KR20190064000A - Binder comprising random copolymer, anode for lithium-ion secondary battery comprising the same, lithium-ion secondary battery comprising the anode, and method for polymerizing the copolymer - Google Patents

Abstract

The present invention provides: a binder comprising a hydroxyalkyl (meth)acrylate monomer and a copolymer of monomers comprising at least one selected from a group consisting of a (meth)acrylic acid monomer and an alkali metal salt monomer of (meth)acrylic acid, wherein the copolymer does not contain a conjugated diene monomer, a fluorine-containing (meth) acrylic acid ester monomer, and a starch; a negative electrode for a lithium-ion secondary battery comprising the same; and the lithium-ion secondary battery comprising the negative electrode. In addition, the present invention provides a method for polymerizing the copolymer. An object of the present invention is to provide a negative electrode for the lithium-ion secondary battery in which cracking of an electrode is significantly reduced and durability is greatly improved by comprising the binder.

Description

TECHNICAL FIELD [0001] The present invention relates to a binder containing a random copolymer, a negative electrode for a lithium ion secondary battery containing the same, a lithium ion secondary battery including the negative electrode, and a polymerization method of the copolymer. THE SAME, LITHIUM-ION SECONDARY BATTERY COMPRISING THE ANODE, AND METHOD FOR POLYMERIZING THE COPOLYMER}

The present invention relates to a binder containing a random copolymer, a negative electrode for a lithium ion secondary battery comprising the same, a lithium ion secondary battery comprising the negative electrode, and a polymerization method of the copolymer.

Recently, interest in energy storage technology is increasing. As the application fields of cell phones, camcorders, notebook PCs and even electric vehicles are expanding, efforts for research and development of electrochemical devices are becoming more and more specified.

The electrochemical device is one of the most attracting attention in this respect, and the development of a rechargeable secondary battery is of particular interest. In recent years, research and development on the design of new electrodes and batteries have been proceeding in order to improve capacity density and specific energy in developing such batteries.

Among the currently applied secondary batteries, lithium secondary batteries are in the spotlight as they have higher operating voltages and energy densities than conventional batteries such as Ni-MH, Ni-Cd, and sulfuric acid-lead batteries using an aqueous electrolyte solution.

Such an electrochemical device generally includes a cathode, an anode, and a separator interposed between the cathode and the anode. At this time, the cathode and the anode are generally formed by a method of forming an electrode active material layer by applying an electrode active material, a polymeric binder, and an electrode active material slurry including a solvent that dissolves the polymeric binder and disperses the electrode active material, .

Among the electrode active materials, the anode active material is volumetrically expanded due to the occlusion and release of lithium. Particularly, when a silicon oxide (SiOx) active material is used as the anode active material, the volume expansion can be further increased.

However, extreme volume expansion (~ 300%) due to alloying / dealloying of Li ions during charge / discharge (~ 300%) promotes electrode degeneration, which is the biggest obstacle to the commercialization of Si materials . This volume change internally causes the fracture of the Si active material to expose a new interface, and the exposed interface creates a new SEI by side reaction with the electrolyte, thereby causing continuous Li ion and electrolyte consumption. In addition, the shattered Si pieces deviate from the original conductive structure and can not participate in charging / discharging, and cause degradation of life.

On the other hand, the binder has been recognized as a component capable of solving the above-mentioned problem of volume expansion as a material essential for the integrity of the electrode and the maintenance of the conductive structure of the active material.

However, the conventional binders (PAA, PAA / CMC, Na-PAA, crosslinked PAA, Alginate, PVA, etc.) It is difficult to expect.

Korean Patent Publication No. 10-2015-0049601

Disclosure of the Invention The present invention has been conceived to solve the above problems of the prior art, and it is an object of the present invention to provide a lithium secondary battery which is excellent in adhesion, effectively suppresses cracking of the electrode due to volume change of active materials such as Si, And to provide a binder which can exhibit the above-mentioned effects even in a state of high loading (> 4.5 mAh / cm < ² >).

It is another object of the present invention to provide a negative electrode for a lithium ion secondary battery in which the occurrence of cracks in the electrode is significantly reduced and durability is greatly improved by including the binder.

Further, the present invention provides a lithium ion secondary battery in which the generation of cracks in electrodes is significantly reduced by including the negative electrode, thereby improving durability, charge / discharge characteristics, and life characteristics.

