Carboxylic acid polymers with upper critical solution temperature-type temperature responsiveness and their applications

Abstract

To provide a carboxylic acid-based polymer exhibiting phase separability of an upper critical solution temperature (USCT) type and a production method thereof, as well as a forward osmosis membrane water treatment system including the polymer and a driving solution used for osmotic pressure power generation.SOLUTION: Provided are: a carboxylic acid-based polymer including a structural unit (A) and a structural unit (B), obtained by copolymerizing a predetermined amount of an aromatic vinyl monomer,and exhibiting UCST type phase separability in which a molecular weight and a molecular weight distribution are controlled in a prescribed range; and a production method thereof. In addition used are: an aqueous solution and a driving solution which are obtained by using the same, phase-separated at a temperature lower than UCST, and become a uniform solution at a temperature equal to or higher than UCST.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a carboxylic acid polymer having upper critical solution temperature (UCST)-type temperature responsiveness, which is useful as a driving solution for a forward osmosis membrane water treatment system or an osmotic pressure power generation system, and to a forward osmosis membrane water treatment system and an osmotic pressure power generation driving solution containing the temperature-responsive carboxylic acid polymer as applications thereof. [Background technology]

[0002] Stimuli-responsive polymers are materials whose physical and chemical properties change in response to external stimuli such as temperature, pH, ionic strength, light irradiation, and application of an electromagnetic field. Temperature-responsive polymers in particular have been widely studied because of their potential applications in fields such as drug delivery systems, gene therapy, separation of metals and cells, bioimaging, catheters, artificial muscles, optical devices, catalysts, forward osmosis water treatment systems, and osmotic power generation (e.g., Patent Documents 1 and 2).

[0003] Thermoresponsive polymers are polymers whose solubility in a solvent changes in response to an external temperature stimulus. Due to the temperature-dependent phase change, they are classified into two types: those that exhibit a lower critical solution temperature (LCST) behavior and those that exhibit an upper critical solution temperature (UCST) behavior.

[0004] LCST polymers, such as poly(N-isopropylacrylamide) (P-NIPAM), have been actively studied, primarily in the biomedical field, and many other LCST polymers have been reported, including P-NIPAM, poly(N-isopropylmethacrylamide), poly(N-vinylcaprolactam), and poly(oligoethylene glycol) acrylate. However, there have only been a few reports of polymers that exhibit UCST behavior, and in particular, there have been almost no reports of ionic polymers that exhibit UCST behavior due to intermolecular interactions over a wide polymer concentration range.

[0005] Known ionic polymers include copolymers of acrylic acid and acrylonitrile (e.g., Non-Patent Document 1), copolymers of acrylic acid and styrene, which has excellent hydrolysis resistance (e.g., Non-Patent Document 2), and homopolymer polyacrylic acid (PAA) and its sodium salt (PAA-Na) (e.g., Non-Patent Document 3). However, of these, only acrylic acid / acrylonitrile copolymers have been reported as temperature-responsive polymers.

[0006] The present inventors have discovered that polyampholites composed of anionic monomers, styrene sulfonic acid, and cationic monomers, 4-vinylbenzyltrimethylammonium, exhibit UCST due to electrostatic and hydrophobic interactions between polymer molecules, and also exhibit high osmotic pressure and hydrolysis resistance, making them useful as working solutions for forward osmosis membrane water treatment systems (e.g., Patent Document 3).

[0007] It is also known that a polymer containing an ester structural unit in which a carboxyl group is aminoalkylated and a structural unit containing a sulfo group, as well as a polymer further having a carboxyl group-containing structural unit, is used in the work solution of the forward osmosis membrane water treatment system (for example, Patent Document 4). [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Patent No. 4069221 [Patent Document 2] Patent No. 6125863 [Patent Document 3] International Publication No. 2020 / 188839 [Patent Document 4] Patent Publication No. 2021-107491 [Non-patent literature]

[0009] [Non-Patent Document 1] Chuanzhuang Zhao et al.; Macromolecules, Vol. 52, pp. 4441-4446, 2019 [Non-patent document 2] S. Toppet et al.; Journal of Polymer Science Part-A, Vol. 13, No. 8, pp. 1879-1887, 1975 [Non-patent document 3] Qingchun Ge et al.;Water Research, Vol. 46, pp. 1318-1326, 2012 Summary of the Invention [Problem to be solved by the invention]

[0010] Non-Patent Document 1 describes that a 0.5 wt % to 2.0 wt % aqueous solution of an acrylic acid copolymer containing 4.5 mol % to 22 mol % of acrylonitrile polymerized units and having a number average molecular weight of 27,000 to 37,000 daltons exhibits UCST properties of 7.2°C to 37.9°C. However, there was no mention of physical properties in the high-concentration range, and there were issues with manifesting UCST behavior across a wide range of polymer concentrations. Because the molecular weight exceeds 20,000, the viscosity and UCST are expected to be quite high in the high-concentration range. Furthermore, because the cyano group in the acrylonitrile polymerization unit hydrolyzes in high-temperature water and converts to a carboxyl group, there is an issue with the UCST decreasing or disappearing over time, making it difficult to achieve stable and reproducible effects.

[0011] Non-Patent Document 2 reports on the copolymerization of acrylic acid and styrene, but does not mention the relationship between polymer structure and temperature responsiveness, such as UCST. The document also reports that the copolymerization of monomers having functional groups capable of forming hydrogen bonds with solvents, such as the carboxyl group of acrylic acid, is significantly affected by the reaction solvent and other factors on the reactivity ratio. Thus, it has been difficult to design molecules capable of producing UCST as an anionic polymer.

[0012] Non-Patent Document 3 indicates that the homopolymer polyacrylic acid sodium salt (PAA-Na) has potential as a working solution for forward osmosis membrane water treatment systems. However, there is no description of a carboxylic acid-based copolymer with temperature responsiveness, and it remains difficult to design specific molecules capable of exhibiting UCST.

[0013] Conventional upper critical solution temperature-responsive polymers (hereinafter sometimes referred to as UCST polymers) have problems such as using expensive cationic monomers as raw materials and having excessively high UCST and aqueous solution viscosity. Therefore, there has been a demand for low-cost UCST polymers with low UCST and aqueous solution viscosity.

[0014] The issues with conventional UCST polymers are explained below using the draw solution (hereinafter sometimes abbreviated as DS) of a forward osmosis membrane water treatment system as an example of its application.

[0015] Figure 1 is a diagram showing a schematic diagram of the phase separation curve of a UCST polymer. The upper part of the solid line in Figure 1 represents the homogeneous phase in which the polymer is dissolved, and the lower part represents the two-phase separation state. Figure 2 shows a schematic diagram of a forward osmosis water treatment system using a UCST (cooling separation) working solution (DS). In Figures 1 and 2, a to e correspond to each other. For example, each of the steps a → b, b → c → d / e, and d → a will be described below. Step a → b: A 40 wt % polymer aqueous solution with sufficient osmotic pressure as the draw solution (DS), shown as a in Figures 1 and 2, draws water from the water to be treated at 50°C through a semipermeable membrane, and as shown as b in Figures 1 and 2, it is diluted to a concentration of 20 wt %, reducing the osmotic pressure (water absorption capacity). Steps b → c → d / e: When the diluted 20 wt % aqueous polymer solution b is cooled to 20°C, as shown by c in Figures 1 and 2, phase separation occurs, and fresh water shown by e in Figures 1 and 2 and a 40 wt % aqueous polymer solution shown by d in Figures 1 and 2 are reproduced. Step d→step a: The regenerated 40 wt % polymer aqueous solution is heated to 50°C, and fresh water is again sucked out from the water to be treated. Figure 2 shows a schematic diagram of the above cycle, and Figure 1 shows the relationship between the polymer concentration and the phase separation temperature in each step.

[0016] The ideal slope of the phase separation curve and the phase separation temperature for the above system cannot be generalized because they vary depending on the temperature of the water to be treated, the operating temperature, the osmotic pressure and viscosity of the work solution, and the physical properties of the semipermeable membrane used. For example, in the case of the slope of the phase separation curve shown in Figure 1, approximately the same amount of fresh water as the work solution can be produced per cycle. In contrast, if the slope of the phase separation curve were, for example, twice as large as in Figure 1, the amount of fresh water produced per cycle would be halved. Conversely, if the slope were halved, the amount of fresh water produced per cycle would increase.

[0017] Furthermore, if the phase separation temperature (dissolution temperature) is too high, it becomes difficult to produce freshwater at around 40 to 60°C from warm seawater or petroleum-produced water. Also, if the viscosity of the aqueous solution is too high, it places a burden on the circulation of the working solution, especially after it has been cooled and regenerated, and furthermore, the water absorption rate decreases due to concentration polarization within the semipermeable membrane (which makes it difficult for the polymer to diffuse into the support layer).

[0018] The polyampholite described in Patent Document 3 (WO 2020 / 188839) had the following problems: (1) in the absence of inorganic salts, the UCST increased dramatically and the slope of the phase separation curve was too steep; (2) the viscosity of the aqueous solution was too high; and (3) the cost was high due to the use of a large amount of expensive cationic monomers. The polymer described in Patent Document 4 (JP 2021-107491 A) is a strong electrolyte polymer in which amino groups and sulfo groups are essential in the polymer side chains, and furthermore, since the above-mentioned ester structural unit is the skeleton, there are concerns about hydrolysis. As described in Patent Documents 3 and 4, copolymerization of monomers having functional groups capable of forming hydrogen bonds with solvents, such as the carboxyl group of acrylic acid, has traditionally been difficult, even in terms of molecular design that would enable the expression of UCST as an anionic polymer. Furthermore, conventional UCST polymers (e.g., Patent Documents 3 and 4) are strong electrolyte polymers, and although they have the advantage of high osmotic pressure, in the absence of inorganic salts they have problems such as an excessively steep slope of the phase separation curve, an excessively high phase separation temperature, and an excessively high aqueous solution viscosity, and therefore there has been a strong demand for improvements.

[0019] Therefore, an object of the present invention is to provide a material that can solve the above-mentioned conventional problems and a system that can utilize the material. Specifically, the present invention provides a carboxylic acid polymer having upper critical solution temperature (UCST)-type temperature responsiveness, which is useful as a driving solution for a forward osmosis membrane water treatment system or an osmotic pressure power generation system, and a driving solution for a forward osmosis membrane water treatment system or an osmotic pressure power generation system, which contains a temperature-responsive carboxylic acid polymer for such use. [Means for solving the problem]

[0020] As a result of extensive research, the present inventors have found that polymers obtained by copolymerizing a specific amount of an aromatic vinyl monomer, such as styrene or vinylbenzoic acid, with a widely available and inexpensive carboxyl group-containing vinyl monomer, such as acrylic acid or methacrylic acid, as the main component, have low solution viscosity within a specific range of molecular weight and molecular weight distribution, and exhibit UCST-type phase separation properties that are more suitable as a working solution for forward osmosis water treatment systems and osmotic power generation, thereby completing the present invention.

