Polymer manufacturing method

Abstract

To provide a method for efficiently producing an N-alkylmaleimide-based polymer.SOLUTION: A method for producing an N-alkylmaleimide-based polymer includes suspension-polymerizing a monomer mixture composed of 20-100 wt.% of N-alkylmaleimide represented by the following general formula (1) and 0-80 wt.% of other copolymerizable monomer, in an aqueous medium containing salt, in the presence of at least one dispersant selected from a group consisting of a nonionic compound which is bonded to oxygen or nitrogen and has no hydrogen, polyacrylate, and polyalkylene glycol, using an oil-soluble radical polymerization initiator. In the general formula (1), (where R represents a 1-12C linear alkyl group, a 3-12C branched alkyl group or a 3-6C cyclic alkyl group).SELECTED DRAWING: None

Description

[Technical Field] 【0001】 This invention relates to a method for producing N-alkylmaleimide polymers, and more specifically, to a method for efficiently producing N-alkylmaleimide polymers. [Background technology] 【0002】 Homopolymers or copolymers obtained from N-alkylmaleimide (N-alkylmaleimide polymers) are known to exhibit higher heat resistance and superior transparency compared to general thermoplastic vinyl polymers. Therefore, N-alkylmaleimide polymers are promising materials as transparent resins that can be used in a variety of applications in the field of optics (see, for example, Non-Patent Document 1). 【0003】 N-alkylmaleimide polymers, including alternating copolymers of N-alkylmaleimide and vinyl aromatic hydrocarbons such as styrene, have been reported to enable the creation of polymer materials with simple compositions, low birefringence, and the ability to maintain this low birefringence over a wide range of ambient temperatures (see, for example, Patent Document 1). 【0004】 N-alkylmaleimide polymers can be produced by radical polymerization. Furthermore, they can be produced by conventionally known methods such as bulk polymerization, suspension polymerization, solution polymerization, and emulsion polymerization. However, bulk polymerization has the problem of requiring special manufacturing equipment due to the difficulty of heat removal and the difficulty of removing unreacted monomers. Emulsion polymerization has the problem of difficulty in removing emulsifiers, resulting in impaired polymer transparency. Solution polymerization has the problem of producing polymers with relatively low molecular weights and insufficient mechanical strength. On the other hand, suspension polymerization is an industrially preferred manufacturing method because it allows for easy temperature control of the polymerization system, yields high molecular weight polymers, and allows for relatively easy removal of monomers and dispersants after polymerization. 【0005】 As an example of suspension polymerization, it has been described that in the polymerization of methacrylic acid, a water-soluble monomer, using an aqueous medium in which a salt is dissolved makes the methacrylic acid poorly soluble in the aqueous medium, resulting in the production of a granular polymer (see, for example, Non-Patent Document 2). In another example, suspension polymerization with the addition of a specific oily medium has been described as an efficient method for producing N-alkylmaleimide polymers with excellent transparency (see, for example, Patent Document 2). [Prior art documents] [Patent Documents] 【0006】 [Patent Document 1] Japanese Patent Publication No. 2020-126229 [Patent Document 2] Japanese Patent Publication No. 2009-215346 [Non-patent literature] 【0007】 [Non-Patent Document 1] Takayuki Otsu, "Future Materials," Vol. 3, No. 1, pp. 74-70. [Non-Patent Document 2] Ashida, Polymer Science, 7(5), 298-302 (1958) [Overview of the Initiative] [Problems that the invention aims to solve] 【0008】 However, even in suspension polymerization, there is a problem in that the polymer yield is impaired when water-soluble monomers are used. In suspension polymerization with the addition of a specific oily medium, as described in Patent Document 2, the yield is still not sufficient, and further improvement is desired. Furthermore, in suspension polymerization using an aqueous medium in which a salt has been dissolved, as described in Non-Patent Document 2, no studies have been conducted regarding the yield. 【0009】 The present invention has been made in view of the above problems, and aims to provide a method for efficiently producing N-alkylmaleimide polymers. [Means for solving the problem] 【0010】 In order to solve the above problems, the inventors of the present invention conducted intensive studies and as a result, found a method for producing an N-alkyl maleimide-based polymer by suspension polymerization using a specific aqueous medium, and thus completed the present invention. 【0011】 That is, aspect 1 of the present invention relates to a method for producing an N-alkyl maleimide-based polymer, characterized in that in an aqueous medium containing a salt, in the presence of at least one dispersant selected from the group consisting of a nonionic compound having no hydrogen bonded to oxygen or nitrogen, a polyacrylate, and a polyalkylene glycol, a monomer mixture composed of 20 to 100% by weight of an N-alkyl maleimide represented by the following general formula (1) and 0 to 80% by weight of other copolymerizable monomers is subjected to suspension polymerization using an oil-soluble radical polymerization initiator. 【0012】 [Chemical formula] 【0013】 Here, R represents a linear alkyl group having 1 to 12 carbon atoms, a branched alkyl group having 3 to 12 carbon atoms, or a cyclic alkyl group having 3 to 6 carbon atoms. 【0014】 In the production method according to aspect 2 of the present invention, in addition to the configuration of aspect 1 described above, the salt is an acetate, chloride, bromide, nitrate, sulfate, chlorate, iodide or thiocyanate. 【0015】 In the production method according to aspect 3 of the present invention, in addition to the configuration of aspect 1 or 2 described above, the dispersant is at least one selected from the group consisting of polyvinylpyrrolidone, sodium polyacrylate and polyethylene glycol. 【0016】 In the production method according to aspect 4 of the present invention, in addition to the configuration of any one of aspects 1 to 3 described above, R is a methyl group, ethyl group, propyl group, isopropyl group, butyl group, isobutyl group, sec-butyl group, tert-butyl group, hexyl group, cyclohexyl group, or octyl group. 