Another object of the present invention is to provide a method for effectively polymerizing a random copolymer contained in the binder.

In order to achieve the above object,

Hydroxyalkyl (meth) acrylate monomers; And

(Meth) acrylic acid monomers, and alkali metal salt monomers of (meth) acrylic acid, wherein the copolymer comprises at least one monomer selected from the group consisting of

Wherein the copolymer does not contain a conjugated diene monomer, a fluorine-containing (meth) acrylic acid ester monomer, and a starch.

Further, according to the present invention,

There is provided a negative electrode for a lithium ion secondary battery, comprising a negative electrode active material, the binder of the present invention, and a conductive material.

Further, according to the present invention,

The negative electrode of the present invention; anode; A separator provided between the anode and the cathode; And an electrolyte.

Further, according to the present invention,

(a) a hydroxyalkyl (meth) acrylate monomer in a polymerization reactor; And a monomer and at least one monomer selected from the group consisting of a (meth) acrylic acid monomer and an alkali metal salt monomer of (meth) acrylic acid and a solvent, and sealing the inlet of the reactor;

(b) subjecting the reactant to nitrogen bubbling to remove oxygen; And

(c) a step of adding a polymerization initiator and a chain transfer agent and reacting the mixture, and a polymerization method of the copolymer for a binder of the present invention.

The binder of the present invention is excellent in adhesion, effectively inhibits cracking of the electrode due to volume change of active materials such as Si, improves the charge / discharge characteristics and lifetime characteristics of the battery, and improves the loading (> 4.5 mAh / cm ² ) The above-described characteristics are exhibited.

Further, the negative electrode for a lithium ion secondary battery of the present invention significantly reduces the occurrence of cracks in the electrode by including the binder, and provides an effect of greatly improving durability.

In addition, since the lithium ion secondary battery of the present invention includes the above-described negative electrode, the occurrence of cracks in the electrode is greatly reduced, thereby providing an effect of greatly improving the durability, charging / discharging characteristics, and lifetime characteristics of the battery.

In addition, the method for producing a copolymer of the present invention provides a method for effectively polymerizing a random copolymer contained in the binder.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram illustrating the structure of one embodiment among the random copolymers included in the binder of the present invention,
2 is a graph showing the initial performance of the lithium ion secondary battery using the conventional binder and the lithium ion secondary battery using the binder of the present invention,
3 is a graph showing lifetime characteristics of a lithium ion secondary battery using a conventional binder and a lithium ion secondary battery using the binder of the present invention, which were conducted in Test Example 1;

Hereinafter, the present invention will be described in detail.

The present invention relates to a composition comprising a hydroxyalkyl (meth) acrylate monomer; And

(Meth) acrylic acid monomers, and alkali metal salt monomers of (meth) acrylic acid, wherein the copolymer comprises at least one monomer selected from the group consisting of

Wherein the copolymer does not contain a conjugated diene monomer, a fluorine-containing (meth) acrylic acid ester monomer, and a starch.

In the case where the copolymer contains the conjugated diene monomer, the solubility of the binder in water, that is, the preparation of the aqueous binder solution, is not preferable. When the fluorine-containing (meth) acrylic acid ester monomer is contained, It is not preferable from the viewpoint of the total interlaminar adhesion, and when it contains starch, it is not preferable from the viewpoint of the viscosity of the slurry.

The hydroxyalkyl acrylate monomer; (Meth) acrylic acid monomers and alkali metal salt monomers of (meth) acrylic acid may be contained in a weight ratio of 5: 5 to 9: 1, more preferably 7: 3 to 8.5: 1.5, 2.5 to 8.3: 1.7 is even more preferable.

When the content range of the monomers satisfies the above-mentioned range, the bonding force between the active materials and between the active material and the current collector can be enhanced. On the other hand, if the content of the hydroxyalkyl (meth) acrylate monomer is deviated from the above-mentioned range, the bond between the active material and the active material and the current collector may be weakened, and when the content of one kind selected from alkali metal salt monomers of (meth) Is insufficient, the adhesion between the active material and the current collector may be weakened, which is not preferable.

The weight average molecular weight of the copolymer may be 200,000 to 10,000,000, preferably 500,000 to 8,000,000, and more preferably 1,000,000 to 5,000,000.

If the weight average molecular weight is less than the above-mentioned range, the adhesive force may be weakened, and if the above range is exceeded, the viscosity of the binder becomes too high.