[0021] That is, the present invention relates to the following inventions. [1] A temperature-responsive carboxylic acid polymer comprising the following structural units (A) and (B), wherein the content of structural unit (B) is 5 mol % to 30 mol % based on the total of structural units (A) and (B), the polymer has a number-average molecular weight Mn of 500 to 20,000 daltons (Da), and a ratio Mw / Mn of number-average molecular weight Mn to weight-average molecular weight Mw of 1.00 to 2.00; the polymer is water-soluble and has an upper critical solution temperature (hereinafter sometimes abbreviated as UCST), and a 50 wt % aqueous solution thereof has a viscosity of 10 mPa s to 10,000 mPa s at 25°C, and is hydrophobic at temperatures below the UCST and hydrophilic at temperatures equal to or higher than the UCST. The structural unit (A) is represented by the following general formula (1): [ka] (In formula (1), R1 represents a hydrogen atom or a carboxyl group, R2 represents a hydrogen atom, a methyl group, an ethyl group, or a carboxymethyl group, and M represents a proton, an ammonium cation, or an alkali metal cation.) The structural unit (B) is represented by the following general formula (2): [ka] (In formula (2), R3 represents a hydrogen atom, a hydroxyl group, or a carboxyl group.) [2] Item [1]: The carboxylic acid polymer according to item [1] further contains the following structural unit (C) in addition to the structural units (A) and (B) of item [1], and the content of the structural unit (C) is 0.1 mol % to 10 mol % based on the total of the structural units (A) to (C). The structural unit (C) is represented by the following general formula (3): [ka] (In formula (3), Q represents a structural unit derived from an acrylamide vinyl monomer.) [3] Item [1] or [2], wherein the number average molecular weight Mn is 500 to 5,000 daltons (Da). [4] The structural unit (A) is a structural unit derived from at least one carboxyl group-containing vinyl monomer selected from the group consisting of acrylic acid, methacrylic acid, ethacrylic acid, itaconic acid, and maleic acid, and the structural unit (B) is a structural unit derived from at least one aromatic vinyl monomer selected from the group consisting of styrene, vinylbenzoic acid, and hydroxystyrene. Item [1] Carboxylic acid polymer. [5] The carboxylic acid polymer according to item [2], wherein the structural unit (A) is a structural unit derived from at least one vinyl monomer selected from the group consisting of acrylic acid, methacrylic acid, ethacrylic acid, itaconic acid, and maleic acid, the structural unit (B) is a structural unit derived from at least one vinyl monomer selected from the group consisting of styrene, vinylbenzoic acid, and hydroxystyrene, and the structural unit (C) is a structural unit derived from at least one acrylamide-based vinyl monomer selected from the group consisting of acrylamide, methacrylamide, 4-acroylmorpholine, and 4-methacryloylmorpholine. [6] The carboxylic acid polymer according to any one of items [1] to [5], wherein the UCST of a 20 wt% aqueous solution is 50°C to 100°C, that of a 30 wt% aqueous solution is 40°C to 90°C, that of a 40 wt% aqueous solution is 30°C to 70°C, and that of a 50 wt% aqueous solution is 10°C to 60°C. [7] The carboxylic acid polymer according to any one of items [1] to [6], wherein the viscosity of a 50 wt % aqueous solution at 40° C. is 10 mPa·s to 5000 mPa·s. [8] Item [1] to [7], wherein the carboxylic acid polymer has a structure in which the terminal of the polymer is a carboxyl group. [9] A method for producing a temperature-responsive carboxylic acid polymer that is hydrophobic below UCST and hydrophilic at or above UCST, comprising polymerizing a monomer mixture containing a carboxyl group-containing vinyl monomer and an aromatic vinyl monomer in a hydrophilic solvent using a hydrophilic azo radical polymerization initiator, or a hydrophilic azo radical polymerization initiator and a RAFT agent or a hydrophilic chain transfer agent, the ratio of the aromatic vinyl monomer to all monomers in the monomer mixture is 9.0 mol % to 20.0 mol %; the total ratio of the hydrophilic azo radical polymerization initiator, the RAFT agent, and the hydrophilic chain transfer agent to all the monomers in the monomer mixture is 0.90 mol % to 13.0 mol %; method.

[10] the carboxyl group-containing vinyl monomer is one or more carboxyl group-containing vinyl monomers selected from the group consisting of acrylic acid, methacrylic acid, ethacrylic acid, itaconic acid, and maleic acid; The aromatic vinyl monomer is one or more aromatic vinyl monomers selected from the group consisting of styrene, vinyl benzoic acid, and hydroxystyrene. The manufacturing method described in item [9].

[11] Item [9] or

[10] , the monomer mixture further contains an acrylamide vinyl monomer, the production method according to item [9] or

[10] , the proportion of the aromatic vinyl monomer to all the monomers in the monomer mixture is 10.0 mol % to 15.0 mol %, and the proportion of the acrylamide vinyl monomer is 0.1 mol % to 5.0 mol %; method.

[12] Item

[11] The method according to item

[11] , wherein the acrylamide vinyl monomer is one or more acrylamide vinyl monomers selected from the group consisting of acrylamide, methacrylamide, 4-acryloylmorpholine, and 4-methacryloylmorpholine.

[13] The method according to any one of items [9] to

[12] , wherein the hydrophilic azo radical polymerization initiator is a hydrophilic polymerization initiator having a carboxyl group, the RAFT agent is a RAFT agent having a carboxyl group, and the hydrophilic chain transfer agent is a hydrophilic chain transfer agent having a carboxyl group.

[14] The method according to any one of items [9] to

[13] , wherein the hydrophilic solvent is one or more selected from the group consisting of methanol, ethanol, propanol, butanol, acetone, and water.

[15] An aqueous solution containing 1 wt % to 70 wt % of the carboxylic acid polymer according to any one of items [1] to [8], which undergoes phase separation below the UCST and becomes a homogeneous solution above the UCST.

[16] Item

[15] . The aqueous solution according to item

[15] , wherein the content of the carboxylic acid polymer is 20% by weight to 50% by weight.

[17] Item

[15] or

[16] , wherein the aqueous solution has a phase separation temperature of 40°C to 60°C.

[18] A work solution for a forward osmosis membrane water treatment system or an osmotic pressure power generation system, comprising the carboxylic acid polymer according to any one of items [1] to [8] as a solute. [Effects of the Invention]

[0022] The UCST-type temperature-responsive polymers of the present invention are water-soluble carboxylic acid-based polymers that are primarily composed of inexpensive, versatile carboxyl-containing vinyl monomers, such as acrylic acid or methacrylic acid, copolymerized with a predetermined amount of aromatic vinyl monomers, such as styrene or vinylbenzoic acid. These polymers have a specific molecular weight and molecular weight distribution. Compared to conventional ionic UCST-type polymers, these polymers exhibit a low UCST and a phase separation curve with a small slope, despite containing no inorganic salts or only small amounts of inorganic salts. Furthermore, because the viscosity of their aqueous solutions is extremely low, they can be used as a driving solution for forward osmosis water treatment systems and osmotic power generation systems. [Brief explanation of the drawings]

[0023] [Figure 1] 1 is a diagram showing a schematic diagram of a phase separation curve of a UCST polymer, in which the horizontal axis indicates the concentration of the polymer aqueous solution (unit: wt% (weight %)) and the vertical axis indicates the phase separation temperature (unit: °C). a to e in Fig. 1 correspond to a to e in Fig. 2, which shows an embodiment of a forward osmosis membrane water treatment system. [Figure 2] A schematic diagram of a forward osmosis membrane water treatment system using a UCST (cooling separation) type working solution (DS) is shown. Figures a to e in Figure 2 correspond to a to e in Figure 1, which shows the phase separation curve of the UCST type polymer. [Figure 3] 1 shows the 13C-NMR spectrum of the acrylic acid / styrene copolymer (after first purification) obtained in Example 6, where the horizontal axis indicates chemical shift (unit: ppm) and the vertical axis indicates relative signal intensity. [Figure 4] This is an enlarged view of the high magnetic field side of the 13C-NMR spectrum of Figure 3 (see Table 1 for the assignment of each peak). [Figure 5] This is an enlarged view of the low magnetic field side of the 13C-NMR spectrum of Figure 3 (see Table 1 for peak assignments). DETAILED DESCRIPTION OF THE INVENTION

[0024] Hereinafter, an embodiment of the present invention (hereinafter referred to as "the present embodiment") will be described in detail. However, the present invention is not limited to the following embodiment. The present invention can be practiced by appropriately modifying it within the scope of its gist.

[0025] The carboxyl group-containing monomer used in the present invention is the main component of the UCST polymer of the present invention, and is a water-soluble monomer necessary for forming the structural unit (A) represented by the above general formula (1).

[0026] Examples of carboxyl group-containing monomers used in the present invention include acrylic acid, methacrylic acid, ethacrylic acid, maleic acid, and itaconic acid. Among these, acrylic acid and methacrylic acid are preferred from the viewpoint of polymerizability, and acrylic acid is more preferred from the viewpoint of the osmotic pressure of the polymer. The carboxyl groups of the structural unit (A) contained in the carboxylic acid polymer of the present invention are advantageously neutralized with an alkali from the viewpoint of osmotic pressure. However, the water solubility changes sharply depending on the degree of neutralization, making it difficult to control the UCST. Therefore, the degree of neutralization (alkali equivalent to the carboxyl group) is basically sufficient at 0%. However, depending on the type and content of the structural unit (B), an excessively high UCST can be reduced by neutralization. Therefore, the degree of neutralization should be greater than 0% and up to 10%, and preferably 0.01% to 10%.

[0027] The aromatic vinyl monomer used in the present invention is a hydrophobic monomer necessary for forming the structural unit (B) represented by the above general formula (2) contained in the carboxylic acid polymer of the present invention, and is not particularly limited. Examples include styrene, chlorostyrene, bromostyrene, fluorostyrene, α-methylstyrene, cyanostyrene, methoxystyrene, para-t-butoxystyrene, para-acetoxystyrene, para-1-ethoxyethoxystyrene, vinylnaphthalene, vinylbenzoic acid, and styrenephosphonic acid. Of these, styrene is preferred from the viewpoint of cost, and vinylbenzoic acid and styrenephosphonic acid are preferred from the viewpoint of a wide range of control over phase separation.

[0028] Furthermore, the monomer necessary to form the structural unit (C) used in the present invention is not particularly limited as long as it can be copolymerized with the above-mentioned carboxyl group-containing vinyl monomer and aromatic vinyl monomer, and examples thereof include acrylamide, methacrylamide, acroylmorpholine, methacryloylmorpholine, N-phenylmaleimide, N-cyclohexylmaleimide, acrylonitrile, vinyl chloride, etc., but from the viewpoint of copolymerizability and phase separability, hydrophilic acrylamide-based monomers such as acrylamide, methacrylamide, acroylmorpholine, and methacryloylmorpholine are preferred.

[0029] The UCST-type phase separation behavior exhibited by the polymer of the present invention is believed to be primarily due to hydrophobic interactions resulting from aromatic rings in the polymer. Therefore, the molar ratio of the structural units (A) and (B) contained in the polymer is extremely important, and the content of the structural unit (B) relative to the total of the structural units (A) and (B) is preferably 5.00 mol % to 30.00 mol %. To control the phase separation temperature within a more appropriate range, the content is preferably 5.00 mol % to 20.00 mol %, more preferably 5.00 mol % to 15.00 mol %.

[0030] The monomer forming the structural unit (C) is used for the purpose of adjusting the phase separation temperature, aqueous solution viscosity, etc., within a range that does not impair the durability or osmotic pressure of the polymer consisting of the structural units (A) and (B). Therefore, the content of the structural unit (C) is preferably 0.10 mol % to 10.00 mol %, and more preferably 1.00 mol % to 5.00 mol %, based on the total of the structural units (A), (B), and (C).

