A method for preparing low molecular weight polyacrylonitrile by hierarchical control of free radical quenchers

Abstract

This invention belongs to the field of polyacrylonitrile synthesis technology, and provides a method for preparing low molecular weight polyacrylonitrile by hierarchical control of free radical quenchers, to solve the problems of insufficiently low molecular weight, low yield, low product purity, and high cost in existing technologies. Based on the traditional aqueous precipitation polymerization method, this invention creatively proposes hierarchical control of free radical quenchers. Free radical quenchers are added in batches at specific time points during the polymerization reaction, along with monomer replenishment. A small amount of free radical quencher preferentially attacks the less active long-chain reactive species in the polymerization system, terminating their growth. Simultaneously, the added acrylonitrile monomer not only activates the initiator to generate more active free radicals but also promotes the growth of short-chain reactive species, resulting in a more uniform molecular weight in the polymerization system. In summary, this invention successfully prepares polyacrylonitrile with a molecular weight below 11000 g / mol through hierarchical control of free radical quenchers. ‑1 The polyacrylonitrile was improved, and the conversion rate was increased to over 70%.

Description

Technical Field

[0001] This invention belongs to the field of polyacrylonitrile synthesis technology, specifically providing a method for preparing low molecular weight polyacrylonitrile through hierarchical control of free radical quenchers. Background Technology

[0002] Polyacrylonitrile (PA) is a crucial polymer material whose excellent physicochemical properties make it widely used in textiles, electronics, aerospace, and other fields. Currently, the market mainly offers high molecular weight and ultra-high molecular weight PA, while low molecular weight PA (molecular weight <12000 g / mol) is less prevalent. -1 There are still many challenges to overcome in the synthesis technology, and large-scale production has not yet been achieved. Low molecular weight polyacrylonitrile (LMP) refers to the form of polyacrylonitrile with a lower molecular weight. Compared with high molecular weight PAP, it has more special and important applications. This material has more unique structural characteristics and physicochemical properties, and is an important precursor for energetic materials. LPP can undergo a [3+2] cycloaddition reaction with sodium azide under the action of a catalyst to generate compounds containing tetrazolium functional groups. Tetrazolium groups are typical high-energy groups, exhibiting excellent stability and safety when applied in energy, explosives, rocket propellants, and other fields. On the other hand, LPP has better solubility and can be used to prepare aqueous resins, micelles, gels, etc. It also has lower viscosity, making it easier to process and modify when applied to polymer materials.

[0003] There are many methods for preparing polyacrylonitrile (PAB). For the synthesis of low molecular weight PAB, 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 produce PAB relatively easily. However, solution polymerization requires the handling of large amounts of organic solvents, which not only increases production costs but also poses potential environmental impacts. Furthermore, the purification of products from solution polymerization is complex; for example, ATRP contains excess metal ions, affecting the product's color and limiting its applications. In addition, solution polymerization requires more precise control of parameters such as temperature, pressure, and solution concentration, making the process more complex and costly. Conversely, aqueous precipitation polymerization has advantages such as lower energy consumption, simpler operation, and lower cost, making it more suitable for industrial production; however, controlling the molecular weight remains a challenge in aqueous precipitation polymerization.

[0004] Aqueous precipitation polymerization is classified into redox-initiated and thermally-initiated polymerization based on different initiation mechanisms. For thermally-initiated polymerization, a method for preparing a spinning solution of acrylonitrile copolymer with low molecular weight distribution is disclosed in Chinese patent document CN 103030736A. The molecular weight of the copolymer is adjusted 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 using this method. For redox-initiated polymerization, such as the redox-initiated aqueous precipitation polymerization disclosed in Chinese patent document CN 110724217A, deionized water and acrylonitrile monomer are added to a container, nitrogen gas is introduced, the temperature is raised to 65-70°C, and then ferrous ammonium sulfate catalyst, potassium persulfate oxidant, and ferrous ammonium sulfate reducing agent are added dropwise. The pH is adjusted, and polymerization is carried out for 90 minutes. The redox-initiated system can generate active free radicals more quickly, so that the acrylonitrile monomer is consumed more quickly, and low molecular weight polyacrylonitrile can be obtained. However, the initiator needs to be added dropwise to the polymerization reaction system, and the process is relatively complicated. In addition, the initiation system is catalyzed by ferrous ions, and the iron ions in the final polyacrylonitrile are difficult to be completely removed, thus limiting its application scenarios. In addition, Chinese patent document CN 110950988A discloses a cationic polymerization method based on aqueous precipitation polymerization to prepare low molecular weight polystyrene. The styrene monomer is cationicly polymerized in an aqueous organic solvent in the presence of a Lewis acid initiator, with water as a co-initiator. The reaction is rapid, but the water content needs to be strictly controlled. Too much water will cause the Lewis acid catalytic performance to fail, while too little water will not play a co-initiating role.