The hydroxyalkyl (meth) acrylate monomer may be at least one selected from the group consisting of hydroxymethyl acrylate, hydroxymethyl methacrylate, hydroxyethyl acrylate, hydroxyethyl methacrylate, hydroxypropyl acrylate, hydroxypropyl methacrylate, Hydroxypropylmethacrylate, hydroxypropylmethacrylate, hydroxypropylmethacrylate, hydroxypropylmethacrylate, hydroxypropylmethacrylate, and the like.

The (meth) acrylic acid monomer may be at least one selected from the group consisting of acrylic acid, methacrylic acid,

The alkali metal salt monomer of (meth) acrylic acid may be at least one lithium, sodium, or potassium salt selected from the group consisting of acrylic acid, methacrylic acid, and the like.

The hydroxyalkyl (meth) acrylate monomer may have a glass transition temperature (Tg) of -100 ° C to 30 ° C, preferably -30 ° C to 0 ° C, Lt; 0 > C can be more preferably used.

When the glass transition temperature of the hydroxyalkyl (meth) acrylate monomer is in the above-mentioned range, the binder can be preferably adhered to the surface of the active material.

In addition, the (meth) acrylic acid monomer preferably has a glass transition temperature (Tg) of 80 to 130 ° C, more preferably 95 to 115 ° C.

When the glass transition temperature of the (meth) acrylic acid monomer satisfies the above-mentioned range, the effect of strengthening the binding force between the active material and the current collector can be obtained, which is preferable.

In the binder of the present invention, the copolymer may be in the form of a random copolymer.

The copolymer may be contained in an amount of 30 to 100% by weight based on the total weight of the binder. That is, the binder may be composed of a copolymer including a solvent together with a copolymer or a copolymer without a solvent.

As the solvent, those known in the art can be used as long as they can dissolve the copolymer. Examples of the known solvent include acetone, tetra hydrofuran, methylenechloride, chloroform, dimethylformamide, N-methyl-2-pyrrolidone (N- methyl-2-pyrrolidone (NMP), cyclohexane, and water. These solvents may be used alone or in combination of two or more. Particularly, water can be preferably used in consideration of environment and physical properties of the binder. As the water, distilled water may be used.

In the binder of the present invention, the copolymer may include a hydroxyalkyl (meth) acrylate monomer; And (meth) acrylic acid monomers and alkali metal salt monomers of (meth) acrylic acid.

The binder of the present invention is preferably an aqueous binder.

The binder may be used for manufacturing a lithium ion secondary battery. That is, the binder can be used for the manufacture of a negative electrode, the manufacture of a positive electrode and the like, and particularly, for the production of a negative electrode. Further, the negative active material may be preferably used for producing the negative electrode including the Si-based active material.

Further, according to the present invention,

(a) a hydroxyalkyl (meth) acrylate monomer in a polymerization reactor; And a monomer and at least one monomer selected from the group consisting of a (meth) acrylic acid monomer and an alkali metal salt monomer of (meth) acrylic acid and a solvent, and sealing the inlet of the reactor;

(b) subjecting the reactant to nitrogen bubbling to remove oxygen; And

(c) adding a polymerization initiator and a chain transfer agent and reacting the mixture.

The content described in the binder can be directly applied to the polymerization method of the copolymer for a binder of the present invention.

As the solvent in the step (a), known solvents capable of dissolving the monomers may be used. Examples of the known solvent include acetone, tetra hydrofuran, methylenechloride, chloroform, dimethylformamide, N-methyl-2-pyrrolidone (N- methyl-2-pyrrolidone (NMP), cyclohexane, and water. These solvents may be used alone or in combination of two or more. In particular, water can be preferably used.

In the step (a), a hydroxyalkyl acrylate monomer; And at least one selected from the group consisting of a (meth) acrylic acid monomer and an alkali metal salt monomer of (meth) acrylic acid is preferably added in a weight ratio of 5: 5 to 9: 1.

As the polymerization initiator and the chain transfer agent in the step (c), those conventionally used in this field may be used.

In the method for polymerizing the copolymer for a binder, a known method in this field can be applied, except for those specifically mentioned above.

The present invention also relates to a negative electrode for a lithium ion secondary battery, which comprises a negative electrode active material, the binder of the present invention, and a conductive material.

As the negative electrode active material, those well known in the art can be used. In particular, a negative active material including a Si-based active material can be preferably used. More specifically, for example, a mixture of SiOx / C and graphite can be used. The SiOx / C includes a carbon coating layer formed on the SiOx surface.

