JP3104049B2 - Method for purifying polymer solution and method for producing polymer - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for purifying an acrylonitrile-based polymer solution and a method for producing a polymer from which impurities such as iron and aluminum have been removed.

[0002]

2. Description of the Related Art Acrylic fiber has properties such as excellent bulkiness, feeling, and clearness of dyeing similar to wool, and is used for a wide range of applications. This acrylic fiber is spun by a spinning method such as a wet method, a dry method, or a dry-wet method through a dissolving step of dissolving an acrylonitrile-based polymer as a raw material in an organic solvent or an inorganic solvent, and becomes a staple or a filament. It has been commercialized. The acrylonitrile-based polymer as a raw material is generally produced by a radical polymerization reaction of an acrylonitrile monomer and a monomer copolymerizable therewith (“Textile Handbook, 1
991 edition "). Further, the acrylonitrile-based fiber obtained by the above method is carbonized into a carbon fiber through a firing step, and this carbon fiber has high performance such as strength, elastic modulus, heat resistance, etc. It is used for various applications.

[0003] The suspension polymerization method, which is a heterogeneous polymerization in an aqueous system, is a method for producing an acrylonitrile-based polymer which has a long history and is widely used. It has advantages such as easy recovery of the monomer and easy management of the whole process (issued by Maruzen Co., Ltd., edited by the Textile Society of Japan, “Textile Handbook, Raw Materials”).

[0004] In the suspension polymerization, a monomer containing an acrylonitrile monomer as a main component is suspended and dispersed in an acidic water such as sulfuric acid and a polymerization reaction is carried out using a polymerization initiator. Polymer particles having a size of about several + micron are formed by the reaction, and an acrylonitrile-based polymer can be obtained in an aqueous dispersion state. After terminating the polymerization by adding a polymerization terminator, the acrylonitrile polymer is obtained by filtering, washing and drying the aqueous dispersion of the polymer.

As a polymerization initiator, an inorganic redox initiator containing iron (for example, a combination of ammonium persulfate-ammonium hydrogen sulfite-ferrous sulfate) has been proposed and used from the viewpoint of polymerization stability.

When a stainless steel or glass lining is used as a polymerization vessel, an acrylonitrile-based polymer adheres to the polymerization vessel, making continuous polymerization substantially impossible. Are proposed and used. That is, it is said that since the inside of the reaction system is an acidic aqueous solution, when aluminum is used as the polymerization vessel, the aluminum surface is corroded and dissolved, thereby preventing scale formation.

As the polymerization terminator, an aqueous solution of an electrolyte such as sodium oxalate, sodium salt of ethylenediaminetetraacetate or sodium bicarbonate is used in consideration of the neutralizing function of the acidic aqueous solution of the reaction system.

[0008]

The particles of the acrylonitrile-based polymer thus obtained are provided with a polymerization initiator,
An electrolyte derived from the polymerization reaction vessel and the polymerization terminator is mixed. In such a polymerization system, the terminal of the acrylonitrile polymer is ionized, and the electrolyte as an impurity is bonded to the polymer as a counter ion. Therefore, it is necessary to wash the polymer particles with water to remove the electrolyte, but even though the surface of the polymer particles can be washed, it is difficult to remove the electrolyte trapped inside the polymer. .

That is, in the conventional production of an acrylonitrile-based polymer by aqueous suspension polymerization, the polymer was transferred to the next drying step with insufficient washing. And since this polymer was dissolved in a solvent to form a polymer solution, which was wet-spun or semi-dry-wet spinning to produce fibers,
There have been various problems due to the electrolyte.

For example, when a dope solution used for spinning is heated and held at about 100 ° C. for a long time, yellow coloring becomes remarkable, and the dope solution itself gels and the fluidity decreases. And because of this, stable spinning could not be secured,
The yarn breakage phenomenon occurred. In addition, the acrylonitrile fiber produced itself also had a yellow coloring phenomenon, and the quality of the fiber was degraded. In addition, the pre-curable fiber is a raw material of the carbon fiber.
When the precursor was used, fluff was generated in the carbonization step of the precursor, and the strength development of the carbon fiber was inhibited.

An object of the present invention is to solve the problems of the aqueous suspension polymerization method and to provide an acrylonitrile polymer solution suitable for producing acrylic fibers.

