TWI592432B - Preparation of maleic anhydride grafted atactic polypropylene - Google Patents

Description

Process for preparing atactic polypropylene grafted with maleic anhydride

The invention relates to a method for preparing a grafted random polypropylene, in particular to a method for preparing atactic polypropylene grafted with maleic anhydride.

Polypropylene has many advantages such as high impact resistance, solvent resistance and corrosion resistance, and is widely used in various products. However, polypropylene still has shortcomings such as poor resistance to low temperature impact, aging, and poor environmental resistance (easy to yellow and become brittle when used outdoors), and it is necessary to modify the above disadvantages through modification. In addition, due to the toxicity of Ziegler-Natta catalysts and other reaction problems, it is difficult for polypropylene to be modified by copolymerization with functional monomers.

For the above reasons, the modification of polypropylene is mostly carried out by grafting, and since polypropylene is easily oxidized and photochemically reacted, polypropylene is usually grafted by a radical reaction. Functional monomers suitable for grafting with polypropylene such as maleic acid and its anhydrides or esters, itaconic acid and its anhydrides or esters, crotonic acid and its anhydrides or esters, acrylic acid, shrinkage Glycidyl methacrylate, vinyl trimethoxysilane, and the like. Among these functional monomers, maleic anhydride or acrylic acid is most commonly used because maleic anhydride or acrylic acid can enhance the interface between the filler and the polymer. The coupling reaction between the polymer and other functional polymers, the adhesion of the composite, and the compatibility of the insoluble polymer blend with the alloy.

Chapter 2 of Reactive Modifiers for Polymers, edited by S. Al-Malaika, mentions that maleic anhydride has lower free radical reactivity due to its structural properties, resulting in the copolymerization of polypropylene with maleic anhydride. The grafting rate is generally low. The effect of the reaction in solid or liquid state and in different solvents on the grafting rate is also mentioned in this book. However, currently commercially available maleic anhydride grafted polypropylene has a graft ratio of less than 1%, so the graft ratio of the maleic anhydride grafted polypropylene still needs to be continuously improved.

Accordingly, it is an object of the present invention to provide a process for the preparation of a maleic anhydride grafted random polypropylene having a relatively high graft ratio and avoiding waste of raw materials.

Therefore, the method for preparing a random polypropylene grafted with maleic anhydride comprises: (a) mixing a random polypropylene, maleic anhydride, a protective agent of a guanamine bond, and a lipophilic solvent to obtain a a mixed solution, wherein the amount of the maleic anhydride is from 2.3 to 9.4 parts by weight based on 10 parts by weight of the random polypropylene; (b) heating the mixture to 80 to 200 ° C, and then adding dropwise a starter solution to obtain a reaction solution; and (c) reacting and purifying the reaction solution to obtain the maleic anhydride Branch of random polypropylene.

The invention has the advantages that the preparation method of the invention can effectively enhance the grafting of maleic anhydride by using the protective agent with a guanamine bond and a lipophilic solvent, and simultaneously regulating the range of the amount of the random polypropylene and maleic anhydride. The graft ratio of the random polypropylene and further enhance the efficacy exhibited by the maleic anhydride grafted random polypropylene in subsequent applications.

Hereinafter, the present invention will be described in detail. This step (a) is a mixture of atactic polypropylene, maleic anhydride, a protective agent having a guanamine bond, and a lipophilic solvent to obtain a mixed liquid.

The protective agent having a guanamine bond is selected from caprolactam, 2-pyrrolidinone or succinimide.

The lipophilic solvent is selected from the group consisting of chlorobenzene, xylene, benzene, toluene or a combination of the foregoing. The lipophilic solvent dissolves the random polypropylene so that a larger amount of the random polypropylene can be grafted with maleic anhydride.

In the step (a), the amount of the maleic anhydride is from 2.3 to 9.4 parts by weight based on 10 parts by weight of the random polypropylene. When the amount of maleic anhydride is less than 2.3 parts by weight, insufficient grafting reaction of maleic anhydride will greatly reduce the graft ratio; when the amount of maleic anhydride is more than 9.4 parts by weight, the grafting cannot be promoted. The rate will also lead to waste of maleic anhydride raw materials and increased costs. For the above reasons, the method of the present invention uses a relatively large amount of atactic polypropylene, and it is considered that the amount of maleic anhydride is limited for the improvement of the graft ratio while avoiding waste of raw materials.

