Method for preparing low-molecular-weight polyacrylonitrile through grading regulation and control of free radical quenching agent

Abstract

The invention belongs to the technical field of polyacrylonitrile synthesis, provides a method for preparing low-molecular-weight polyacrylonitrile through grading regulation and control of a free radical quenching agent, and aims to solve the problems of low molecular weight, low yield, low product purity, high cost and the like of a polymerization product in the prior art. On the basis of a traditional water phase precipitation polymerization method, grading regulation and control of the free radical quenching agent are creatively provided, the free radical quenching agent is added in batches at the specific time point of a polymerization reaction, monomers are supplemented, a small amount of the free radical quenching agent preferentially attacks long-chain active species with weak activity in a polymerization system, growth of the long-chain active species is stopped, and meanwhile, the long-chain active species with the weak activity are separated from the polymerization system. The supplemented acrylonitrile monomer not only can activate the initiator to generate more active free radicals, but also can promote the growth of short-chain active species, so that the molecular weight of the polymerization system is more average. In conclusion, polyacrylonitrile with the molecular weight lower than 11000 g mol <-1 > is successfully prepared through grading regulation and control of the free radical quenching agent, and the conversion rate is increased to 70% or above.

Description

Technical Field

[0001] The invention belongs to the technical field of polyacrylonitrile synthesis, and specifically provides a method for preparing low-molecular-weight polyacrylonitrile by graded regulation of a free radical quencher. Background Art

[0002] Polyacrylonitrile is a vital polymer material. Its excellent physical and chemical properties make it widely used in textiles, electronics, aerospace and other fields. At present, the market is mainly high molecular weight and ultra-high molecular weight polyacrylonitrile, while low molecular weight polyacrylonitrile (molecular weight <12000g mol -1 ) There are still many problems to be overcome in terms of synthesis technology, and it has not yet been mass-produced. Low molecular weight polyacrylonitrile refers to a form of polyacrylonitrile with a lower molecular weight. Compared with high molecular weight polyacrylonitrile, it has more special and important uses. This material has more unique structural characteristics and physicochemical properties, and is an important precursor of energetic materials; low molecular weight polyacrylonitrile can undergo a [3+2] cycloaddition reaction with sodium azide under the action of a catalyst to generate a compound containing a tetrazole functional group. The tetrazole group is a typical high-energy group and has excellent stability and safety when used in the fields of energy, explosives, rocket propellants, etc.; on the other hand, low molecular weight polyacrylonitrile has better solubility and can be used to prepare water-based resins, micelles, gels, etc. It also has a lower viscosity and can be more easily processed and modified when applied to polymer materials.

[0003] There are many methods for preparing polyacrylonitrile. For the synthesis of low molecular weight polyacrylonitrile, solution polymerization methods such as atom transfer radical polymerization (ATRP), reversible addition-fragmentation chain transfer polymerization (RAFT), and iodine transfer radical polymerization (ITP) can theoretically easily prepare low molecular weight polyacrylonitrile. However, solution polymerization requires the treatment of a large amount of organic solvents, which not only increases production costs, but also has the possibility of environmental impact; and solution polymerization is more complicated for product purification. For example, ATRP contains excessive metal ions, which affects the color of the product and limits the use of the product; in addition, solution polymerization requires more precise control of parameters such as temperature, pressure, and solution concentration, and the process is more complicated and the operating cost is higher. On the contrary, aqueous precipitation polymerization has the advantages of lower energy consumption, simpler operation, and low cost, and is more suitable for industrial production; however, aqueous precipitation polymerization is still a challenge for controlling molecular weight.