[0012]

The gist of the present invention is that an acrylonitrile-based monomer is obtained by subjecting an acrylonitrile-based monomer to aqueous suspension polymerization using an iron-containing inorganic redox initiator in an aluminum polymerization vessel. Purification of a polymer solution comprising producing a polymer, and then contacting the polymer solution obtained by dissolving the polymer in a solvent with an ion-exchange substance to remove an iron compound or its ions and an aluminum compound or its ions. In the way. The gist of the present invention is a method for producing an acrylonitrile-based polymer in which the content of the iron compound or its ion and the content of the aluminum compound or its ion are reduced by the above-mentioned method.

Hereinafter, the present invention will be described in detail. The present invention
It is intended for an acrylonitrile-based polymer obtained by aqueous suspension polymerization using an iron-containing inorganic redox initiator. The inorganic redox initiator can be selected from ordinary oxidizing agents and reducing agents. Representative oxidizing agents include ammonium persulfate, potassium persulfate, sodium persulfate and the like. Further, as a reducing agent, sodium sulfite, ammonium sulfite, sodium bisulfite, ammonium bisulfite, sodium thiosulfate,
Examples include ammonium thiosulfate, sodium dithionite, sodium formaldehyde sulfoxylate, L-alcorbic acid, dextrose and the like. In addition, ferrous sulfate or the like is used as a redox assistant. Among them, a combination of ammonium persulfate-sodium bisulfite (ammonium) -ferrous sulfate is preferred. Any ratio of reducing agent / oxidizing agent is possible, but in order to promote the polymerization more efficiently, the equivalent ratio of reducing agent / oxidizing agent should be 1 to 4
Is preferable.

The acrylonitrile polymer used in the present invention is obtained from an acrylonitrile monomer or a monoolefinic monomer copolymerizable with the acrylonitrile monomer. Here, the acrylonitrile-based polymer preferably contains at least 60% by weight of an acrylonitrile monomer component. If the content of the acrylonitrile monomer is less than 60% by weight, the acrylonitrile-based synthetic fiber cannot have the inherent fiber function. Examples of the copolymerizable monoolefinic monomer include acrylic acid, methacrylic acid and esters thereof, acrylamide, vinyl acetate, styrene, vinyl chloride, vinylidene chloride, maleic anhydride, N-substituted maleimide, and butadiene. , Isoprene and the like. Further, P-sulfonylmethallyl ether, methallylsulfonic acid, allylsulfonic acid, styrenesulfonic acid, 2-acrylamido-2-methylpropanesulfonic acid, 2-sulfoethylmethacrylate and salts thereof are also copolymerized. Can be used as a possible monomer.

The polymerization of the acrylonitrile monomer is carried out as follows. That is, the polymerization reaction temperature is 30 to 80.
C. is preferred. If the polymerization temperature exceeds 80 ° C., acrylonitrile evaporates and separates out of the reaction system, lowering the polymerization conversion rate. On the other hand, when the temperature is lower than 30 ° C., the polymerization rate is lowered, and not only the productivity is lowered but also the polymerization stability is impaired. As the water as the polymerization medium, it is preferable to use ion-exchanged water. Further, the ratio of ion-exchanged water to the monomer (hereinafter, referred to as water / monomer ratio) may be any ratio, but is preferably in the range of 1.0 to 5.0. The average residence time of the monomer in the polymerization reactor is
It may be the usual time employed when producing an acrylonitrile-based polymer by an aqueous suspension polymerization method. The hydrogen ion concentration in the polymerization reactor may be within a range in which the catalyst used causes an oxidation / reduction reaction promptly.
An acidic region of 0 to 3.5 is preferred.

The reaction is stopped by adding a polymerization terminator to the aqueous polymer solution taken out of the polymerization vessel. There is no problem with the terminator of the polymerization reaction as long as it is generally used when producing an acrylonitrile-based polymer by aqueous suspension polymerization. After the addition of the polymerization terminator, the unreacted monomer is recovered from the aqueous polymer solution. As a method for recovering the unreacted monomer, there is a method of directly distilling the aqueous polymer solution, or a method of once dehydrating and separating the unreacted monomer from the polymer and then distilling, and both methods can be adopted. As the dehydration washer in the latter, a rotary vacuum filter, a centrifugal dehydrator, or the like, which is a generally known filtration dehydrator, is used. In separating the polymer from the aqueous polymer solution using these devices, a coagulant such as ammonium sulfate, aluminum sulfate, sodium sulfate or the like is added in order to perform the polymerization more efficiently, and the polymer is used in the sense of promoting the aggregation of the polymer. Operations such as raising the temperature of the aqueous solution can also be performed. The water remaining in the polymer is removed by a usual drying method.