【0017】 In the manufacturing method according to embodiment 5 of the present invention, in addition to the configuration of any one embodiment from embodiments 1 to 4 described above, the other copolymerizable monomer is a vinyl aromatic hydrocarbon, an olefin, an alkyl acrylate, an alkyl methacrylate, or a vinyl carboxylate. 【0018】 In the manufacturing method according to embodiment 6 of the present invention, in addition to the configuration of any one embodiment from embodiments 1 to 5 described above, the weight-average molecular weight of the N-alkylmaleimide polymer is 200,000 to 2,000,000. [Effects of the Invention] 【0019】 The manufacturing method of the present invention allows for the efficient production of N-alkylmaleimide polymers. [Modes for carrying out the invention] 【0020】 The various aspects of the present invention will be described in detail below. In this specification, numerical ranges indicated using "~" represent a range that includes the numbers before and after "~" as the minimum and maximum values, respectively. 【0021】 A method for producing an N-alkylmaleimide polymer according to one aspect of the present invention involves suspension polymerization of a monomer mixture consisting of 20 to 100% by weight of an N-alkylmaleimide represented by the following general formula (1) and 0 to 80% by weight of other copolymerizable monomers in an aqueous medium containing a salt, in the presence of at least one dispersant selected from the group consisting of nonionic compounds that do not have hydrogen bonded to oxygen or nitrogen, polyacrylates, and polyalkylene glycols, using an oil-soluble radical polymerization initiator. 【0022】 [ka] 【0023】 The manufacturing method according to this embodiment allows for the efficient production of N-alkylmaleimide polymers. Specifically, the manufacturing method according to this embodiment improves the yield, shortens the polymerization time required to obtain the same yield, or both, compared to conventional suspension polymerization using an aqueous medium. Furthermore, although the manufacturing method according to this embodiment uses suspension polymerization, which is simpler to operate than bulk polymerization, films formed using polymers obtained by this manufacturing method exhibit excellent optical properties (transparency, low birefringence), heat resistance, and mechanical strength, similar to films formed using polymers obtained by bulk polymerization. Therefore, N-alkylmaleimide polymers with superior optical properties, heat resistance, and mechanical strength can be produced more efficiently compared to bulk polymerization. 【0024】 In one aspect of the present invention, the salt is a salt that dissolves in an aqueous medium at the polymerization temperature. In suspension polymerization, in which a monomer mixture consisting of N-alkylmaleimide and other copolymerizable monomers is polymerized using an oil-soluble radical polymerization initiator in the presence of at least one dispersant selected from the group consisting of nonionic compounds without hydrogen bonded to oxygen or nitrogen, polyacrylates, and polyalkylene glycols, using an aqueous medium in which the salt is dissolved makes the water-soluble monomer mixture poorly soluble in the aqueous medium without causing precipitation of the dispersant. This allows for an increase in the concentration of the monomer mixture in the oil phase, thereby improving the yield. It should be noted that the improvement in yield by using an aqueous medium in which a salt is dissolved has not been reported before. 【0025】 Examples of salts in one aspect of the present invention include alkali metal salts of inorganic acids, alkaline earth metal salts of inorganic acids, ammonium salts of inorganic acids, alkali metal salts of organic acids, alkaline earth metal salts of organic acids, and ammonium salts of organic acids. The salt is preferably an alkali metal salt of an inorganic acid, an alkaline earth metal salt of an inorganic acid, an alkali metal salt of an organic acid, or an alkaline earth metal salt of an organic acid, and is particularly preferably an alkali metal salt of an inorganic acid. Using these preferred salts, the dispersant dissolved in the aqueous medium at the polymerization temperature is less likely to precipitate, and an N-alkylmaleimide polymer can be produced as granular particles with good handling properties. Only one type of salt may be used, or multiple types may be used in mixture form. 【0026】 Of these salts, it is preferable to use acetate as the salt of an organic acid (e.g., alkali metal salt or alkaline earth metal salt), and it is preferable to use chloride salt, bromide salt, nitrate salt, sulfate salt, chlorate salt, iodide salt, or thiocyanate salt as the salt of an inorganic acid (e.g., alkali metal salt or alkaline earth metal salt). When these preferred salts are used, the dispersant dissolved in the aqueous medium is less likely to precipitate at the polymerization temperature, and an N-alkylmaleimide polymer can be produced as granular particles with good handling properties. 【0027】 Examples of alkali metal salts of inorganic acids include: alkali metal sulfates such as lithium sulfate, sodium sulfate, potassium sulfate, rubidium sulfate, and cesium sulfate; alkali metal chlorides such as lithium chloride, sodium chloride, potassium chloride, rubidium chloride, and cesium chloride; alkali metal bromides such as lithium bromide, sodium bromide, potassium bromide, rubidium bromide, and cesium bromide; alkali metal nitrates such as lithium nitrate, sodium nitrate, potassium nitrate, rubidium nitrate, and cesium nitrate; alkali metal chlorates such as lithium chlorate, sodium chlorate, potassium chlorate, rubidium chlorate, and cesium chlorate; alkali metal iodides such as lithium iodide, sodium iodide, potassium iodide, rubidium iodide, and cesium iodide; and alkali metal thiocyanates such as lithium thiocyanate, sodium thiocyanate, potassium thiocyanate, rubidium thiocyanate, and cesium thiocyanate. As an alkali metal salt of an inorganic acid that is less likely to precipitate as a dispersant, it is preferable to use sodium chloride, potassium chloride, rubidium chloride, cesium chloride, sodium bromide, potassium bromide, rubidium bromide, cesium bromide, sodium nitrate, potassium nitrate, rubidium nitrate, cesium nitrate, sodium sulfate, potassium sulfate, rubidium sulfate, cesium sulfate, sodium chlorate, potassium chlorate, rubidium chlorate, cesium chlorate, sodium iodide, potassium iodide, rubidium iodide, cesium iodide, sodium thiocyanate, potassium thiocyanate, rubidium thiocyanate, or cesium thiocyanate, and it is particularly preferable to use sodium chloride. 