[0004] Aqueous precipitation polymerization is divided into redox initiation and thermal initiation according to different initiation mechanisms. For thermal initiation, for example, a method for preparing acrylonitrile copolymer spinning solution with low molecular weight distribution disclosed in Chinese patent document No. CN 103030736A adjusts the molecular weight of the copolymer by adding acrylonitrile monomer and comonomer at one time or by adding acrylonitrile in batches. This process is relatively simple, but it is difficult to synthesize low molecular weight polymers. For redox initiated polymerization, such as a redox initiated aqueous precipitation polymerization disclosed in a Chinese patent document with publication number CN 110724217A, deionized water and acrylonitrile monomer are added to a container, nitrogen is introduced, the temperature is raised to 65-70°C, and the catalyst ammonium ferrous sulfate, the oxidant potassium persulfate, and the reducing agent ammonium ferrous sulfate are added dropwise, the pH is adjusted, and the polymerization is carried out for 90 minutes; the redox initiation system can generate active free radicals faster, so that the monomer acrylonitrile is consumed faster, and low molecular weight polyacrylonitrile can be obtained; however, the initiator needs to be added dropwise into the polymerization reaction system, and the process is relatively complicated; in addition, the initiation system is catalyzed by ferrous ions, and the ferric ions in the final polyacrylonitrile are difficult to completely remove, thereby limiting its application scenarios. In addition, a Chinese patent document with publication number CN 110950988A discloses a cationic polymerization method based on aqueous precipitation polymerization to prepare low molecular weight polystyrene. Styrene monomer is cationic polymerized in an aqueous organic solvent in the presence of a Lewis acid initiator, and water is used as a co-initiator. The reaction is rapid, but the moisture content needs to be strictly controlled. Too much moisture will make the Lewis acid catalytic performance ineffective, and too little moisture will not have a co-initiation effect.

[0005] From the current research status, it can be seen that the existing low molecular weight polyacrylonitrile preparation method has problems such as low molecular weight not low enough, low yield, low product purity and high cost. Therefore, it is of great significance to develop a new low molecular weight polyacrylonitrile preparation method so that the purity of the prepared low molecular weight polyacrylonitrile can meet the needs of national key areas such as energy, explosives, and rocket propellants, and the preparation method meets the requirements of green and efficient production process. Summary of the invention

[0006] The purpose of the present invention is to provide a method for preparing low molecular weight polyacrylonitrile by graded regulation of a free radical quencher, so as to solve the problems of insufficient molecular weight, low yield, low product purity and high cost in the existing low molecular weight polyacrylonitrile preparation method. The present invention is based on the traditional aqueous precipitation polymerization method, creatively proposes graded regulation of a free radical quencher, adopts graded regulation and gradient quenching, adds a free radical quencher in batches at a specific time point of the polymerization reaction, and supplements monomers, so as to achieve molecular weight control of suppressing long chains and increasing short chains, so that the prepared low molecular weight polyacrylonitrile has the advantages of low molecular weight, high yield and the like.

[0007] To achieve the above object, the technical solution adopted by the present invention is:

[0008] A method for preparing low molecular weight polyacrylonitrile by graded regulation of a free radical quencher, characterized in that it comprises the following steps:

[0009] Step 1. Add the solvent into the polymerization container, heat it to 25-40°C, then add acrylonitrile monomer and initiator, introduce inert gas for 15-30 minutes, seal the polymerization container, then heat it to 65-75°C, polymerize for 20-40 minutes, and complete the primary polymerization;

[0010] Step 2. Add a free radical quencher and acrylonitrile monomer into the polymerization container, seal the polymerization container and continue polymerization for 30 to 50 minutes to complete the secondary polymerization;

[0011] Step 3. Add the free radical quencher and acrylonitrile monomer to the polymerization container again, seal the polymerization container and continue polymerization for 30 to 50 minutes to complete the tertiary polymerization;

[0012] Step 4. After the polymerization is completed, deionized water is added to cool the polymerization system, and then the polymerization product is filtered to obtain the polymerization product; the polymerization product is washed with deionized water, filtered again, and dried to obtain low molecular weight polyacrylonitrile.

