Trifluoromethyl-initiated acrylic ester monomer reversible addition-fragmentation chain transfer free radical polymerization method

Abstract

The invention provides a trifluoromethyl-initiated acrylic ester monomer reversible addition-fragmentation chain transfer free radical polymerization method, and belongs to the technical field of high polymer chemistry. The method comprises the following steps: mixing an acrylate monomer, trifluoromethylsulfonyl chloride, bis (dodecyl sulfur alkyl thiocarbonyl) disulfide and a nitrogen-based catalyst, and reacting in a nitrogen atmosphere and blue light to obtain the polymer. According to the method, the trifluoromethylsulfonyl chloride is directly used as a trifluoromethyl source to directly initiate polymerization, so that pre-synthesis of an initiator or a chain transfer agent is avoided, and controllable free radical polymerization is realized. Nitrogen-bene is selected as a catalyst, so that the method has the advantages of high stability, easiness in synthesis, low toxicity, low cost and the like. A catalytic optical activity strategy of RAFT polymerization is further introduced, trifluoromethyl free radicals are directly used for light-operated RAFT polymerization from widely available and stable trifluoromethylsulfonyl chloride for the first time, and synthesis of the polymer is simplified.

Description

Technical Field

[0001] The invention relates to the technical field of polymer chemistry, and in particular to a trifluoromethyl-initiated reversible addition-fragmentation chain transfer free radical polymerization method for acrylic ester monomers. Background Art

[0002] Since its introduction in 1998, reversible addition-fragmentation chain transfer (RAFT) polymerization (Macromolecules, 1998, 31, 5559–5562.) has developed into a key method for the precise synthesis of polymers with specific structures and topological configurations, and its applications are widely covered in materials science, biomedicine and other fields (Chem. Soc. Rev., 2014, 43, 496–505.). In recent years, visible light-mediated RAFT polymerization has further promoted the development of this technology, realizing the photo-activated activation and precise temporal regulation of the chain growth process, providing a new way to design highly specific and multifunctional polymer materials (Chem. Soc. Rev., 2023, 52, 3035–3097.).

[0003] The mechanism of photoinduced RAFT reaction mainly involves oxidation / reduction quenching process (J.Am.Chem.Soc.,2022,144,19942–19952.) and energy transfer (Chem 2020,6,1888–1903.). In this process, the chain transfer agent (CTA) undergoes homolysis through single electron transfer or induced cleavage through energy transfer. Although pre-functionalized CTA is widely used in cutting-edge research, its practical application is still limited by high cost, limited commercial availability, cumbersome synthesis steps and constraints on polymer grafting methods (Chem.Soc.Rev.,2018,47,8998–9014.). Ideally, if selective polymer grafting can be achieved directly from structurally stable and functional group-rich substrate molecules, the synthesis process will be significantly simplified. However, research on photocontrolled RAFT polymerization based on such efficient and direct strategies is still rarely reported. It is worth noting that the Fors team (J.Am.Chem.Soc., 2020, 142, 4581–4585.) successfully achieved controlled radical polymerization of C–H bonds through the hydrogen atom transfer (HAT) RAFT pathway; the Hooper team (Angew.Chem., Int.Ed., 2024, 63, e202317071.) used the reduction quenching mechanism to achieve direct grafting of alkyl carboxylic acids onto RAFT polymers. These breakthroughs have laid the foundation for the development of new photocontrol strategies. Despite this, how to achieve direct polymer grafting of diverse functionalized substrates through innovative RAFT mechanisms remains a key challenge that needs to be solved in this field, and also provides an important direction for the future development of polymer chemistry.

[0004] The trifluoromethyl (CF3) group has become a privileged structural unit in drug molecule design due to its unique physicochemical properties, such as high lipophilicity, anti-metabolic degradation ability, strong electronegativity, and excellent pharmacokinetic properties (Chem. Soc. Rev., 2008, 37, 320 - 330.). In recent years, the development of photoinduced trifluoromethylation reactions has provided efficient methods for the introduction of CF3 groups (Chem. Rev., 2011, 111, 4475–4521.). However, although CF3 radicals are easily generated and widely used, there has been no report on the research of controlled polymer grafting via RAFT polymerization. It is worth noting that previous studies have shown that the CF3 groups at the polymer termini can significantly improve the hydrophobic / oil-repellent properties of materials, but there is still a lack of a general strategy for efficiently introducing this group. Summary of the Invention

[0005] Based on the above technical problems, the present invention aims to develop an efficient method to precisely introduce CF3 groups into the polymer backbone, so as to expand the polymer synthesis toolkit and provide new ideas for the design of high-performance materials. To solve the above problems, the present invention provides a novel photo-controlled reversible addition-fragmentation chain transfer (RAFT) polymerization system, which uses trifluoromethyl (CF3) radicals to initiate polymerization and achieves precise regulation through a photoactive complex.