【0028】 Examples of alkaline earth metal salts of inorganic acids include: alkaline earth metal sulfates such as magnesium sulfate, calcium sulfate, strontium sulfate, and barium sulfate; alkaline earth metal chlorides such as magnesium chloride, calcium chloride, strontium chloride, and barium chloride; alkaline earth bromides such as magnesium bromide, calcium bromide, strontium bromide, and barium bromide; alkaline earth nitrates such as magnesium nitrate, calcium nitrate, strontium nitrate, and barium nitrate; alkaline earth chlorates such as magnesium chlorate, calcium chlorate, strontium chlorate, and barium chlorate; alkaline earth iodides such as magnesium iodide, calcium iodide, strontium iodide, and barium iodide; and alkaline earth thiocyanates such as magnesium thiocyanate, calcium thiocyanate, strontium thiocyanate, and barium thiocyanate. It is preferable to use calcium chloride, strontium chloride, barium chloride, calcium bromide, strontium bromide, barium bromide, calcium nitrate, strontium nitrate, barium nitrate, calcium sulfate, strontium sulfate, barium sulfate, calcium chlorate, strontium chlorate, barium chlorate, calcium iodide, strontium iodide, barium iodide, calcium thiocyanate, strontium thiocyanate, or barium thiocyanate as the alkaline earth metal salt of the inorganic acid that is less likely to precipitate as a dispersant. 【0029】 Examples of alkali metal salts of organic acids include: alkali metal salts of citrate such as lithium citrate, sodium citrate, potassium citrate, rubidium citrate, and cesium citrate; alkali metal salts of tartrate such as lithium tartrate, sodium tartrate, potassium tartrate, rubidium tartrate, and cesium tartrate; and alkali metal salts of acetate such as lithium acetate, sodium acetate, potassium acetate, rubidium acetate, and cesium acetate. It is preferable to use sodium acetate, potassium acetate, rubidium acetate, or cesium acetate as alkali metal salts of organic acids that are less likely to precipitate as a dispersant. 【0030】 Examples of alkaline earth metal salts of organic acids include: alkaline earth metal salts of citrate such as magnesium citrate, calcium citrate, strontium citrate, and barium citrate; alkaline earth metal salts of tartrate such as magnesium tartrate, calcium tartrate, strontium tartrate, and barium tartrate; and alkaline earth metal salts of acetate such as magnesium acetate, calcium acetate, strontium acetate, and barium acetate. It is preferable to use calcium acetate, strontium acetate, or barium acetate as the alkaline earth metal salt of the organic acid that is less likely to precipitate as a dispersant. 【0031】 Examples of ammonium salts of inorganic acids include ammonium sulfate, ammonium chloride, ammonium bromide, ammonium nitrate, ammonium chlorate, ammonium iodide, and ammonium thiocyanate. Examples of ammonium salts of organic acids include ammonium citrate, ammonium tartrate, and ammonium acetate. 【0032】 Furthermore, the salt content in the aqueous medium may be any amount as long as the dispersant dissolved in the aqueous medium at the polymerization temperature does not precipitate and the manufacturing method of the present invention can be carried out. In particular, in order to produce N-alkylmaleimide polymers in high yield and to efficiently remove residual salts in the produced N-alkylmaleimide polymers, the total salt content in the aqueous medium relative to the total weight of the aqueous medium and salts is preferably 1 to 40% by weight, more preferably 1 to 35% by weight, and especially preferably 1 to 30% by weight. If the aqueous medium contains multiple types of salts, the salt content is expressed as the total content of all multiple types of salts. 【0033】 In one aspect of the present invention, the dispersant is a nonionic compound that does not have hydrogen bonded to oxygen or nitrogen. That is, the compound does not have hydrogen bonded to oxygen or nitrogen. Nonionic compounds that do not have hydrogen bonded to oxygen or nitrogen have weak interactions with N-alkylmaleimide polymers and can be efficiently removed by washing after polymerization. Because they can be efficiently removed by washing after polymerization, suspension polymerization using this compound yields N-alkylmaleimide polymers with excellent optical properties and mechanical strength. 【0034】 Examples of nonionic compounds without hydrogen bonded to oxygen or nitrogen in one aspect of the present invention include polyoxyethylene alkyl ethers, poly(N-vinylamide), and polyoxypropylene / polyoxyethylene block copolymers. 【0035】 Examples of polyoxyethylene alkyl ethers include polyoxyethylene lauryl ether, polyoxyethylene cetyl ether, polyoxyethylene stearyl ether, polyoxyethylene oleyl ether, polyoxyethylene myristyl ether, and polyoxyethylene octyldodecyl ether. 【0036】 Examples of poly(N-vinylamide) include poly(N-vinylformamide), poly(N-vinylacetamide), poly(N-vinylisobutylamide), polyvinylpyrrolidone, and polyvinylcaprolactam. Among these, poly(N-vinylformamide), poly(N-vinylacetamide), and polyvinylpyrrolidone are particularly preferred, as they yield N-alkylmaleimide polymers in granular form and allow for more efficient removal of poly(N-vinylamide) after polymerization, resulting in N-alkylmaleimide polymers with superior optical properties and mechanical strength. Poly(N-vinylformamide), poly(N-vinylacetamide), and polyvinylpyrrolidone are particularly preferred. 【0037】 When polyvinylpyrrolidone is used as a nonionic compound that does not contain hydrogen bonded to oxygen or nitrogen, the viscosity property value (K value) calculated by Fikentscher's formula shown in formula (2) below is preferably 10 to 130. In particular, the obtained N-alkylmaleimide polymer is obtained as granular particles, and the water-soluble poly(N-vinylamide) after polymerization can be removed more efficiently, resulting in an N-alkylmaleimide polymer with superior optical properties and mechanical strength. Therefore, the K value is more preferably 20 to 125, and particularly preferably 30 to 120. K = (1.5 × logη) rel -1) / (0.15+0.003×c)+{300×c×logη rel +(c+1.5×c×logη rel ) 2} 1 / 2 / (0.15×c+0.003×c 2 ) (2) η rel : Relative viscosity of polyvinylpyrrolidone aqueous solution with respect to water c: Concentration of polyvinylpyrrolidone in an aqueous solution of polyvinylpyrrolidone (%). 