[0031] As described above, in the present invention, the molar ratio of the structural unit (A) derived from a carboxyl group-containing vinyl monomer to the structural unit (B) derived from an aromatic vinyl monomer contained in the UCST polymer is the most important factor affecting temperature responsiveness. Furthermore, the molecular weight of the polymer is an important factor affecting the concentration polarization and viscosity within the semipermeable membrane, which affect the phase separation temperature, osmotic pressure, and water permeability. Furthermore, as described below, the terminal structure of the polymer also affects phase separation, so a structure having a carboxyl group at at least one of the polymer terminals is preferred.

[0032] The number-average molecular weight and weight-average molecular weight of the UCST polymer of the present invention can be measured by gel permeation chromatography (GPC). The number-average molecular weight is preferably 500 to 20,000 daltons (Da). If the number-average molecular weight is less than 500 daltons, the polymer may dissolve in water even at low temperatures. On the other hand, if the number-average molecular weight exceeds 20,000 daltons, the polymer may not dissolve in water even when heated. Furthermore, considering the phase separation property and whether the viscosity of the aqueous solution may become too high, resulting in poor operability, the number-average molecular weight is preferably 800 to 13,000 daltons, and more preferably 1,000 to 5,000 daltons. Furthermore, the narrower the molecular weight distribution, the better (sharper) the temperature responsiveness and phase separation, so the smaller the value obtained by dividing the weight average molecular weight by the number average molecular weight (Mw / Mn), the better. A value in the range of 1.00 to 2.00 is acceptable, but a value in the range of 1.00 to 1.50 is preferable, and a value in the range of 1.00 to 1.20 is even more preferable.

[0033] Furthermore, even if the molecular weight measured by GPC is the same, the viscosity of an aqueous solution containing a polymer varies depending on the degree of neutralization. A lower viscosity of the aqueous solution is preferable, but because the polymer of the present invention is an interactive polymer, a certain degree of increase in viscosity or high viscosity is unavoidable. Viscosity can be measured using a Brookfield viscometer. The viscosity of a 50 wt. % aqueous polymer solution may be 10 mPa·s to 10,000 mPa·s at 25°C, preferably 10 mPa·s to 10,000 mPa·s at 25°C and 10 mPa·s to 5,000 mPa·s at 40°C and 60°C, and more preferably 100 mPa·s to 6,000 mPa·s at 25°C and 30 mPa·s to 2,000 mPa·s at 40°C and 60°C.

[0034] As described above, the phase separation properties of the UCST polymer of the present invention are controlled by the copolymer composition, molecular weight, and molecular weight distribution, but considering use as a driving solution in a forward osmosis membrane water treatment system or an osmotic power generation system, it is preferable that the UCST of a 20 wt % aqueous polymer solution be in the range of 50° C. to 100° C., the UCST of a 30 wt % aqueous polymer solution be in the range of 40° C. to 90° C., the UCST of a 40 wt % aqueous polymer solution be in the range of 30° C. to 70° C., and the UCST of a 50 wt % aqueous polymer solution be in the range of 10° C. to 60° C. Temperatures outside these ranges may make it difficult to efficiently obtain fresh water within the practical temperature range of 40° C. to 60° C. The principle of osmotic power generation is similar to that of freshwater production. For example, electricity is generated by the difference in osmotic pressure between freshwater (low osmotic pressure) and a working solution (high osmotic pressure) heated using factory waste heat or solar heat, and the diluted working solution is cooled and reused.

[0035] The present invention also includes an aqueous solution containing 1 to 70% by weight of the above-mentioned UCST-type temperature-responsive carboxylic acid polymer. The polymer concentration is preferably 1 to 70% by weight, more preferably 10 to 70% by weight, and even more preferably 20 to 50% by weight. Of course, when actually circulating and absorbing water as a driving solution for a forward osmosis membrane water treatment system or osmotic power generation, a polymer concentration of 20 to 70% by weight is preferred due to osmotic pressure.

[0036] The phase separation temperature of an aqueous solution containing a UCST-type temperature-responsive carboxylic acid polymer is not particularly limited, but is preferably 10°C to 100°C, more preferably 30°C to 70°C, and even more preferably 40°C to 60°C, from the viewpoints of operational efficiency and temperature control when actually implementing a forward osmosis membrane water treatment system or osmotic power generation.

[0037] The upper critical solution type temperature-responsive polymer of this embodiment can be produced by producing a copolymer by radical polymerization or anionic polymerization, and then removing low-molecular-weight components derived from unreacted monomers, initiators, molecular weight modifiers, etc. by methods such as vacuum distillation, phase separation utilizing the UCST property of the polymer, or ultrafiltration. When using carboxyl-containing vinyl monomers such as acrylic acid or methacrylic acid, traditional radical polymerization or living radical polymerization is preferred. When copolymerizing acrylic acid esters or methacrylic acid esters and then hydrolyzing them to regenerate the carboxyl groups, anionic polymerization as well as radical polymerization can be used. Among radical polymerizations, living radical polymerization methods such as atom transfer polymerization, reversible addition-fragmentation transfer polymerization, iodine transfer polymerization, and stable nitroxyl-mediated polymerization are preferred from the perspective of obtaining polymers with narrow molecular weight distributions, and known methods can be used (e.g., Yamako et al., Journal of the Society of Rubber Science and Technology of Japan, Vol. 82, No. 8, pp. 363-369, 2009; Uegai et al., Network Polymer, Vol. 30, No. 5, pp. 234-249, 2009).

[0038] Among the above-mentioned living radical polymerization methods, reversible addition-fragmentation transfer polymerization, so-called RAFT polymerization, will be described in detail as an example. For example, a monomer solution containing a carboxyl group-containing vinyl monomer such as acrylic acid or methacrylic acid, an aromatic vinyl monomer such as styrene or vinylbenzoic acid, and, if necessary, a monomer copolymerizable with these monomers such as acrylamide, a radical polymerization initiator, and a polymerization control agent such as a dithioester compound (a so-called RAFT agent used in reversible addition-fragmentation transfer polymerization) are charged into a reaction vessel, and polymerization is carried out under an inert gas atmosphere at 40°C to 100°C for 5 hours to 48 hours, thereby obtaining the carboxylic acid polymer of the present invention.

[0039] In a monomer solution in which a carboxyl group-containing vinyl monomer, an aromatic vinyl monomer, and, if necessary, a monomer copolymerizable therewith, such as acrylamide, are dissolved, the proportion of the aromatic vinyl monomer relative to all monomers in the monomer solution is preferably 5.00 mol % to 30.00 mol %, more preferably 9.00 mol % to 20.00 mol %.

[0040] The amount of the RAFT agent used may be adjusted depending on the molecular weight of the target polymer, and is usually 0.01 mol % to 100.00 mol % relative to the number of moles of all monomers. However, from the viewpoint of practicality, the amount is preferably 0.10 mol % to 30.00 mol %, and more preferably 0.50 mol % to 5.00 mol %. The amount of radical polymerization initiator used is usually 0.01 mol % to 100.00 mol % relative to the number of moles of all monomers, but in consideration of molecular weight controllability, it is preferably 0.01 mol % to 20.00 mol %, more preferably 0.10 mol % to 15.00 mol %. As the RAFT agent, known compounds commercially available from Wako Pure Chemical Industries, Ltd., Tokyo Chemical Industry Co., Ltd., Boron Molecular, Inc., etc. can be used. For example, RAFT agents that are dithioester compounds such as 4-cyano-4-(dodecylsulfanylthiocarbonyl)sulfanylpentanoic acid, 3-((((1-carboxyethyl)thio)carbonothioyl)thio)propanoic acid, 4-cyano-4-(thiobenzoylthio)pentanoic acid, 2-cyanopropan-2-yl benzodithioate, 4-cyano-4-[(thiobenzoyl)sulfanyl]pentanoic acid, 2-methyl-2-[(dodecylsulfanylthiocarbonyl)sulfanyl]propanoic acid, benzyl dithiobenzoate, 2-cyanoprop-2-yldithiobenzoate, S,S-dibenzyl trithiocarbonate, cyanomethyl(3,5-dimethyl-1H-pyrazole)carbodithioate, and cyanomethyl N-methyl-N-phenyldithiocarbamate can be used. Among these, from the viewpoint of the phase separation properties of the carboxylic acid polymer of the present invention, preferred RAFT agents are dithioester compounds having a carboxyl group, such as 4-cyano-4-(dodecylsulfanylthiocarbonyl)sulfanylpentanoic acid, 3-((((1-carboxyethyl)thio)carbonothioyl)thio)propanoic acid, 4-cyano-4-(thiobenzoylthio)pentanoic acid, 2-cyanopropan-2-yl benzodithioate, 4-cyano-4-[(thiobenzoyl)sulfanyl]pentanoic acid, and 2-methyl-2-[(dodecylsulfanylthiocarbonyl)sulfanyl]propanoic acid.

[0041] Examples of the radical polymerization initiator include di-t-butyl peroxide, dicumyl peroxide, t-butylcumyl peroxide, benzoyl peroxide, dilauryl peroxide, cumene hydroperoxide, t-butyl hydroperoxide, 1,1-bis(t-butylperoxy)-3,5,5-trimethylcyclohexane, 1,1-bis(t-butylperoxy)-cyclohexane, cyclohexanone peroxide, t-butyl peroxybenzoate, t-butylperoxyisobutyrate, t-butylperoxy Peroxide compounds such as 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile), 2,2'-azobis(2,4-dimethylvaleronitrile), 2,2'-azobis(2-methylpropionitrile), 2,2'-azobis(2-methylbutyronitrile), 1,1'-azobis(cyclohexane) -1-carbonitrile), 1-[(1-cyano-1-methylethyl)azo]formamide, dimethyl 2,2'-azobis(2-methylpropionate), 4,4'-azobis(4-cyanovaleric acid), 2,2'-azobis(2,4,4-trimethylpentane), 2,2'-azobis{2-methyl-N-[1,1'-bis(hydroxymethyl)-2-hydroxyethyl]propionamide}, 2,2'-azobis{2-(2-imidazolin-2-yl)propane]dihydrochloride, 2,2'-azobis{2-(2-imidazoline- 2-yl)propane] disulfate dihydrate, 2,2'-azobis{2-[1-(2-hydroxyethyl)-2-imidazolin-2-yl)propane]} dihydrochloride, 2,2'-azobis(1-imino-1-pyrrolidino-2-methylpropane) dihydrochloride, 2,2'-azobis(2-methylpropionamidine) dihydrochloride, 2,2'-azobis[N-(2-carboxyethyl)-2-methylpropionamidine] tetrahydrate, 1,1'-azobis(1-acetoxy-1-phenylmethane), 4,Examples include azo compounds such as 4'-diazenediylbis(4-cyanopentanoic acid)·α-hydro-ω-hydroxypoly(oxyethylene) polycondensates, and stable nitroxyl radicals such as N-tert-butyl-O-[1-[4-(chloromethyl)phenyl]ethyl]-N-(2-methyl-1-phenylpropyl)hydroxylamine, 2,2,6-tetramethylpiperidine 1-oxyl, and N-tert-butyl-N-(2-methyl-1-phenylpropyl)-O-(1-phenylethyl)hydroxylamine. Among these, when using a RAFT agent, from the viewpoint of molecular weight control, 2,2'-azobis{2-methyl-N-[1,1'-bis(hydroxymethyl)-2-hydroxyethyl]propionamide}, 2,2'-azobis{2-(2-imidazolin-2-yl)propane]dihydrochloride, 2,2'-azobis{2-(2-imidazolin-2-yl)propane]disulfate dihydrate, 2,2'-azobis{2-[1-(2-hydroxyethyl)-2-imidazoline-2 azo compounds such as 2,2'-azobis(2-methylpropionamidine) dihydrochloride, 2,2'-azobis[N-(2-carboxyethyl)-2-methylpropionamidine]tetrahydrate, 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile), 2,2'-azobis(2,4-dimethylvaleronitrile), 2,2'-azobis(isobutyronitrile), and 4,4'-azobis(4-cyanovaleric acid) From the viewpoint of phase separation, 2,2'-azobis{2-methyl-N-[1,1'-bis(hydroxymethyl)-2-hydroxyethyl]propionamide}, 2,2'-azobis{2-(2-imidazolin-2-yl)propane]dihydrochloride, 2,2'-azobis{2-(2-imidazolin-2-yl)propane]disulfate dihydrate, 2,2'-azobis{2-[1-(2-hydroxyethyl)-2-imidazolin-2-yl]propane} Hydrophilic azo radical polymerization initiators such as 2,2'-azobis[N-(2-carboxyethyl)-2-methylpropionamidine]dihydrochloride, 2,2'-azobis(2-methylpropionamidine)dihydrochloride, 2,2'-azobis[N-(2-carboxyethyl)-2-methylpropionamidine]tetrahydrate, and 4,4'-azobis(4-cyanovaleric acid) are more preferred, and hydrophilic azo radical polymerization initiators containing a carboxyl group such as 4,4'-azobis(4-cyanovaleric acid) are even more preferred.