[0006] The present invention uses N-heterocyclic nitrenium ion (NHN), which is simple to synthesize and has excellent cost-effectiveness, as a photocatalyst, and at the same time selects commercially available and stable trifluoromethanesulfonyl chloride (CF3SO2Cl) as the CF3 radical precursor. This reaction system not only successfully realizes polymer grafting based on CF3 radicals, but more importantly, establishes a new activation mechanism for photo-controlled RAFT polymerization through the photoactive complex strategy.

[0007] To achieve the above object, the present invention provides the following technical solutions:

[0008] The present invention provides a method for trifluoromethyl-initiated reversible addition-fragmentation chain transfer radical polymerization of acrylate monomers, comprising the following steps:

[0009] Mix an acrylate monomer, trifluoromethanesulfonyl chloride, bis(dodecylthio)trithiocarbonate, and a nitrenium catalyst, and react under a nitrogen atmosphere and blue light to obtain a polymer;

[0010] The monomer is any one of the compounds shown in Formula I:

[0011]

[0012] The trifluoromethanesulfonyl chloride is shown in Formula II:

[0013]

[0014] The bis(dodecylsulfanylthiocarbonyl) disulfide is as shown in Formula III:

[0015]

[0016] The nitrene catalyst is as shown in Formula IV:

[0017]

[0018] Preferably, the wavelength of the blue light is 460 nm.

[0019] Preferably, the molar ratio of the acrylate monomer, trifluoromethanesulfonyl chloride and bis(dodecylsulfanylthiocarbonyl) disulfide is (200 - 1000):1:(0.6 - 2.0).

[0020] Preferably, the molar ratio of the catalyst NHN to the monomer is 1:(1000 - 2000).

[0021] Preferably, the reaction conditions include: the time is 20 h, and the temperature is 20 - 30 °C.

[0022] Preferably, the reaction is carried out with stirring under a nitrogen atmosphere.

[0023] Beneficial effects: The present invention provides a method for reversible addition-fragmentation chain transfer radical polymerization of acrylate monomers initiated by trifluoromethyl, directly using trifluoromethanesulfonyl chloride as the trifluoromethyl source to directly initiate polymerization, avoiding the pre-synthesis of initiators or chain transfer agents and realizing controlled radical polymerization. Secondly, nitrene is selected as the catalyst, which not only has high stability and is easy to synthesize, but also has advantages such as low toxicity and low cost. In addition, compared with traditional methods relying on oxidation, reduction quenching and energy transfer, a catalytic photoactivity strategy of RAFT polymerization is introduced. Finally, the present invention first uses the highly valuable functional group trifluoromethyl radical directly from widely available and stable trifluoromethanesulfonyl chloride for photocontrolled RAFT polymerization, simplifying the synthesis of polymer conjugates. The method of the present invention not only successfully realizes polymer grafting based on CF3 radicals, but more importantly, establishes a completely new activation mechanism for photocontrolled RAFT polymerization through a photoactive complex strategy. Brief Description of the Drawings

[0024] Figure 1 It is a synthetic route diagram of polyacrylate in a specific embodiment of the present invention. Detailed Embodiments

[0025] The present invention provides a method for reversible addition-fragmentation chain transfer radical polymerization of acrylate monomers initiated by trifluoromethyl, comprising the following steps:

[0026] After mixing an acrylate monomer, trifluoromethanesulfonyl chloride, bis(dodecylthioalkylthio-carbonyl) disulfide and a nitrene catalyst, the reaction is carried out under a nitrogen atmosphere and blue light to obtain a polymer;

[0027] The monomer is any one of the compounds shown in Formula I:

[0028]

[0029] The trifluoromethanesulfonyl chloride is shown in Formula II:

[0030]