[0013] Furthermore, in step 1, the amount of acrylonitrile monomer added is 10% to 25% of the mass of the solvent, and the amount of initiator added is 1% to 5% of the mass of the acrylonitrile monomer in the same step.

[0014] Furthermore, in step 1, the solvent is deionized water or a mixture of deionized water and an organic solvent, and the organic solvent is preferably one or a mixture of dimethyl sulfoxide, isopropanol, and tert-butanol.

[0015] Furthermore, in step 1, the initiator is one or a mixture of ammonium persulfate, potassium persulfate, hydrogen peroxide, and azobisisobutyronitrile.

[0016] Furthermore, in step 1, the inert gas is nitrogen.

[0017] Furthermore, in step 2 and step 3, the amount of acrylonitrile monomer added is 10% to 50% of the amount of acrylonitrile monomer added in step 1, and the amount of free radical quencher added is 0.1% to 0.6% of the mass of acrylonitrile monomer in the same step.

[0018] Furthermore, in step 2 and step 3, the free radical quencher is one or a mixture of butylated hydroxytoluene (BHT), butylated hydroxyanisole (BHA), tannic acid (TA), and gossypol (G).

[0019] Furthermore, in step 4, drying is performed in a vacuum drying oven at 50°C to 70°C.

[0020] Further, in step 4, the molecular weight of the low molecular weight polyacrylonitrile is 8000 g mol -1 ~11000g mol -1 .

[0021] It should be noted that the polymerization reactions in steps 1 to 3 are all carried out under stirring, and the temperature of the polymerization reactions in steps 2 and 3 is the same as that in step 1.

[0022] In terms of working principle:

[0023] The invention provides a method for preparing low molecular weight polyacrylonitrile by graded regulation of a free radical quencher. The method is based on a traditional aqueous phase precipitation polymerization method. An initiator initiates polymerization after being thermally decomposed into active free radicals. During the polymerization process, the invention introduces a free radical quencher to achieve polymerization reaction control. When the average molecular weight of the polyacrylonitrile reaches an expected value, a small amount of free radical quencher is introduced. The small amount of free radical quencher preferentially attacks long-chain active species with weaker activity in the polymerization system to terminate their growth. Meanwhile, monomer acrylonitrile is added, which can not only activate the initiator to generate more active free radicals, but also promote the growth of short-chain active species, so that the molecular weight distribution of the polymerization system is narrower. In addition, the invention adopts a method of adding the free radical quencher in batches during the polymerization process to achieve graded regulation of the polymerization reaction, so that the average molecular weight of the polymerization system is controlled within an expected range, and finally a polyacrylonitrile with a molecular weight lower than 11000 g mol is prepared. -1 of polyacrylonitrile and increase the conversion rate to over 70%.

[0024] Based on the above technical solution and working principle, the beneficial effects of the present invention are:

[0025] 1) The present invention accurately controls the molecular weight of polyacrylonitrile through graded regulation of free radical quenchers, while effectively improving the conversion rate;

[0026] 2) The present invention uses a simple and easy-to-operate aqueous phase precipitation polymerization method to stably synthesize a molecular weight of less than 11000 g mol -1 The polyacrylonitrile fills the gap of low molecular weight polyacrylonitrile in the current market and provides a reference for the design of synthesis process of other low molecular weight polymers.

[0027] 3) The polyacrylonitrile synthesized by the present invention has high purity and can be used as a precursor of important tetrazole compounds, which has a significant role in promoting the research of key materials for national aerospace and national defense industries;

[0028] 4) The present invention adopts aqueous phase precipitation polymerization to prepare low molecular weight polyacrylonitrile, the product is easy to separate, and it is environmentally friendly and suitable for industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 The scanning electron microscope (SEM) images of polyacrylonitrile in Example 1, Example 2 and Comparative Example 1 are shown.