[0042] The solvent used for polymerization is not particularly limited as long as it can uniformly dissolve the monomer mixture, and examples thereof include hydrophilic solvents such as alcohols such as methanol, ethanol, propanol, butanol, cellosolves such as methoxyethanol, ethoxyethanol, hydrophilic solvents such as acetonitrile, acetone, tetrahydrofuran, dioxane, N-methylpyrrolidone, dimethylformamide, dimethyl sulfoxide, dimethylacetamide, and mixtures of these with water, hydrocarbon solvents such as benzene, toluene, xylene, cyclohexane, methylcyclohexane, heptane, and halogenated solvents such as chloroform, tetrachloromethane, etc. Among these, considering the polymerization rate and the solubility of the polymerization initiator and chain transfer agent, hydrophilic solvents such as methanol, ethanol, propanol, butanol, acetone, and mixtures of these with water are preferred.

[0043] In order to increase the polymerization rate and conversion rate, it is preferable that the monomer concentration is as high as possible. In consideration of molecular weight controllability, the monomer concentration is preferably 5.0 wt % to 50.0 wt %, and in consideration of reaction temperature control, the monomer concentration is more preferably 5.0 wt % to 30.0 wt %, and even more preferably 9.0 wt % to 20.0 wt %.

[0044] Because polymerization solutions often contain unreacted monomers, impurities derived from the polymerization initiator and RAFT agent, and oligomers with molecular weights lower than the target molecular weight, it is preferable to purify them by utilizing the UCST properties of the polymer. For example, the polymer can be dissolved in water at a low concentration by heating, then cooled to separate into a concentrated aqueous polymer solution and a supernatant containing low-molecular-weight impurities, and the concentrated aqueous polymer solution can then be recovered. Repeating this process can improve the purity and monodispersity of the polymer. Alternatively, impurities can be removed from the polymer solution using an ultrafiltration membrane.

[0045] In addition to living radical polymerization, such as the RAFT polymerization described above, traditional radical polymerization can also be used to produce the carboxylic acid polymer of the present invention. Examples include a batch addition polymerization method in which a monomer solution containing a carboxyl group-containing vinyl monomer such as acrylic acid or methacrylic acid, an aromatic vinyl monomer such as styrene or vinylbenzoic acid, and optionally a copolymerizable monomer such as acrylamide, a radical polymerization initiator, or a radical polymerization initiator and a chain transfer agent (also known as a molecular weight modifier) ​​is charged into a reaction vessel and polymerized under an inert gas atmosphere at 50°C to 120°C for 3 to 20 hours; and a sequential addition method in which the monomer and molecular weight modifier or a solution thereof, and a polymerization initiator are continuously supplied to a reaction vessel while polymerization is carried out. Among these, the sequential addition method is preferred in terms of excellent removal of polymerization heat and molecular weight controllability, and among the sequential addition methods, the so-called power feed polymerization method, in which the composition of the added monomers is continuously changed, is preferred in terms of excellent uniformity of the copolymer composition.

[0046] The radical polymerization initiator used here is the same as that described above, but a hydrophilic azo radical polymerization initiator is particularly preferred. When a peroxide is used as the radical polymerization initiator, a reducing agent such as ascorbic acid, erythorbic acid, aniline, tertiary amine, Rongalite, hydrosulfite, sodium sulfite, sodium hydrogen sulfite, sodium thiosulfate, or sodium hypophosphite may be used in combination. The amount of the radical polymerization initiator used depends on the type of radical initiator and the polymerization method, but is usually 0.01 mol % to 100.00 mol % based on the total monomers. However, taking into consideration the purity of the resulting polymer, the amount is preferably 0.01 mol % to 20.00 mol %, and more preferably 0.10 mol % to 15.00 mol %.

[0047] The chain transfer agent (molecular weight regulator) is not particularly limited, and examples thereof include thioglycolic acid, thiomalic acid, 2-mercaptopropionic acid, 3-mercaptopropionic acid, thiosalicylic acid, 3-mercaptobenzoic acid, 4-mercaptobenzoic acid, thiomalonic acid, dithiosuccinic acid, thiomaleic acid, thiomaleic anhydride, dithiomaleic acid, thioglutaric acid, cysteine, homocysteine, 5-mercaptotetrazoleacetic acid, 3-mercapto-1-propanesulfonic acid, 3-mercaptopropane-1,2-diol, mercaptoethanol, 1-mercapto-2-methylpropanol ... Mercaptans such as 2-dimethylmercaptoethane, 2-mercaptoethylamine hydrochloride, 6-mercapto-1-hexanol, 2-mercapto-1-imidazole, 3-mercapto-1,2,4-triazole, cysteine, N-acylcysteine, glutathione, N-butylaminoethanethiol, N,N-diethylaminoethanethiol, diisopropyl xanthogen disulfide, diethyl xanthogen disulfide, diethylthiuram disulfide, 2,2'-dithiodipropionic acid, 3,3'-dithiodi Examples of the alkyl iodide include disulfides such as propionic acid, 4,4'-dithiodibutanoic acid, and 2,2'-dithiobisbenzoic acid; halogenated hydrocarbons such as iodoform; alkyl iodide compounds such as α-iodobenzyl cyanide, 1-iodoethylbenzene, ethyl 2-iodo-2-phenylacetate, 2-iodo-2-phenylacetic acid, 2-iodopropanoic acid, and 2-iodoacetic acid; diphenylethylene, p-chlorodiphenylethylene, p-cyanodiphenylethylene, α-methylstyrene dimer, organic tellurium compounds, and sulfur. Among these, thioglycolic acid, thiomalic acid, 2-mercaptopropionic acid, 3-mercaptopropionic acid, thiomalonic acid, dithiosuccinic acid, thiomaleic acid, thiomaleic anhydride, dithiomaleic acid, thioglutaric acid, cysteine, homocysteine, 5-mercaptotetrazoleacetic acid, 3-mercapto-1-propanesulfonic acid, 3-mercaptopropane-1,2-diol, mercaptoethanol, 1 Hydrophilic chain transfer agents such as 2-dimethylmercaptoethane, 2-mercaptoethylamine hydrochloride, 6-mercapto-1-hexanol, 2-mercapto-1-imidazole, 3-mercapto-1,2,4-triazole, cysteine, N-acylcysteine, glutathione, N-butylaminoethanethiol, and N,N-diethylaminoethanethiol are preferred, and hydrophilic chain transfer agents containing a carboxyl group such as thioglycolic acid, thiomalic acid, 2-mercaptopropionic acid, 3-mercaptopropionic acid, thiomalonic acid, dithiosuccinic acid, thiomaleic acid, thiomaleic anhydride, dithiomaleic acid, thioglutaric acid, cysteine, homocysteine, 5-mercaptotetrazoleacetic acid, and 3-mercapto-1-propanesulfonic acid are more preferred from the viewpoint of improving phase separation.

[0048] The total proportion of the polymerization initiator, RAFT agent, and chain transfer agent relative to all monomers is preferably 0.90 mol % to 13.00 mol % in order to achieve the targeted UCST behavior and low viscosity, and more preferably 2.00 mol % to 13.00 mol % in order to achieve an even lower viscosity.

[0049] The polymerization solvent can be the same as that described above for RAFT polymerization.

[0050] Additionally, the polymers obtained by the traditional radical polymerization described above can be purified by the methods described above for RAFT polymerization.

[0051] Another method for producing carboxylic acid polymers involves radical or anionic copolymerization of a vinyl monomer containing a carboxylic acid ester group, such as an acrylic acid ester, a methacrylic acid ester, or a maleic acid ester, with an aromatic vinyl monomer, followed by hydrolysis of the carboxylic acid ester with an acid or base to regenerate the carboxyl group. Alternatively, radical or anionic copolymerization of acrylic acid, methacrylic acid, or their esters with an aromatic vinyl monomer, such as para-t-butoxystyrene, para-acetoxystyrene, or para-1-ethoxyethoxystyrene, followed by hydrolysis with an acid such as hydrochloric acid or sulfuric acid can regenerate the carboxyl group or hydroxystyrene structural unit. For anionic copolymerization, solvents containing no active hydrogen, such as toluene, xylene, hexane, cyclohexane, tetrahydrofuran, dioxane, or mixtures thereof, are used. Known initiators, such as alkyllithium, sodium naphthalene, and alkyllithium / bis(2,6-di-t-butylphenoxy)methylaluminum, can be used. The polymerization initiator is adjusted according to the target polymer molecular weight. The polymerization temperature is -30°C to 40°C, and more preferably -30°C to 10°C in order to suppress side reactions.

[0052] In the case of radical polymerization, it is preferable to use a polymerization initiator, molecular weight modifier, or chain transfer agent having a carboxyl group, as described above. The same is true in the case of anionic polymerization, and it is preferable from the viewpoint of phase separation to introduce a carboxyl group by a method such as treating the polymer growing end with carbon dioxide gas.

[0053] The narrower the molecular weight distribution of the polymer and the fewer low molecular weight impurities it contains, the better the temperature response and phase separation properties will be, so it is advisable to purify it using the method described above.

[0054] When the carboxylic acid polymer of the present invention is used as a driving solution for a forward osmosis membrane water treatment system or an osmotic pressure power generation system, the viscosity and UCST of the driving solution can be adjusted depending on the operating conditions by adding a surfactant, a water-soluble polymer, or the like. [Example]

[0055] The present invention will be explained in more detail with reference to the following examples, but the present invention is not limited to these examples in any way.