[0031] The bis(dodecylthioalkylthio-carbonyl) disulfide is shown in Formula III:

[0032]

[0033] The nitrene catalyst is shown in Formula IV:

[0034]

[0035] In the present invention, the wavelength of the blue light is preferably 460 nm. In the present invention, a blue lamp is used to provide the wavelength, with a power of 10 W and a model: from ROGER, RLH-18CU. In the present invention, the molar ratio of the acrylate monomer, trifluoromethanesulfonyl chloride and bis(dodecylthioalkylthio-carbonyl) disulfide is (200-1000):1:(0.6-2.0), and the molar ratio of the catalyst NHN to the monomer can also be 1:(1000-2000); and it has been verified that when the molar ratio of the acrylate monomer, trifluoromethanesulfonyl chloride, bis(dodecylthioalkylthio-carbonyl) disulfide and the nitrene catalyst is 200:1:1.5:0.1, the effect is the best. The conditions of the reaction include: the time is 20 h and the temperature is 20-30 °C. The reaction is carried out with stirring under a nitrogen atmosphere.

[0036] To further illustrate the present invention, the present invention will be described in detail below with reference to the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0037] Example 1

[0038] According to Figure 1 The synthesis route diagram shown, P1 is synthesized as follows:

[0039] Under a nitrogen atmosphere, MA (5 mmol), CF3SO2Cl (0.025 mmol, 500 mM in DEC), DS (0.0375 mmol), NHN (0.0025 mmol, 25 mM in MeCN), and 350 μL of DEC were added to a 10 mL reaction tube. A 10 W blue LED (460 nm) lamp was used to stir the mixture at room temperature for 24 hours. After the reaction was completed, the reaction solution was directly taken for 1 1H NMR and GPC analyses. The monomer conversion was measured by 1 1H NMR, and the molecular weight and molecular weight distribution (Đ) were determined by GPC, where M n,theo n = Mw CF3SO2Cl + 200 × Mw MA × Conv.%.

[0040]

[0041] The experimental results are shown in Table 1:

[0042] Table 1 Experimental Results

[0043]

[0044]

[0045] Comparative Example 1

[0046] P1 was synthesized according to the Figure 1 synthesis route diagram shown, where [MA]:[CF3SO2Cl]:[DS]:[NHN] = 200:1:1.5:0.1. The specific steps are as follows:

[0047] Under a nitrogen atmosphere, MA (5 mmol), CF3SO2Cl (0.025 mmol, 500 mM in DEC), DS (0.0375 mmol), NHN (0.0025 mmol, 25 mM in MeCN), and 350 μL of DEC were added to a 10 mL reaction tube. A 10 W blue LED (460 nm) lamp was used to stir the mixture at room temperature for 24 hours. After the reaction was completed, the reaction solution was directly taken for 1 1H NMR and GPC analyses. The monomer conversion was measured by 1 1H NMR, and the molecular weight and molecular weight distribution (Đ) were determined by GPC.

[0048]

[0049] The experimental results are shown in Table 2:

[0050] Table 2 Experimental Results

[0051] Conv. (%) <![CDATA[M n,theo / (g / mol)]]> <![CDATA[M n,GPC / (g / mol)]]> <![CDATA[D(M w / M n )]]> 89 15700 16600 1.34

[0052] Example 2

[0053] According to Figure 1 the synthetic route diagram shown below, synthesize P2. The specific steps are as follows:

[0054] Under a nitrogen atmosphere, add EA (5 mmol), CF3SO2Cl (0.025 mmol, 500 mM in DEC), DS (0.0375 mmol), NHN (0.0025 mmol, 25 mM in MeCN), and 350 μL of DEC to a 10 mL reaction tube. Use a 10 W blue LED (460 nm) lamp and stir at room temperature for 24 hours. After the reaction is completed, directly take the reaction solution for 1 1H NMR and GPC analysis. Through 1 the monomer conversion rate measured by 1H NMR, and the molecular weight and molecular weight distribution (D) are determined by GPC, M n,theo = MW CF3SO2Cl + 200 × MW EA × Conv.%.