Preferably, the amount of the random polypropylene is 10 parts by weight. The amount of the protective agent having a guanamine bond is in the range of 2.6 to 16.2 parts by weight. When the amount of the protective agent having a guanamine bond is less than 2.6 parts by weight, the graft ratio is lowered; when the amount of the protective agent having a guanamine bond is more than 16.2 parts by weight, in addition to the inability to increase the graft ratio, It also leads to excessive unreacted protective agent residues, which increases the cost. It should be noted that when other reagents which facilitate the reaction are added to the reaction, the amount of the amide-protecting agent may be selectively reduced or increased as needed.

The mixing temperature of the step (a) can be controlled within a range in which the mixed liquid exhibits a uniform state. Preferably, the step (a) is carried out at a temperature ranging from 20 ° C to 50 ° C.

Preferably, the initiator solution of the step (b) contains an initiator and a lipophilic solvent. The initiator is used to initiate a radical reaction, and an initiator suitable for use with maleic anhydride or polypropylene, such as, but not limited to, 2,2-azobisisobutyronitrile [2,2-azobis ( Isobutyronitrile), AIBN], t- butylperoxy isopropylcarbonate (BPIC), dibenzoyl peroxide (BPO), dicumyl peroxide, 2,3-dimethyl-2,3-diphenyl-butane (DMDPB), di- t- butyl peroxide, DTBP), 2,5-bis( t- butylperoxy)-2,5-dimethylhexane [2,5-bis( t- butylperoxy)-2,5-dimethylhexane,DTBPH], 2,5 - bis( t- butylperoxy)-2,5-dimethylhexyne [2,5-bis( t- butyl-peroxy)-2,5-dimethylhexyne, DTBPHY], bis ( t -butyl) Oxidized isopropyl)benzene [bis( t- butylperoxyisopropyl)benzene, DTBPIB], bis(2-ethylhexyl)peroxydicarbonate (EHP), dilauroyl peroxide ( Dilauroyl peroxide, LPO), t- butylcumyl peroxid e, TBCP), t -butyl hydroperoxide (TBHP), t -butyl peroxybenzoate (TBPB), tert-butylperoxide-2-B T- butylperoxy-2-ethylhexanoate (TBPEH), t- butyl peroxypivalate (TBPP), and the like.

The lipophilic solvent in the starter solution is the same as described in paragraph [0011].

The amount of the initiator is adjusted according to the amount of maleic anhydride and random polypropylene. Preferably, the amount of the initiator is from 2.8 to 17.3, based on 10 parts by weight of the random polypropylene. Parts by weight.

Preferably, the starter solution further contains a mercaptan. The thiol is mainly used as a chain transfer agent for transferring a maleic anhydride radical which is less active and less reactive, so that the radical can be reused. The thiol is selected from the group consisting of 1-hexanethiol, dodecanethiol, 1,4-butanedithiol, 1,6-hexane. 1,6-hexanedithiol, pentaerythritol tetra(3-mercapto butyrate), pentaerythritol tris (3-mercapto butyrate), pentaerythritol quinone [pentaerythritol bis (3-mercapto butyrate)] or pentaerythritol mono (3-mercapto butyrate). When the initiator solution also contains a thiol When the amount of the protective agent having a guanamine bond is decreased, the amount of the protective agent having a guanamine bond is preferably 0.1 to 10 parts by weight based on 10 parts by weight of the random polypropylene.

Preferably, the total weight of the reaction liquid in the step (b) is 100% by weight, and the reactants in the reaction liquid (including atactic polypropylene, maleic anhydride, a protective agent having a guanamine bond, an initiator, The concentration of the selectively added thiol) ranges from 5 to 25 wt%.

Preferably, the mixture of the step (b) is heated to 100 to 200 °C. More preferably, when the initiator solution is composed of a starter and a lipophilic solvent, the mixture of the step (b) is heated to 100 to 150 ° C; when the initiator solution is a starter When the thiol and the lipophilic solvent are combined, the mixture of the step (b) is heated to 150 to 200 °C.

Preferably, the mercaptan is used in an amount ranging from 0.04 to 0.5 parts by weight based on 10 parts by weight of the random polypropylene.