The acrylonitrile-based polymer obtained as described above is dissolved in a solvent. Examples of the solvent include dimethylformamide, dimethylacetamide, dimethylsulfoxide and the like. The solution concentration is not limited,
5-35% by weight of acrylonitrile-based polymer and solvent 95-
It is preferred to have a solution consisting of 65% by weight. If the amount of the polymer is less than 5% by weight, satisfactory spinnability cannot be secured because the concentration of the polymer is too low, and if it exceeds 35% by weight, the viscosity of the polymer solution becomes too high. This is because spinnability cannot be ensured.

In the present invention, an iron compound or its ion and an aluminum compound or its ion are to be removed. This is because an inorganic redox initiator containing iron is used at the time of polymerization, and an acidic aqueous solution is used. This is due to the fact that aluminum is corroded and dissolved using an aluminum reactor in the middle. That is, the above-mentioned metal or their ions are present as impurities in the formed acrylonitrile-based polymer and the polymer solution obtained by dissolving the same in a solvent.

These impurities in the polymer solution are removed and purified using an ion exchange material. The iron compound or its ion in the purified polymer is preferably at most 3 ppm, more preferably at most 1 ppm.
The aluminum compound or its ion is 10p
pm or less, and more preferably 5 ppm or less.

If the iron compound or its ion as an impurity exceeds 3 ppm and the aluminum compound or its ion exceeds 10 ppm, the thermal stability of the polymer solution may be reduced. And the decrease in thermal stability of the polymer solution
As described above, this has various adverse effects on the production process of the acrylic fiber and has a great adverse effect on the quality of the acrylic fiber.

Examples of the ion-exchange substance include a cross-linkable ion-exchange resin insoluble in an organic solvent and a cross-linkable ion-exchange fiber. Among them, a cross-linkable ion-exchange resin composed of divinylbenzene-styrene is generally used. Is preferable. The ion exchange resin must have an affinity for the organic solvent in order to make the acrylonitrile polymer in a dope solution in an organic solvent and to purify and remove impurities from the polymer with a crosslinked ion exchange resin. And it should be insoluble in the organic solvent.

Examples of the functional group having an ion exchange ability include a strong cation exchange resin having a sulfonic acid group. When the ionic species purified by ion exchange are both cation and anion species, use both a strong cation exchange resin having a sulfonic acid group and a strong anion exchange resin having a quaternary ammonium group. I do. To capture heavy metals, the iminediacetate type and polyamine type
Resin. Further, a weak cation exchange resin composed of methacrylic acid and acrylic acid, a weak anion exchange resin composed of primary, secondary, and tertiary amines, and the like can be given. Further, both the weak cation exchange resin and the weak anion exchange resin can be used. Among them, it is particularly preferable to use a strong cation exchange resin having a sulfonic acid group, or both a strong cation exchange resin having a sulfonic acid group and a strong anion exchange resin having a quaternary ammonium group.

The ion exchange resin is preferably used in an amount of 0.01 to 100% by weight based on 100% by weight of the polymer solution. When the amount is less than 0.01% by weight, the ion exchange capacity is significantly reduced, and is not practical. If it exceeds 100% by weight, it is not practical because it is economically disadvantageous and it takes more time than necessary to recover the ion exchange resin.

For purifying the polymer solution with an ion exchange resin, either a tubular reactor or a tank reactor may be used. It is practical to fill a tubular reactor with an ion exchange resin, continuously supply a polymer solution into the resin, and pass the polymer solution while contacting to purify the polymer solution. Purification of the polymer solution with an ion exchange resin is performed in a temperature range of 15 to 150 ° C. If the temperature is lower than 15 ° C., the viscosity of the polymer solution increases, and the polymer solution is inferior in fluidity, so that the ion exchange reaction hardly proceeds. On the other hand, if the temperature exceeds 150 ° C., the thermal stability of the ion exchange resin used is insufficient, so that it cannot be used for a long time.

When an acrylonitrile-based polymer is produced by aqueous suspension polymerization, a glass container is used, and when a metal compound such as an iron compound is not used as a polymerization aid, an impurity as an impurity is contained in the polymer. The iron compound or its ion, the aluminum compound or its ion are substantially absent. However, in practice, it is impossible to carry out aqueous suspension continuous polymerization by this method, and therefore, only the examination in a laboratory is performed. However, the thermal stability of the polymer solution obtained by the above method is good. there were.

[0026]

The present invention will be described below in detail with reference to examples. Parts and% represent parts by weight and% by weight. In the examples, each physical property was measured by the following method. (1) Measurement of polymerization conversion: An aqueous dispersion after polymerization was collected, and the polymer was collected by filtration to obtain a polymer yield, which was defined as a polymerization conversion. (2) Measurement of specific viscosity of polymer: A polymer having a concentration of 0.5 g was dissolved in 100 ml of dimethylformamide,
At C, the solution viscosity (Ubbel-de-type viscometer) was measured. (3) Measurement of sodium ion content: The polymer and the fiber were burned, and the ash content was measured by flame measurement.

(4) Measurement of the amount of ammonium ion: The polymer and the fiber were heated with water to extract ammonium ion, which was measured by a colorimetric method. (5) Iron and aluminum content measurement: The polymer and fiber were burned, and the ash content was measured by atomic absorption spectrometry. (6) Measurement of polymer solution viscosity: Using a dope having the indicated polymer concentration, the ball drop time was measured in an oil bath at 85 ° C., and the solution viscosity was measured. Changes over time were similarly performed. (7) Measurement of coloration of polymer solution: After heating to 85 ° C in an oil bath at 85 ° C using a dope having the indicated polymer concentration, the coloration was immediately visually evaluated. (8) Measurement of polymer composition: The polymer composition was determined using a proton NMR measuring instrument.

EXAMPLE 1 Acrylic nitrile 98.5 was charged with 35 L of ion-exchanged water (set to pH = 3) in a polymerization reactor having a capacity of 80 L with stirring (the reaction vessel was an aluminum vessel and the stirring blade was an aluminum-coated turbine type). Parts, 1.5 parts of methacrylic acid,
1.5 part of ammonium persulfate, 4.5 parts of ammonium bisulfite, ferrous sulfate _{(FeSO 4 · 7H 2 0)} 0.00
005 parts and 0.085 parts of sulfuric acid were dissolved in ion-exchanged water, respectively, and supply was started continuously. Further, ion-exchanged water was separately supplied so that the total amount of ion-exchanged water was 400 parts. The polymerization temperature was maintained at 60 ° C., sufficient stirring was carried out, and the average reaction time was 80 minutes to continuously supply the raw materials to carry out the polymerization reaction.

A polymer aqueous dispersion was continuously taken out from the overflow port of the reactor, and 0.5 part of ammonium oxalate was added thereto.
An aqueous solution of a polymerization terminator prepared by dissolving 1.5 parts of ammonium bicarbonate in 100 parts of ion-exchanged water was added at a ratio of 0.2% to the aqueous polymer dispersion, and further added 10 times to the aqueous polymer dispersion. After adding an amount of ion-exchanged water, the unreacted monomer and the residue of the excessive polymerization aid were removed and washed by a rotary vacuum filter. The obtained wet polymer was formed into a pellet by a screw type extruder, and then dried by a through-air drier to obtain an acrylonitrile polymer shown in Table 1.

22 parts of the above dried polymer was dissolved in 78 parts of dimethylformamide to form a polymer solution, and 100 parts of the polymer was added to a strong ion exchange resin having a sulfonic acid group [Diaion HPK55H Mitsubishi Chemical Co., Ltd.] (stock)
The mixture was mixed and stirred in a kneader at 80 ° C. for 30 minutes and then filtered through a 100-mesh wire net to remove the ion-exchange resin to obtain an acrylonitrile polymer solution shown in Table 1. The thermal stability of this acrylonitrile polymer solution was evaluated. The results are shown in Table 1. Using this acrylonitrile polymer solution, a width of 10 mm and a thickness of 0.1 mm were used.
A thin film was formed in a 55% aqueous solution of dimethylformamide (30 ° C.) through a 1 mm die, dried by heat setting at 115 ° C., and a film was obtained. The performance of the acrylonitrile polymer obtained by pulverizing this film was evaluated. The results are shown in Table 1.

