in water, and the monomers contained in the syrup are copolymerized in a suspended state, and the monomers contained in the copolymer chains obtained in the first step are copolymerized. a second step of imidizing maleic anhydride residues with an aromatic amine. Method. 2. In the first step, copolymerize while maintaining the proportion of maleic anhydride in the monomer mixture of vinyl aromatic compound monomer and maleic anhydride present in the polymerization system at 5% by weight or less. A method for producing a copolymer comprising an N-aromatic maleimide, a vinyl aromatic compound, and a vinyl cyanide compound according to claim 1, characterized by: