WO2025183050A1 - Absorbent for seawater-derived carbon dioxide - Google Patents

Abstract

The purpose of the present invention is to provide a carbon dioxide absorbent for efficiently absorbing carbon dioxide contained in seawater. The present invention relates to a novel polymer including a guanidino group or a cyclic guanidino group. The present invention also relates to a carbon dioxide absorbent for absorbing carbon dioxide contained in seawater, the carbon dioxide absorbent including the polymer.

Description

Seawater-derived carbon dioxide absorbent

The present invention relates to a novel polymer containing a guanidino group or a cyclic guanidino group. The present invention also relates to a carbon dioxide absorbent containing the polymer for absorbing carbon dioxide in seawater and recovering it from the ocean.

In recent years, active research has been conducted into technologies for capturing carbon dioxide (hereinafter sometimes referred to as "CO ₂ ") from exhaust gases or the atmosphere and storing it underground or on the seabed. These technologies are called CCS (Carbon dioxide-Capture-and-Storage) (Non-Patent Document 1) and DAC (Direct-Air-Capture) (Non-Patent Documents 2 and 3), and are known as CO ₂ reduction technologies. Furthermore, DOC (Direct-Ocean-Capture), a technology for directly capturing carbon dioxide in the ocean, has recently been attracting attention. The reason for this is that if the CO ₂ concentration in the ocean continues to rise in conjunction with the rise in atmospheric CO ₂ concentration, the pH in the ocean will change, raising concerns about the resulting impact on ecosystems.

Water-soluble amine compounds such as ethylenediamine have been commonly used as carbon dioxide absorbents in the past. However, in DOC conducted in the ocean, where _CO2 concentrations are lower than in the atmosphere, it has been difficult to use existing water-soluble amine compounds as carbon dioxide absorbents because they are miscible with water and cause marine pollution. Furthermore, because the carbon dioxide concentration in seawater is lower than in the atmosphere, compounds with high carbon dioxide absorption performance are required.

Recently, the present inventors have discovered that alkylamines in which a hydrophobic phenyl group has been introduced near the amino group can selectively and efficiently absorb and release atmospheric carbon dioxide (Patent Documents 1-4, Non-Patent Documents 4 and 5). This method, whether the alkylamine itself or an aqueous solution is used as a carbon dioxide absorbent, results in a solid that contains almost no water after carbon dioxide absorption, enabling solid-liquid separation. This has the advantage that the energy required to heat water is not required when releasing the absorbed carbon dioxide, and carbon dioxide can be efficiently generated under mild temperature conditions. However, it is unclear whether the alkylamines can absorb low concentrations of carbon dioxide in seawater, which contains large amounts of various salts. Even if they could, the alkylamine compounds would disperse in the ocean after carbon dioxide absorption, making their separation and recovery from the ocean difficult, and they may not be suitable for use in seawater.

Meanwhile, molecular sieves are also known as adsorbents for carbon dioxide from exhaust gases (Patent Document 5), but because they also adsorb water vapor, it is expected that their _CO2 adsorption efficiency will decrease in water. Patent Document 6 also describes that a carbon dioxide absorbent obtained by physically supporting or impregnating (spraying and then drying) a cyclic guanidine compound on inorganic particles such as silica or activated carbon, or on fibers, can absorb carbon dioxide in various organic solvents, but does not even suggest its use in water, and therefore it is unclear whether it can withstand use in water, particularly seawater, which is rich in various salts.

As mentioned above, to date, there have been no reports of chemical absorbents that can efficiently capture carbon dioxide in seawater and recover it from the ocean.

Japanese Patent Application Laid-Open No. 2017-31046 Japanese Patent Application Laid-Open No. 2017-31062 Japanese Patent Application Laid-Open No. 2019-127417 International Publication No. 2022/176534 Special Publication No. 2002-519188 JP 2011-206671 A

Iijima, M. and Nakatani, S., Kagaku Kogaku, 2013, Vol. 77, pages 300-303. Baciocchi, R., Storti, G., and Mazzotti, M. Chemical Engineering and Processing, 2006, Vol.45, pages 1047-1058. Kiani, A., Jiang, K., and Feron, P., frontiers in Energy Research, 2020, Vol.8, Article 92. Inagaki, F., Matsumoto, C., Iwata, T., and Mukai, C., J. Am. Chem. Soc., 2017, Vol.139, pages 4639-4642. Murakami, R., Kawamitsu, H., Otsuka, R., Tanishima, H., and Inagaki, F., Adv. Mater. Interfaces., 2024, Vol. 11, 2300881.

The object of the present invention is to provide a compound that can efficiently absorb carbon dioxide at low concentrations in seawater and can be easily recovered from the ocean after absorbing carbon dioxide.

Under these circumstances, the present inventors conducted extensive research and found that by using a polymer having a guanidino group in the side chain, particularly a polymer having a polystyrene main chain, as a carbon dioxide absorbent, high carbon dioxide absorption performance can be achieved even in a low _CO2 concentration range such as that found in seawater, and that the absorbed carbon dioxide can be easily recovered from the ocean, thereby completing the present invention.

That is, the present invention is as follows.
[1] Formula (1):

[In the formula,
^R1 and ^R2 each independently represent a hydrogen atom or an optionally substituted alkyl group, or ^R1 and ^R2 bond to each other to form an optionally substituted 5- to 8-membered ring together with the nitrogen atom to which they are attached; and ^R3 represents a hydrogen atom or an optionally substituted alkyl group.
and a repeating structural unit (1) represented by formula (2):

A polymer (hereinafter sometimes referred to as the "polymer of the present invention") comprising a repeating structural unit (2) represented by the following formula: wherein each repeating unit is contained randomly or in a block.
[2] The polymer according to the above [1], which comprises the repeating structural unit (1) and the repeating structural unit (2).
[3] The polymer according to the above [1] or [2], wherein R ¹ , R ² and R ³ are each independently a hydrogen atom or a methyl group.
[4] The polymer according to the above [1] or [2], wherein R ¹ and R ² are bonded to each other to form, together with the nitrogen atom to which they are bonded, imidazoline or 1,4,5,6-tetrahydropyrimidine.
[5] The polymer according to any one of the above [1] to [4], which is a random copolymer in which the repeating structural unit (1) accounts for 50 to 99 mol % and the repeating structural unit (2) accounts for 1 to 50 mol % of all structural units of the polymer.
[2'] The polymer is represented by formula (3):

[Where R is the following formula:

(In the formula, the wavy line indicates the bonding position to the main chain, ^R4 represents a _C1-6 alkyl group, and n represents an integer of 1 to 10.)
and R' represents a hydrogen atom or a methyl group.
The polymer according to the above [1], further comprising a repeating structural unit (3) represented by the following formula:
[3'] The polymer according to the above [2'], wherein R ¹ , R ² and R ³ are each independently a hydrogen atom or a methyl group.
[4'] The polymer according to the above [2'], wherein R ¹ and R ² are bonded to each other to form, together with the nitrogen atom to which they are bonded, an imidazoline or a 1,4,5,6-tetrahydropyrimidine.
[5'] The polymer according to any one of the above [2'] to [4'], which is a random copolymer in which, of all the structural units of the polymer, the repeating structural unit (1) accounts for 35 to 96 mol%, the repeating structural unit (2) accounts for 2 to 63 mol%, and the repeating structural unit (3) accounts for 2 to 63 mol%.
[6] A carbon dioxide absorbent containing the polymer according to any one of the above [1] to [5] and [2'] to [5'] (hereinafter, also referred to as "the carbon dioxide absorbent of the present invention").
[7] The carbon dioxide absorbent according to the above [6] for absorbing carbon dioxide in seawater.
[8] Formula (I):

[Each symbol in the formula has the same meaning as defined above.]
and a compound represented by formula (II):

The method for producing the polymer according to the above [1], characterized by reacting a compound represented by the following formula (I) with a polymerization initiator under heating.
[9] The method according to the above [8], wherein the polymerization initiator is 2,2'-azobis(2,4-dimethylvaleronitrile) (ADVN) or azobisisobutyronitrile (AIBN).
[8'] Formula (I):

[Each symbol in the formula has the same meaning as defined above.]
a compound represented by formula (II):

and a compound represented by formula (III):

[Each symbol in the formula has the same meaning as defined above.]
The method for producing a polymer according to the above [2'], characterized by reacting a compound represented by the following formula with a polymerization initiator under heating.
[9'] The method according to the above [8'], wherein the polymerization initiator is 2,2'-azobis(2,4-dimethylvaleronitrile) (ADVN) or azobisisobutyronitrile (AIBN).