Example 2 A polymer was obtained in the same manner as in Example 1, and 15 parts of this polymer was dissolved in 85 parts of dimethylacetamide to obtain an acrylonitrile polymer solution. Incorporation of 5 disks filled with a strong ion-exchange resin having a sulfonic acid group (Diaion PK228LH) (a stainless steel disk having a thickness of 5 mm and a diameter of 50 mm, in which 50 through-holes having a diameter of 5 mm are arranged almost equally). The acrylonitrile-based polymer solution was fed to the polymer dope purification device at a rate of 150 g / min for purification to obtain an acrylonitrile-based polymer solution shown in Table 1. The distance between the disks was 20 mm, and the filling amount of the ion exchange resin in each disk was 10 mm.
The disk filled in 35 holes at the peripheral portion and the disk filled in 35 holes in the central portion were alternately arranged. Using this polymer solution, polymers shown in Table 1 were obtained in the same manner as in Example 1.

Example 3 Polymer dope purification incorporating 10 discs filled with a strong ion exchange resin having sulfonic acid groups (Amberlyst 15, manufactured by Rohm and Haas Company) in the same manner as in Example 2. The same acrylonitrile-based polymer solution as in Example 1 was fed to the apparatus at a rate of 80 g / min for purification, whereby an acrylonitrile-based polymer solution shown in Table 1 was obtained. Using this polymer solution, polymers shown in Table 1 were obtained in the same manner as in Example 1.

Example 4 As in Example 2, five disks having a sulfonic acid group-containing strong ion exchange resin (Diaion PK228LH) filled in 35 holes around the disk and having a quaternary ammonium group Example 2 A polymer dope purifying apparatus in which 10 g of a strong anion exchange resin (Diaion PA312) were alternately assembled with 5 disks filled with 35 holes at the center of the disk as in Example 2 was used. The same acrylonitrile-based polymer solution was supplied at a rate of 150 g / min and purified to obtain an acrylonitrile-based polymer solution shown in Table 1. Using this polymer solution, polymers shown in Table 1 were obtained in the same manner as in Example 1.

Example 5 In the same polymerization reactor as in Example 1, ion-exchanged water (pH = 3) was added.
35L) and acrylonitrile 98.5
Parts, sodium methallylsulfonate 1.5 parts, ammonium persulfate 0.5 parts, ammonium bisulfite 1.5 parts
Parts, ferrous sulfate _{(FeSO 4 · 7H 2 0)} 0.00002
Parts and sulfuric acid 0.27 parts were dissolved in ion-exchanged water, and the supply was started continuously. Further, ion-exchanged water was separately supplied so that the total amount of ion-exchanged water was 325 parts. The polymerization temperature was maintained at 55 ° C., sufficient stirring was carried out, and the average reaction time was 90 minutes to continuously supply the raw materials to carry out the polymerization reaction.

A polymer aqueous dispersion was continuously taken out from the overflow port of the reactor, and 0.5 part of sodium oxalate and 0.0 part of disodium ethylenediaminetetraacetate were added thereto.
7 parts and 1.5 parts of sodium bicarbonate dissolved in 100 parts of ion-exchanged water were added at a rate of 0.2% to the aqueous polymer dispersion and further added to the aqueous polymer dispersion. After adding 10 times the volume of ion-exchanged water, the unreacted monomer and excess polymerization aid residue were removed and washed with a rotary vacuum filter. The aqueous polymer solution from which unreacted monomers and excess polymerization aid were removed was dehydrated with a centrifugal dehydrator to obtain a wet polymer containing about 55% by weight of water.

The obtained wet polymer was formed into pellets by a screw type extruder, and dried by a through-air drier.
An acrylonitrile polymer shown in Table 1 was obtained. 22 parts of the dried polymer was dissolved in 78 parts of dimethylformamide to obtain a polymer solution, which was purified and the like in the same manner as in Example 2 to obtain a polymer solution and a polymer shown in Table 1.

Comparative Example 1 A polymer shown in Table 1 was obtained in the same manner as in Example 1 except that the ion exchange purification was not carried out.

Comparative Example 2 A polymer shown in Table 1 was obtained in the same manner as in Example 4 except that the ion exchange purification was not carried out.