[0056] The analytical instruments and measurement methods used in this example are listed below. 1. Measurement of polymerization conversion and molecular weight by gel permeation chromatography (GPC) The reaction solutions before and after polymerization were dissolved in the following eluent, and GPC measurement was carried out under the following conditions. The polymerization conversion was calculated from a calibration curve prepared using each monomer, and the molecular weight of the polymer was calculated from a calibration curve prepared using standard polystyrene sodium sulfonate or standard polyethylene oxide. Model: Tosoh Corporation HLC-8320 Column: TSK guard column AW-H / TSK AW-6000 / TSK AW-3000 / TSK AW-2500 Eluent: 0.2 M aqueous sodium sulfate / acetonitrile = 65 / 35 (volume ratio) solution Flow rate: 0.6 ml / min, injection volume: 10 μl, column temperature: 40°C Detector: UV detector (wavelength 210 nm) or RI detector Polymer calibration curve: Created from peak top molecular weight and elution time using standard polystyrene sodium sulfonate (manufactured by Sowa Scientific). For polyampholite, standard polyethylene oxide (manufactured by Sigma-Aldrich) was used.

[0057] 2. Polymer 13 Structural analysis by C-NMR Polymer under the following conditions 13 C-NMR analysis was performed to confirm the copolymer composition and terminal structure. Model: Bruker AVANCE NEO 700 Probe: 10mmφPABBO BB Observation kernel: 13 C(176.07MHz) Solvent: dimethyl sulfoxide-d6 Concentration: about 10% Accumulation count: 2048 times Temperature: room temperature Standard: Tetramethylsilane (TMS) Measurement mode: Inverse gated decoupling (quantitative measurement method)

[0058] 3. Measuring UCST of Aqueous Polymer Solutions The polymer and ion exchanger were placed in a glass bottle to prepare a polymer aqueous solution of a specified concentration. The contents were heated in an oil bath while stirring with a magnetic stirrer. When the solution became hydrophilic and transparent, it was cooled to become hydrophobic and opaque. It was then reheated, and the temperature at which it became hydrophilic again and transparent was determined as the UCST.

[0059] 4. Osmotic pressure measurement of aqueous polymer solutions The water activity value of the polymer aqueous solution was converted to osmotic pressure (bar) using the following conversion formula [see Divina D.; Separation and Purification Technology 138 (2014) 92-97].

number

[0060] 5. Viscosity of aqueous polymer solutions Measurement was carried out at a predetermined temperature using a Brookfield viscometer LVDV2T (manufactured by Eiko Seiki Co., Ltd.).

[0061] <Reagents used> The compounds described in the examples below were used, but the present invention is not limited to these examples in any way. AA: Acrylic acid (purity 99%, Fujifilm Wako Pure Chemical Industries, Ltd.) MAA: methacrylic acid (purity 99%, Fujifilm Wako Pure Chemical Industries, Ltd.) St: Styrene (purity 99%, manufactured by FUJIFILM Wako Pure Chemical Corporation) αMSt: α-Methylstyrene (purity 99%, manufactured by Tokyo Chemical Industry Co., Ltd.) 4-VBA: 4-Vinylbenzoic acid (purity 97%, manufactured by FUJIFILM Wako Pure Chemical Corporation) AM: Acrylamide (purity 99%, manufactured by FUJIFILM Wako Pure Chemical Corporation) MAM: Methacrylamide (purity 97%, manufactured by FUJIFILM Wako Pure Chemical Corporation) NaSS: Sodium para-styrenesulfonate (purity 98%, manufactured by Tokyo Chemical Industry Co., Ltd.) VBTAC: Vinylbenzyltrimethylammonium chloride (purity 99%, manufactured by Sigma-Aldrich) V-501: 4,4’-Azobis-(4-cyanopentanoic acid) (purity 98%, manufactured by FUJIFILM Wako Pure Chemical Corporation) AIBN: 2,2’-Azobis(isobutyronitrile) (purity 98%, 2,2’-Azobis(isobutyronitrile)) HP: Hydrogen peroxide solution (purity 30%, manufactured by FUJIFILM Wako Pure Chemical Corporation) TGL: 3-Mercapto-1,2-propanediol (purity 97%, manufactured by FUJIFILM Wako Pure Chemical Corporation) TMA: Thiomalic acid (purity 98%, manufactured by FUJIFILM Wako Pure Chemical Corporation) RAFT agent-1: 3-((((1-Carboxyethyl)thio)carbonothioyl)thio)propanoic acid (purity 95%, manufactured by Sigma-Aldrich)

[0062] The structural units (A), structural units (B), and structural units (C) shown below respectively mean the structural units in the above formulas (1), formula (2), and formula (3).

[0063] I Example 1 <Synthesis of AA / St copolymer> Acrylic acid (AA) (41.60 g, 571.52 mmol), styrene (St) (9.10 g, 86.46 mmol), RAFT agent-1 (3.50 g, 13.07 mmol), initiator V-501 (0.30 g, 1.05 mmol), isopropanol (150.00 g), and ion-exchanged water (150.00 g) were charged into a 1000 mL glass four-neck flask equipped with a nitrogen inlet tube, a three-way stopcock, and a Dimroth condenser, and dissolved to form a homogeneous solution (the amount of St charged relative to the total of AA and St = 13.14 mol%). This solution was thoroughly degassed by repeatedly reducing the pressure with an aspirator and introducing nitrogen, and then polymerized for 20 hours at 70°C under a nitrogen atmosphere while stirring with a magnetic stirrer. The polymerization conversion at the end of the reaction, as measured by GPC, was AA = 82%, St = 100%, number average molecular weight Mn = 3300, weight average molecular weight Mw = 4700, and Mw / Mn = 1.42. That is, based on the polymerization conversion of each monomer, the content of St (structural unit (B)) relative to the total of AA (structural unit (A)) and St (structural unit (B)) contained in the polymer was 15 mol%. The polymerization solution was vacuum dried at 85 °C for 5 hours to remove the solvent and unreacted monomer, yielding 44.26 g of powdered polymer.

[0064] <Physical properties of copolymer> The relationship between the concentration of the polymer in aqueous solution and the UCST is shown in Table 1. It is clear that the UCST was lower and the slope of the phase separation curve was smaller than those of the polyampholite of Comparative Example 1, which utilizes strong electrostatic interactions; Comparative Example 3, which has a high content of structural unit (B) in the polymer; Comparative Example 4, which has a broad molecular weight distribution; Comparative Example 5, which has a high molecular weight; and Comparative Examples 6 and 7, which used a different polymerization initiator. On the other hand, Comparative Example 2, which contained too little styrene (a hydrophobic component), was soluble in water over a wide concentration range and did not exhibit the target UCST. The viscosity of a 50 wt % aqueous solution of the polymer was measured at various temperatures (No. 3 rotor, 60 rpm), and the results are shown in Table 2 below. It is clear that the viscosity is significantly lower than that of the polyampholite of Comparative Example 1, which utilizes strong electrostatic interactions. When the polymer is a weak electrolyte type, although its osmotic pressure is expected to be lower than that of the conventional strong electrolyte type polyampholyte (Comparative Example 1), it has an extremely low viscosity (it is expected that concentration polarization is difficult to occur), so it can be expected to be used as a driving solution for a forward osmosis membrane water treatment system or osmotic pressure power generation.

[0065] Example 2 <Synthesis of AA / VBA copolymer> Acrylic acid (AA) (41.55 g, 570.84 mmol), 4-vinylbenzoic acid (VBA) (15.60 g, 105.08 mmol), RAFT agent -1 (3.90 g, 14.57 mmol), initiator V-501 (0.60 g, 2.10 mmol), isopropanol (225.00 g) and ion-exchanged water (225.00 g) were charged into a 1000 mL four-necked glass flask equipped with a nitrogen inlet tube, a three-way cock, and a Dimroth condenser and dissolved to form a uniform solution (the charged amount of VBA relative to the total of AA and VBA = 15.55 mol%). After sufficiently degassing this solution by repeating aspirator decompression and nitrogen introduction, it was polymerized at 70 °C for 20 hours while stirring with a magnetic stirrer under a nitrogen atmosphere. The polymerization conversion rate at the end of the reaction measured by GPC was AA = 87%, VBA = 100%, number average molecular weight Mn = 3800, weight average molecular weight Mw = 5200, Mw / Mn = 1.37. That is, from the polymerization conversion rate of each monomer, the content of VBA (structural unit (B)) relative to the total of AA (structural unit (A)) and VBA (structural unit (B)) contained in the polymer was 17 mol%. The polymerization solution was vacuum dried at 85 °C for 5 hours to remove the solvent and unreacted monomers, and 52.96 g of a powdery polymer was obtained.

[0066] <Physical properties of copolymer> Table 2 shows the relationship between the aqueous solution concentration of the polymer and UCST. It is clear that the UCST is lower, the slope of the phase separation curve is smaller, and the viscosity of the 50 wt% aqueous polymer solution is significantly lower compared with Comparative Examples 1 and 2 to 8 described later. When the polymer is a weak electrolyte type, although its osmotic pressure is expected to be lower than that of a conventional strong electrolyte type polyampholyte (Comparative Example 1), it has an extremely low viscosity (it is expected that concentration polarization is difficult to occur), so it can be expected to be used as a driving solution for a forward osmosis membrane water treatment system or osmotic pressure power generation.

[0067] Example 3 <Synthesis of AA / VBA copolymer> Acrylic acid (AA) (42.00 g, 577.02 mmol), 4-vinylbenzoic acid (VBA) (9.00 g, 60.62 mmol), RAFT agent - 1 (1.00 g, 3.74 mmol), initiator V - 501 (0.60 g, 2.10 mmol), isopropanol (225.00 g) and ion-exchanged water (225.00 g) were charged into a 1000 mL four-necked glass flask equipped with a nitrogen inlet tube, a three-way cock, and a Dimroth condenser and dissolved to form a homogeneous solution (the charged amount of VBA relative to the total of AA and VBA = 9.51 mol%). After sufficiently degassing this solution by repeating aspirator vacuum and nitrogen introduction, it was polymerized at 70 °C for 20 hours while stirring with a magnetic stirrer under a nitrogen atmosphere. The polymerization conversion rate at the end of the reaction measured by GPC was AA = 91%, VBA = 100%, number average molecular weight Mn = 12900, weight average molecular weight Mw = 20500, Mw / Mn = 1.59. That is, from the polymerization conversion rate of each monomer, the content of VBA (structural unit (B)) relative to the total of AA (structural unit (A)) and VBA (structural unit (B)) contained in the polymer was 10 mol%. The polymerization solution was vacuum dried at 85 °C for 5 hours to remove the solvent and unreacted monomers, and 46.81 g of a powdery polymer was obtained.

[0068] <Physical properties of the copolymer> Table 2 shows the relationship between the aqueous solution concentration of the polymer and UCST. It is clear that compared with Comparative Examples 1 and 2 - 8, the UCST is lower, the slope of the phase separation curve is smaller, and the viscosity of the 50 wt% aqueous polymer solution is significantly lower. When the polymer is a weak electrolyte type, although its osmotic pressure is expected to be lower than that of the conventional strong electrolyte type polyampholyte (Comparative Example 1), it has an extremely low viscosity (it is expected that concentration polarization is difficult to occur), so it can be expected to be used as a driving solution for a forward osmosis membrane water treatment system or osmotic pressure power generation.