[0030] Figure 2 The following is the Fourier infrared absorption spectrum (FT-IR) of polyacrylonitrile in Example 1, Example 2 and Example 3.

[0031] Figure 3 is the hydrogen nuclear magnetic resonance spectrum of polyacrylonitrile in Example 1 ( 1 H-NMR).

[0032] Figure 4 The diagram is a comparison of the state changes of polyacrylonitrile in Example 1, Example 2 and Comparative Example 1 in a visual melting point apparatus.

[0033] Figure 5 The solubility comparison of polyacrylonitrile in N,N-dimethylformamide (DMF) in Example 1, Example 2 and Comparative Example 1 (a: 20 mg mL -1 , b: 10 mg mL -1 ). DETAILED DESCRIPTION

[0034] In order to make the purpose, technical scheme and beneficial effects of the present invention more clearly understood, the technical scheme of the present invention is fully and clearly described below in conjunction with the embodiments and drawings; the raw materials used in the following embodiments, unless otherwise specified, are all commercially available products.

[0035] Example 1

[0036] This embodiment provides a method for preparing low molecular weight polyacrylonitrile by graded regulation of free radical quencher, and the specific steps are as follows:

[0037] 90 g of deionized water was added to the polymerization container, stirring was started and nitrogen was introduced, the polymerization system temperature was maintained at 35° C., then 0.15 g of ammonium persulfate and 9 g of acrylonitrile were added, nitrogen was continuously introduced for 20 min, then the introduction of nitrogen was stopped and the polymerization container was sealed, the polymerization system temperature was raised to 70° C., polymerization was carried out for 30 min, and the first-stage polymerization was completed; 0.0035 g of free radical quencher dibutyl hydroxytoluene and 3 g of acrylonitrile monomer were added, polymerization was carried out for 45 min, the polymerization container was sealed, and the second-stage polymerization was completed; 0.0035 g of free radical quencher dibutyl hydroxytoluene and 3 g of acrylonitrile monomer were added, polymerization was carried out for 45 min, and ... second-stage polymerization was completed; 0.0035 g of free radical quencher dibutyl hydroxytoluene and 3 g of acrylonitrile monomer were added, polymerization was carried out for 45 min, and the second-stage polymerization was carried out; 0.0035 g of free radical quencher dibutyl hydroxytoluene and 0.0035g of free radical quencher dibutyl hydroxytoluene and 2g of acrylonitrile monomer were placed in a sealed polymerization container and reacted for 50min, and the tertiary polymerization was completed; 80g of deionized water was added to rapidly cool the polymerization system, and the polymerization product was filtered using a funnel, and then the polymerization product was washed once with 100g of deionized water, and filtered again. After removing the volatile matter in a vacuum drying oven at 60°C, 10.2g of white powdered polyacrylonitrile was obtained, and the conversion rate was 72.5%. The viscosity average molecular weight was 11505g mol by viscosity method test. -1 .

[0038] Example 2

[0039] This embodiment provides a method for preparing low molecular weight polyacrylonitrile by graded regulation of free radical quencher, and the specific steps are as follows:

[0040] 80 g of deionized water was added to the polymerization container, stirring was started and nitrogen was introduced, the temperature of the polymerization system was maintained at 30° C., then 0.28 g of potassium persulfate and 8 g of acrylonitrile were added, nitrogen was continuously introduced for 25 min, then the introduction of nitrogen was stopped and the polymerization container was sealed, the temperature of the polymerization system was raised to 70° C., polymerization was carried out for 30 min, and the first-stage polymerization was completed; 0.0024 g of free radical quencher butyl hydroxyanisole and 2.2 g of acrylonitrile monomer were added, polymerization was carried out for 30 min, the polymerization container was sealed, and the second-stage polymerization was completed; 0.0024 g of free radical quencher butyl hydroxyanisole and 2.2 g of acrylonitrile monomer were added, polymerization was carried out for 30 min, and ... second-stage polymerization was completed; 0.0 0.0024g of free radical quencher butyl hydroxyanisole and 1.5g of acrylonitrile monomer were placed in a sealed polymerization container and reacted for 45min, and the tertiary polymerization was completed; 80g of deionized water was added to quickly cool the polymerization system, and the polymerization product was filtered using a funnel to obtain a polymerization product, which was then washed once with 100g of deionized water, filtered again, and 7.98g of white powdery polyacrylonitrile was obtained after removing volatiles in a vacuum drying oven at 60°C, with a conversion rate of 68.2%. The viscosity average molecular weight was 9568gmol as determined by viscosity method. -1 .