Example 6 The amount of acrylonitrile charged into the polymerization reactor was 96.0.
Parts, acrylamide, 3.0 parts of 1.0 parts of itaconic acid, ammonium persulfate, 1.5 parts, 4.5 parts of ammonium bisulfite, ferrous sulfate _{(FeSO 4 · 7H 2 O)} 0.00
005 parts, sulfuric acid 0.085 parts, and the polymerization temperature is 55
C., and the other conditions were the same as in Example 1 to obtain a polymer. Next, a polymer solution was prepared by dissolving 22 parts of this polymer in 78 parts of dimethylacetamide, and was fed to the same polymer purification apparatus as in Example 2 at a rate of 200 g / min for purification. Further, a film was formed in the same manner as in Example 1, and this was pulverized to obtain a polymer.

Example 7 A polymer was obtained in the same manner as in Example 6 except that methacrylic acid was used instead of itaconic acid. A purified polymer solution and a purified polymer were obtained in the same manner as in Example 6, except that the solvent was changed to 78 parts of dimethylformamide.

Example 8 The amount of acrylonitrile charged into the polymerization reactor was 98.0.
Parts, itaconic acid 2.0 parts, ammonium persulfate 1.5
Part, ammonium bisulfite 4.5 parts, ferrous sulfate (F
eSO ₄ · 7H ₂ O) 0.00005 parts of sulfuric 0.085
And the polymerization temperature was 55 ° C., and the other conditions were the same as in Example 1 to obtain a polymer. Next, a polymer solution in which 23 parts of this polymer was dissolved in 77 parts of dimethylformamide was prepared, and 2 parts of the polymer were dissolved in the same polymer purification apparatus as in Example 2.
The solution was sent at a rate of 00 g / min for purification. Example 1
A film was formed in the same manner as described above, and this was pulverized to obtain a polymer.

Comparative Example 3 A polymer was obtained in the same manner as in Example 6, except that the ion exchange purification of the polymer solution was not performed.

Comparative Example 4 A polymer was obtained in the same manner as in Example 7, except that the ion exchange purification of the polymer solution was not carried out.

Comparative Example 5 A polymer was obtained in the same manner as in Example 8, except that the ion exchange purification of the polymer solution was not carried out.

[0045]

[Table 1]

[0046]

[Table 2]

As is clear from Tables 1 and 2, the acrylonitrile-based polymer obtained by the production method of the present invention has a low content of electrolytes as impurities, a distinctly low color tone, and this polymer solution Excellent heat stability.
In addition, the purification method of the present invention has an excellent effect of removing impurities.

[0048]

According to the present invention, it is possible to efficiently remove electrolytes, iron compounds or ions thereof and aluminum compounds or ions thereof as impurities contained in an acrylonitrile polymer produced by an aqueous suspension polymerization method. it can. In addition, an acrylonitrile-based polymer solution having good heat stability and less coloring can be prepared. From this polymer solution, high-quality acrylic fibers can be produced without causing thread breakage. Further, when this polymer is a raw material of carbon fiber, since high-quality acrylic fiber can be produced, generation of fluff can be suppressed in the carbonization step, and effective production of high-performance carbon fiber is possible. .

Continuing from the front page (72) Inventor Osamu Kato 20-1 Miyukicho, Otake City, Hiroshima Prefecture Mitsubishi Rayon Co., Ltd. Otake Works (72) Inventor Yoshio Manabe 20-1 Miyukicho, Otake City, Hiroshima Prefecture Mitsubishi Rayon Co., Ltd. (56) References JP 5-179508 (JP, A) JP 5-295614 (JP, A) JP 5-295621 (JP, A) (58) Fields studied (Int .Cl. ⁷ , DB name) C08F 6/06 C08F 20/44

Claims (2)

(57) [Claims] 1. An acrylonitrile-based polymer is produced by subjecting an acrylonitrile-based monomer to aqueous suspension polymerization using an iron-containing inorganic redox initiator in an aluminum polymerization vessel to produce an acrylonitrile-based polymer. A method for purifying a polymer solution, comprising contacting a polymer solution obtained by dissolving the compound in a solvent with an ion-exchange substance to remove an iron compound or its ion and an aluminum compound or its ion. 2. A method for producing an acrylonitrile polymer having a reduced content of an iron compound or an ion thereof and an aluminum compound or an ion thereof according to the method of claim 1.