[0055]

[0056] The experimental results are shown in Table 3:

[0057] Table 3 Experimental Results

[0058] Conv. (%) <![CDATA[M n,theo / (g / mol)]]> <![CDATA[M n,GPC / (g / mol)]]> <![CDATA[D(M w / M n )]]> 67 13800 13500 1.30

[0059] Example 3

[0060] According to Figure 1 the synthetic route diagram shown below, synthesize P3. The specific steps are as follows:

[0061] Under a nitrogen atmosphere, add nBuA (5 mmol), CF3SO2Cl (0.025 mmol, 500 mM in DEC), DS (0.0375 mmol), NHN (0.0025 mmol, 25 mM in MeCN), and 350 μL of DEC to a 10 mL reaction tube. Use a 10 W blue LED (460 nm) lamp and stir at room temperature for 24 hours. After the reaction is completed, directly take the reaction solution for 1 1H NMR and GPC analysis. Through 1 the monomer conversion rate measured by 1H NMR, and the molecular weight and molecular weight distribution (D) are determined by GPC,

[0062]

[0063] The experimental results are shown in Table 4 as follows:

[0064] Table 4 Experimental Results

[0065] Conv. (%) <![CDATA[M n,theo / (g / mol)]]> <![CDATA[M n,GPC / (g / mol)]]> <![CDATA[D(M w / M n ) <!-- 6 -->]]> 93 24200 20400 1.38

[0066] Example 4

[0067] According to Figure 1 the synthesis route diagram shown, P4 was synthesized, and the specific steps are as follows:

[0068] Under a nitrogen atmosphere, iBuA (5 mmol), CF3SO2Cl (0.025 mmol, 500 mM in DEC), DS (0.0375 mmol), NHN (0.0025 mmol, 25 mM in MeCN), and 350 μL of DEC were added to a 10 mL reaction tube. Using a 10 W blue LED (460 nm) lamp, the mixture was stirred at room temperature for 24 hours. After the reaction was completed, the reaction solution was directly taken for 1 1H NMR and GPC analysis. The monomer conversion rate measured by 1 1H NMR, and the molecular weight and molecular weight distribution (D) were determined by GPC.

[0069]

[0070] The experimental results are shown in Table 5 as follows:

[0071] Table 5 Experimental Results

[0072] Conv. (%) <![CDATA[M n,theo / (g / mol)]]> <![CDATA[M n,GPC / (g / mol)]]> <![CDATA[D(M w / M n )]]> 78 20300 16300 1.33

[0073] Example 5

[0074] According to Figure 1 the synthesis route diagram shown, P5 was synthesized, and the specific steps are as follows:

[0075] Under a nitrogen atmosphere, tBuA (5 mmol), CF3SO2Cl (0.025 mmol, 500 mM in DEC), DS (0.0375 mmol), NHN (0.0025 mmol, 25 mM in MeCN), and 350 μL of DEC were added to a 10 mL reaction tube. Using a 10 W blue LED (460 nm) lamp, the mixture was stirred at room temperature for 24 hours. After the reaction was completed, the reaction solution was directly taken for 1 1H NMR and GPC analysis. The monomer conversion rate measured by 1 1H NMR, and the molecular weight and molecular weight distribution (D) were determined by GPC.

[0076]

[0077] The experimental results are shown in Table 6 as follows:

[0078] Table 6 Experimental Results

[0079] Conv. (%) <![CDATA[M n,theo / (g / mol)]]> <![CDATA[M n,GPC / (g / mol)]]> <![CDATA[D(M w / M n )]]> 75 19600 16100 1.30

[0080] Example 6

[0081] According to Figure 1 the synthesis route diagram shown below, synthesize P6. The specific steps are as follows:

[0082] Under a nitrogen atmosphere, add EHA (5 mmol), CF3SO2Cl (0.025 mmol, 500 mM in DEC), DS (0.0375 mmol), NHN (0.0025 mmol, 25 mM in MeCN), and 350 μL of DEC to a 10 mL reaction tube. Use a 10 W blue LED (460 nm) lamp and stir at room temperature for 24 hours. After the reaction is completed, directly take the reaction solution for 1 1H NMR and GPC analysis. The monomer conversion rate measured by 1 1H NMR, and the molecular weight and molecular weight distribution (D) are determined by GPC

[0083]

[0084] The experimental results are shown in Table 7 as follows:

[0085] Table 7 Experimental Results

[0086] Conv. (%) <![CDATA[M n,theo / (g / mol)]]> <![CDATA[M n,GPC / (g / mol)]]> <![CDATA[D(M w / M n )]]> 87 32400 20600 1.34