The carbon dioxide absorbent of the present invention is easy to synthesize, stable and easy to handle even in water, particularly seawater where various salts are present, and has a high carbon dioxide absorption capacity. Therefore, it has the advantage of being able to efficiently absorb and immobilize carbon dioxide, even at room temperature and atmospheric pressure, even when carbon dioxide is diluted and present in the ocean at low concentrations. Furthermore, the carbon dioxide absorbent of the present invention not only makes it easy to recover carbon dioxide after immobilization, but also allows the immobilized carbon dioxide to be released under mild conditions and effectively used as a carbon source. Therefore, the present invention can provide a new and effective method of DOC (Direct-Ocean-Capture).

FIG. 1a shows the change in carbon dioxide concentration (mg/L) over time in distilled water at room temperature and atmospheric pressure with the addition of the polymer of the present invention (compound (1-1) and compound (1-2)), molecular sieve 3 Å, and molecular sieve 4 Å. FIG. 1b shows the results of an experiment conducted on a scale 10 times larger than that of the experiment in FIG. 1a using the polymer of the present invention (compound (1-2)) and molecular sieve 4 Å. FIG. 2a shows the change in carbon dioxide concentration (mg/L) over time in seawater at room temperature and atmospheric pressure due to the addition of a polymer of the present invention (compound (1-2)) and a molecular sieve 4Å, and FIG. 2b shows the results of an experiment conducted on a scale 10 times larger than that of the experiment in FIG. 2a using a polymer of the present invention (compound (1-2)) and a molecular sieve 4Å. FIG. 3 shows the change in the amount of carbon dioxide released (ppm) over time from the polymer of the present invention (compound (1-2)) after carbon dioxide absorption due to heating. FIG. 4 shows the change in carbon dioxide concentration (mg/L) over time due to the addition of a polymer of the present invention (compound (1-2), compound (1-3), compound (1-4), compound (1-5), or compound (1-6)) in seawater (50 mL) at room temperature and atmospheric pressure. FIG. 5 shows the change in carbon dioxide concentration (mg/L) over time due to the addition of the polymer of the present invention (compound (1-2) or compound (1-7)) in seawater (500 mL) at room temperature and atmospheric pressure. FIG. 6 shows a schematic diagram of an experimental apparatus for examining the amount of carbon dioxide released over time from the polymer of the present invention after carbon dioxide absorption due to heating. FIG. 7 shows the change in the amount of carbon dioxide released (ppm) over time from the polymer of the present invention (compound (1-2) or compound (1-7)) after carbon dioxide absorption due to heating.

The present invention is described in detail below.

(definition)
In this specification, "room temperature" means about 15°C to about 25°C.

In this specification, "normal pressure" means 1 atmosphere (1013 hPa).

In this specification, "approximately" is defined as ±5°C for temperature, ±10 minutes for time, and ±10% for weight, volume, and concentration.

In this specification, "bicarbonate" refers to a hydrogen carbonate salt of a substituted guanidine or cyclic guanidine formed by reacting a substituted guanidino group or cyclic guanidino group contained in the polymer of the present invention with carbon dioxide.

In this specification, "absorption" refers to a state in which a substituted guanidino group or cyclic guanidino group contained in the polymer of the present invention chemically reacts with carbon dioxide to form a substituted guanidine or cyclic guanidine bicarbonate salt, which is then incorporated into the polymer of the present invention. Carbon dioxide absorbed into the polymer of the present invention is easily desorbed and released from the polymer of the present invention by subjecting it to conditions such as heating.

In this specification, "adsorption" refers to the state in which carbon dioxide is physically absorbed by van der Waals forces, etc.

The polymer of the present invention (guanidino group-containing polymer) is a random copolymer or block copolymer containing repeating structural units (1) and (2), and is preferably a random copolymer. In the copolymer, each of the repeating structural units (1) and (2) may be of only one type, or of two or more types. Furthermore, the copolymer may contain structural units other than the structural units (1) and (2). Below, the groups contained in the repeating structural units will be explained in order.

In this specification, the "alkyl group" in the "optionally substituted alkyl group" means a linear or branched alkyl group having 1 or more carbon atoms, and when there is no particular limitation on the range of the carbon atoms, means a _C1-20 alkyl group. Among them, a _C1-6 alkyl group is more preferred, and a _C1-3 alkyl group is particularly preferred.

In the present specification, the term "C _1-20 alkyl group" means a straight-chain or branched-chain alkyl group having 1 to 20 carbon atoms, and examples thereof include methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl, tert-butyl, pentyl, isopentyl, neopentyl, 1-ethylpropyl, hexyl, isohexyl, 1,1-dimethylbutyl, 2,2-dimethylbutyl, 3,3-dimethylbutyl, 2-ethylbutyl, heptyl, octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, eicosyl, and the like.

In the present specification, the term "C _1-6 alkyl group" means a straight-chain or branched-chain alkyl group having 1 to 6 carbon atoms, and examples thereof include methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl, tert-butyl, pentyl, isopentyl, neopentyl, 1-ethylpropyl, hexyl, isohexyl, 1,1-dimethylbutyl, 2,2-dimethylbutyl, 3,3-dimethylbutyl, 2-ethylbutyl, and the like.

In the present specification, the term "C _1-3 alkyl group" means a straight or branched chain alkyl group having 1 to 3 carbon atoms, and examples thereof include methyl, ethyl, propyl, and isopropyl.

In the present specification, examples of the "optionally substituted 5- to 8-membered ring formed by bonding together with the nitrogen atom to which they are bonded" that R ¹ and R ² may form include rings of the following formula:

(In the formula, the wavy line indicates the bonding site with the NH group, and ^R3 is as defined above.)
Among these, imidazoline or 1,4,5,6-tetrahydropyrimidine is preferred, and imidazoline is particularly preferred.

In this specification, unless otherwise specified, the term "optionally substituted" means that the compound may have one or more substituents, and examples of the "substituents" include: (1) a halogen atom, (2) a nitro group, (3) a cyano group, (4) a hydroxy group, (5) an amino group optionally substituted by a protecting group, (6) a C _3-8 cycloalkyl group, (7) a C _1-6 alkoxy group, (8) a C _1-6 alkoxy-C _1-6 alkoxy group, (9) a C _6-14 aryl group, (10) a C _6-14 aryloxy group, (11) a formyl group, (12) a carbamoyl group optionally substituted by 1 or 2 C _1-6 alkyl groups, (13) a sulfamoyl group optionally substituted by 1 or 2 C _1-6 alkyl groups, (14) a C _1-6 alkyl-carbonyl group, (15) a C _1-6 alkoxy-carbonyl group, (16) a C _3-8 cycloalkyl-carbonyl group, (17) a C Examples of the alkyl group include a _6-14 aryl-carbonyl group, (18) a 5- to 10-membered heteroarylcarbonyl group, (19) a 3- to 14-membered heterocyclylcarbonyl group, (20) a C _1-6 alkylsulfonyl group, (21) a C _6-14 arylsulfonyl group, (22) a 5- to 10-membered heteroarylsulfonyl group, (23) a 3- to 14-membered heterocyclylsulfonyl group, (24) an azido group, (25) a tri-substituted silyl group, (26) a tri-substituted silyloxy group, (27) a 5- to 10-membered heteroaryl group, and (28) a 3- to 14-membered heterocyclyl group. Among these, halogen, cyano, hydroxy, amino, C _1-6 alkoxy, C _1-6 alkyl-carbonyl, C _1-6 alkoxy-carbonyl, C _6-10 aryl, carbamoyl, sulfamoyl, or a 5- to 10-membered heteroaryl group is preferred. When multiple substituents are present, the substituents may be the same or different.

The above substituents may be further substituted with the above substituents. The number of substituents is not particularly limited as long as it is a substitutable number, but is preferably 1 to 5, and more preferably 1 to 3. When multiple substituents are present, the respective substituents may be the same or different.

In this specification, unless otherwise specified, the term "C _1-6 alkoxy group" means a straight-chain or branched-chain alkoxy group having 1 to 6 carbon atoms, such as methoxy, ethoxy, propoxy, isopropoxy, butoxy, isobutoxy, sec-butoxy, and tert-butoxy.

In the present specification, the term "C _3-8 cycloalkyl group" means a monocyclic saturated hydrocarbon ring group having 3 to 8 carbon atoms, and examples thereof include cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, cyclooctyl and the like.

As used herein, the term "C _6-14 aryl group" refers to a monocyclic or polycyclic (fused) aromatic hydrocarbon group, and examples thereof include phenyl, naphthyl, anthryl, phenanthryl, acenaphthylenyl, biphenylyl, etc. Furthermore, the "C _6-14 aryl group" may be fused with another ring, and examples thereof include fluorenyl, dihydronaphthyl, tetrahydronaphthyl, etc. Of these, a C _6-10 aryl group is preferred, and a phenyl group is particularly preferred.