【0038】 Furthermore, in one embodiment of the present invention, the dispersant is a polyacrylate salt. Polyacrylate salts do not have hydrogen bonded to oxygen or nitrogen in the polyacrylic acid main chain excluding the cation portion, and therefore have weak interactions with N-alkylmaleimide polymers, and can be efficiently removed by washing after polymerization. As a result of being efficiently removed by washing after polymerization, suspension polymerization using this polyacrylate salt yields N-alkylmaleimide polymers with excellent optical properties and mechanical strength. 【0039】 Examples of polyacrylates in one aspect of the present invention include sodium polyacrylate, potassium polyacrylate, calcium polyacrylate, magnesium polyacrylate, and ammonium polyacrylate. Among these, sodium polyacrylate is particularly preferred because the resulting N-alkylmaleimide polymer is obtained as granular particles, and the polyacrylate can be removed more efficiently after polymerization, resulting in an N-alkylmaleimide polymer with superior optical properties and mechanical strength. 【0040】 When sodium polyacrylate is used as the polyacrylate, there are no particular restrictions on the weight-average molecular weight (Mw) of sodium polyacrylate. However, a weight-average molecular weight (Mw) of 100,000 to 5,000,000 is preferred, more preferably 500,000 to 3,000,000, and most preferably 1,000,000 to 3,000,000, as this allows the resulting N-alkylmaleimide polymer to be obtained as granular particles and enables efficient removal of sodium polyacrylate after polymerization, thereby obtaining an N-alkylmaleimide polymer with superior optical properties and mechanical strength. 【0041】 Furthermore, in one embodiment of the present invention, the dispersant is polyalkylene glycol. Polyalkylene glycol is a polymer of alkylene oxide having 2 to 4 carbon atoms and has hydrogen atoms bonded to oxygen, but it has weak interaction with N-alkylmaleimide polymers and can be efficiently removed by washing after polymerization. As it can be efficiently removed by washing after polymerization, suspension polymerization using this polyalkylene glycol yields N-alkylmaleimide polymers with excellent optical properties and mechanical strength. 【0042】 Examples of polyalkylene glycols in one aspect of the present invention include polyethylene glycol, polypropylene glycol, polybutylene glycol, and polyoxyethylene polyoxypropylene glycol. Among these, polyethylene glycol is particularly preferred because the resulting N-alkylmaleimide polymer is obtained as granular particles, and the polyalkylene glycol after polymerization can be removed more efficiently, resulting in an N-alkylmaleimide polymer with superior optical properties and mechanical strength. 【0043】 When polyethylene glycol is used as the polyalkylene glycol, there are no particular restrictions on the viscosity-average molecular weight of polyethylene glycol. However, a viscosity-average molecular weight of 20,000 to 10,000,000 is preferred, more preferably 150,000 to 8,000,000, and most preferably 1,000,000 to 5,000,000, as this allows the resulting N-alkylmaleimide polymer to be obtained as granular particles and enables efficient removal of the polyalkylene glycol after polymerization, thereby obtaining an N-alkylmaleimide polymer with superior optical properties and mechanical strength. 【0044】 In one embodiment of the present invention, the dispersant is one or more selected from the group consisting of nonionic compounds that do not contain hydrogen bonded to oxygen or nitrogen, polyacrylates, and polyalkylene glycols. The dispersant may be used alone or in combination of multiple types. 【0045】 In one aspect of the present invention, the amount of at least one dispersant selected from the group consisting of nonionic compounds without hydrogen bonded to oxygen or nitrogen, polyacrylates, and polyalkylene glycols is preferably 0.01 to 25 parts by weight per 100 parts by weight of the monomer mixture. In particular, the amount of dispersant added is more preferably 0.01 to 20 parts by weight, and especially preferably 0.01 to 15 parts by weight, because the resulting N-alkylmaleimide polymer is obtained as granular particles and the dispersant can be efficiently removed after polymerization, resulting in an N-alkylmaleimide polymer with superior optical properties and mechanical strength. 【0046】 In one embodiment of the present invention, R in general formula (1) represents a linear alkyl group having 1 to 12 carbon atoms, a branched alkyl group having 3 to 12 carbon atoms, or a cyclic alkyl group having 3 to 6 carbon atoms. 【0047】 Examples of linear alkyl groups having 1 to 12 carbon atoms include methyl, ethyl, propyl, butyl, pentyl, hexyl, octyl, decyl, and dodecyl groups. Examples of branched alkyl groups having 3 to 12 carbon atoms include isopropyl, isobutyl, sec-butyl, and tert-butyl groups. Examples of cyclic alkyl groups having 3 to 6 carbon atoms include cyclopropyl, cyclobutyl, and cyclohexyl groups. Among these, it is preferable that R is a methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl, tert-butyl, hexyl, cyclohexyl, or octyl group. N-alkylmaleimides having these preferred functional groups as R are soluble in conventional aqueous media but poorly soluble in aqueous media containing salts; that is, they show a large change in solubility depending on the presence or absence of salts. Therefore, by using such N-alkylmaleimides, N-alkylmaleimide polymers can be produced more efficiently. Furthermore, because N-alkylmaleimides have high safety as compounds, reducing the need for safety measures in equipment, and can be manufactured efficiently and economically, ethyl groups, tert-butyl groups, and cyclohexyl groups are preferred as R in general formula (1). The N-alkylmaleimide represented by general formula (1) may be used alone or as a mixture of multiple types. 【0048】 Specific examples of N-alkylmaleimides represented by general formula (1) include N-methylmaleimide, N-ethylmaleimide, N-propylmaleimide, N-isopropylmaleimide, N-butylmaleimide, N-isobutylmaleimide, N-sec-butylmaleimide, N-tert-butylmaleimide, N-pentylmaleimide, N-hexylmaleimide, N-octylmaleimide, N-decylmaleimide, N-dodecylmaleimide, N-cyclopropylmaleimide, N-cyclobutylmaleimide, and N-cyclohexylmaleimide. Among these, N-methylmaleimide, N-ethylmaleimide, N-propylmaleimide, N-isopropylmaleimide, N-butylmaleimide, N-isobutylmaleimide, N-sec-butylmaleimide, N-tert-butylmaleimide, N-hexylmaleimide, N-cyclohexylmaleimide, and N-octylmaleimide are preferred as N-alkylmaleimides. These preferred N-alkylmaleimides are soluble in conventional aqueous media but are poorly soluble in aqueous media containing salts, meaning they show a significant change in solubility depending on the presence or absence of salts. Therefore, by using such N-alkylmaleimides, N-alkylmaleimide polymers can be produced more efficiently. Furthermore, N-ethylmaleimide, N-tert-butylmaleimide, and N-cyclohexylmaleimide are even more preferred because they have high safety as compounds, reduce safety measures on equipment, and can be produced efficiently and economically. 