[0087] Example 7

[0088] According to Figure 1 the synthesis route diagram shown below, synthesize P7. The specific steps are as follows:

[0089] Under a nitrogen atmosphere, add CyA (5 mmol), CF3SO2Cl (0.025 mmol, 500 mM in DEC), DS (0.0375 mmol), NHN (0.0025 mmol, 25 mM in MeCN), and 350 μL of DEC to a 10 mL reaction tube. Use a 10 W blue LED (460 nm) lamp and stir at room temperature for 24 hours. After the reaction is completed, directly take the reaction solution for 1 1H NMR and GPC analysis. The monomer conversion rate measured by 1 1H NMR, and the molecular weight and molecular weight distribution (D) are determined by GPC

[0090]

[0091] The experimental results are shown in Table 8 as follows:

[0092] Table 8 Experimental Results

[0093] Conv. (%) <![CDATA[M n,theo / (g / mol)]]> <![CDATA[M n,GPC / (g / mol)]]> <![CDATA[D(M w / M n )]]> 96 30000 18700 1.34

[0094] Example 8

[0095] According to Figure 1 the shown synthesis route diagram, P8 was synthesized, and the specific steps are as follows:

[0096] Under a nitrogen atmosphere, PhA (5 mmol), CF3SO2Cl (0.025 mmol, 500 mM in DEC), DS (0.0375 mmol), NHN (0.0025 mmol, 25 mM in MeCN), and 350 μL of DEC were added to a 10 mL reaction tube. Using a 10 W blue LED (460 nm) lamp, it was stirred at room temperature for 24 hours. After the reaction was completed, the reaction solution was directly taken for 1 1H NMR and GPC analyses. The monomer conversion rate measured by 1 1H NMR, and the molecular weight and molecular weight distribution (D) were determined by GPC.

[0097]

[0098] The experimental results are shown in Table 9 as follows:

[0099] Table 9 Experimental Results

[0100] Conv. (%) <![CDATA[M n,theo / (g / mol)]]> <![CDATA[M n,GPC / (g / mol)]]> <![CDATA[D(M w / M n )]]> 86 25800 15700 1.32

[0101] Example 9

[0102] According to Figure 1 the shown synthesis route diagram, P9 was synthesized, and the specific steps are as follows:

[0103] Under a nitrogen atmosphere, BnA (5 mmol), CF3SO2Cl (0.025 mmol, 500 mM in DEC), DS (0.0375 mmol), NHN (0.0025 mmol, 25 mM in MeCN), and 350 μL of DEC were added to a 10 mL reaction tube. Using a 10 W blue LED (460 nm) lamp, it was stirred at room temperature for 24 hours. After the reaction was completed, the reaction solution was directly taken for 1 1H NMR and GPC analyses. By 1Monomer conversion measured by \(^1\)H NMR, molecular weight and molecular weight distribution (\(D\)) determined by GPC

[0104]

[0105] The experimental results are shown in Table 10:

[0106] Table 10 Experimental Results

[0107] Conv. (%) <![CDATA[M n,theo / (g / mol)]]> <![CDATA[M n,GPC / (g / mol)]]> <![CDATA[D(M w / M n )]]> 97 31800 17500 1.36

[0108] Example 10

[0109] According to Figure 1 the synthetic route diagram shown, synthesize P10. The specific steps are as follows:

[0110] Under a nitrogen atmosphere, add MEA (5 mmol), CF₃SO₂Cl (0.025 mmol, 500 mM in DEC), DS (0.0375 mmol), NHN (0.0025 mmol, 25 mM in MeCN), and 350 μL of DEC to a 10 mL reaction tube. Use a 10 W blue LED (460 nm) lamp and stir at room temperature for 24 hours. After the reaction is completed, directly take the reaction solution for 1 \(^1\)H NMR and GPC analysis. By 1 Monomer conversion measured by \(^1\)H NMR, molecular weight and molecular weight distribution (\(D\)) determined by GPC

[0111]

[0112] The experimental results are shown in Table 11:

[0113] Table 11 Experimental Results

[0114] Conv. (%) <![CDATA[M n,theo / (g / mol)]]> <![CDATA[M n,GPC / (g / mol)]]> <![CDATA[D(M w / M n )]]> 90 23800 18700 1.33