As used herein, the term "heteroaryl (group)" refers to an aromatic heterocyclic ring (group), and examples thereof include 5- to 10-membered (preferably 5- or 6-membered) monocyclic heteroaryl groups and fused heteroaryl groups containing, in addition to carbon atoms, 1 to 4 heteroatoms selected from oxygen, sulfur, and nitrogen atoms as ring-constituting atoms. Examples of such fused heteroaryl groups include groups derived from a ring corresponding to these 5- or 6-membered monocyclic heteroaryl groups fused with one or two rings selected from a 5- or 6-membered monocyclic heteroaryl ring containing one or two nitrogen atoms (e.g., pyrrole, imidazole, pyrazole, pyrazine, pyridine, pyrimidine, etc.), a 5-membered heteroaryl ring containing one sulfur atom (e.g., thiophene), and a benzene ring.

Suitable examples of the heteroaryl group include:
5- or 6-membered monocyclic heteroaryl groups such as furyl, thienyl, pyridyl, pyrimidinyl, pyridazinyl, pyrazinyl, pyrrolyl, imidazolyl, pyrazolyl, thiazolyl, isothiazolyl, oxazolyl, isoxazolyl, oxadiazolyl, thiadiazolyl, triazolyl, tetrazolyl, triazinyl, etc.;
Examples thereof include 8- to 10-membered fused heteroaryl groups such as quinolyl, isoquinolyl, quinazolyl, quinoxalyl, benzofuranyl, benzothienyl, benzoxazolyl, benzisoxazolyl, benzothiazolyl, benzimidazolyl, benzotriazolyl, indolyl, indazolyl, carbazolyl, pyrrolopyrazinyl, imidazopyridyl, thienopyridyl, imidazopyrazinyl, pyrazolopyridyl, pyrazolothienyl, pyrazolotriazinyl, pyridopyridyl, and thienopyridyl.

As used herein, the term "heterocyclyl (group)" refers to a non-aromatic heterocyclic (group), and examples include 3- to 8-membered (preferably 5- or 6-membered) monocyclic heterocyclyl groups and fused heterocyclyl groups, as well as 7- to 14-membered bridged heterocyclyl groups, each containing, in addition to carbon atoms, 1 to 4 heteroatoms selected from oxygen, sulfur, and nitrogen atoms. Examples of such fused heterocyclyl groups include groups derived from rings formed by fusing a ring corresponding to these 3- to 8-membered monocyclic heterocyclyl groups with one or two rings selected from a 5- or 6-membered monocyclic heteroaryl ring containing one or two nitrogen atoms (e.g., pyrrole, imidazole, pyrazole, pyrazine, pyridine, pyrimidine, etc.), a 5-membered monocyclic heteroaryl ring containing one sulfur atom (e.g., thiophene), and a benzene ring, as well as groups obtained by partial saturation of such groups.

Suitable examples of heterocyclyl groups include:
3- to 7-membered monocyclic heterocyclyl groups such as aziridinyl, azetidinyl, pyrrolidinyl, piperidyl, morpholinyl, thiomorpholinyl, piperazinyl, hexamethyleneiminyl, oxazolidinyl, thiazolidinyl, imidazolidinyl, oxazolinyl, thiazolinyl, imidazolinyl, dioxolyl, dioxolanyl, dihydrooxadiazolyl, pyranyl, tetrahydropyranyl, thiopyranyl, tetrahydrothiopyranyl, tetrahydrofuryl, pyrazolidinyl, pyrazolinyl, tetrahydropyrimidinyl, dihydrotriazolyl, and tetrahydrotriazolyl;
9- to 14-membered fused heterocyclyl groups such as dihydroindolyl, dihydroisoindolyl, dihydrobenzofuranyl, dihydrobenzodioxenyl, dihydrobenzodioxepinyl, tetrahydrobenzofuranyl, chromenyl, dihydrochromenyl, dihydroquinolyl, tetrahydroquinolyl, dihydroisoquinolyl, tetrahydroisoquinolyl, and dihydrophthalazinyl;
etc.

In this specification, examples of "7- to 14-membered bridged heterocyclyl group" include quinuclidinyl, 7-azabicyclo[2.2.1]heptanyl, etc.

In the present specification, the term "tri-substituted silyl group" means a silyl group substituted by three identical or different substituents (for example, a C _1-6 alkyl group, a C _6-14 aryl group, etc.), and preferred examples of the group include trialkylsilyl groups such as a trimethylsilyl group, a triethylsilyl group, a triisopropylsilyl group, and a tert-butyldimethylsilyl group (preferably a tri-C _1-6 alkylsilyl group), a tert-butyldiphenylsilyl group, and a triphenylsilyl group.

In this specification, the term "tri-substituted silyloxy group" refers to a group in which a tri-substituted silyl group is bonded to an oxygen atom. Examples of such groups include trimethylsiloxy groups, triethylsiloxy groups, triisopropylsiloxy groups, and tert-butyldimethylsiloxy groups.

In this specification, the term "amino group optionally substituted with a protecting group" refers to an amino group in which one or two of the hydrogen atoms of the amino group may be substituted with a "protecting group." Examples of such "protecting groups" that can be used include those described in P. G. Wuts, "Protective Groups in Organic Synthesis," 5th Edition, Wiley, 2014. Specific examples of such amino protecting groups include methyl, acetyl, trifluoroacetyl, pivaloyl, benzoyl, tert-butoxycarbonyl, and benzyloxycarbonyl.

In this specification, the term "structural unit derived from a styrene derivative" refers to a structural unit formed by polymerization of the carbon-carbon double bond of a styrene derivative which may be substituted with one or more substituents other than an amidino group or a cyclic amidino group (for example, a hydroxy group, a sulfanyl group, an amino group, a polyethylene glycol (PEG) group, a triethylene glycol (TEG) group, an alkyl group, etc.). The "structural unit derived from a styrene derivative" may be composed of only one type of styrene derivative, or may be composed of two or more types of styrene derivatives.
By selecting the substituents of the styrene derivative, it is possible to adjust the swelling degree of the polymer of the present invention as required.

In this specification, "structural units derived from acrylic acid derivatives" refers to structural units formed by polymerization of carbon-carbon double bonds of acrylic acid derivatives (e.g., acrylic acid esters, acrylonitrile, acrylamide, methacrylic acid esters, methacrylic acid amides, etc.). "Structural units derived from acrylic acid derivatives" may consist of only one type of acrylic acid derivative, or may consist of two or more types of acrylic acid derivatives. Among these, those represented by formula (3):

[Where R is the following formula:

(In the formula, the wavy line indicates the bonding position to the main chain, ^R4 represents a _C1-6 alkyl group, and n represents an integer of 1 to 10.)
and R' represents a hydrogen atom or a methyl group.
The repeating structural unit (3) represented by the following formula is preferred.

In this specification, "structural unit derived from a diacrylic acid derivative" refers to a cross-linkable structural unit formed by polymerization of two carbon-carbon double bonds of a diacrylic acid derivative.

In this specification, examples of "diacrylic acid derivatives" include di(meth)acrylates such as methylene di(meth)acrylate, ethylene di(meth)acrylate, propylene di(meth)acrylate, ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, tetraethylene glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, dipropyleneethylene glycol di(meth)acrylate, tripropylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, butanediol di(meth)acrylate, hexanediol di(meth)acrylate, and nonanediol di(meth)acrylate. p) Acrylates: Examples include methylene di(meth)acrylamide, ethylene di(meth)acrylamide, propylene di(meth)acrylamide, ethylene glycol di(meth)acrylamide, diethylene glycol di(meth)acrylamide, triethylene glycol di(meth)acrylamide, tetraethylene glycol di(meth)acrylamide, polyethylene glycol di(meth)acrylamide, propylene glycol di(meth)acrylamide, dipropyleneethylene glycol di(meth)acrylamide, tripropylene glycol di(meth)acrylamide, polypropylene glycol di(meth)acrylamide, butanediol di(meth)acrylamide, hexanediol di(meth)acrylamide, and nonanediol di(meth)acrylamide. The "structural unit derived from a diacrylic acid derivative" may be composed of only one type of diacrylic acid derivative, or may be composed of two or more types of diacrylic acid derivatives.

In this specification, "structural units derived from vinyl derivatives" refers to structural units formed by polymerization of carbon-carbon double bonds of vinyl derivatives (e.g., vinyl halides, N-vinyl amides, vinyl ethers, vinylidene halides, α-olefins, etc.). A "structural unit derived from a vinyl derivative" may be composed of only one type of vinyl derivative, or may be composed of two or more types of vinyl derivatives.

In this specification, "α-olefins" refers to alkenes in which the carbon-carbon double bond is at the α-position, i.e., the terminal.

In this specification, "structural units derived from 1,3-diene derivatives" refers to cross-linkable structural units formed by polymerization of the carbon-carbon double bonds of 1,3-diene derivatives (e.g., 1,3-butadiene, isoprene, chloroprene, etc.). A "structural unit derived from a 1,3-diene derivative" may be composed of only one type of 1,3-diene derivative, or may be composed of two or more types of 1,3-diene derivatives.