[0005] Current research shows that existing methods for preparing low molecular weight polyacrylonitrile (LMP) suffer from problems such as insufficient molecular weight, low yield, low product purity, and high cost. Therefore, developing a new method for preparing LPP is of great significance, enabling the prepared LPP to meet the purity requirements of key national sectors such as energy, explosives, and rocket propellants, while also meeting the requirements of green and efficient production processes. Summary of the Invention

[0006] The purpose of this invention is to provide a method for preparing low molecular weight polyacrylonitrile by hierarchical control of free radical quenchers, in order to solve the problems of insufficient molecular weight, low yield, low product purity and high cost in existing low molecular weight polyacrylonitrile preparation methods. Based on the traditional aqueous precipitation polymerization method, this invention creatively proposes hierarchical control of free radical quenchers. By using hierarchical control and gradient quenching, free radical quenchers are added in batches at specific time points in the polymerization reaction, and monomers are added to achieve molecular weight control that inhibits long chains and increases short chains. This results in low molecular weight polyacrylonitrile with the advantages of both low molecular weight and high yield.

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

[0008] A method for preparing low molecular weight polyacrylonitrile using a graded control of free radical quenchers, characterized by comprising the following steps:

[0009] Step 1. Add the solvent to the polymerization container, heat to 25-40℃, then add acrylonitrile monomer and initiator, purge with inert gas for 15-30 minutes, seal the polymerization container, then heat to 65-75℃ and polymerize for 20-40 minutes to complete the first-stage polymerization.

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

[0012] Step 4. After polymerization, add deionized water to cool the polymerization system, then filter to obtain the polymerization product; wash the polymerization product with deionized water, filter again and dry to obtain low molecular weight polyacrylonitrile.

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

[0014] Furthermore, in step 1, the solvent is deionized water or a mixture of deionized water and an organic solvent, 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 several of the following: ammonium persulfate, potassium persulfate, hydrogen peroxide, and azobisisobutyronitrile.

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

[0017] Furthermore, in steps 2 and 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 steps 2 and 3, the free radical quencher is one or a mixture of several of butylated hydroxytoluene (BHT), butylated hydroxyanisole (BHA), tannic acid (TA), and gossypol (G).

[0019] Furthermore, in step 4, the drying is completed in a vacuum drying oven at 50℃~70℃.

[0020] Furthermore, in step 4, the low molecular weight polyacrylonitrile has a molecular weight of 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 polymerization temperatures in steps 2 and 3 are the same as those in step 1.

[0022] In terms of working principle:

[0023] This invention provides a method for the graded control of low molecular weight polyacrylonitrile (PBU) preparation using a free radical quencher. Based on the traditional aqueous precipitation polymerization method, the initiator initiates polymerization after thermal decomposition into active free radicals. During polymerization, this invention introduces a free radical quencher to control the polymerization reaction. When the average molecular weight of the PBU reaches the desired value, a small amount of free radical quencher is introduced. This small amount of free radical quencher preferentially attacks the less active long-chain species in the polymerization system, terminating their growth. Simultaneously, the monomer acrylonitrile is added, which not only activates the initiator to generate more active free radicals but also promotes the growth of short-chain active species, resulting in a narrower molecular weight distribution in the polymerization system. Furthermore, this invention employs a batch-wise addition of the free radical quencher during polymerization to achieve graded control of the polymerization reaction, keeping the average molecular weight of the polymerization system within the expected range, ultimately producing a PBU with a molecular weight below 11000 g / mol. -1 The polyacrylonitrile was improved, and the conversion rate was increased to over 70%.