【0049】 Examples of other copolymerizable monomers in one aspect of the present invention include vinyl aromatic hydrocarbons such as styrene and α-methylstyrene; olefins such as ethylene, propylene, 1-butene, and isobutene; alkyl acrylates such as methyl acrylate, ethyl acrylate, and butyl acrylate; alkyl methacrylates such as methyl methacrylate, ethyl methacrylate, and butyl methacrylate; and vinyl carboxylates such as vinyl acetate, vinyl propionate, and vinyl pivalate. Among these, vinyl aromatic hydrocarbons such as styrene and α-methylstyrene; and alkyl methacrylates such as methyl methacrylate, ethyl methacrylate, and butyl methacrylate are preferred because they allow for the realization of N-alkylmaleimide polymers with simple compositions, low birefringence, and the ability to maintain this low birefringence over a wide range of ambient temperatures. The other copolymerizable monomers may be used individually or in combination of multiple types. 【0050】 In one embodiment of the present invention, the mixing ratio of the monomer mixture consisting of N-alkylmaleimide represented by general formula (1) and other copolymerizable monomers is 20 to 100% by weight of N-alkylmaleimide and 0 to 80% by weight of other copolymerizable monomers. Among these, a polymer with particularly excellent heat resistance and optical properties can be obtained, so it is preferably 35 to 100% by weight of N-alkylmaleimide and 0 to 65% by weight of other copolymerizable monomers, and more preferably 50 to 100% by weight of N-alkylmaleimide and 0 to 50% by weight of other copolymerizable monomers. Furthermore, since good photoelastic coefficient and intrinsic birefringence are obtained, it is particularly preferable to have 55 to 80% by weight of N-alkylmaleimide and 20 to 45% by weight of other copolymerizable monomers. 【0051】 In one aspect of the present invention, oil-soluble radical polymerization initiators include benzoyl peroxide, lauryl peroxide, octanoyl peroxide, acetyl peroxide, di-tert-butyl peroxide, tert-butylcumyl peroxide, dicumyl peroxide, tert-butyl peroxyacetate, tert-butyl peroxybenzoate, tert-butyl peroxy-2-ethylhexanoate, tert-hexyl peroxy-2-ethylhexanoate, 2,5-dimethyl-2,5-di(2-ethylhexanoylperoxy)hexane, 1,1, Examples include organic peroxides such as 3,3-tetramethylbutylperoxy-2-ethylhexanoate, tert-butylperoxypivalate, tert-hexylperoxypivalate, tert-butylperoxyneodecanoate, and tert-hexylperoxyneodecanoate, as well as azo initiators such as 2,2'-azobis(2,4-dimethylvaleronitrile), 2,2'-azobis(2-butyronitrile), 2,2'-azobisisobutyronitrile, dimethyl-2,2'-azobisisobutyrate, and 1,1'-azobis(cyclohexane-1-carbonnitrile). 【0052】 Furthermore, the amount of oil-soluble radical polymerization initiator used can be set as appropriate, for example, 0.0001 to 2 parts by weight per 100 parts by weight of monomer mixture. In particular, the resulting N-alkylmaleimide polymer exhibits superior optical properties and mechanical strength, so the amount of oil-soluble radical polymerization initiator used is preferably 0.001 to 1 part by weight, and more preferably 0.01 to 0.5 parts by weight. 【0053】 In one embodiment of the present invention, the aqueous medium may be any medium that is normally treated as an aqueous medium, such as water, industrial water, deionized water, and distilled water. The amount of aqueous medium used is, for example, 100 to 500 parts by weight per 100 parts by weight of monomer mixture, and among these, 125 to 400 parts by weight is preferred, and particularly preferred is 150 to 300 parts by weight, in order to efficiently produce granular N-alkylmaleimide polymers. 【0054】 In a manufacturing method according to one aspect of the present invention, when the N-alkylmaleimide is in a solid state, an oily medium may be added to efficiently produce the N-alkylmaleimide polymer by charging the N-alkylmaleimide in a solution state during the monomer charging process. As the oily medium, at least one selected from the group consisting of aromatic hydrocarbons, ethers, esters, ketones, and halogen compounds can be used. 【0055】 Examples of aromatic hydrocarbons include toluene and xylene. Examples of ethers include diethyl ether and diisopropyl ether. Examples of esters include ethyl acetate, butyl acetate, and dimethyl carbonate. Examples of ketones include methyl ethyl ketone and methyl isobutyl ketone. Examples of halogen compounds include chloroform, methylene chloride, and 1,2-dichloroethane. Among these, aromatic hydrocarbons, ethers, esters, and ketones are preferred, with toluene and xylene being particularly preferred, in order to efficiently produce N-alkylmaleimide polymers with excellent mechanical strength. 【0056】 Furthermore, when an oily medium is added, the amount used may be any amount as long as the manufacturing method of the present invention can be carried out. In particular, in order to efficiently produce an N-alkylmaleimide polymer having excellent mechanical strength, the amount used is preferably 0.01 to 70 parts by weight, more preferably 0.1 to 50 parts by weight, and especially preferably 5 to 25 parts by weight per 100 parts by weight of the monomer mixture. 【0057】 In one embodiment of the manufacturing method of the present invention, the polymerization temperature can be appropriately set according to the decomposition temperature of the oil-soluble radical polymerization initiator. In particular, in order to efficiently produce an N-alkylmaleimide polymer having excellent optical properties and mechanical strength, the polymerization temperature is preferably in the range of 40 to 150°C, more preferably in the range of 50 to 90°C, and especially preferably in the range of 60 to 80°C. 【0058】 In a manufacturing method according to one aspect of the present invention, in order to efficiently and economically produce an N-alkylmaleimide polymer having excellent optical properties and mechanical strength in granular form, it is preferable to include filtering the N-alkylmaleimide polymer obtained by suspension polymerization, washing the N-alkylmaleimide polymer with a solvent that dissolves a dispersant without dissolving the N-alkylmaleimide polymer, and washing the N-alkylmaleimide polymer with a solvent that dissolves unreacted monomers without dissolving the N-alkylmaleimide polymer. 