In this specification, the term "polymerization initiator" is not particularly limited, but is preferably a compound that generates radical species upon heating. Specific examples include azo compounds such as 2,2'-azobisisobutyronitrile (AIBN), 2,2'-azobis(2-methylbutyronitrile), 2,2'-azobis(2,4-dimethylvaleronitrile) (ADVN), 1,1'-azobis(1-cyclohexanecarbonitrile), 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile), and 2-(carbamoylazo)isobutyronitrile, as well as dibenzoyl peroxide, dilauroyl peroxide, distearoyl peroxide, and 1,1-di(tert-butylperoxide). Examples of suitable peroxides include 1,1-di(tert-hexylperoxy)-2-methylcyclohexane, 1,1-di(tert-hexylperoxy)-3,3,5-trimethylcyclohexane, 1,1-di(tert-hexylperoxy)cyclohexane, 1,1-di(tert-butylperoxy)cyclohexane, di-tert-hexyl peroxide, tert-butylcumyl peroxide, di-tert-butyl peroxide, 1,1,3,3-tetramethylbutylperoxy-2-ethylhexanoate, tert-hexylperoxy-2-ethylhexanoate, tert-butylperoxy-2-ethylhexanoate, and tert-butylperoxyisopropyl monocarbonate. Among these, ADVN and AIBN are preferred. The amount of polymerization initiator to be added is not particularly limited, and a person skilled in the art would be able to select an appropriate amount.

(Polymer of the present invention)
The polymer of the present invention has the formula (1):

[In the formula,
^R1 and ^R2 each independently represent a hydrogen atom or an optionally substituted alkyl group, or ^R1 and ^R2 bond to each other to form an optionally substituted 5- to 8-membered ring together with the nitrogen atom to which they are attached; and ^R3 represents a hydrogen atom or an optionally substituted alkyl group.
and a repeating structural unit (1) represented by formula (2):

It is a polymer containing repeating structural units (2) represented by the formula:

The following describes each group in the repeating structural unit (1) (i.e., formula (1)) of the polymer of the present invention.

^R1 and ^R2 each represent a hydrogen atom or an optionally substituted alkyl group, or ^R1 and ^R2 are bonded to each other to form an optionally substituted 5- to 8-membered ring together with the nitrogen atom to which they are attached.

R ¹ and R ² are preferably each independently a hydrogen atom or a C _1-4 alkyl group, more preferably each independently a hydrogen atom or a methyl group, and particularly preferably both are hydrogen atoms.

In another preferred embodiment of ^R1 and ^R2 , ^R1 and ^R2 are bonded to each other to form, together with the nitrogen atom to which they are bonded, an optionally substituted 5- or 6-membered ring (e.g., imidazoline or 1,4,5,6-tetrahydropyrimidine), more preferably an unsubstituted imidazoline.

^R3 represents a hydrogen atom or an optionally substituted alkyl group.

^R3 is preferably a hydrogen atom or a _C1-4 alkyl group, more preferably a hydrogen atom or a methyl group, and particularly preferably a hydrogen atom.

The following are suitable as structural unit (1):

[Structural unit (1A)]
A structural unit (1) in which R ¹ and R ² are each independently a hydrogen atom or a C _1-4 alkyl group; and R ³ is a hydrogen atom or a C _1-4 alkyl group.

[Structural unit (1B)]
A structural unit (1) in which ^R1 and ^R2 , together with the nitrogen atom to which they are attached, bond to each other to form an optionally substituted 5- or 6-membered ring (e.g., imidazoline or 1,4,5,6-tetrahydropyrimidine); and ^R3 is a hydrogen atom or a _C1-4 alkyl group.

[Structural unit (1C)]
The structural unit (1) in which R ¹ and R ² are each independently a hydrogen atom or a methyl group; and R ³ is a hydrogen atom or a methyl group.

[Structural unit (1D)]
A structural unit (1) in which R ¹ and R ² are bonded to each other and, together with the nitrogen atom to which they are bonded, form an imidazoline or 1,4,5,6-tetrahydropyrimidine; and R ³ is a hydrogen atom or a methyl group.

[Structural Unit (1E)]
The structural unit (1) in which R ¹ and R ² are both hydrogen atoms; and R ³ is a hydrogen atom.

[Structural unit (1F)]
The structural unit (1) in which R ¹ and R ² are bonded to each other and, together with the nitrogen atom to which they are attached, form an imidazoline; and R ³ is a hydrogen atom.

Furthermore, the monomer corresponding to structural unit (1) used in the method for producing the polymer of the present invention described below is represented by formula (I):

[Each symbol in the formula has the same meaning as defined above.]
The compound is represented by the formula (hereinafter referred to as compound (I)).

Suitable compound (I) includes monomer compounds in which the groups in formula (I) correspond to the groups shown in the structural units (1A), (1B), (1C), (1D), (1E), and (1F). Only one type of compound (I) may be used, or two or more types may be used in combination.

Particularly preferred examples of compound (I) include compound (I-1) and compound (I-2) described in the Examples below.

The structural unit (2) is represented by the formula (2):

Examples of the repeating structural unit (2) include those represented by the following formula (2A):

Or the following formula (2B):

Repeating structural unit (2) represented by the formula:

Furthermore, the monomer corresponding to structural unit (2) used in the method for producing the polymer of the present invention described below is represented by formula (II):

The compound represented by the formula (hereinafter referred to as compound (II)) is

Suitable examples of compound (II) include monomer compounds corresponding to the aforementioned structural unit (2A) or structural unit (2B). Compound (II) may be used alone or in combination of two or more types.

A particularly suitable example of compound (II) is p-divinylbenzene or m-divinylbenzene, and specific examples include the commercially available mixture of p-divinylbenzene and m-divinylbenzene (compound (II-1)) used in the examples described below.

The polymer of the present invention is preferably a random copolymer consisting of structural unit (1) and structural unit (2).

When the polymer of the present invention is composed of structural units (1) and (2), the amount of structural unit (1) is 50 to 99 mol%, preferably 70 to 97 mol%, and more preferably 80 to 95 mol%, relative to 100 mol% of the total amount of all structural units, in order to maintain high absorption capacity as a carbon dioxide absorbent in seawater.

When the polymer of the present invention is composed of structural unit (1) and structural unit (2), the amount of structural unit (2) is 1 to 50 mol %, preferably 3 to 30 mol %, and more preferably 5 to 20 mol %, relative to 100 mol % of the total amount of all structural units, in order to improve affinity for water.

The polymer of the present invention can adjust its affinity for water and carbon dioxide absorption capacity by adjusting the ratio of each structural unit.

The polymer of the present invention may contain repeating structural units other than structural units (1) and (2) to adjust the degree of swelling, etc., as needed, as long as its carbon dioxide absorption capacity is not impaired. The content of structural units other than structural units (1) and (2) is preferably 63 mol % or less, relative to the total amount of all structural units in the polymer of the present invention (100 mol %).

The structural units other than structural units (1) and (2) are not particularly limited, but suitable examples include the aforementioned structural units derived from styrene derivatives, structural units derived from acrylic acid derivatives, structural units derived from diacrylic acid derivatives, structural units derived from vinyl derivatives, and structural units derived from 1,3-diene derivatives. As structural units other than structural units (1) and (2), structural units derived from acrylic acid derivatives are preferred, and are represented by formula (3):

[Where R is the following formula:

(In the formula, the wavy line indicates the bonding position to the main chain, ^R4 represents a _C1-6 alkyl group, and n represents an integer of 1 to 10.)
and R' represents a hydrogen atom or a methyl group.
The structural unit (3) represented by the following formula is more preferred.

The structural unit (3) is represented by the following formula (3A):

, the following formula (3B):

, the following formula (3C):

, or the following formula (3D):

Repeating structural unit (3) represented by the formula:

Furthermore, the monomer corresponding to structural unit (3) used in the method for producing the polymer of the present invention described below is represented by formula (III):

[Each symbol in the formula has the same meaning as defined above.]
The compound is represented by the formula (hereinafter referred to as compound (III)).

Suitable examples of compound (III) include monomer compounds corresponding to the structural units (3A) to (3D) described above. Compound (III) may be used alone or in combination of two or more types.

Particularly suitable examples of compound (III) include the commercially available products methyl methacrylate (III-1), acrylamide (III-2), N,N-dimethylacrylamide (III-3), and poly(ethylene glycol) monomethyl ether monomethacrylate (III-4), which were used in the examples described below.

When the polymer of the present invention is composed of structural units (1), (2), and (3), the amount of structural unit (1) is 35 to 96 mol%, and preferably 40 to 92 mol%, relative to 100 mol% of the total amount of all structural units, in order to maintain high absorption capacity as a carbon dioxide absorbent in seawater.