[0041] Example 3

[0042] This embodiment provides a method for preparing low molecular weight polyacrylonitrile by graded regulation of free radical quencher, and the specific steps are as follows:

[0043] 95 g of deionized water was added to the polymerization container, stirring was started and nitrogen was introduced, the polymerization system temperature was maintained at 40° C., then 0.15 g of ammonium persulfate, 0.15 g of potassium persulfate and 8 g of acrylonitrile were added, nitrogen was continuously introduced for 30 min, then the introduction of nitrogen was stopped and the polymerization container was sealed, the polymerization system temperature was raised to 75° C., polymerization was carried out for 30 min, and the first-stage polymerization was completed; 0.0041 g of free radical quencher tannic acid and 3 g of acrylonitrile monomer were added, the polymerization container was sealed, polymerization was carried out for 33 min, and the second-stage polymerization was completed; and then 0.0041g of free radical quencher tannic acid and 2.1g of acrylonitrile monomer were added, the polymerization container was sealed, and the reaction was carried out for 40min, and the tertiary polymerization was completed; 80g of deionized water was added to quickly cool the polymerization system, and the polymerization product was filtered using a funnel, and the polymerization product was washed once with 90g of deionized water, and filtered again. After removing the volatile matter in a vacuum drying oven at 65°C, 8.52g of white powdered polyacrylonitrile was obtained, and the conversion rate was 65.1%. The viscosity average molecular weight was 7985gmol as determined by the viscosity method. -1 .

[0044] Example 4

[0045] This embodiment provides a method for preparing low molecular weight polyacrylonitrile by graded regulation of free radical quencher, and the specific steps are as follows:

[0046] 80 g of deionized water and 12 g of tert-butyl alcohol were added to a polymerization container, stirring was started and nitrogen was introduced, the polymerization system temperature was maintained at 35° C., then 0.19 g of hydrogen peroxide and 11 g of acrylonitrile were added, nitrogen was continuously introduced for 20 min, then the introduction of nitrogen was stopped and the polymerization container was sealed, the polymerization system temperature was raised to 65° C., polymerization was carried out for 30 min, and the primary polymerization was completed; 0.0038 g of free radical quencher cottonyphenol and 2.9 g of acrylonitrile monomer were added, the polymerization container was sealed, polymerization was carried out for 35 min, and the secondary polymerization was completed; 0.0028 g of free radical quencher cottonyphenol and 1.1 g of acrylonitrile monomer were added again, and the polymerization container was sealed and reacted for 40 min, and the tertiary polymerization was completed; 80 g of deionized water was added to quickly cool the polymerization system, and the polymerization product was filtered using a funnel to obtain a polymerization product, and the polymerization product was washed once with 10 g of deionized water, and filtered again. After removing the volatile matter in a vacuum drying oven at 70° C., 11.1 g of white powdered polyacrylonitrile was obtained, and the conversion rate was 69.4%. The viscosity average molecular weight was 8962 g mol by viscosity method test. -1 .