【0059】 A solvent that dissolves a dispersant without dissolving the N-alkylmaleimide polymer may also dissolve unreacted monomers as well as the dispersant. Similarly, a solvent that dissolves unreacted monomers without dissolving the N-alkylmaleimide polymer may also dissolve the dispersant as well as the unreacted monomers. That is, the solvent used may be one in which the N-alkylmaleimide polymer is insoluble and (i) only the dispersant is soluble, (ii) only the unreacted monomers are soluble, or (iii) both the dispersant and the unreacted monomers are soluble. Such solvents are not particularly limited and examples include water, methanol, ethanol, methanol / toluene mixed solvent, ethanol / toluene mixed solvent, methanol / water mixed solvent, and ethanol / water mixed solvent. For example, water is soluble in both the dispersant and the unreacted N-alkylmaleimide monomers. On the other hand, methanol, ethanol, methanol / toluene mixed solvent, ethanol / toluene mixed solvent, methanol / water mixed solvent, and ethanol / water mixed solvent are soluble in dispersants, unreacted N-alkylmaleimide monomers, and unreacted other copolymerizable monomers. 【0060】 If the solvent in which the N-alkyl maleimide polymer is insoluble and both the dispersant and the unreacted monomer are soluble, the dispersant and the unreacted monomer can be removed simultaneously with a single solvent. Also, different solvents may be used to separately remove the dispersant and the unreacted monomer. When separately removing the dispersant and the unreacted monomer, after washing the N-alkyl maleimide polymer with a solvent that dissolves the dispersant, the N-alkyl maleimide polymer may be washed with a solvent that dissolves the unreacted monomer, or after washing with a solvent that dissolves the unreacted monomer, it may be washed with a solvent that dissolves the dispersant. From the viewpoint of washing the dispersant in a state where the N-alkyl maleimide polymer is swollen with the unreacted monomer in order to efficiently remove the dispersant, it is preferable to wash the N-alkyl maleimide polymer with a solvent that dissolves the dispersant and then wash the N-alkyl maleimide polymer with a solvent that dissolves the unreacted monomer. 【0061】 Also, in one aspect of the present invention, if necessary, a chain transfer agent such as an alkyl mercaptan, or a hindered phenol-based or phosphorus-based antioxidant may be used at the initial stage of polymerization, during polymerization, or after polymerization. 【0062】 The weight average molecular weight (M w ) of the N-alkyl maleimide polymer obtained by the production method in one aspect of the present invention is not particularly limited, but in order to produce an N-alkyl maleimide polymer having excellent optical properties and mechanical strength, it is preferably 200,000 to 2,000,000, more preferably 500,000 to 1,800,000, and particularly preferably 800,000 to 1,500,000. Note that the weight average molecular weight (M w ) of the N-alkyl maleimide polymer can be controlled by the polymerization temperature. 【0063】 The ratio of units derived from N-alkylmaleimide in the N-alkylmaleimide polymer obtained by the production method in one embodiment of the present invention is in the range of 20 to 100% by weight, and among these, it is preferably 35 to 100% by weight, and more preferably 50 to 100% by weight, in order to obtain an N-alkylmaleimide polymer with excellent heat resistance and optical properties. 【0064】 There are no particular restrictions on the particle shape of the N-alkylmaleimide polymer obtained by the manufacturing method in one aspect of the present invention. However, by efficiently removing the dispersant, unreacted monomers, or both, an N-alkylmaleimide polymer with excellent optical properties and mechanical strength can be obtained. Therefore, it is preferably granular, with an average particle size in the range of 20 to 2,000 μm, more preferably in the range of 30 to 1,000 μm, and particularly preferably in the range of 50 to 900 μm. In this specification, the average particle size is the particle size at which the cumulative particle amount by volume reaches 50%, as determined by laser diffraction-scattering. 【0065】 The glass transition temperature (Tg) of an N-alkylmaleimide polymer serves as an indicator of its heat resistance. While there are no particular limitations on the glass transition temperature (Tg) of an N-alkylmaleimide polymer obtained by one embodiment of the manufacturing method of the present invention, it is preferably 120°C or higher, more preferably 135°C or higher, and particularly preferably 150°C or higher, as this yields an N-alkylmaleimide polymer with excellent heat resistance. 【0066】 The tensile stress of an N-alkylmaleimide polymer serves as an indicator of its mechanical strength. While there are no particular limitations on the tensile stress of an N-alkylmaleimide polymer obtained by one embodiment of the manufacturing method of the present invention, it is preferably 30 MPa or higher, more preferably 35 MPa or higher, and particularly preferably 40 MPa or higher, as this yields an N-alkylmaleimide polymer with excellent mechanical strength. Note that the tensile stress of an N-alkylmaleimide polymer is M w It can be controlled by [this method]. 【0067】 The tensile elongation of an N-alkylmaleimide polymer serves as an indicator of its mechanical strength. While there are no particular limitations on the tensile elongation of an N-alkylmaleimide polymer obtained by the production method according to one embodiment of the present invention, it is preferably 1.5% or more, more preferably 2% or more, even more preferably 2.5% or more, and particularly preferably 3% or more, since an N-alkylmaleimide polymer with excellent mechanical strength can be obtained. 【0068】 The haze of an N-alkylmaleimide polymer is one of the indicators of its optical properties. The haze of an N-alkylmaleimide polymer obtained by one embodiment of the manufacturing method of the present invention is not particularly limited, but it is preferably less than 3%, more preferably less than 2.5%, and especially preferably less than 2%, in order to obtain an N-alkylmaleimide polymer with excellent transparency. The haze of an N-alkylmaleimide polymer can be controlled by the type of dispersant, the amount of dispersant remaining in the N-alkylmaleimide polymer, and the amount of monomer remaining. 