During the reaction of the step (c), the protective agent having a guanamine bond is mainly used for protecting the free radical generated by the reaction of the maleic anhydride with the initiator solution, and can act as a free radical during the reaction. The capture agent helps to increase the grafting rate by forming an unstable transition state structure with free radicals and rapidly transferring the exchange.

The reaction of the step (c) is also carried out at a temperature of from 80 to 200 ° C. When the temperature is not within the above range, the maleic anhydride-grafted random polypropylene has a lower graft ratio.

The purification of the step (c) can be carried out according to a known method. Preferably, the purification is carried out by adding a precipitation solvent to the reaction liquid after the reaction. Wash it. Preferably, the precipitation solvent is selected from the group consisting of methanol, ethanol, acetone or a combination of the foregoing.

The maleic anhydride-grafted random polypropylene obtained by the method of the invention is suitable for use as a crystallization aid, a compatibilizer, an intercalation agent, a slip agent and the like. The maleic anhydride-grafted random polypropylene has the effects of improving crystallinity and dyeability, and when used as an additive, it not only improves material compatibility, but also increases the crystallinity and dyeability of the composite material.

Other features and effects of the present invention will be apparent from the embodiments of the present invention, wherein: FIG. 1 is a photograph illustrating the appearance structure test result of the application example 1 of the present invention; FIG. 2 is a photograph illustrating The appearance structure test result of the application example 2 of the present invention; FIG. 3 is a photograph illustrating the appearance structure test result of the comparative application example 1 of the present invention; and FIG. 4 is a photograph showing the fracture surface test result of the application example 1 of the present invention. Fig. 5 is a photograph showing the fracture surface test result of Application Example 2 of the present invention; Fig. 6 is a photograph showing the fracture surface test result of Application Example 3 of the present invention; and Fig. 7 is a photograph showing the comparison of the present invention. The fracture surface test result of Application Example 1; Figure 8 is a photograph showing the results of the fine structure test of Application Example 2 of the present invention; Figure 9 is a photograph showing the results of the fine structure test of Application Example 3 of the present invention; and Figure 10 is a photograph showing the comparison of the present invention. The fine structure test results of Application Example 1.

The invention is further illustrated by the following examples, but it should be understood that this embodiment is intended to be illustrative only and not to be construed as limiting.

[Examples 1 to 9] Atactic polypropylene grafted with maleic anhydride

Examples 1 to 9 were prepared according to the following steps and the temperature and lipophilic solvent types listed in Table 1: (a) 30 g (10 parts by weight) of atactic polypropylene (purchased from Formosa Petrochemical Co., Ltd., product name aPP), 14.01 g (4.67 parts by weight) of maleic anhydride, 24.23 g (8.08 parts by weight) of caprolactam (as a protective agent with a guanamine bond) and 500 mL of a lipophilic solvent were mixed at 30 ° C, A mixed solution was obtained; (b) 17.25 g (5.75 parts by weight) of benzamidine peroxide and 100 mL of a lipophilic solvent were mixed at 30 ° C to obtain a starter solution. According to the temperature of Table 1 below, the mixture is heated to the temperature of the carrier, and the initiator solution is added dropwise to obtain a reaction solution; and (c) the reaction solution is allowed to react at the temperature of the step (b). After the reaction is completed, the reaction solution is poured into 1500 mL of methanol (as a precipitation solvent). In this case, solids are precipitated at this time, and the solid is obtained by suction filtration and washed with methanol to obtain the maleic anhydride-grafted random polypropylene.

[test]

The maleic anhydride-grafted random polypropylene prepared in Examples 1 to 9 was dissolved in xylene and formulated into a 5 wt% sample. The sample was placed in an infrared spectrometer (purchased from Lily Company, model Varian 2000, wave number range 1000~3500 cm ^-1 ) for penetration test, and the calculated infrared spectrum was calculated to be 1780 cm ^- The area ratio of the absorption peak of ¹ (the C=O group absorption peak of maleic anhydride) and 1167 cm ^-1 (the CH ₃ absorption peak of atactic polypropylene) (A ₁₇₈₀ /A ₁₁₆₇ ), and the results are summarized in Table 1. The higher the area ratio, the higher the grafting rate of maleic anhydride.

[Comparative Example 1]

Comparative Example 1 was carried out in accordance with the procedure of Example 3 to carry out the preparation of a maleic anhydride-grafted random polypropylene, except that the amount of maleic anhydride was changed to 71.05 g (23.68 parts by weight). Finally, the test was carried out in accordance with the test procedure of Example 3, and the results were summarized in Table 1 below.