[0047] Example 5

[0048] This embodiment provides a method for preparing low molecular weight polyacrylonitrile by graded regulation of free radical quencher, and the specific steps are as follows:

[0049] 100 g of deionized water was added to the polymerization container, stirring was started and nitrogen was introduced, the polymerization system temperature was maintained at 30° C., then 0.15 g of azobisisobutyronitrile and 13 g of acrylonitrile were added, nitrogen was continuously introduced for 20 min, then the introduction of nitrogen was stopped and the polymerization container was sealed, the polymerization system temperature was raised to 70° C., polymerization was carried out for 35 min, and the first-stage polymerization was completed; 0.0032 g of free radical quencher tannic acid and 3.1 g of acrylonitrile monomer were added, the polymerization container was sealed, polymerization was carried out for 38 min, and the second-stage polymerization was completed; 0.0032 g of free radical quencher tannic acid and 2.6 g of acrylonitrile monomer were added, and the polymerization container was sealed and reacted for 40 min, and the tertiary polymerization was completed; 80 g of deionized water was added to quickly cool the polymerization system, and the polymerization product was filtered using a funnel to obtain a polymerization product, and then the polymerization product was washed once with 10 g of deionized water, and filtered again. After removing the volatile matter in a vacuum drying oven at 75° C., 13.2 g of white powdered polyacrylonitrile was obtained, and the conversion rate was 70.8%. The viscosity average molecular weight was 10623 g mol by viscosity method test. -1 .

[0050] Comparative Example 1

[0051] This comparative example provides a method for preparing low molecular weight polyacrylonitrile, using a traditional aqueous precipitation polymerization method, and the specific steps are as follows:

[0052] 90 g of deionized water was added to the polymerization container, stirring was started, the polymerization system temperature was maintained at 35° C., then 0.3 g of ammonium persulfate and 8 g of acrylonitrile were added, nitrogen was continuously introduced for 20 min, then the introduction of nitrogen was stopped and the polymerization container was sealed, the polymerization system temperature was raised to 70° C., and the polymerization was kept warm for 120 min. After the polymerization was completed, 50 g of deionized water was added to quickly lower the polymerization system temperature, and then the polymerization product was filtered through a funnel to obtain a polymerization product. Finally, the polymerization product was washed once with 100 g of deionized water, filtered again, and vacuum dried in a vacuum drying oven at 65° C. to remove volatiles, to obtain 4.31 g of white powdered polyacrylonitrile, with a conversion rate of 53.9%. The viscosity average molecular weight was 56042 g mol as determined by the viscosity method. -1 .

[0053] Comparative Example 2

[0054] This comparative example provides a method for preparing low molecular weight polyacrylonitrile, using a primary regulation method of introducing a free radical quencher, and the specific steps are as follows:

[0055] 100 g of deionized water and 10 g of acrylonitrile were added to a polymerization container, stirring was started, the polymerization system temperature was maintained at 35° C., then 0.3 g of ammonium persulfate was added, nitrogen was continuously introduced for 20 min, then the introduction of nitrogen was stopped and the polymerization container was sealed, the polymerization system temperature was raised to 70° C., the temperature was maintained for 40 min, 0.0022 g of free radical quencher tannic acid was added, the polymerization container was sealed, and the polymerization was continued for 30 min. After the polymerization was completed, the polymerization product was filtered through a funnel to obtain a polymerization product, and then the polymerization product was washed once with 100 g of deionized water, filtered again, and vacuum dried at 65° C. in a vacuum drying oven to remove volatiles, to obtain 3.51 g of white powdered polyacrylonitrile, with a conversion rate of 35.1%. The viscosity average molecular weight was 11042 g mol as determined by viscosity method. -1 .

[0056] The beneficial effects of the present invention are described in detail below in conjunction with the performance tests of polyacrylonitrile in the above Examples 1 to 5 and Comparative Examples 1 to 2.