When the polymer of the present invention is composed of structural units (1), (2), and (3), the amount of structural unit (2) is 2 to 63 mol%, and preferably 4 to 56 mol%, relative to 100 mol% of the total amount of all structural units, in order to improve affinity for water.

When the polymer of the present invention is composed of structural units (1), (2), and (3), the amount of structural unit (3) is 2 to 63 mol %, and preferably 4 to 56 mol %, relative to 100 mol % of the total amount of all structural units, in order to improve affinity for water.

The molecular weight of the polymer of the present invention is not particularly limited, but the weight average molecular weight (Mw) is preferably in the range of 10,000 to 1,000,000. The weight average molecular weight (Mw) can be measured using high-temperature gel permeation chromatography (GPC)/size exclusion chromatography (SEC) and a refractive index (RI) detector.

For example, when structural units (1) and (2) are polymerized in a 10:1 ratio, the proportion of guanidino groups or cyclic guanidino groups supported per 1 g of polymer is approximately 5.2 meq/g (milliequivalents of guanidino groups or cyclic guanidino groups per 1 g of polymer). In another embodiment, when structural units (1), (2), and (3) are polymerized in a 10:1:10 ratio, the proportion is approximately 2.0 to 3.5 meq/g (milliequivalents of guanidino groups or cyclic guanidino groups per 1 g of polymer). Here, the milliequivalents of guanidino groups or cyclic guanidino groups per 1 g of polymer (meq/g) correspond to the number of millimoles of guanidino groups or cyclic guanidino groups per 1 g of polymer.

(Method for producing the polymer of the present invention)
The polymer of the present invention can be produced by thermal radical polymerization (radical suspension polymerization) of commercially available monomers, or the compound (I) and the compound (II), which are monomers produced according to the methods described in the Examples below or methods analogous thereto, and, if necessary, other monomers such as the above-mentioned styrene derivatives, acrylic acid derivatives, diacrylic acid derivatives, vinyl derivatives, α-olefins, and 1,3-diene derivatives (preferably the compound (III)), in the presence of a polymerization initiator in a solvent that does not affect the reaction. The polymer of the present invention is obtained as a solid.

Polymerization initiators used in the production of the polymer of the present invention include those mentioned above, with 2,2'-azobis(2,4-dimethylvaleronitrile) (ADVN) or 2,2'-azobisisobutyronitrile (AIBN) being particularly preferred. The amount of polymerization initiator added is not particularly limited, but when ADVN is used, for example, it is preferably 0.1 to 20 mol%, more preferably 0.5 to 10 mol%, and even more preferably 1 to 5 mol%, based on the total amount of all monomers used.

There are no particular restrictions on the solvent used in producing the polymer of the present invention, and solvents commonly used in the field of thermal radical polymerization can also be used in the present invention. Specific examples include ethers such as 1,4-dioxane, diethyl ether, and cyclopentyl methyl ether; amides such as dimethylformamide; nitriles such as acetonitrile; halogenated hydrocarbons such as dichloromethane, chloroform, and carbon tetrachloride; aromatic hydrocarbons such as benzene and toluene; water; or mixed solvents of these, with water being preferred.

The temperature during the polymerization reaction is usually 60 to 120°C, preferably 65 to 90°C, and more preferably 70 to 80°C.
The reaction time is usually 1 to 24 hours.

(Carbon dioxide absorbent of the present invention and carbon dioxide absorption method using the same)
Since the polymer of the present invention is obtained as a solid, the polymer can be used alone as a carbon dioxide absorbent in water or seawater. Furthermore, the carbon dioxide absorbent of the present invention may contain additives and the like in addition to the polymer of the present invention, or may be used in combination with other carbon dioxide absorbents.

A carbon dioxide absorbent containing the polymer of the present invention can immobilize carbon dioxide on the polymer of the present invention in water or seawater by reacting carbon dioxide with the substituted guanidino group or cyclic guanidino group contained in the polymer to form a bicarbonate salt of substituted guanidine or cyclic guanidine.

Carbon dioxide absorbents containing the polymers of the present invention are stable, easy to handle, and easy to scale up. Furthermore, recovery of the polymer after carbon dioxide absorption can be easily carried out by solid-liquid separation. Furthermore, since they exhibit high carbon dioxide absorption capacity even in seawater, which is rich in various salts (cations, anions, etc.) in addition to carbon dioxide, they are expected to be suitable for use not only at the laboratory level as described in the test examples below, but also on a pilot scale in the ocean using similar methods.

By using the carbon dioxide absorbent of the present invention, it is possible to reduce the carbon dioxide concentration per unit volume in seawater by up to approximately one-fifth.

The carbon dioxide absorbent of the present invention can naturally be used in water, particularly seawater, but can also be used in waste liquids or waste gases containing high concentrations of carbon dioxide, or in the atmosphere.

(Method for releasing carbon dioxide absorbed (fixed) in the carbon dioxide absorbent of the present invention)
The bicarbonate of the polymer of the present invention, obtained by absorbing (immobilizing) carbon dioxide in a carbon dioxide absorbent containing the polymer of the present invention, can be easily regenerated by heating it in water (distilled water or seawater) at about 100 to 120°C according to or in accordance with the method described in Test Example 2 below. The regenerated polymer of the present invention can be used again as a carbon dioxide absorbent in water, particularly seawater.

Compared to conventional methods that generally require heating conditions at a high temperature of 900°C (specifically, conventional technologies that use aqueous sodium hydroxide or hydroxyethylamine solutions as carbon dioxide absorbents), the carbon dioxide release method of the present invention uses the carbon dioxide again in water, particularly seawater, after it has been released (desorbed), eliminating the need to remove water. Carbon dioxide is released under extremely mild conditions, allowing the polymer of the present invention to be regenerated in an energy-efficient manner.

By repeating the above series of processes (carbon dioxide absorption step (a method for absorbing carbon dioxide using the polymer of the present invention), carbon dioxide generation step (a method for releasing carbon dioxide absorbed (immobilized) in the carbon dioxide absorbent of the present invention), and carbon dioxide absorbent regeneration step of the present invention), it is possible to improve the efficiency of carbon dioxide absorption from seawater, and when utilizing the absorbed and immobilized carbon dioxide, it is possible to realize an environmentally friendly process that significantly reduces the amount of external energy used.

The present invention will be explained in more detail below with reference to the following reference examples, examples, and test examples. However, the present invention is not limited to these examples, and may be changed within the scope of the present invention. % indicates mol/mol% for yield, and % by weight for other values unless otherwise specified. Furthermore, room temperature indicates a temperature between 15°C and 30°C unless otherwise specified.

¹ H (500 MHz)-NMR and ¹³ C (125 MHz)-NMR spectra were measured on an AVANCE III 500 manufactured by Bruker BioSpin Inc. Chemical shift values for ¹ H and ¹³ C-NMR spectra are based on tetramethylsilane. Chemical shifts are reported in δ ppm.
IR spectra were measured using a Nicoleti STM5FT-IR manufactured by Thermo Scientific.
High-resolution mass spectra were measured on a Bruker micrOTOF-Q mass spectrometer.
TLC analysis was carried out on commercially available glass plates with a 0.25 mm layer of Merck Silicagel 60F ₂₅₄ .
The _CO2 concentration was measured using a CGP-31 (DKK-TOA Corporation) (closed space). The oil-free air compressor used was the SA2000S manufactured by SILENTAIR, and the mass flow controller used was the FCS-T1000L manufactured by Fujikin.
The commercially available products used in the examples were potassium phthalimide (Tokyo Chemical Industry Co., Ltd.), 4-vinylbenzyl chloride (Tokyo Chemical Industry Co., Ltd.), divinylbenzene (Tokyo Chemical Industry Co., Ltd.), thiourea (Fujifilm Wako Pure Chemical Industries, Ltd.), ethylene thiourea (Tokyo Chemical Industry Co., Ltd.), iodomethane (Fujifilm Wako Pure Chemical Industries, Ltd.), hydrazine monohydrate (Tokyo Chemical Industry Co., Ltd.), and polystyrene resin (crosslinking agent: 1% divinylbenzene) (Tokyo Chemical Industry Co., Ltd.), and were used as they were.
The molecular sieve 4 Å powder and molecular sieve 3 Å powder (manufactured by Nacalai Tesque) used in the test examples were commercially available products that were dried at 130° C. for 6 hours before use.
All other reagents and solvents were purchased commercially and used as received.
The seawater used in the _CO2 absorption experiment was collected from the surface layer of Kobe Port.