[Comparative Example 2]

In addition to changing the amount of maleic anhydride to 71.05 g (23.68 parts by weight), Comparative Example 2 was that no caprolactam was added in the step (a), and the remaining steps were carried out by grafting the maleic anhydride according to Example 3. Preparation of gauge polypropylene. Finally, the test was carried out in accordance with the test procedure of Example 3, and the results were summarized in Table 1 below.

Table 1

From the results of Table 1, it was found that although Comparative Example 1 used a larger amount of maleic anhydride, the area ratios of Examples 1 to 9 were higher than Comparative Example 1, demonstrating that the process of the present invention can be effectively effective by using an appropriate amount of maleic anhydride. Increase the grafting rate. Comparing the results of Examples 1 to 9 with Comparative Example 2, it was found that the area ratios of Examples 2 to 9 were higher than those of Comparative Example 2, and it was also confirmed that the method of the present invention can be effectively lifted by using a protective agent having a guanamine bond. Branch rate. It should be noted that the area ratio of Example 1 is slightly lower than that of Comparative Example 2 because the reaction temperature of Example 1 is lower; and the area ratio of Example 5 is slightly higher than that of Comparative Example 2, Because of the lower reaction temperature.

In order to obtain a more accurate graft ratio, the maleic anhydride-grafted random polypropylene obtained in Examples 3, 7 and 9 was additionally analyzed by an elemental analyzer, and the results are shown in Table 2 below.

Table 2

The calculation of the above graft amount is exemplified in Example 3: O: 100% - 84.34% - 13.97% = 1.69%; C: O (Molar ratio) = (84.34 / 12): (1.69 / 16) = 199.67:3; calculation of grafting ratio: (199.67-7)/3=64.22 and [1/(1+64.22)]×100%=1.53%; calculation of grafting amount: [(1.53×99)/(100× 42)] × 100% = 3.6%.

From the results of Table 2, it was confirmed that the process of the present invention can effectively increase the graft ratio and graft amount of maleic anhydride. Further, when the lipophilic solvent is benzene, a more preferable graft ratio and graft amount can be obtained.

[Examples 10 to 13] Atactic polypropylene grafted with maleic anhydride

Examples 10 to 13 were prepared according to the following steps and the temperature and lipophilic solvent types listed in Table 3:

(a) 30 g (10 parts by weight) of atactic polypropylene (available from Formosa Petrochemical Company, trade name aPP), 7.00 g (2.33 parts by weight) of maleic anhydride, 1.01 g (0.34 parts by weight) of The indoleamine (as a protective agent with a guanamine bond) and 70 g of a lipophilic solvent were mixed at 30 ° C to obtain a mixed solution.

(b) 9.66 g (3.22 parts by weight) of diisopropyl peroxide at 30 ° C Benzene, 0.195 g (0.07 part by weight) of pentaerythritol ruthenium (mercaptobutyrate) and 20 g of a lipophilic solvent were mixed to obtain a starter solution. According to the temperature of Table 3, the mixture is heated to the temperature of the carrier, and the initiator solution is added dropwise to obtain a reaction solution;

(c) reacting the reaction solution at the temperature of the step (b). After the reaction is completed, the reaction solution is poured into 1500 mL of methanol (as a precipitation solvent), at which time solids are precipitated, and then filtered by suction. The solid was obtained and washed with methanol to obtain the maleic anhydride-grafted random polypropylene.

It can be seen from Table 3 that the area ratios of Examples 10 to 13 are significantly higher than those of Examples 1 to 9 and Comparative Examples 1 and 2, and it is further verified that the method of the present invention is applied by using the protective agent having a guanamine bond and the addition of a thiol molecule. It can really improve the grafting rate.