[0057] like Figure 1 The following are scanning electron microscope images (SEM) of polyacrylonitrile in Example 1, Example 2 and Comparative Example 1. It can be seen from the figure that the molecular weight determines the particle size of polyacrylonitrile. In Example 3, polyacrylonitrile with the lowest molecular weight was synthesized, and the particle size was also the smallest. Figure 2 The Fourier transform infrared absorption spectra (FT-IR) of polyacrylonitrile in Example 1, Example 2 and Example 3 are shown. It can be seen from the figure that the three groups of samples have peaks at the same position. The analysis of the results shows that 1452cm -1 The peaks are caused by the carbon-hydrogen single bond, namely the in-plane bending vibration peaks of CH; 2244cm -1 There is an obvious sharp peak at 1075cm, which is the C≡N stretching vibration peak unique to cyano group. -1 The absorption peak at is the stretching vibration peak of C-CN. Therefore, through the analysis of the spectrum, it can be concluded that the product is a polyacrylonitrile homopolymer structure. Figure 3 Shown is the nuclear magnetic resonance hydrogen spectrum of polyacrylonitrile in Example 1 ( 1 H-NMR), as can be seen from the figure, there are two obvious signal peaks in the spectrum, δ2.04 and δ3.15, which reflect hydrogen atoms with different chemical shifts, represented by peak a and peak b respectively. Peak a and peak b correspond to two different types of hydrogen atoms in -CH2 and -CH on the PAN chain. Figure 4The following is a comparison of the state changes of polyacrylonitrile in Example 1, Example 2 and Comparative Example 1 in a visual melting point instrument. It can be seen from the figure that the three groups of samples all begin to turn yellow in the range of 170°C to 190°C, which may be due to the partial oxidation of PAN and the beginning of cyclization of the polymer chain. It is obvious that the color of the example with the lowest molecular weight begins to change the fastest, and it can be seen that the lower the molecular weight, the higher the activity of the polymer. Figure 5 The solubility comparison diagram of polyacrylonitrile in N,N-dimethylformamide (DMF) in Example 1, Example 2 and Comparative Example 1 is shown (a: 20 mg mL -1 , b: 10 mg mL -1 ), as can be seen from the figure, the solubility of low molecular weight polyacrylonitrile in DMF is very good. The molecular weight difference between Example 1 and Example 2 is not large, and the solubility difference is not large either. Compared with Comparative Example 1, it is obvious that the larger the molecular weight, the worse the solubility.

[0058] Furthermore, the molecular weight of polyacrylonitrile in Examples 1 to 5 and Comparative Examples 1 to 2 was tested by a viscosity method, and the conversion rate was calculated by a weighing method to obtain the viscosity average molecular weight and conversion rate test results of the polymers, as shown in Table 1.

[0059] Table 1

[0060] Group <![CDATA[M η (g mol -1 )]]> Conversion rate (%) Example 1 11505 72.5 Example 2 9568 68.2 Example 3 7985 65.1 Example 4 8962 69.4 Example 5 10623 70.8 Comparative Example 1 56042 53.9 Comparative Example 2 11042 35.1

[0061] As can be seen from the above, Examples 1 to 5 successfully prepared low molecular weight polyacrylonitrile by graded regulation of free radical quencher. Compared with the traditional aqueous precipitation polymerization method in Comparative Example 1, the molecular weight of polyacrylonitrile prepared by the present invention can be lower than 8000 gmol. -1 ; Compared with the primary regulation in Comparative Example 2, the conversion rate of the present invention is significantly improved, and it is more suitable for industrial production; and, from the comparison of Examples 1 to 3, it can be seen that the hierarchical regulation idea proposed by the present invention is based on the most basic principle of polymerization, and the molecular weight is adjusted by the polymerization time. The present invention introduces a free radical quencher, and a small amount of quencher will preferentially attack the long-chain active species with weaker activity and terminate its growth. At the same time, the addition of monomer acrylonitrile can not only activate the initiator to produce more active free radicals, but also promote the growth of short-chain active species; finally, the present invention adds free radical quenchers in batches during the polymerization process, and regulates the reaction in a hierarchical manner, so that the overall molecular weight of the polymerization system is maintained within the expected range.