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

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

[0026] 2) This invention uses a simple and easy-to-operate aqueous precipitation polymerization method to stably synthesize molecules with a molecular weight lower than 11000 g / mol. -1 The resulting polyacrylonitrile fills the current market gap for low molecular weight polyacrylonitrile and provides a reference for the design of synthesis processes for other low molecular weight polymers.

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

[0028] 4) This invention uses aqueous precipitation polymerization to prepare low molecular weight polyacrylonitrile, which is easy to separate the product and is environmentally friendly, making it suitable for industrial production. Attached Figure Description

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

[0030] Figure 2 The Fourier transform infrared absorption spectra (FT-IR) of polyacrylonitrile in Examples 1, 2 and 3 are shown.

[0031] Figure 3 The 1H NMR spectrum of the polyacrylonitrile in Example 1 is shown below. 1 H-NMR).

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

[0033] Figure 5 A comparative graph showing the solubility of polyacrylonitrile in N,N-dimethylformamide (DMF) in Examples 1, 2, and Comparative Example 1 (a: 20 mg / mL) -1 b: 10mg / mL -1 ). Detailed Implementation

[0034] To make the objectives, technical solutions, and beneficial effects of the present invention clearer, the technical solutions of the present invention will be fully and clearly described below in conjunction with the embodiments and accompanying drawings; unless otherwise specified, the raw materials used in the following embodiments are all commercially available products.

[0035] Example 1

[0036] This embodiment provides a method for preparing low molecular weight polyacrylonitrile by hierarchical control of free radical quenchers, the specific steps of which are as follows:

[0037] Add 90g of deionized water to the polymerization container, start stirring and purge with nitrogen to maintain the polymerization system temperature at 35℃. Then add 0.15g of ammonium persulfate and 9g of acrylonitrile, continue purging with nitrogen for 20min, then stop purging with nitrogen and seal the polymerization container. Raise the polymerization system temperature to 70℃ and polymerize for 30min to complete the first-stage polymerization. Add 0.0035g of free radical quencher dibutylhydroxytoluene and 3g of acrylonitrile monomer, polymerize for 45min, seal the polymerization container, and complete the second-stage polymerization. Add 0.15g of deionized water to the polymerization container again. 0.0035g of free radical quencher dibutylhydroxytoluene and 2g of acrylonitrile monomer were reacted in a sealed polymerization container for 50 minutes, completing the tertiary polymerization. 80g of deionized water was added to rapidly cool the polymerization system. The mixture was filtered through a funnel to obtain the polymerization product. The product was then washed once with 100g of deionized water, filtered again, and the volatile components were removed in a vacuum drying oven at 60℃, yielding 10.2g of white powdered polyacrylonitrile with a conversion rate of 72.5%. Viscosity testing determined the viscosity-average molecular weight to be 11505 g / mol. -1 .

[0038] Example 2

[0039] This embodiment provides a method for preparing low molecular weight polyacrylonitrile by hierarchical control of free radical quenchers, the specific steps of which are as follows:

[0040] Add 80g of deionized water to the polymerization container, start stirring and purge with nitrogen to maintain the polymerization system temperature at 30℃. Then add 0.28g of potassium persulfate and 8g of acrylonitrile, continue purging with nitrogen for 25min, then stop purging with nitrogen and seal the polymerization container. Raise the polymerization system temperature to 70℃ and polymerize for 30min to complete the first-stage polymerization. Add 0.0024g of butylated hydroxyanisole (BHA) and 2.2g of acrylonitrile monomer, polymerize for 30min, seal the polymerization container, and complete the second-stage polymerization. Add 0.28g of potassium persulfate and 8g of acrylonitrile monomer again. 0.0024g of butylated hydroxyanisole (BHA) and 1.5g of acrylonitrile monomer were mixed in a sealed polymerization container and reacted for 45 minutes, completing the tertiary polymerization. 80g of deionized water was added to rapidly cool the polymerization system. The mixture was filtered through a funnel to obtain the polymerization product. The product was then washed once with 100g of deionized water, filtered again, and the volatile components were removed in a vacuum drying oven at 60℃ to obtain 7.98g of white powdered polyacrylonitrile, with a conversion rate of 68.2%. Viscosity-average molecular weight was determined to be 9568 g / mol. -1 .