The negative electrode active material, the binder of the present invention, and the conductive material form a negative electrode active material layer. The negative electrode active material layer is prepared by preparing a slurry containing the above components according to a conventional method, applying the slurry to a current collector, And is included in the cathode.

The negative electrode active material may be included in an amount of 70 to 98% by weight based on the total weight of the negative electrode active material layer, the conductive material may be included in an amount of 1 to 20% by weight, and the binder may be included in an amount of 1 to 10% It is not.

The slurry for preparing the negative electrode active material layer may further contain a conventional dispersant.

The conductive material is not particularly limited, but conductive materials such as graphite materials such as KS6, carbon-based materials such as Super-P, denka black and carbon black, and conductive materials such as polyaniline, polythiophene, polyacetylene, and polypyrrole Can be used alone or in combination.

The present invention also relates to a cathode according to the present invention; anode; A separator provided between the anode and the cathode; And an electrolyte. The present invention also relates to a lithium ion secondary battery comprising the same.

As for the negative electrode, the contents described above can be applied as it is.

In the lithium ion secondary battery of the present invention, the positive electrode is not particularly limited, and the positive electrode active material may be bound to the current collector according to a conventional method known in the art.

As the cathode active material, a known cathode active material can be used. For example, lithium manganese oxide, lithium cobalt oxide, lithium nickel oxide, lithium iron oxide, LiNi _x Mn _y Co _z O ₂ (NMC), which is a three- A lithium complex oxide in which these are combined can be used. Examples of the current collector include foil produced by aluminum, nickel or a combination thereof.

The separator positioned between the positive and negative electrodes separates or insulates the positive and negative electrodes from each other and enables lithium ion transport between the positive and negative electrodes. The separator may be made of a porous nonconductive or insulating material, but is not limited thereto, Separators known in the art can be used.

The separator may be an independent member such as a film, or may be a coating layer added to the anode and / or the cathode. The material of the separator includes, for example, a polyolefin such as polyethylene and polypropylene, a glass fiber filter paper, and a ceramic material, but is not limited thereto. The thickness of the separator is about 5 탆 to about 50 탆, Lt; / RTI >

As the electrolyte, those known in the art may be used, and for example, an electrolyte including a lithium salt and an organic solvent may be used. The electrolyte may be impregnated into a negative electrode, a positive electrode and a separator.

As the organic solvent contained in the electrolyte, for example, a single solvent or a mixed organic solvent of two or more may be used. When two or more mixed organic solvents are used, at least one solvent may be selected from two or more of the weak polar solvent group, the strong polar solvent group, and the lithium metal protective solvent group. Wherein said weak polar solvent is defined as a solvent having a dielectric constant of less than 15 that is capable of dissolving a sulfur element in an aryl compound, bicyclic ether, or acyclic carbonate, said strong polar solvent being selected from the group consisting of bicyclic carbonates, sulfoxide compounds, A lithium metal protective solvent is defined as a solvent having a dielectric constant higher than 15 that is capable of dissolving lithium polysulfide in a compound, a ketone compound, an ester compound, a sulfate compound, or a sulfite compound, and the lithium metal protective solvent is a saturated ether compound, an unsaturated ether compound, , A cyclic compound having a cyclic efficiency of not less than 50% to form a stable SEI (Solid Electrolyte Interface) on a lithium metal such as a heterocyclic compound containing O, S, or a combination thereof.

The lithium ion secondary battery may be constructed by applying techniques known in the art, except for the characteristic features of the present invention described above.

It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the invention as set forth in the appended claims. Such changes and modifications are intended to be within the scope of the appended claims.

Example 1: Preparation of a binder comprising a HEA-AA random copolymer

5 g of 2-hydroxyethyl acrylate (HEA), 1.25 g of acrylic acid (AA) and 55 g of distilled water were placed in a 100 mL round bottom flask, and the inlet was sealed. After the oxygen was removed by bubbling with nitrogen for 30 minutes and the reaction flask was placed in an oil bath heated to 65 ° C, 6 mg of initiator (VA-65, Wako Chem) and 3.5 mg of CTA (2-mercaptoethanol) was administered to initiate the reaction. The reaction was allowed to proceed for about 20 hours and then terminated to produce a HEA-AA random copolymer. The conversion rate calculated for the sum of the monomers HEA and AA applied in the above reaction was found to be about 99%.