[0115] Example 11

[0116] According to Figure 1 the synthetic route diagram shown, synthesize P11. The specific steps are as follows:

[0117] Under a nitrogen atmosphere, add EEA (5 mmol), CF₃SO₂Cl (0.025 mmol, 500 mM in DEC), DS (0.0375 mmol), NHN (0.0025 mmol, 25 mM in MeCN), and 350 μL of DEC to a 10 mL reaction tube. Use a 10 W blue LED (460 nm) lamp and stir at room temperature for 24 hours. After the reaction is completed, directly take the reaction solution for 11H NMR and GPC analyses. By 1 The monomer conversion rate measured by 1H NMR, the molecular weight and molecular weight distribution (D) were determined by GPC,

[0118]

[0119] The experimental results are shown in Table 12:

[0120] Table 12 Experimental Results

[0121] Conv. (%) <![CDATA[M n,theo / (g / mol)]]> <![CDATA[M n,GPC / (g / mol)]]> <![CDATA[D(M w / M n )]]> 93 27200 19500 1.40

[0122] Example 12

[0123] According to Figure 1 the synthetic route diagram shown, P12 was synthesized as follows:

[0124] Under a nitrogen atmosphere, LA (5 mmol), CF3SO2Cl (0.025 mmol, 500 mM in DEC), DS (0.0375 mmol), NHN (0.0025 mmol, 25 mM in MeCN), and 350 μL of DEC were added to a 10 mL reaction tube. Using a 10 W blue LED (460 nm) lamp, it was stirred at room temperature for 24 hours. After the reaction was completed, the reaction solution was directly taken for 1 1H NMR and GPC analyses. By 1 The monomer conversion rate measured by 1H NMR, the molecular weight and molecular weight distribution (D) were determined by GPC,

[0125]

[0126] The experimental results are shown in Table 13:

[0127] Table 13 Experimental Results

[0128] Conv. (%) <![CDATA[M n,theo / (g / mol)]]> <![CDATA[M n,GPC / (g / mol)]]> <![CDATA[D(M w / M n )]]> 68 33000 21400 1.36

[0129] Although the above embodiments have described the present invention in detail, they are only a part of the embodiments of the present invention, rather than all embodiments. People can also obtain other embodiments based on this embodiment without creative efforts, and these embodiments all fall within the protection scope of the present invention.

Claims

1. A reversible addition-fragmentation chain transfer radical polymerization method for trifluoromethyl-initiated acrylate monomers, characterized in that, Comprising the following steps: After mixing an acrylate monomer, trifluoromethanesulfonyl chloride, bis(dodecylthio)disulfide and a nitrene catalyst, carry out a reaction under a nitrogen atmosphere and blue light to obtain a polymer.

2. The reversible addition-fragmentation chain transfer radical polymerization method according to claim 1, wherein The acrylate monomer is any one of the compounds shown in Formula I:

3. The reversible addition-fragmentation chain transfer radical polymerization method according to claim 1, wherein The structure of the trifluoromethanesulfonyl chloride is shown in Formula II:

4. The reversible addition-fragmentation chain transfer radical polymerization method according to claim 1, wherein The structure of the bis(dodecylthio)disulfide is shown in Formula III:

5. The reversible addition-fragmentation chain transfer radical polymerization method according to claim 1, characterized in that, The nitrene catalyst is shown in Formula IV:

6. The reversible addition-fragmentation chain transfer radical polymerization method according to claim 1, wherein The molar ratio of the acrylate monomer to trifluoromethanesulfonyl chloride and bis(dodecylthio)disulfide is (200~1000):1:(0.6~2.0).

7. The reversible addition-fragmentation chain transfer radical polymerization method according to claim 1, characterized in that, The molar ratio of the catalyst NHN to the monomer is 1:(1000~2000).

8. The reversible addition-fragmentation chain transfer radical polymerization method according to claim 1, wherein The wavelength of the blue light is 460 nm.

9. The reversible addition-fragmentation chain transfer radical polymerization method according to claim 1, characterized in that The time of the reaction is 20 h, and the temperature is 20~30 °C.

10. The reversible addition-fragmentation chain transfer radical polymerization method according to claim 8, characterized in that, During the reaction, stir under a nitrogen atmosphere.

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