[0062] In summary, the present invention is based on the traditional aqueous precipitation polymerization method, adopts free radical quencher graded regulation, successfully prepares low molecular weight polyacrylonitrile, and can effectively control the molecular weight of polyacrylonitrile and improve the conversion rate, which is conducive to industrial production.

[0063] The above description is only a specific implementation mode of the present invention. Any feature disclosed in this specification, unless otherwise stated, can be replaced by other alternative features that are equivalent or have similar purposes; all the disclosed features, or all the steps in the methods or processes, except for mutually exclusive features and / or steps, can be combined in any way.

Claims

1. A method for preparing low molecular weight polyacrylonitrile by graded regulation of free radical quencher, characterized in that: The following steps are involved: Step 1. Add the solvent into the polymerization container, heat it to 25-40°C, then add acrylonitrile monomer and initiator, introduce inert gas for 15-30 minutes, seal the polymerization container, then heat it to 65-75°C, polymerize for 20-40 minutes, and complete the primary polymerization; Step 2. Add a free radical quencher and acrylonitrile monomer into the polymerization container, seal the polymerization container and continue polymerization for 30 to 50 minutes to complete the secondary polymerization; Step 3. Add the free radical quencher and acrylonitrile monomer to the polymerization container again, seal the polymerization container and continue polymerization for 30 to 50 minutes to complete the tertiary polymerization; Step 4. After the polymerization is completed, deionized water is added to cool the polymerization system, and then the polymerization product is filtered to obtain the polymerization product; the polymerization product is washed with deionized water, filtered again, and dried to obtain low molecular weight polyacrylonitrile.

2. The method for preparing low molecular weight polyacrylonitrile by graded regulation of free radical quencher according to claim 1, characterized in that: In step 1, the amount of acrylonitrile monomer added is 10% to 25% of the mass of the solvent, and the amount of initiator added is 1% to 5% of the mass of the acrylonitrile monomer in the same step.

3. The method for preparing low molecular weight polyacrylonitrile by graded regulation of free radical quencher according to claim 1, characterized in that: In step 1, the solvent is deionized water or a mixture of deionized water and an organic solvent, and the organic solvent is preferably one or a mixture of dimethyl sulfoxide, isopropanol, and tert-butanol.

4. The method for preparing low molecular weight polyacrylonitrile by graded regulation of free radical quencher according to claim 1, characterized in that: In step 1, the initiator is one or a mixture of ammonium persulfate, potassium persulfate, hydrogen peroxide, and azobisisobutyronitrile.

5. The method for preparing low molecular weight polyacrylonitrile by graded regulation of free radical quencher according to claim 1, characterized in that: In step 1, the inert gas is nitrogen.

6. The method for preparing low molecular weight polyacrylonitrile by graded regulation of free radical quencher according to claim 1, characterized in that: In step 2 and step 3, the amount of acrylonitrile monomer added is 10% to 50% of the amount of acrylonitrile monomer added in step 1, and the amount of free radical quencher added is 0.1% to 0.6% of the mass of acrylonitrile monomer in the same step.

7. The method for preparing low molecular weight polyacrylonitrile by graded regulation of free radical quencher according to claim 1, characterized in that: In step 2 and step 3, the free radical quencher is one or a mixture of butylated hydroxytoluene (BHT), butylated hydroxyanisole (BHA), tannic acid (TA), and gossypol (G).

8. The method for preparing low molecular weight polyacrylonitrile by graded regulation of free radical quencher according to claim 1, characterized in that: In step 4, drying is performed in a vacuum drying oven at 50°C to 70°C.

9. The method for preparing low molecular weight polyacrylonitrile by graded regulation of free radical quencher according to claim 1, characterized in that: In step 4, the molecular weight of the low molecular weight polyacrylonitrile is 8000 g mol -1 ~11000g mol -1 .

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