Example 2: Preparation of a binder containing Na-substituted HEA-AA random copolymer

The copolymer prepared in Example 1 above was treated with a 1N NaOH aqueous solution A Na-substituted HEA-AA random copolymer was prepared by replacing the hydrogen (H) in the carboxylic acid group of acrylic acid (AA) with Na ions.

Example 3: Preparation of negative electrode for lithium ion secondary battery

0.08 g of the above carbon black and 2.83 g of the binder (6%) prepared in Example 1 were mixed and sufficiently mixed at 1500 rpm for 3 minutes in a paste mixer capable of revolving and rotating. Then, 2 g of water was further added thereto and the mixture was stirred at 1500 rpm for 3 minutes The viscosity was adjusted by mixing.

Further, 4.52 g of graphite and 0.24 g of SiOx / C (having a carbon coating layer on the surface of SiOx) were added and mixed at 1500 rpm for 3 minutes. Then, 0.3 g of water was further added and mixed at 1500 rpm for 3 minutes to control the viscosity. At this time, if a lot of heat was generated during mixing, the mixture was sufficiently cooled at room temperature and then mixed again to prepare a slurry composition.

The slurry prepared above was coated on a Cu foil having a thickness of 20 탆 using a Mathiscoater to a thickness of 50 to 150 탆 and then subjected to a primary drying at 60 캜 for 30 minutes using a convection oven. A constant portion of the loading was taken and secondary drying was carried out in a vacuum oven at 100 ° C for 10 hours to prepare a cathode (design capacity 400 mAh / g).

Example 4: Preparation of negative electrode for lithium ion secondary battery

Except that the Na-substituted HEA-AA random copolymer prepared in Example 2 was used instead of the HEA-AA random copolymer prepared in Example 1 in Example 3, .

Comparative Example 1: Preparation of negative electrode for lithium ion secondary battery

0.08 g of the above carbon black and 5 g of CMC (1.1%) were mixed and sufficiently mixed at 1500 rpm for 3 minutes in a paste mixer capable of revolving and rotating.

Further, 4.52 g of graphite and 0.24 g of SiOx / C (having a carbon coating layer on the SiOx surface) were charged and mixed at 1500 rpm for 3 minutes. Then, SBR (40%) 0.28 was further added thereto and mixed at 1500 rpm for 3 minutes to control the viscosity .

The slurry prepared above was coated on a Cu foil having a thickness of 20 탆 using a Mathiscoater to a thickness of 50 to 150 탆 and then subjected to a primary drying at 60 캜 for 30 minutes using a convection oven. And the secondary drying was performed at 100 캜 for 10 hours in a vacuum oven to prepare a negative electrode.

Example 5: Preparation of lithium ion secondary battery

In the embodiment in which the negative electrode prepared in 4 (rolling also control the porosity about 30%) using a lithium metal as a reference and a counter electrode, the electrolyte, and EC: DEC = 3: 7, 10wt% FEC, 1.3M LiPF 6 composition And 20 micron polyolefin was used as a separator to prepare a coin cell.

Comparative Example 2: Preparation of lithium ion secondary battery

A coin cell was prepared in the same manner as in Example 5 except that the negative electrode prepared in Comparative Example 1 was used.

Test Example 1: Evaluation of characteristics of lithium ion secondary battery

The charging and discharging characteristics, initial efficiency, and lifespan characteristics of the battery of Example 5 and the battery of Comparative Example 2 were evaluated in the following manner, and the results are shown in Table 2, and Figs. 2 and 3, respectively.

<Initial efficiency evaluation method>

The cells were charged and discharged three times under the conditions shown in Table 1 to evaluate the initial efficiency of the cells.

1st cycle Charge 0.1C rate, 0.005V, 0.02C (CC-CV) / discharge 0.1C rate, 1.5V (CC) 2nd cycle Charge 0.1C rate, 0.005V, 0.02C (CC-CV) / discharge 0.1C rate, 1.0V (CC) 3rd cycle Charge 0.5C rate, 0.005V, 0.02C (CC-CV) / discharge 0.5C rate, 1.5V (CC)

<Life performance evaluation method>

Charging and discharging of 50 cycles were performed under the conditions of the 1st cycle shown in Table 1 to evaluate the life characteristics.

battery The battery of Comparative Example 2 The cell of Example 5 Used binders SBR / CMC
(Comparative Example 1) Na-substituted HEA-AA
(Example 2) Loading (mAh / cm ² ) 5.25 4.86 Charging (mAh / g) 376.4 417.5 Discharge (mAh / g) 331.6 361.8 Initial efficiency (%) 88.1 86.7 life span (%) 62.7 76.6

 As can be seen from the above Table 2, the battery of Example 5 using the binder of the present invention showed the same degree of initial efficiency as the battery of Comparative Example 2 using the conventional binder, but the initial charge- Was found to be noticeably superior. Further, it was confirmed that the life characteristic was extremely excellent.