[Examples 14 and 15] Atactic polypropylene grafted with maleic anhydride

Examples 14 and 15 were prepared according to the following steps and the temperature and lipophilic solvent types listed in Table 4: (a) 30 g (10 parts by weight) of random polypropylene (purchased from Formosa Petrochemical Co., Ltd., product name aPP), 7.00 g (2.33 parts by weight) of maleic anhydride, Caprolactam (as a protective agent with a guanamine bond) and 70 g of a lipophilic solvent were mixed at 30 ° C to obtain a mixed solution; (b) at 30 ° C, 9.66 g (3.22 parts by weight) Dicumyl peroxide, 0.195 g (0.07 part by weight) of pentaerythritol hydrazine (mercaptobutyrate) and 20 g of a lipophilic solvent were mixed to obtain a starter solution. According to the temperature in Table 4, the mixture is heated to the temperature of the carrier, and the initiator solution is added dropwise to obtain a reaction solution; and (c) the reaction solution is reacted at a temperature of 160 ° C until the reaction After completion, the reaction solution was poured into 1500 mL of methanol (as a precipitation solvent), at which time solids were precipitated, and the solid was taken up by suction filtration and washed with methanol to obtain the grafted maleic anhydride. Atactic polypropylene.

From the area ratios of Examples 14 and 15 of Table 4, it can be found that the addition of the protective agent and the mercaptan can effectively increase the grafting rate, and the grafting amount is increased as the amount of the protective agent is decreased. This is because sulfur The alcohol reacts with the protective agent having a guanamine bond. This reaction will simultaneously inhibit the protective ability of the protective agent and the chain transfer ability of the thiol. Therefore, when the protective agent and the thiol are used at the same time, the ratio of the two needs to be appropriately adjusted. .

[Application Examples 1 to 3] Compositions containing the maleic anhydride-grafted random polypropylene

The co-production method of Application Examples 1 to 3 is as follows: the maleic anhydride-grafted random polypropylene obtained in Example 4 is used according to the amount of the maleic anhydride-grafted random polypropylene in Table 5 below. 90 parts by weight of polypropylene and 10 parts by weight of nylon 6 were mixed into a spectrometer (manufactured by Brabender, model: Plasti-corder PL2000, temperature control at 240 ° C, rotation speed of 100 rpm) to prepare a composition. .

[Comparative Application Example 1]

The composition was prepared according to the common method of Application Examples 1 to 3 except that the maleic anhydride-grafted random polypropylene was not added, to obtain a composition of Comparative Application Example 1.

[test]

The compositions of Application Examples 1 to 3 and Comparative Application Example 1 were respectively subjected to the following tests:

1. Thermal property analysis: The thermal properties of the composition were analyzed by a differential scanning calorimeter (DSC) according to the following heating or cooling process, and the results are shown in Table 3 below. The heating process was carried out by raising the temperature to 260 ° C at a heating rate of 10 ° C / min at 25 ° C. The cooling process is: cooling from 260 ° C to a temperature of 10 ° C / min to 25 ° C.

2. Appearance structure analysis: 2 to 5 mg of the composition was taken, and the appearance and structure of the composition were observed by a polarizing microscope. The results are shown in Figures 1 to 3. 1 is the result of Application Example 1, FIG. 2 is the result of Application Example 2, and FIG. 3 is the result of Comparative Application Example 1.

3. Analysis of fracture surface: freeze the composition with liquid nitrogen, wait 3 minutes, then The frozen composition was taken out and broken to obtain a sample. The appearance and structure of the sample were observed using a polarizing microscope, and the results are shown in Figures 4-7. 4 is the result of Application Example 1, FIG. 5 is the result of Application Example 2, FIG. 6 is the result of Application Example 3, and FIG. 7 is the result of Comparative Application Example 1.

4. Microstructure analysis: About 10-20 mg of the composition was placed in an epoxy resin for embedding, solidification and slicing to prepare a sample. The fine structure of the sample was observed by a transmission microscope (TEM), and the results are shown in Figs. 8 is a result of Application Example 2, FIG. 9 is a result of Application Example 3, and FIG. 10 is a result of Comparative Application Example 1.

△H _m represents melting enthalpy

From the results of Table 5, it was found that when 5 parts by weight or more of the maleic anhydride-grafted random polypropylene was added, higher ΔH _m and T _m temperatures were obtained, indicating that the maleic anhydride grafted random Polypropylene can act as a nucleating agent to enhance the crystallinity of polypropylene.

From the results of FIGS. 1 to 3, it can be found that the compositions of Application Example 1 and Application Example 2 start to produce a large amount of dense and small-sized crystal particles at 120 ° C, and Comparative Application Example 1 needs to be produced at 110 ° C. A large amount of crystal particles, and the crystal grain size (about 50 μm) of Comparative Application Example 1 was larger than that of Application Example 1 and Application Example 2 (about 10 μm), and the maleic anhydride-grafted random polypropylene was also confirmed. Can be used as a nucleating agent to enhance the crystallinity of polypropylene.