Other abbreviations used in the text have the following meanings:
THF: tetrahydrofuran CDCl ₃ : deuterated chloroform DMSO-d ₆ : deuterated dimethyl sulfoxide DMF: N,N-dimethylformamide MS: molecular sieves

Reference Example 1: Synthesis of 4-vinylbenzylamine (4)

4-Vinylbenzyl chloride (1) (28 mL, 200 mmol) was added to potassium phthalimide (2) (37 g, 200 mmol) and potassium carbonate (K ₂ CO ₃ ) (33 g, 240 mmol) in DMF (200 mL), and the mixture was stirred at room temperature for 12 hours. Ethyl acetate/hexane and water were then added to the reaction mixture, and the aqueous phase was separated and extracted with ethyl acetate. The combined organic phase was washed successively with water (3×100 mL) and saturated brine, dried over magnesium sulfate, filtered, and concentrated under reduced pressure using an evaporator. The residue was dried in vacuo at room temperature for 12 hours to give N-(4-vinylbenzyl)phthalimide (3):

(45.4 g, 172 mmol, yield: 86%) was obtained as a white powder.
^1H NMR (500 MHz, CDCl ₃ , 25℃): δ 4.83 (s, 2H), 5.22 (d, 1H, J _cis = 10.9 Hz), 5.71 (d, 1H, J _trans = 17.6 Hz), 6.67 (dd, 1H, J _cis = 10.9 Hz, J _trans = 17.6 Hz), 7.35 (d, 2H, J = 8.2 Hz), 7.40 (d, 2H, J = 8.0 Hz), 7.70-7.85 (m, 4H);
^13C NMR (125 MHz, CDCl ₃ , 25℃): δ 41.5, 114.3, 123.5, 126.6, 129.0, 132.2, 134.1, 136.0, 136.4, 137.3, 168.2.

The obtained N-(4-vinylbenzyl)phthalimide (3) (26.3 g, 100 mmol), hydrazine monohydrate (12.7 mL, 400 mmol), and ethanol (200 mL) were placed in a 500 mL eggplant-shaped flask equipped with a stirrer and stirred overnight under reflux (70°C), after which the solvent was removed under reduced pressure. The residue was filtered through a glass filter and dried under vacuum at room temperature for 12 hours to obtain 4-vinylbenzylamine (4) (9.5 g, 71 mmol, yield: 71%) as a yellow oil.
^1H NMR (500 MHz, CDCl ₃ , 25℃): δ 3.73 (s, 2H), 5.11 (dd, 1H, J _gem = 0.8 Hz, J _cis = 10.9 Hz), 5.62 (dd, 1H, J _gem = 0.8, J _trans = 17.6 Hz), 6.60 (dd, 1H, J _cis = 10.9 Hz, J _trans = 17.6 Hz), 7.15 (d, 2H, J = 8.1 Hz), 7.27 (d, 2H, J = 8.2 Hz);
^13C NMR (125 MHz, CDCl ₃ , 25℃): δ 46.3, 113.5, 126.4, 127.3, 136.3, 136.6, 143.0.

Reference Example 2: Synthesis of isothiouronium iodide (6)

(The symbols in the formula have the same meanings as above.)

According to the method described in the literature (Aoyagi, N. et al., Synlett, 2014, 25(07), 983; Aoyagi, N. and Endo, T., Synth Commun., 2017, 47, 442), a solution of methyl iodide (1.46 mL, 120 mmol) was added dropwise over approximately 5 minutes to a solution of thiourea (5) (100 mmol) in methanol (100 mL) at room temperature while stirring. The mixture was stirred at room temperature for 12 hours and then concentrated under reduced pressure using an evaporator. The residue was washed with diethyl ether (3 x 50 mL), filtered, and dried under vacuum at room temperature to obtain the isothiouronium iodide derivative (6).

(1) When compound (5a) (in the formula 5, R ¹ , R ² and R ³ are all hydrogen atoms) is used as thiourea (5), S-methylisothiouronium iodide (6a):

was obtained as a pale yellow powder (20.7 g, 95 mmol, 95%).
^1H NMR (500 MHz, DMSO-d ₆ , 25℃): δ 8.88 (brs, 4H), 2.56 (s, 3H);
^13C NMR (125 MHz, DMSO-d ₆ , 25℃): δ 13.4, 171.1.

(2) When compound (5b) (in the formula 5, ^R3 is a hydrogen atom, and ^R1 and ^R2 together form a bond to form a dihydroimidazole ring together with the nitrogen atom to which they are attached) is used as thiourea (5), 2-methylthio-4,5-dihydro-1H-imidazole hydroiodide (6b) is obtained.

was obtained as a white powder (22.0 g, 90 mmol, 90%).
^1H NMR (500 MHz, DMSO-d ₆ , 25℃): δ 2.62 (s, 3H), 3.86 (s, 4H), 9.98 (brs, 2H);
^13C NMR (125 MHz, DMSO-d ₆ , 25℃): δ 13.5, 45.2, 170.5.

Reference Example 3: Synthesis of 4-vinylbenzylguanidine (I)

(The symbols in the formula have the same meanings as above.)

According to the method described in the literature (Aoyagi, N. et al., Synlett, 2014, 25(07), 983; Aoyagi, N. and Endo, T., Synth Commun., 2017, 47, 442), 4-vinylbenzylamine (4) (100 mmol) synthesized in Reference Example 1, isothiouronium iodide (6) (100 mmol) synthesized in Reference Example 2, and THF (100 mL) were placed in a flask (200 mL) equipped with a stirrer, and the mixture was stirred overnight under reflux (70°C). The solvent was removed from the reaction mixture under reduced pressure. The residue was washed with diethyl ether and dried under vacuum at room temperature. The resulting solid was then neutralized with aqueous sodium hydroxide (40 wt%) to obtain 4-vinylbenzylguanidine (I).

(1) When S-methylisothiouronium iodide (6a) obtained in Reference Example 2 (1) is used as isothiouronium iodide (6), 4-vinylbenzylguanidine (I-1):

was obtained as a white powder (yield: 72%).
^1H NMR (500 MHz, CDCl ₃ , 25℃): δ 4.21(s, 2H), 5.22 (d, 1H, J _cis = 10.9 Hz), 5.72 (d, 1H, J _trans = 17.6 Hz), 6.68 (dd, 1H, J _cis = 10.9 Hz, J _trans = 17.6 Hz), 7.24 (d, 2H, J = 8.0 Hz), 7.35 (d, 2H, J = 8.1 Hz);
^13C NMR (125 MHz, CDCl ₃ , 25℃): δ 46.3, 77.4, 113.9, 126.6, 127.6, 136.5, 136.8, 138.5.

(2) When 2-methylthio-4,5-dihydro-1H-imidazole hydroiodide (6b) obtained in Reference Example 2 (2) is used as isothiouronium iodide (6), 2-(4-vinylbenzylamino)-4,5-dihydro-1H-imidazole (I-2) is obtained.

was obtained as a white powder (yield: 77%).
^1H NMR (500 MHz, CDCl ₃ , 25℃): δ 3.50 (s, 4H), 4.31 (s, 2H), 5.22 (dd, 1H, J _gem = 0.7 Hz, J _cis = 10.9 Hz), 5.71 (dd, 1H, J _gem = 0.8, J _trans = 17.6 Hz), 6.68 (dd, 1H, J _cis = 10.9 Hz, J _trans = 17.6 Hz), 7.25 (d, 2H, J = 8.5 Hz), 7.35 (d, 2H, J = 8.1 Hz);
^13C NMR (125 MHz, CDCl ₃ , 25℃): δ 47.5, 77.36, 133.8, 126.5, 127.72, 136.54, 136.7, 139.1, 161.7.

Example 1: Synthesis of guanidino group-containing polystyrene (compound (1-1)) An aqueous solution of acacia gum (6.3 g) and sodium chloride (7.9 g) was placed in a round-bottom flask equipped with a stirrer, and nitrogen gas was bubbled through for 30 minutes to deoxygenate the mixture. After deoxygenation, 4-vinylbenzylguanidine (I-1) (20 mmol) obtained in Reference Example 3(1), divinylbenzene (II-1) (2 mmol), and 2,2-azobis(2,4-dimethylvaleronitrile) (ADVN) (0.27 mmol) were added, and the mixture was stirred at 70°C for 18 hours under a nitrogen atmosphere. After cooling to room temperature, the mixture was filtered through a glass filter, washed successively with water, methanol, toluene, and THF, and dried under vacuum at 60°C for 12 hours to obtain compound (1-1) (2.3 g, yield: 60%) as a pale yellow bead-like solid.
IR (ATR): 3340, 2918, 1683, 1653, 1559, 1457, 1322 cm ^-1