[0069] Example 4 <Synthesis of AA / VBA copolymer> Acrylic acid (AA) (30.00 g, 412.16 mmol), 4-vinylbenzoic acid (VBA) (15.00 g, 101.04 mmol), RAFT agent - 1 (5.00 g, 18.68 mmol), initiator V - 501 (1.00 g, 2.10 mmol), isopropanol (150.00 g) and ion-exchanged water (150.00 g) were charged into a 1000 mL four-necked glass flask equipped with a nitrogen inlet tube, a three-way cock, and a Dimroth condenser and dissolved to form a homogeneous solution (the charged amount of VBA relative to the total of AA and VBA = 19.69 mol%). After sufficiently degassing this solution by repeating aspirator decompression and nitrogen introduction, it was polymerized at 70 °C for 20 hours while stirring with a magnetic stirrer under a nitrogen atmosphere. The polymerization conversion rate at the end of the reaction measured by GPC was AA = 72%, VBA = 100%, number average molecular weight Mn = 2500, weight average molecular weight Mw = 4300, Mw / Mn = 1.72. That is, from the polymerization conversion rate of each monomer, the content of VBA (structural unit (B)) relative to the total of AA (structural unit (A)) and VBA (structural unit (B)) contained in the polymer was 25 mol%. The polymerization solution was vacuum dried at 85 °C for 5 hours to remove the solvent and unreacted monomers, and 39.25 g of a powdery polymer was obtained.

[0070] <Physical properties of the copolymer> Table 2 shows the relationship between the aqueous solution concentration of the polymer and UCST. It is clear that the UCST is lower, the slope of the phase separation curve is smaller, and the viscosity of the 50 wt% aqueous polymer solution is significantly lower compared with Comparative Examples 1 and 2 - 8. When the polymer is a weak electrolyte type, although its osmotic pressure is expected to be lower than that of a conventional strong electrolyte type polyampholyte (Comparative Example 1), it has an extremely low viscosity (it is expected that concentration polarization is difficult to occur), so it can be expected to be used as a driving solution for a forward osmosis membrane water treatment system or osmotic pressure power generation.

[0071] Example 5 <Synthesis of AA / AM / VBA copolymer> Into a 1000 mL four-necked glass flask equipped with a nitrogen inlet tube, a three-way cock, and a Dimroth condenser, acrylic acid (AA) (40.00 g, 549.54 mmol), 4-vinylbenzoic acid (VBA) (7.00 g, 66.51 mmol), acrylamide (AM) (2.00 g, 27.86 mmol), RAFT agent -1 (3.30 g, 12.33 mmol), initiator V-501 (0.30 g, 1.05 mmol), isopropanol (150.00 g) and ion-exchanged water (150.00 g) were charged and dissolved to form a homogeneous solution (the charged amount of VBA relative to the total of AA and VBA = 10.80 mol%, the charged amount of AM relative to the total of all monomers = 4.33 mol%). After thoroughly degassing this solution by repeating aspirator decompression and nitrogen introduction, it was polymerized at 70 °C for 20 hours while stirring with a magnetic stirrer under a nitrogen atmosphere. The polymerization conversion rate at the end of the reaction measured by GPC was AA = 81%, AM = 83%, VBA = 100%, number average molecular weight Mn = 3800, weight average molecular weight Mw = 5400, Mw / Mn = 1.42. That is, from the polymerization conversion rate of each monomer, the content of VBA (structural unit (B)) relative to the total of AA (structural unit (A)) and VBA (structural unit (B)) contained in the polymer was 13 mol%, and the content of AM (structural unit (C)) relative to the total of structural units (A) to C was 4 mol%. The polymerization solution was vacuum dried at 85 °C for 5 hours to remove the solvent and unreacted monomers, and 44.42 g of a powdery polymer was obtained.

[0072] <Physical properties of the copolymer> Table 2 shows the relationship between the aqueous solution concentration of the polymer and the UCST. It is clear that the UCST is lower, the slope of the phase separation curve is smaller, and the viscosity of the 50 wt% aqueous polymer solution is significantly lower compared to Comparative Examples 1 and 2 - 8. Table 1 shows the results of measuring the viscosity of a 50 wt% aqueous solution of the polymer at different temperatures (No. 3 rotor, 60 rpm). It is clear that the viscosity is significantly lower compared to the polyampholyte of Comparative Example 1. The polymer is a weak electrolyte type, and although its osmotic pressure is expected to be lower than that of conventional strong electrolyte type polyampholytes (Comparative Example 1), due to its extremely low viscosity (it is expected that concentration polarization is difficult to occur), it can be expected to be used as a driving solution for forward osmosis membrane water treatment systems and osmotic pressure power generation.

[0073] Example 6 <Synthesis of AA / St copolymer> A monomer solution consisting of acrylic acid (AA) (43.71 g, 600.51 mmol), styrene (St) (7.06 g, 67.08 mmol), and 1-propanol (217.00 g), and an initiator solution consisting of initiator V-501 (17.97 g, 62.85 mmol), 1-propanol (285.35 g), and ion-exchanged water (111.82 g) were sufficiently degassed by repeating aspirator vacuum and nitrogen introduction. A 1000 mL glass four-neck flask equipped with a nitrogen introduction tube, a three-way cock, a Dimroth condenser, and a magnetic stirrer was immersed in an oil bath at 96 °C, and the above monomer solution and initiator solution were dropped in over 4.5 hours, and polymerization was continued by heating for another 1 hour (the charged amount of St with respect to the total of AA and St = 10.05 mol%). The polymerization conversion rate at the end of the reaction measured by GPC was AA = 96%, St = 100%, number average molecular weight Mn = 1800, weight average molecular weight Mw = 3100, and Mw / Mn = 1.72. That is, from the polymerization conversion rate of each monomer, the content of St (structural unit (B)) with respect to the total of AA (structural unit (A)) and St (structural unit (B)) contained in the polymer was 10 mol%. The polymerization solution was vacuum dried at 85 °C for 5 hours to remove the solvent and unreacted monomers, and 45.04 g of a powdery polymer was obtained.

[0074] <Purification of copolymer> The entire amount of the powdered polymer was added to a glass bottle with ion-exchanged water to prepare a 20 wt% aqueous solution. The solution was heated to 85°C for dissolution, cooled to 25°C, and left to stand overnight. The supernatant was discarded, and the concentrated solution in the lower layer was recovered. Approximately 1 g of the concentrated solution was precisely weighed into a weighing bottle and vacuum-dried at 85°C for 7 hours. As a result, the solid content was 53 wt% and the polymer recovery rate was approximately 80 wt%.

[0075] <Copolymer 13 C-NMR structural analysis> The above once purified polymer 13 The C-NMR spectrum was measured (FIGS. 3, 4, and 5), and the peaks were assigned as shown in Table 1 below based on simulation (simulation software: ACD / C+H NMR Predictors). [Table 1]

[0076] The integral per carbon of the acrylic acid structural unit is G-(B+D) / 2-(A+C) / 2=92.74 The integral value per carbon of the styrene structural unit was (E+F) / 6=12.02, so the molar ratio of the acrylic acid structural unit to the styrene structural unit was 89:11 mol%, which was almost consistent with the composition ratio calculated from the GPC conversion rate. Furthermore, of the two peaks derived from the methyl and cyano groups of the polymerization initiator V-501, the broad peak was presumed to be derived from the initiator fragment (molecular weight 126.13) attached to the polymer end, as shown in formula (5). Therefore, the molar ratio of the acrylic acid structural unit, styrene structural unit, and initiator fragment attached to the polymer end was 92.74:12.02:5.53 = 16.8:2.2:1. The desired UCST behavior was likely due to the effect of the carboxyl group attached to the polymer end, as well as the polymer composition and molecular weight.

[0077] [ka]

[0078] Moreover, the peaks derived from the ester structure in Table 3 should not be contained in the raw materials, and the reason for the peak appearance is unknown. Also, the initiator residue is derived from the decomposition product that did not bind to the polymer end.

[0079] <Physical Properties of the Copolymer> Table 1 shows the relationship between the aqueous solution concentration of the above-mentioned polymer purified once and the UCST. It is clear that the UCST is lower, the slope of the phase separation curve is smaller, and the viscosity of the 50 wt% polymer aqueous solution is significantly lower compared to Comparative Examples 1 and 2 to 8. The polymer is a weak electrolyte type, and although the osmotic pressure is expected to be lower compared to conventional strong electrolyte type polyampholytes (Comparative Example 1), due to its extremely low viscosity (it is expected that concentration polarization is difficult to occur), it can be expected to be used as a driving solution for a forward osmosis membrane water treatment system or osmotic pressure power generation.

[0080] Example 7 <Synthesis of AA / St Copolymer> A monomer solution consisting of acrylic acid (AA) (40.00 g, 549.54 mmol), styrene (St) (8.70 g, 82.66 mmol), and 1-propanol (230.00 g), and an initiator solution consisting of initiator V-50 (5.00 g, 18.07 mmol), thiomalic acid (4.50 g, 29.37 mmol), and ion-exchanged water (100.00 g) were sufficiently degassed by repeating aspirator decompression and nitrogen introduction. A 1000 mL four-neck glass flask equipped with a nitrogen inlet tube, a three-way cock, a Dimroth condenser, and a magnetic stirrer was immersed in an oil bath at 96 °C, and the above-mentioned monomer solution and initiator solution were dropped in over 5 hours, and the polymerization was continued by heating for another 1 hour (the charged amount of St with respect to the total of AA and St = 13.07 mol%). The polymerization conversion rate at the end of the reaction measured by GPC was AA = 97%, St = 100%, number average molecular weight Mn = 2700, weight average molecular weight Mw = 4800, and Mw / Mn = 1.78. That is, from the polymerization conversion rate of each monomer, the content of St (structural unit (B)) with respect to the total of AA (structural unit (A)) and St (structural unit (B)) contained in the polymer was 13 mol%. By vacuum drying the polymerization solution at 85 °C for 5 hours, the solvent and unreacted monomers were removed, and 45.01 g of powdery polymer was obtained.

[0081] <Physical properties of the copolymer> Table 2 shows the relationship between the aqueous solution concentration of the polymer and the UCST. It is clear that the UCST is lower, the slope of the phase separation curve is smaller, and the viscosity of the 50 wt% aqueous polymer solution is significantly lower compared to Comparative Examples 1 and 2 - 8. Table 1 shows the results of measuring the viscosity of the 50 wt% aqueous solution of the polymer at different temperatures (No. 3 rotor, 60 rpm). It is clear that the viscosity is higher compared to Examples 1, 2 and 5, but significantly lower than that of the polyampholyte in Comparative Example 1. The polymer is of the weak electrolyte type, and although it is expected to have a lower osmotic pressure compared to the conventional strong electrolyte type polyampholyte (Comparative Example 1), due to its extremely low viscosity (it is expected that concentration polarization is difficult to occur), it can be expected to be used as a driving solution for a forward osmosis membrane water treatment system or osmotic pressure power generation.

[0082] Example 8 <Synthesis of MAA / St copolymer - 1> A monomer solution consisting of methacrylic acid (MAA) (52.20 g, 600.49 mmol), styrene (St) (7.00 g, 66.51 mmol), and 1-propanol (200.00 g) and an initiator solution consisting of initiator V-50 (23.00 g, 83.12 mmol), 1-propanol (350.00 g), and ion-exchanged water (135.00 g) were thoroughly degassed by repeatedly aspirating and introducing nitrogen. A 1000 mL glass four-neck flask equipped with a nitrogen inlet tube, a three-way stopcock, a Dimroth condenser, and a magnetic stirrer was immersed in an oil bath at 96 °C. The monomer solution and initiator solution were added dropwise over 6 hours, and the mixture was heated for another hour to polymerize (the amount of St relative to the total amount of MAA and St = 9.97 mol%). The polymerization conversion rates at the end of the reaction, as measured by GPC, were MAA = 92%, St = 100%, number average molecular weight Mn = 1900, weight average molecular weight Mw = 2400, and Mw / Mn = 1.26. That is, based on the polymerization conversion rates of each monomer, the content of St (structural unit (B)) relative to the total of MAA (structural unit (A)) and St (structural unit (B)) contained in the polymer was 11 mol%. The polymerization solution was vacuum dried at 85°C for 5 hours to remove the solvent and unreacted monomer, yielding 55.01 g of powdered polymer.