Claims (18)

Hydroxyalkyl (meth) acrylate monomers; And
(Meth) acrylic acid monomers, and alkali metal salt monomers of (meth) acrylic acid, wherein the copolymer comprises at least one monomer selected from the group consisting of
Wherein the copolymer does not contain a conjugated diene monomer, a fluorine-containing (meth) acrylic acid ester monomer, and a starch. The method according to claim 1,
A hydroxyalkyl acrylate monomer contained in the copolymer; And at least one selected from the group consisting of a (meth) acrylic acid monomer and an alkali metal salt monomer of (meth) acrylic acid is contained in a weight ratio of 5: 5 to 9: 1. The method according to claim 1,
Wherein the weight average molecular weight of the copolymer is 200,000 to 10,000,000. The method according to claim 1,
The hydroxyalkyl (meth) acrylate monomer may be at least one selected from the group consisting of hydroxymethyl acrylate, hydroxymethyl methacrylate, hydroxyethyl acrylate, hydroxyethyl methacrylate, hydroxypropyl acrylate, hydroxypropyl methacrylate, Wherein the binder is at least one selected from the group consisting of hydroxyethyl acrylate, hydroxyethyl acrylate, hydroxyethyl acrylate, hydroxyethyl acrylate, hydroxyethyl acrylate, hydroxyethyl acrylate, hydroxyisopropyl acrylate, and hydroxyisopropyl methacrylate. The method according to claim 1,
The (meth) acrylic acid monomer is at least one selected from the group consisting of acrylic acid and methacrylic acid,
Wherein the alkali metal salt monomer of (meth) acrylic acid is at least one lithium, sodium, or potassium salt selected from the group consisting of acrylic acid and methacrylic acid. The method according to claim 1,
Wherein the hydroxyalkyl (meth) acrylate monomer has a glass transition temperature (Tg) of -100 ° C. to 30 ° C. and the (meth) acrylic acid monomer has a glass transition temperature (Tg) of 80 ° C. to 130 ° C. bookbinder. The method according to claim 1,
Wherein the copolymer is a random copolymer. The method according to claim 1,
Wherein the copolymer is contained in an amount of 30 to 100% by weight based on the total weight of the binder. The method according to claim 1,
The copolymer may be selected from the group consisting of hydroxyalkyl (meth) acrylate monomers; And at least one member selected from the group consisting of (meth) acrylic acid monomers and alkali metal salt monomers of (meth) acrylic acid. The method according to claim 1,
Wherein the binder is an aqueous binder. The method according to claim 1,
Wherein the binder is for producing a lithium ion secondary battery. 12. The method of claim 11,
Wherein the binder is for producing a negative electrode. 13. The method of claim 12,
Wherein the binder is for producing a negative electrode containing an Si-based active material. A negative electrode for a lithium ion secondary battery, comprising a negative electrode active material, a binder according to claim 1, and a conductive material. A cathode according to claim 14; anode; A separator provided between the anode and the cathode; And
A lithium ion secondary battery comprising an electrolyte. (a) a hydroxyalkyl (meth) acrylate monomer in a polymerization reactor; And a monomer and at least one monomer selected from the group consisting of a (meth) acrylic acid monomer and an alkali metal salt monomer of (meth) acrylic acid and a solvent, and sealing the inlet of the reactor;
(b) subjecting the reactant to nitrogen bubbling to remove oxygen; And
(c) adding a polymerization initiator and a chain transfer agent and reacting the mixture. 17. The method of claim 16,
Wherein the solvent in step (a) is water. 17. The method of claim 16,
In the step (a), a hydroxyalkyl acrylate monomer; And at least one member selected from the group consisting of a (meth) acrylic acid monomer and an alkali metal salt monomer of (meth) acrylic acid is added in a weight ratio of 5: 5 to 9: 1.

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