Example 2: Synthesis of cyclic guanidino group-containing polystyrene (compound (1-2)) An aqueous solution of acacia gum (6.3 g) and sodium chloride (7.9 g) was placed in a round-bottom flask equipped with a stirrer, and nitrogen gas was bubbled through for 30 minutes to deoxygenate the mixture. After deoxygenation, 2-(4-vinylbenzylamino)-4,5-dihydro-1H-imidazole (I-2) (20 mmol) obtained in Reference Example 3(2), divinylbenzene (II-1) (2 mmol), and 2,2-azobis(2,4-dimethylvaleronitrile) (ADVN) (0.27 mmol) were added, and the mixture was stirred at 70°C for 18 hours under a nitrogen atmosphere. After cooling to room temperature, the mixture was filtered through a glass filter, washed successively with water, methanol, toluene, and THF, and dried under vacuum at 60°C for 12 hours to obtain compound (1-2) (3.0 g, yield: 69%) as a pale yellow bead-like solid.
IR (ATR): 3293, 2919, 1663, 1654, 1559, 1508, 1457, 1285 cm ^-1

Example 3: Synthesis of cyclic guanidino group-containing copolymer (compound (1-3)) An aqueous solution (30 mL) of gum acacia (0.45 g) and sodium chloride (1.35 g) was placed in a round-bottom flask equipped with a stirrer, and nitrogen gas was bubbled through for 30 minutes to deoxygenate the mixture. After deoxygenation, 2-(4-vinylbenzylamino)-4,5-dihydro-1H-imidazole (I-2) (15 mmol) obtained in Reference Example 3(2), divinylbenzene (II-1) (0.2 g; 1.5 mmol), methyl methacrylate (III-1) (1.6 mL; 15 mmol), and 2,2-azobis(2,4-dimethylvaleronitrile) (ADVN) (74.5 mg; 0.3 mmol) were added, and the mixture was stirred under a nitrogen atmosphere at 70°C for 18 hours. After cooling to room temperature, the mixture was filtered through a glass filter, washed successively with water, methanol, toluene, and THF, and dried under vacuum at 40°C for 12 hours to obtain compound (1-3) (4.48 g, yield: 94%) as a pale yellow solid.
IR (ATR): 3648, 3177, 1670, 1558, 1507, 1287 cm ^-1

Example 4: Synthesis of cyclic guanidino group-containing copolymer (compound (1-4)) An aqueous solution (30 mL) of gum acacia (0.45 g) and sodium chloride (1.35 g) was placed in a round-bottom flask equipped with a stirrer, and nitrogen gas was bubbled through for 30 minutes to deoxygenate the mixture. After deoxygenation, 2-(4-vinylbenzylamino)-4,5-dihydro-1H-imidazole (I-2) (15 mmol) obtained in Reference Example 3(2), divinylbenzene (II-1) (0.2 g; 1.5 mmol), acrylamide (III-2) (1.07 g; 15 mmol), and 2,2-azobis(2,4-dimethylvaleronitrile) (ADVN) (74.5 mg; 0.3 mmol) were added, and the mixture was stirred at 70°C for 18 hours under a nitrogen atmosphere. After cooling to room temperature, the mixture was filtered through a glass filter, washed successively with water, methanol, toluene, and THF, and dried under vacuum at 40°C for 12 hours to obtain compound (1-4) (2.76 g, yield: 64%) as a pale yellow solid.
IR (ATR): 1653, 1569, 1558, 1395 cm ^-1

Example 5: Synthesis of cyclic guanidino group-containing copolymer (compound (1-5)) An aqueous solution (18 mL) of gum acacia (0.27 g) and sodium chloride (0.81 g) was placed in a round-bottom flask equipped with a stirrer, and nitrogen gas was bubbled through for 30 minutes to deoxygenate the mixture. After deoxygenation, 2-(4-vinylbenzylamino)-4,5-dihydro-1H-imidazole (I-2) (9 mmol) obtained in Reference Example 3(2), divinylbenzene (II-1) (0.12 g; 0.9 mmol), N,N-dimethylacrylamide (III-3) (0.89 g; 9 mmol), and 2,2-azobis(2,4-dimethylvaleronitrile) (ADVN) (44.7 mg; 0.18 mmol) were added, and the mixture was stirred at 70°C for 18 hours under a nitrogen atmosphere. After cooling to room temperature, the mixture was filtered through a glass filter, washed successively with water, methanol, toluene, and THF, and dried under vacuum at 40°C for 12 hours to obtain compound (1-5) (1.71 g, yield: 44%) as a pale yellow solid.
IR (ATR): 2911, 1653, 1576, 1558, 1507, 1395 cm ^-1

Example 6: Synthesis of cyclic guanidino group-containing copolymer (compound (1-6)) An aqueous solution (30 mL) of gum acacia (0.45 g) and sodium chloride (1.35 g) was placed in a round-bottom flask equipped with a stirrer, and nitrogen gas was bubbled through for 30 minutes to deoxygenate the mixture. After deoxygenation, 2-(4-vinylbenzylamino)-4,5-dihydro-1H-imidazole (I-2) (15 mmol) obtained in Reference Example 3(2), divinylbenzene (II-1) (0.2 g; 1.5 mmol), poly(ethylene glycol) monomethyl ether monomethacrylate (III-4) (n = 4) (3.9 mL; 15 mmol), and 2,2-azobis(2,4-dimethylvaleronitrile) (ADVN) (74.5 mg; 0.3 mmol) were added, and the mixture was stirred at 70°C for 18 hours under a nitrogen atmosphere. After cooling to room temperature, the mixture was filtered through a glass filter, washed successively with water, methanol, toluene, and THF, and dried under vacuum at 40°C for 12 hours to obtain compound (1-6) (5.15 g, yield: 69%) as a pale yellow solid.
IR (ATR): 3157, 2892, 1670, 1558, 1457, 1398, 1287 cm ^-1

Example 7 Synthesis of Cyclic Guanidino Group-Containing Copolymer (Compound (1-7)) [0123] A suspension polymerization reaction was carried out under the same conditions as in Example 5, except that the amounts of 2-(4-vinylbenzylamino)-4,5-dihydro-1H-imidazole (I-2) were changed to 10 mmol, divinylbenzene (II-1) to 1 mmol, N,N-dimethylacrylamide (III-3) to 1 mmol, and 2,2-azobis(2,4-dimethylvaleronitrile) (ADVN) to 0.2 mmol, to obtain compound (1-7) (2.49 g, yield: >99%) as a pale yellow solid.
IR (ATR): 3689, 3156, 1868, 1663, 1541, 1507, 1287 cm ^-1

Test Example 1: _CO2 absorption experiment in distilled water using the polymer and molecular sieve of the present invention (experimental procedure)
(1) A stirrer and distilled water (50 mL) were added to a 50 mL dedicated cell, and a carbon dioxide gas concentration meter was immersed in the solution and allowed to stabilize for approximately 10 minutes. After numerical stabilization, the polymers of the present invention (compound (1-1), compound (1-2)), molecular sieve 3 Å powder, and molecular sieve 4 Å powder were each added to separate cells, and the change in _CO2 concentration in each cell was measured over time. The amount of the polymers of the present invention used was 100 equivalents of the amount of _CO2 in the cell, calculated as amino groups, and the amount of molecular sieve powder used was the same as the amount of the polymer of the present invention used. In addition, the polymers of the present invention were vacuum dried at 60°C for approximately 12 hours before use to completely remove the solvent before use.
(2) The same procedure as in (1) above was followed, except that the scale was increased to 10 times the amount in (1) above, a 500-ml Erlenmeyer flask was used instead of the 50-mL dedicated cell, and the amount of the polymer of the present invention used was 10 equivalents of the amount of _CO in the cell, calculated as amino groups, to measure the change in _CO concentration over time due to the addition of the polymer of the present invention (compound (1-2)) and molecular sieve 4 Å powder.

(Experimental results)
The results of the change in _CO2 concentration over time for each sample are shown in Table 1 (volume 50 mL), Table 2 (volume 500 mL), Figure 1a (volume 50 mL), and Figure 1b (volume 500 mL) below. Figure 1 confirms that in distilled water, the polymers of the present invention (compound (1-1) and compound (1-2)), molecular sieve 3 Å powder, and molecular sieve 4 Å powder all exhibit good carbon dioxide absorption capacity, and a similar tendency was observed when the experiment was scaled up.

Test Example 2: _CO2 absorption experiment in seawater using the polymer and molecular sieve of the present invention (experimental procedure)
(1) A stirrer and seawater (50 mL) were added to a 50 mL dedicated cell, and a carbon dioxide gas concentration meter was immersed in the solution and allowed to stabilize for approximately 10 minutes. After numerical stabilization, the polymer of the present invention (compound (1-2)) and molecular sieve 4Å powder were added to separate cells, and the change in _CO2 concentration in each cell was measured over time. The amount of the polymer of the present invention used was 100 equivalents of the amount of _CO2 in the cell, calculated as amino groups, and the amount of molecular sieve powder used was the same as the amount of the polymer of the present invention used. In addition, the polymer of the present invention was vacuum dried at 60°C for approximately 12 hours before use to completely remove the solvent before use.
(2) The same procedure as in (1) above was followed, except that the scale was increased to 10 times the amount in (1) above, a 500 ml Erlenmeyer flask was used instead of the 50 ml dedicated cell, and the amount of the polymer of the present invention used was 10 equivalents of the amount of _CO in the cell in terms of amino groups. The changes in _CO concentration due to the addition of the polymer of the present invention (compound (1-2)) and molecular sieve 4 Å powder were measured over time.