【0069】 Photoelastic constant (C) and intrinsic birefringence (Δn) of N-alkylmaleimide polymers 0 The photoelastic constant (C) and intrinsic birefringence (Δn) of the N-alkylmaleimide polymer obtained by one embodiment of the manufacturing method of the present invention are one of the indicators of optical properties. 0 While there are no particular restrictions on the absolute values ​​of each, a low birefringence N-alkylmaleimide polymer is obtained, so preferably the absolute value of C is 50 × 10 -12 Pa -1 The following, and Δn 0 The absolute value is 20 × 10 -3 The following, and more preferably, the absolute value of C is 10 × 10 -12 Pa -1 The following, and Δn 0 The absolute value is 5 × 10 -3 The following is the case, and in particular, the absolute value of C is 2 × 10 -12 Pa -1 The following, and Δn 0 The absolute value is 1 × 10 -3 The following applies. Note that C and Δn of the N-alkylmaleimide polymer. 0This is a constant that can be controlled by the type of monomer and the ratio of monomer units in the N-alkylmaleimide polymer. 【0070】 The temperature constant (dΔn) of the intrinsic birefringence of N-alkylmaleimide polymers. 0 The temperature constant (dΔn) of the intrinsic birefringence of an N-alkylmaleimide polymer obtained by one embodiment of the manufacturing method of the present invention is one of the indicators of optical properties. 0 While there are no particular restrictions on the absolute value of ( / dT), a value of 2 × 10 is preferred because it yields an N-alkylmaleimide polymer that can maintain low birefringence over a wide range of ambient temperatures. -5 ℃ -1 The following, and more preferably 1 × 10 -5 ℃ -1 The following applies. Note that the dΔn of N-alkylmaleimide polymers 0 / dT is a constant that can be controlled by the type of monomer and the ratio of monomer units in the N-alkylmaleimide polymer. 【0071】 The N-alkylmaleimide polymer obtained by the manufacturing method according to one embodiment of the present invention exhibits excellent optical properties, mechanical strength, and heat resistance, and can therefore be used in various optical components, optical lenses, optical sheets, and optical films. [Examples] 【0072】 The present invention will be described below based on examples, but the present invention is not limited in any way by these examples. Unless otherwise specified, commercially available reagents were used. Polyvinylpyrrolidone (PVP) K30 was special grade from Fujifilm Wako Pure Chemical Corporation. Polyethylene glycol (PEG) was first grade from Fujifilm Wako Pure Chemical Corporation. Tert-butylperoxy-2-ethylhexanoate was Perbutyl O from NOF Corporation. 【0073】 The following describes the evaluation and measurement methods for N-alkylmaleimide polymers obtained in the examples. 【0074】 <Average particle size> 150 mg of N-alkylmaleimide polymer was dispersed in 20 g of deionized water and measured using a particle size distribution analyzer MT-3300 manufactured by Nikkiso Co., Ltd. The shape was set to a perfect sphere, and the refractive index was set to 1.549. 【0075】 <Particle form> Using a Hitachi High-Tech TM3030Plus desktop microscope, the image signal was set to secondary electrons, and observations were performed in standard mode at magnifications of 30 to 100 times. 【0076】 <N-alkylmaleimide content in the polymer> A predetermined amount of N-alkylmaleimide polymer was dissolved in deuterated chloroform (approximately 5% by weight), and the resulting solution was analyzed using a 400 MHz NMR spectrometer (JNM-ECZ400S / L1) from JEOL Ltd. 1 1H-NMR measurements were performed. For copolymers of N-alkylmaleimide and styrene, the N-alkylmaleimide content was calculated from the integral ratio of the peaks between 5.0 and 0.3 ppm to the peaks between 8.0 and 5.5 ppm. 【0077】 <Molecular weight of polymer> Molecular weight distribution was measured using a Tosoh GPC (HLC-8320GPC). Two Tosoh TSKgel Super HM-H columns were used, the column temperature was set to 40°C, and tetrahydrofuran was used as the eluent. A molecular weight calibration curve was created using Tosoh standard polystyrene with known molecular weight. 【0078】 Example 1 In a 200 mL four-necked flask equipped with a stirrer, nitrogen inlet tube, and thermometer, 0.170 g of PVP K30, 69.2 g of 20 wt% sodium chloride aqueous solution, 19.83 g of N-ethylmaleimide, 6.10 g of styrene, 3.7 g of toluene, and 0.0202 g of tert-butylperoxy-2-ethylhexanoate were added. After 1 hour of nitrogen bubbling, suspension polymerization was carried out by holding at 70°C for 5 hours while stirring at 600 rpm. After the suspension polymerization was complete, the contents were introduced into 500 g of distilled water stirred with a magnetic stirrer and filtered to recover the resulting N-alkylmaleimide polymer particles. The recovered polymer particles were washed six times with 500 g of distilled water, and then five times with 316.4 g of methanol. Subsequently, the polymer particles were washed once with a mixed solvent of 97.4 g of toluene and 227.4 g of methanol, followed by five washes with 316.4 g of methanol. Furthermore, they were washed once with a mixed solvent of 165.4 g of toluene and 165.4 g of methanol, followed by five washes with 316.4 g of methanol. The washed polymer particles were placed in a drying oven set to 95°C and dried under vacuum. Drying was stopped when there was no further weight loss due to drying, yielding 19.3 g of N-ethylmaleimide / styrene copolymer. Observation of the polymer particle morphology using a bench microscope revealed that the particle size ranged from approximately 50 μm to approximately 2,000 μm. Table 1 shows the starting composition for polymerization, yield, N-ethylmaleimide unit content in the copolymer, Mw, and Mw / Mn. 【0079】 Example 2 In a 500 mL four-necked flask equipped with a stirrer, nitrogen inlet tube, and thermometer, 0.074 g of polyethylene glycol, 346.1 g of 5 wt% sodium chloride aqueous solution, 103.3 g of N-ethylmaleimide, 30.2 g of styrene, 14.8 g of toluene, and 0.166 g of tert-butylperoxy-2-ethylhexanoate were added. After 1 hour of nitrogen bubbling, suspension polymerization was carried out by holding at 70°C for 3 hours while stirring at 600 rpm. After the suspension polymerization was complete, the contents were introduced into 2,500 g of distilled water stirred with a magnetic stirrer, and the resulting N-alkylmaleimide polymer particles were recovered by filtration. The recovered polymer particles were washed six times with 2,000 g of distilled water, and then five times with 1,582 g of methanol. Subsequently, the polymer particles were washed once with a mixed solvent of 487 g of toluene and 1,137 g of methanol, followed by five washes with 1,582 g of methanol. Furthermore, they were washed once with a mixed solvent of 827 g of toluene and 827 g of methanol, followed by five washes with 1,582 g of methanol. The washed polymer particles were placed in a drying oven set to 95°C and dried under vacuum. Drying was stopped when there was no further weight loss due to drying, yielding 92.1 g of N-ethylmaleimide / styrene copolymer. Observation of the polymer particle morphology using a bench microscope revealed that the particle size ranged from approximately 50 μm to approximately 500 μm. Table 1 shows the starting composition for polymerization, yield, N-ethylmaleimide unit content in the copolymer, Mw, and Mw / Mn. 