[0083] <Physical properties of copolymer> The relationship between the concentration of the polymer in aqueous solution and the UCST is shown in Table 2. Compared with the acrylic acid-based solutions of Examples 1 to 7, the UCST and viscosity of the aqueous solution were higher, but compared with Comparative Examples 1 and 2 to 8, the UCST was lower, the slope of the phase separation curve was smaller, and furthermore, it is clear that the viscosity of a 50 wt% aqueous polymer solution was significantly lower. This polymer is a weak electrolyte and is expected to have a lower osmotic pressure than conventional strong electrolyte polyampholite (Comparative Example 1). However, because it has extremely low viscosity (concentration polarization is expected to be less likely to occur), it is expected to be used as a driving solution for forward osmosis membrane water treatment systems and osmotic pressure power generation.

[0084] [Table 2] TIFF0007770175000008.tif156147

[0085] Comparative Example 1 <Synthesis of NaSS / VBTAC Copolymer> 55.00 g of ion-exchanged water was charged into a 1000 mL four-necked glass flask equipped with a nitrogen inlet tube, a three-way cock, and a Dimroth condenser. Here, 4-vinylbenzyltrimethylammonium chloride (VBTAC) (54.98 g, 254.48 mmol), sodium styrenesulfonate (NaSS) (59.17 g, 254.82 mmol), thioglycerol (TGL) (1.54 g, 13.95 mmol), and a water-soluble azo initiator V-50 (6.80 g, 24.57 mmol) were dissolved in 551.30 g of ion-exchanged water, and the monomer solution was degassed by repeating aspirator suction and nitrogen introduction, and then heated and polymerized at 85 °C while dropping it over 3 hours, and further aged at 85 °C for 2 hours. The polymerization conversion rate at the end of the reaction measured by GPC was VBTAC = 100%, NaSS = 100%, number average molecular weight Mn = 860, weight average molecular weight Mw = 2200, and Mw / Mn = 2.56. The polymerization solution was concentrated with a rotary evaporator, and the physical properties of the polymer were confirmed.

[0086] <Physical Properties of the Copolymer> Table 1 shows the results of measuring the viscosity of a 50 wt% aqueous solution of the polymer at different temperatures (No. 4 rotor, 30 rpm). It is clear that, although it has a low molecular weight and contains NaCl as a counter ion, it has a significantly higher viscosity compared to the examples. The polymer contains NaCl equivalent to the anions and cations in the polymer, but if NaCl is removed by a dissolution / phase separation operation, the shielding effect of the charge by NaCl disappears, and the electrostatic interaction between the polymers increases, so the solution viscosity is considered to be even higher. Although the polymer does exhibit a UCST, since it contains NaCl, if it is used directly as a driving agent for a forward osmosis membrane water treatment system, at least initially, water containing NaCl derived from the polymer rather than fresh water is obtained. Therefore, the same volume of ion-exchanged water was added to a 50 wt% aqueous solution of the polymer, stirred and heated at 85 °C, allowed to stand for phase separation, and then the supernatant containing NaCl was discarded. The same volume of ion-exchanged water was added to the remaining concentrated layer, stirred and heated again at 85 °C, allowed to stand for phase separation, and then the supernatant was discarded. After repeating this purification operation a total of 4 times, the UCST of the polymer was confirmed and is shown in Table 3. It is clear that the UCST is higher and the slope of the phase separation curve is larger compared to the examples.

[0087] Comparative Example 2 <Synthesis of AA / St copolymer> Acrylic acid (AA) (48.00 g, 659.45 mmol), styrene (St) (3.00 g, 28.50 mmol), RAFT agent - 1 (3.20 g, 11.95 mmol), initiator AIBN (0.20 g, 1.19 mmol), isopropanol (150.00 g) and ion-exchanged water (150.00 g) were charged into a 1000 mL four-necked glass flask equipped with a nitrogen inlet tube, a three-way cock and a Dimroth condenser and dissolved to form a homogeneous solution (the charged amount of St relative to the total of AA and St = 4.14 mol%). After thoroughly degassing this solution by repeating aspirator vacuum and nitrogen introduction, it was polymerized at 70 °C for 20 hours while stirring with a magnetic stirrer under a nitrogen atmosphere. The polymerization conversion rate at the end of the reaction measured by GPC was AA = 93%, St = 100%, number average molecular weight Mn = 4200, weight average molecular weight Mw = 6200, Mw / Mn = 1.48. That is, from the polymerization conversion rate of each monomer, the content of St (structural unit (B)) relative to the total of AA (structural unit (A)) and St (structural unit (B)) contained in the polymer is 4 mol%. The polymerization solution was vacuum dried at 85 °C for 5 hours to remove the solvent and unreacted monomers, and 46.46 g of a powdery polymer was obtained.

[0088] <Physical properties of the copolymer> As shown in Table 3, 30 wt% to 50 wt% aqueous solutions of the polymer all became homogeneous solutions at room temperature, and no UCST was observed above room temperature. This is because the structural unit (B) in the polymer was less than that in the examples (Table 1), and the hydrophobic interaction was too weak. Comparative Example 3 <Synthesis of AA / VBA copolymer> Acrylic acid (AA) (12.00 g, 164.86 mmol), 4-vinylbenzoic acid (VBA) (15.00 g, 101.04 mmol), RAFT agent-1 (1.50 g, 5.60 mmol), initiator V-501 (0.15 g, 0.52 mmol), isopropanol (150.00 g) and ion-exchanged water (150.00 g) were charged into a 1000 mL four-necked glass flask equipped with a nitrogen inlet tube, a three-way cock, and a Dimroth condenser, and dissolved to form a homogeneous solution (the charged amount of VBA relative to the total of AA and VBA = 38.00 mol%). After thoroughly degassing this solution by repeating aspirator vacuum and nitrogen introduction, it was polymerized at 70 °C for 20 hours while stirring with a magnetic stirrer under a nitrogen atmosphere. The polymerization conversion rate at the end of the reaction measured by GPC was AA = 71%, VBA = 100%, number average molecular weight Mn = 4800, weight average molecular weight Mw = 7500, Mw / Mn = 1.56. That is, from the polymerization conversion rate of each monomer, the content of VBA (structural unit (B)) relative to the total of AA (structural unit (A)) and VBA (structural unit (B)) contained in the polymer was 46 mol%. The polymerization solution was vacuum dried at 85 °C for 5 hours to remove the solvent and unreacted monomers, and 24.98 g of a powdery polymer was obtained.

[0089] <Physical properties of the copolymer> As shown in Table 3, the polymer showed temperature responsiveness, but it was clear that the UCST was too high compared to the examples. This is because the structural unit (B) in the polymer was too much and the hydrophobic interaction was too strong.

[0090] Comparative Example 4 <Synthesis of AA / VBA copolymer> Into a 1000 mL four-neck glass flask equipped with a nitrogen inlet tube, a three-way cock, and a Dimroth cooler, acrylic acid (AA) (41.55 g, 570.84 mmol), 4-vinylbenzoic acid (VBA) (15.60 g, 105.08 mmol), RAFT agent-1 (0.70 g, 2.61 mmol), initiator V-501 (0.60 g, 2.10 mmol), isopropanol (225.00 g), and ion-exchanged water (225.00 g) were charged and dissolved to form a homogeneous solution (the charged amount of VBA relative to the total of AA and VBA = 15.55 mol%). This solution was thoroughly degassed by repeating aspirator vacuum and nitrogen introduction, and then polymerized at 70 °C for 20 hours while stirring with a magnetic stirrer under a nitrogen atmosphere. The polymerization conversion rate at the end of the reaction measured by GPC was AA = 89%, VBA = 100%, number average molecular weight Mn = 20800, weight average molecular weight Mw = 33100, and Mw / Mn = 1.59. That is, from the polymerization conversion rate of each monomer, the content of VBA (structural unit (B)) relative to the total of AA (structural unit (A)) and VBA (structural unit (B)) contained in the polymer was 17 mol%. The polymerization solution was vacuum dried at 85 °C for 5 hours to remove the solvent and unreacted monomers, and 52.98 g of a powdery polymer was obtained.

[0091] <Physical properties of the copolymer> As shown in Table 3, the polymer showed temperature responsiveness, but it was clear that the UCST was too high compared to the examples. This is because there were many structural units (B) in the polymer and the hydrophobic interaction was too strong.

[0092] Comparative Example 5 <Synthesis of AA / St copolymer> An aqueous solution consisting of acrylic acid (AA) (8.02 g, 110.18 mmol), styrene (St) (1.74 g, 16.53 mmol), initiator 30% hydrogen peroxide solution (15.04 g, 132.67 mmol), 1-propanol (30.36 g) and ion-exchanged water (29.71 g), which had been degassed in advance by the above method, was slowly dropped into a 200 mL four-necked glass flask equipped with a nitrogen inlet tube, a three-way cock and a Dimroth condenser over 10 hours while polymerizing at 97 °C for 20 hours (the charged amount of St relative to the total of AA and St = 13.05 mol%). The polymerization conversion rate at the end of the reaction measured by GPC was AA = 70%, St = 100%, number average molecular weight Mn = 7000, weight average molecular weight Mw = 16300, and Mw / Mn = 2.33. That is, from the polymerization conversion rate of each monomer, the content of St (structural unit (B)) relative to the total of AA (structural unit (A)) and St (structural unit (B)) contained in the polymer was 18 mol%. By vacuum drying the polymerization solution at 85 °C for 5 hours, the solvent and unreacted monomers were removed to obtain 7.26 g of a powdery polymer.

[0093] <Physical properties of the copolymer> As shown in Table 3, the 40 wt% - 50 wt% aqueous solution of the polymer did not dissolve even when heated to 80 °C and did not show a UCST. Because the molecular weight distribution was broad, it was considered that the UCST was raised by the slightly contained high molecular weight components.

[0094] Comparative Example 6 <Synthesis of AA / St copolymer> A monomer solution consisting of acrylic acid (AA) (41.50 g, 570.15 mmol), styrene (St) (6.90 g, 65.56 mmol) and 1-propanol (250.00 g), and an initiator solution consisting of water-soluble initiator APS (16.50 g, 71.58 mmol) and ion-exchanged water (220.00 g) were thoroughly degassed by repeating aspirator vacuum and nitrogen introduction. A 1000 mL four-neck glass flask equipped with a nitrogen inlet tube, a three-way cock, a Dimroth condenser and a magnetic stirrer was immersed in an oil bath at 96 °C, and the above monomer solution and initiator solution were dropped therein over 5 hours, and heating was continued for an additional 1 hour for polymerization (charge amount of St with respect to the total of AA and St = 10.31 mol%) The polymerization conversion rate at the end of the reaction measured by GPC was AA = 100%, St = 100%, number average molecular weight Mn = 1000, weight average molecular weight Mw = 1900, Mw / Mn = 1.90. That is, from the polymerization conversion rate of each monomer, the content of St (structural unit (B)) with respect to the total of AA (structural unit (A)) and St (structural unit (B)) contained in the polymer was 10 mol%. The polymerization solution was vacuum dried at 85 °C for 5 hours to remove the solvent and unreacted monomers, and 49.52 g of a powdery polymer was obtained.