(Experimental results)
The results of the change in _CO2 concentration over time for each sample are shown in Table 3 (volume 50 mL), Table 4 (volume 500 mL), Figure 2a (volume 50 mL), and Figure 2b (volume 500 mL). Figure 2 shows that in seawater, the molecular sieve 4Å powder hardly absorbed carbon dioxide, while the polymer of the present invention (compound (1-2)) exhibited good carbon dioxide absorption capacity similar to that in distilled water. It was also confirmed that the polymer of the present invention can maintain good carbon dioxide absorption capacity even when scaled up.

Test Example 3: Experiment on carbon dioxide release from the polymer of the present invention (compound (1-2)) after carbon dioxide absorption under heating (100°C) (Experimental procedure)
The polymer of the present invention (compound (1-2)) (2 g) that had absorbed carbon dioxide in Test Example 1 was placed in a two-necked flask (100 mL), and the flask was heated in an oil bath (100°C) while nitrogen gas was flowed into the flask at a rate of 100 mL/min using a mass flow controller (MFC), and the _CO2 concentration at the outlet side was measured over time. The same experiment was repeated twice.

(Experimental results)
The results of the change in _CO2 concentration (ppm) over time are shown in Table 5 below and Figure 3. As shown in Figure 3, it was confirmed that the polymer of the present invention (compound (1-2)) that had absorbed carbon dioxide efficiently released carbon dioxide in a short time under heated conditions, and the polymer of the present invention was regenerated with good reproducibility.

Test Example 4: _CO2 absorption experiment in seawater using the polymers of the present invention (compounds (1-2), (1-3), (1-4), (1-5), and (1-6)) (experimental procedure)
A stirrer and seawater (50 mL) were added to a dedicated cell with a capacity of 50 mL, and a carbon dioxide concentration meter was immersed in the solution and allowed to stabilize for approximately 10 minutes. After numerical stabilization, the polymers of the present invention (i.e., Compound (1-2), Compound (1-3), Compound (1-4), Compound (1-5), or Compound (1-6)) were added to separate cells, and the change in _CO2 concentration in each cell was measured over time. The amount of the polymer used was 100 equivalents of the amount of _CO2 in the cell, calculated as amino groups. Furthermore, the polymers of the present invention were vacuum dried at 40°C for approximately 12 hours before use to completely remove the solvent before use.

(Experimental results)
The results of the change in _CO2 concentration over time for each sample are shown in Table 6 below and Figure 4. Figure 4 confirms that all of the polymers of the present invention exhibit good carbon dioxide absorption capacity in seawater. Among them, compound (1-5) was confirmed to exhibit high carbon dioxide absorption capacity.

Test Example 5: _CO2 absorption experiment in seawater using the polymer of the present invention (compound (1-2) or compound (1-7)) (experimental procedure)
A stirrer and seawater (500 mL) were added to a 500 mL Erlenmeyer flask, and a carbon dioxide gas concentration meter was immersed in the solution and allowed to stabilize for approximately 10 minutes. After numerical stabilization, 1 g of the polymer of the present invention (i.e., compound (1-2) or compound (1-7)) was added to each separate cell, and the change in _CO2 concentration in each cell was measured over time. The amount of the polymer of the present invention used was 100 equivalents of the amount of _CO2 in the cell, calculated as amino groups. Furthermore, the polymer of the present invention was vacuum dried at 40°C for approximately 12 hours before use to completely remove the solvent before use.

(Experimental results)
The results of the change in _CO2 concentration over time for each sample are shown in Table 7 below and Figure 5. Figure 5 confirms that in seawater, the polymers of the present invention can all maintain good carbon dioxide absorption capacity even when scaled up.

Test Example 6: Experiment on carbon dioxide release from the polymer of the present invention (compound (1-2) or compound (1-7)) after carbon dioxide absorption under heating (100°C) (Experimental procedure)
Compound (1-2) that had absorbed carbon dioxide in Test Example 2, or compound (1-7) (1 g) that had absorbed carbon dioxide in Test Example 5, was placed in a two-necked flask (100 mL), and heated in an oil bath (100°C) while flowing nitrogen gas into the two-necked flask at a rate of 40 mL/min using a mass flow controller (MFC). The _CO2 concentration on the outlet side was measured over time and recorded every 30 seconds. A schematic diagram of the apparatus used in the carbon dioxide release experiment is shown in Figure 6.

(Experimental results)
The results of the change in _CO2 concentration (ppm) over time are shown in Table 8 below and Figure 7. Figure 7 confirms that the polymer of the present invention (compound (1-2) or compound (1-7)) that had absorbed carbon dioxide efficiently released carbon dioxide in a short period of time under heated conditions, and the polymer of the present invention was regenerated with good reproducibility.

The carbon dioxide absorbent of the present invention is easy to synthesize, stable and easy to handle even in water, particularly seawater where various salts are present, and has a high carbon dioxide absorption capacity. Therefore, it has the advantage of being able to efficiently absorb and immobilize carbon dioxide, even at room temperature and atmospheric pressure, even when carbon dioxide is diluted and present in the ocean at low concentrations. Furthermore, the carbon dioxide absorbent of the present invention not only makes it easy to recover carbon dioxide after immobilization, but also allows the immobilized carbon dioxide to be released under mild conditions and effectively used as a carbon source. Therefore, the present invention can provide a new and effective method of DOC (Direct-Ocean-Capture).

This application is based on patent application No. 2024-030458, filed in Japan on February 29, 2024, the contents of which are incorporated in their entirety herein.

Claims (14)

Formula (1):
[In the formula,
^R1 and ^R2 each independently represent a hydrogen atom or an optionally substituted alkyl group, or ^R1 and ^R2 bond to each other to form an optionally substituted 5- to 8-membered ring together with the nitrogen atom to which they are attached; and ^R3 represents a hydrogen atom or an optionally substituted alkyl group.
and a repeating structural unit (1) represented by formula (2):
A polymer comprising a repeating structural unit (2) represented by the following formula: wherein each repeating unit is contained randomly or in blocks. The polymer described in claim 1, wherein the polymer consists of repeating structural units (1) and repeating structural units (2). The polymer according to claim 1 or 2, wherein R ¹ , R ² and R ³ are each independently a hydrogen atom or a methyl group. 3. The polymer of claim 1, wherein R ¹ and R ² are bonded to each other and together with the nitrogen atom to which they are attached form an imidazoline or a 1,4,5,6-tetrahydropyrimidine. The polymer has the formula (3):
wherein R is a group represented by the following formula:
(In the formula, the wavy line indicates the bonding position to the main chain, ^R4 represents a _C1-6 alkyl group, and n represents an integer of 1 to 10.)
and R' represents a hydrogen atom or a methyl group.
The polymer according to claim 1, further comprising a repeating structural unit (3) represented by: The polymer of claim 5 , wherein R ¹ , R ² and R ³ are each independently a hydrogen atom or a methyl group. 6. The polymer of claim 5, wherein R ¹ and R ² are bonded to each other and, together with the nitrogen atom to which they are attached, form an imidazoline or a 1,4,5,6-tetrahydropyrimidine. The polymer according to any one of claims 5 to 7, which is a random copolymer in which, of all the structural units of the polymer, repeating structural unit (1) accounts for 35 to 96 mol%, repeating structural unit (2) accounts for 2 to 63 mol%, and repeating structural unit (3) accounts for 2 to 63 mol%. A carbon dioxide absorbent containing the polymer described in any one of claims 1 to 8. The carbon dioxide absorbent according to claim 9, for absorbing carbon dioxide in seawater. Formula (I):
[Each symbol in the formula has the same meaning as defined above.]
and a compound represented by formula (II):
2. A method for producing a polymer according to claim 1, characterized by reacting a compound represented by the following formula (I) with a polymerization initiator under heating. The method of claim 8, wherein the polymerization initiator is 2,2'-azobis(2,4-dimethylvaleronitrile) (ADVN) or azobisisobutyronitrile (AIBN). Formula (I):
[Each symbol in the formula has the same meaning as defined above.]
a compound represented by formula (II):
and a compound represented by formula (III):
[Each symbol in the formula has the same meaning as defined above.]
6. The method for producing a polymer according to claim 5, wherein the compound represented by the formula (I) is reacted with a polymerization initiator under heating. The method of claim 13, wherein the polymerization initiator is 2,2'-azobis(2,4-dimethylvaleronitrile) (ADVN) or azobisisobutyronitrile (AIBN).