【0080】 Example 3 The N-ethylmaleimide / styrene copolymer was prepared in the same manner as in Example 2, except that the aqueous medium was changed from a 5% by weight sodium chloride aqueous solution to a 2% by weight sodium sulfate aqueous solution. As a result, 90.8 g of N-alkylmaleimide polymer was obtained. Observation of the polymer particle morphology using a bench microscope revealed that the particle size ranged from approximately 50 μm to approximately 500 μm. The polymerization starter composition, yield, N-ethylmaleimide unit content in the copolymer, Mw, and Mw / Mn are shown in Table 1. 【0081】 Reference example 1 The N-ethylmaleimide / styrene copolymer was prepared in the same manner as in Example 1, except that distilled water was used instead of a 20% by weight sodium chloride aqueous solution as the aqueous medium. As a result, 15.9 g of N-alkylmaleimide polymer was obtained. Table 1 shows the polymerization composition, yield, average particle size of N-alkylmaleimide polymer particles, N-ethylmaleimide unit content in the N-alkylmaleimide polymer, Mw, and Mw / Mn. Reference Example 1 showed a lower yield than Example 1. 【0082】 Reference example 2 The N-ethylmaleimide / styrene copolymer was prepared in the same manner as in Example 2, except that distilled water was used instead of a 5% by weight sodium chloride aqueous solution as the aqueous medium. As a result, 87.5 g of N-alkylmaleimide polymer was obtained. Table 1 shows the polymerization composition, yield, average particle size of N-alkylmaleimide polymer particles, N-ethylmaleimide unit content in the N-alkylmaleimide polymer, Mw, and Mw / Mn. Reference Example 2 showed a lower yield than Example 2. 【0083】 Comparative Example 1 3.8 g of N-ethylmaleimide and 1.2 g of styrene were placed in a test tube and mixed. After sealing the test tube, the contents were mixed by shaking thoroughly. The test tube was left to stand in a 70°C water bath for 24 hours to allow bulk polymerization to occur. The resulting N-alkylmaleimide polymer was a cylindrical mass adhered to the inside of the test tube. To remove unreacted monomers, the cylindrical mass was removed from the test tube, crushed into approximately 1 cm cubes, and placed in methylene chloride to prepare a clear N-alkylmaleimide polymer solution. Then, the obtained N-alkylmaleimide polymer solution was added dropwise to methanol to precipitate the N-alkylmaleimide polymer. After filtration with filter paper to remove the filtrate containing dissolved unreacted monomers, the N-alkylmaleimide polymer remaining on the filter paper was collected and dried under reduced pressure in a desiccator for 3 hours. The polymer was then further dried under reduced pressure in a vacuum dryer at 105°C for 24 hours to obtain 4.0 g of N-alkylmaleimide polymer. The particle morphology of the N-alkylmaleimide polymer particles was visually amorphous. The starting composition for polymerization, yield, N-ethylmaleimide unit content, Mw, and Mw / Mn in the N-alkylmaleimide polymer are shown in Table 1. 【0084】 In Comparative Example 1, an N-alkylmaleimide polymer was produced by bulk polymerization, resulting in a polymer having the same N-ethylmaleimide unit content as the polymer obtained in Example 1. However, unlike the production method of Example 1, the production method of Comparative Example 1 required additional steps to remove unreacted monomers: removing the produced N-alkylmaleimide polymer from the test tube, pulverizing it and dissolving it in a solvent, and precipitating the N-alkylmaleimide polymer from the resulting solution. Furthermore, in Comparative Example 1, the polymerization time required to obtain the same yield as in Example 1 was 4.8 times the polymerization time of Example 1, indicating that the production efficiency of Example 1 is superior to that of Comparative Example 1. 【0085】 [Table 1]

Claims

[Claim 1] In an aqueous medium containing a salt, in the presence of at least one dispersant selected from the group consisting of nonionic compounds without hydrogen bonded to oxygen or nitrogen, polyacrylates, and polyalkylene glycols, a monomer mixture consisting of 35 to 100% by weight of N-alkylmaleimide represented by the following general formula (1) and 0 to 65% by weight of other copolymerizable monomers is subjected to suspension polymerization using an oil-soluble radical polymerization initiator. A method for producing an N-alkylmaleimide polymer, characterized in that the salt is an acetate, chloride, bromide, nitrate, sulfate, chlorate, iodide, or thiocyanate. 【Chemistry 1】 (Here, R represents a linear alkyl group having 1 to 12 carbon atoms, a branched alkyl group having 3 to 12 carbon atoms, or a cyclic alkyl group having 3 to 6 carbon atoms.) [Claim 2] The method for producing an N-alkylmaleimide polymer according to claim 1, characterized in that the dispersant is at least one selected from the group consisting of polyvinylpyrrolidone, sodium polyacrylate, and polyethylene glycol. [Claim 3] A method for producing an N-alkylmaleimide polymer according to claim 1 or 2, characterized in that R is a methyl group, ethyl group, propyl group, isopropyl group, butyl group, isobutyl group, sec-butyl group, tert-butyl group, hexyl group, cyclohexyl group, or octyl group. [Claim 4] The method for producing an N-alkylmaleimide polymer according to claim 1 or 2, characterized in that the other copolymerizable monomers are vinyl aromatic hydrocarbons, olefins, alkyl acrylates, alkyl methacrylates, or vinyl carboxylates. [Claim 5] A method for producing an N-alkylmaleimide polymer according to claim 1 or 2, characterized in that the weight-average molecular weight of the N-alkylmaleimide polymer is 200,000 to 2,000,000.

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