From the results of Figs. 4 to 7, it was found that the surface of Fig. 7 (compared with the result of Application Example 1) had fine particles due to the phase separation phenomenon caused by the incompatibility of polypropylene with nylon 6. In contrast, in Figures 4 to 6, there are no fine particles at all, which proves that the maleic anhydride-grafted random polypropylene can act as a compatibilizer and slow down the phase separation of polypropylene and nylon 6, thereby improving The compatibility of the two.

From the results of FIGS. 8 and 9, it can be found that the nylon 6 can be uniformly dispersed in the polypropylene substrate; on the contrary, as a result of FIG. 10, it is found that the nylon 6 is present in the presence of larger particles and is not uniformly dispersed. It is also proved that the maleic anhydride-grafted random polypropylene can act as a compatibilizer and can slow the phase separation of polypropylene and nylon 6, thereby improving the compatibility of the two.

[Application Example 4] A composition containing the maleated anhydride-grafted random polypropylene

According to the amount of the maleic anhydride grafted random polypropylene of Application Example 2 of Table 5 above, the maleic anhydride grafted random obtained in Example 4 was used. Polypropylene, 90 parts by weight of polypropylene and 10 parts by weight of nylon 6 are added to a twin-screw extruder, and the temperature is controlled to be mixed and extruded between 220 and 240 ° C, and then pelletized to form a composition of plastic pellets. Further, the manufacturer is asked to spin the fiber to obtain the fiber.

[test]

The dyeing test is carried out with the dye of Taitang Industrial Co., Ltd., and the steps include the steps of preparing dye solution, dyeing bath, temperature dyeing, water washing, etc., and the detailed parts thereof are shown by the color card of Taitang Industrial Co., Ltd. The fibers obtained by using the pure polypropylene fibers and the composition of Application Example 4 were subjected to the above dyeing step.

It can be found that pure polypropylene fibers can hardly be dyed, and the fibers obtained by using the composition of Example 4 can be dyed significantly and are not easily faded after washing.

In summary, the method of the present invention can effectively enhance the grafting of maleic anhydride by using the protective agent with a guanamine bond and a lipophilic solvent to simultaneously control the amount of the random polypropylene and maleic anhydride. The grafting rate of the random polypropylene, and further enhancing the efficacy exhibited in the subsequent applications, can indeed achieve the object of the present invention.

The above is only the preferred embodiment of the present invention, and the scope of the present invention is not limited thereto, that is, the simple equivalent changes and modifications made by the patent application scope and patent specification content of the present invention, All remain within the scope of the invention patent.

Claims (8)

A method for preparing a random polypropylene grafted with maleic anhydride, comprising: (a) mixing a random polypropylene, maleic anhydride, a protective agent of a guanamine bond, and a lipophilic solvent to obtain a mixed solution Wherein, the amount of the maleic anhydride is in the range of 2.3 to 9.4 parts by weight and the amount of the protective agent having the indoleamine bond is in the range of 2.6 to 16.2 parts by weight, based on 10 parts by weight of the random polypropylene. The protective agent having a guanamine bond is selected from the group consisting of caprolactam, 2-pyrrolidone or succinimide; (b) heating the mixture to 80-200 ° C, and then adding a starter solution to obtain a a reaction solution; and (c) reacting and purifying the reaction liquid to obtain the maleic anhydride-grafted random polypropylene. The process of claim 1, wherein the lipophilic solvent is selected from the group consisting of chlorobenzene, xylene, benzene, toluene or a combination thereof. The process of claim 1, wherein the step (a) is carried out at a temperature ranging from 20 ° C to 50 ° C. The method of claim 1, wherein the initiator solution contains a starter and a lipophilic solvent. The process of claim 4, wherein the amount of the starter is from 2.8 to 17.3 parts by weight based on 10 parts by weight of the random polypropylene. The method of claim 4, wherein the starter solution further contains a mercaptan. The process according to claim 1, wherein the total weight of the reaction liquid in the step (b) is 100% by weight, and the reactant concentration in the reaction liquid ranges from 5 to 25% by weight. The process of claim 1, wherein the purification of the step (c) is carried out by adding a precipitation solvent to the reaction solution after the reaction.