[0095] <Physical properties of the copolymer> As shown in Table 3, although the polymer had a low molecular weight and a water-soluble radical polymerization initiator was used, its temperature responsiveness was unclear and it was hardly soluble in water. The reason is not clear, but it is suggested that the difference in the polymer terminal structure, that is, a hydrophobic group derived from the initiator is introduced at the polymer terminal and it does not contain a carboxyl group.

[0096] Comparative Example 7 <Synthesis of AA / St copolymer> A monomer solution consisting of acrylic acid (AA) (42.10 g, 578.39 mmol), styrene (St) (6.80 g, 64.61 mmol), and 1-propanol (220.00 g), and an initiator solution consisting of oil-soluble initiator V-65 (20.00 g, 78.91 mmol) and 1-propanol (220.00 g) were thoroughly degassed by repeating aspirator vacuum and nitrogen introduction. A 1000 mL four-neck glass flask equipped with a nitrogen inlet tube, a three-way cock, a Dimroth condenser, and a magnetic stir bar was immersed in an oil bath at 96 °C, and the above monomer solution and initiator solution were added dropwise over 5 hours, followed by continued heating for 1 hour to polymerize (the charged amount of St with respect to the total of AA and St = 10.05 mol %). The polymerization conversion rate at the end of the reaction measured by GPC was AA = 94%, St = 100%, number average molecular weight Mn = 700, weight average molecular weight Mw = 1000, and Mw / Mn = 1.43. That is, from the polymerization conversion rate of each monomer, the content of St (structural unit (B)) with respect to the total of AA (structural unit (A)) and St (structural unit (B)) contained in the polymer was 11 mol %. The polymerization solution was vacuum dried at 85 °C for 5 hours to remove the solvent and unreacted monomers, and 45.38 g of a powdery polymer was obtained.

[0097] <Physical properties of the copolymer> As shown in Table 3, although the molecular weight of the polymer was small, its temperature responsiveness was unclear and it was difficult to dissolve in water. The reason is not necessarily clear, but it is suggested that a hydrophobic group derived from the initiator was introduced at the polymer end, that is, the polymer end does not contain a carboxyl group. Comparative Example 8 <Synthesis of AA / St copolymer> A monomer solution consisting of acrylic acid (AA) (50.10 g, 688.30 mmol), styrene (St) (11.00 g, 104.51 mmol), and 1-propanol (220.00 g) and an initiator solution consisting of oil-soluble initiator V-65 (10.00 g, 39.46 mmol), thioglycerol (4.30 g, 38.96 mmol), and 1-propanol (250.00 g) were thoroughly degassed by repeatedly aspirating and introducing nitrogen. A 1000 mL four-neck glass flask equipped with a nitrogen inlet tube, a three-way stopcock, a Dimroth condenser, and a magnetic stirrer was immersed in an oil bath at 96 °C. The monomer solution and initiator solution were added dropwise over 5 hours, and the mixture was heated for another hour to polymerize (the amount of St relative to the total amount of AA and St = 13.18 mol%). The polymerization conversion rates at the end of the reaction, as measured by GPC, were AA = 93%, St = 96%, number average molecular weight Mn = 1400, weight average molecular weight Mw = 1500, and Mw / Mn = 1.07. That is, based on the polymerization conversion rates of each monomer, the content of St (structural unit (B)) relative to the total of AA (structural unit (A)) and St (structural unit (B)) contained in the polymer was 14 mol%. The polymerization solution was vacuum dried at 85°C for 5 hours to remove the solvent and unreacted monomers, yielding 58.05 g of powdered polymer.

[0098] <Physical properties of copolymer> As shown in Table 3, despite the polymer's small molecular weight, its temperature response was unclear and it was difficult to dissolve in water. The reason for this is not entirely clear, but it is suggested that the polymer terminals lack hydrophilicity, i.e., they do not contain carboxyl groups.

[0099] [Table 3] TIFF0007770175000010.tif161148 [Industrial Applicability]

[0100] The carboxylic acid-based upper critical solution temperature (UCST) polymer of the present invention is an ionic polymer with excellent hydrolysis resistance and low viscosity, and is therefore expected to be used as a driving solution in forward osmosis membrane water treatment systems and osmotic power generation systems.

Claims

1. It consists of the following structural units (A) and (B): The content of the structural unit (B) is 5 mol % to 30 mol % based on the total of the structural units (A) and (B); A number average molecular weight Mn of 500 Daltons to 20,000 Daltons (Da) and a polymer having a ratio Mw / Mn of number average molecular weight Mn to weight average molecular weight Mw of 1.00 to 2.00; the polymer is water soluble and has an upper critical solution temperature (UCST); a viscosity of a 50 wt % aqueous solution containing the polymer is 10 mPa s to 10,000 mPa s at 25°C; It is hydrophobic at temperatures below UCST and hydrophilic at temperatures above UCST. Temperature-responsive carboxylic acid-based polymers. The structural unit (A) is represented by the following general formula (1): 【Transformation 6】 A structural unit derived from a carboxyl group-containing vinyl monomer represented by the formula (1), １ represents a hydrogen atom or a carboxyl group, R ２ represents a hydrogen atom, a methyl group, an ethyl group, or a carboxymethyl group, and M represents a proton, an ammonium cation, or an alkali metal cation; The structural unit (B) is represented by the following general formula (2): 【Transformation 7】 A structural unit derived from an aromatic vinyl monomer represented by the formula (2), ３ represents a hydrogen atom, a hydroxyl group, or a carboxyl group.

2. A polymer comprising the following structural units (A), (B), and (C): The content of the structural unit (B) is 5 mol % to 30 mol % based on the total of the structural units (A) and (B); the content of the structural unit (C) is 0.1 mol % to 10 mol % based on the total of the structural units (A) to (C); A number average molecular weight Mn of 500 Daltons to 20,000 Daltons (Da) and a polymer having a ratio Mw / Mn of number average molecular weight Mn to weight average molecular weight Mw of 1.00 to 2.00; the polymer is water soluble and has an upper critical solution temperature (UCST); a viscosity of a 50 wt % aqueous solution containing the polymer is 10 mPa s to 10,000 mPa s at 25°C; It is hydrophobic at temperatures below UCST and hydrophilic at temperatures above UCST. Temperature-responsive carboxylic acid-based polymers. The structural unit (A) is represented by the following general formula (1): 【Transformation 6】 (in formula (1), R 1 represents a hydrogen atom or a carboxyl group, R 2 represents a hydrogen atom, a methyl group, an ethyl group or a carboxymethyl group, and M represents a proton, an ammonium cation or an alkali metal cation), The structural unit (B) is represented by the following general formula (2): 【Transformation 7】 In formula (2), R 3 represents a hydrogen atom, a hydroxyl group, or a carboxyl group. The structural unit (C) is represented by the following general formula (3): 【Transformation 8】 (In formula (3), Q represents a structural unit derived from an acrylamide vinyl monomer.)

3. 3. The carboxylic acid polymer according to claim 1, wherein the number average molecular weight Mn is 500 to 5,000 daltons (Da).

4. 2. The carboxylic acid polymer according to claim 1, wherein the structural unit (A) is a structural unit derived from at least one carboxyl group-containing vinyl monomer selected from the group consisting of acrylic acid, methacrylic acid, ethacrylic acid, itaconic acid, and maleic acid, and the structural unit (B) is a structural unit derived from at least one aromatic vinyl monomer selected from the group consisting of styrene, vinylbenzoic acid, and hydroxystyrene.

5. The carboxylic acid polymer according to claim 2, wherein the structural unit (A) is a structural unit derived from at least one vinyl monomer selected from the group consisting of acrylic acid, methacrylic acid, ethacrylic acid, itaconic acid, and maleic acid; the structural unit (B) is a structural unit derived from at least one vinyl monomer selected from the group consisting of styrene, vinylbenzoic acid, and hydroxystyrene; and the structural unit (C) is a structural unit derived from at least one acrylamide-based vinyl monomer selected from the group consisting of acrylamide, methacrylamide, 4-acroylmorpholine, and 4-methacryloylmorpholine.

6. The carboxylic acid polymer according to any one of claims 1 to 5, wherein the UCST of a 20 wt% aqueous solution is 50°C to 100°C, that of a 30 wt% aqueous solution is 40°C to 90°C, that of a 40 wt% aqueous solution is 30°C to 70°C, and that of a 50 wt% aqueous solution is 10°C to 60°C.

7. The carboxylic acid polymer according to any one of claims 1 to 6, wherein the viscosity of a 50 wt% aqueous solution at 40°C is 10 mPa·s to 5000 mPa·s.

8. The carboxylic acid polymer according to any one of claims 1 to 7, wherein the polymer has a structure in which a terminal of the polymer is a carboxyl group.

9. A method for producing a temperature-responsive carboxylic acid polymer that is hydrophobic below UCST and hydrophilic at or above UCST, comprising polymerizing a monomer mixture containing a carboxyl group-containing vinyl monomer and an aromatic vinyl monomer in a hydrophilic solvent using a hydrophilic azo radical polymerization initiator, or a hydrophilic azo radical polymerization initiator and a RAFT agent or a hydrophilic chain transfer agent, the ratio of the aromatic vinyl monomer to all monomers in the monomer mixture is 9.0 mol % to 20.0 mol %; the total ratio of the hydrophilic azo radical polymerization initiator, the RAFT agent, and the hydrophilic chain transfer agent to all the monomers in the monomer mixture is 0.90 mol % to 13.0 mol %; method

10. the carboxyl group-containing vinyl monomer is one or more carboxyl group-containing vinyl monomers selected from the group consisting of acrylic acid, methacrylic acid, ethacrylic acid, itaconic acid, and maleic acid; 10. The method according to claim 9, wherein the aromatic vinyl monomer is one or more aromatic vinyl monomers selected from the group consisting of styrene, vinyl benzoic acid, and hydroxystyrene.

11. The method according to claim 9 or 10, wherein the monomer mixture further contains an acrylamide vinyl monomer, the ratio of the aromatic vinyl monomer to all the monomers in the monomer mixture is 10.0 mol% to 15.0 mol%, and the ratio of the acrylamide vinyl monomer is 0.1 mol% to 5.0 mol%; method.

12. 12. The method according to claim 11, wherein the acrylamide vinyl monomer is one or more acrylamide vinyl monomers selected from the group consisting of acrylamide, methacrylamide, 4-acroylmorpholine, and 4-methacryloylmorpholine.

13. The production method according to any one of claims 9 to 12, wherein the hydrophilic azo radical polymerization initiator is a hydrophilic polymerization initiator having a carboxyl group, the RAFT agent is a RAFT agent having a carboxyl group, and the hydrophilic chain transfer agent is a hydrophilic chain transfer agent having a carboxyl group.

14. The method according to any one of claims 9 to 13, wherein the hydrophilic solvent is one or more hydrophilic solvents selected from the group consisting of methanol, ethanol, propanol, butanol, acetone, and water.

15. 9. An aqueous solution containing 1% by weight to 70% by weight of the carboxylic acid polymer according to claim 1, which undergoes phase separation below the UCST and becomes a homogeneous solution at or above the UCST.

16. The aqueous solution according to claim 15, wherein the content of the carboxylic acid polymer according to any one of claims 1 to 8 is 20% by weight to 50% by weight.

17. 17. The aqueous solution according to claim 15 or 16, wherein the phase separation temperature is 40°C to 60°C.

18. A driving solution for a forward osmosis membrane water treatment system or an osmotic power generation system, comprising the carboxylic acid polymer according to any one of claims 1 to 8 as a solute.

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