[0041] Example 3

[0042] This embodiment provides a method for preparing low molecular weight polyacrylonitrile by hierarchical control of free radical quenchers, the specific steps of which are as follows:

[0043] Add 95g of deionized water to the polymerization container, start stirring and purge with nitrogen to maintain the polymerization system temperature at 40℃. Then add 0.15g of ammonium persulfate, 0.15g of potassium persulfate, and 8g of acrylonitrile, and continue purging with nitrogen for 30 minutes. Then stop purging with nitrogen and seal the polymerization container. Raise the polymerization system temperature to 75℃ and polymerize for 30 minutes to complete the first-stage polymerization. Add 0.0041g of free radical quencher tannic acid and 3g of acrylonitrile monomer, seal the polymerization container, and polymerize for 33 minutes to complete the second-stage polymerization. The following steps were performed: Add 0.0041g of tannic acid (a free radical quencher) and 2.1g of acrylonitrile monomer to the polymer container. The reaction was carried out in a sealed container for 40 minutes, completing the tertiary polymerization. Add 80g of deionized water to rapidly cool the polymerization system. Filter the mixture using a funnel to obtain the polymer product. Wash the polymer product once with 90g of deionized water, filter again, and remove volatiles in a vacuum drying oven at 65℃ to obtain 8.52g of white powdered polyacrylonitrile with a conversion rate of 65.1%. Viscosity testing revealed a viscosity-average molecular weight of 7985 g / mol. -1 .

[0044] Example 4

[0045] This embodiment provides a method for preparing low molecular weight polyacrylonitrile by hierarchical control of free radical quenchers, the specific steps of which are as follows:

[0046] Add 80g of deionized water and 12g of tert-butanol to the polymerization container, start stirring and purge with nitrogen to maintain the polymerization system temperature at 35℃. Then add 0.19g of hydrogen peroxide and 11g of acrylonitrile, continue purging with nitrogen for 20min, then stop purging with nitrogen and seal the polymerization container. Raise the polymerization system temperature to 65℃ and polymerize for 30min to complete the first-stage polymerization. Add 0.0038g of free radical quencher gossypol and 2.9g of acrylonitrile monomer, seal the polymerization container, and polymerize for 35min to complete the second-stage polymerization. Add 0.0028g of free radical quencher gossypol and 1.1g of acrylonitrile monomer again, seal the polymerization container, and react for 40 minutes to complete the tertiary polymerization. Add 80g of deionized water to rapidly cool the polymerization system, filter using a funnel to obtain the polymerization product, wash the product once with 10g of deionized water, filter again, and remove volatiles in a vacuum drying oven at 70℃ to obtain 11.1g of white powdered polyacrylonitrile with a conversion rate of 69.4%. The viscosity-average molecular weight was determined to be 8962 g / mol using the viscosity method. -1 .

[0047] Example 5

[0048] This embodiment provides a method for preparing low molecular weight polyacrylonitrile by hierarchical control of free radical quenchers, the specific steps of which are as follows:

[0049] Add 100g of deionized water to the polymerization container, start stirring and purge with nitrogen to maintain the polymerization system temperature at 30℃. Then add 0.15g of azobisisobutyronitrile (AIBN) and 13g of acrylonitrile, continue purging with nitrogen for 20min, then stop purging with nitrogen and seal the polymerization container. Raise the polymerization system temperature to 70℃ and polymerize for 35min to complete the first-stage polymerization. Add 0.0032g of the free radical quencher tannic acid and 3.1g of acrylonitrile monomer, seal the polymerization container, and polymerize for 38min to complete the second-stage polymerization. Add 0.0032g of tannic acid (a free radical quencher) and 2.6g of acrylonitrile monomer, seal the polymerization container, and react for 40 minutes to complete the tertiary polymerization. Add 80g of deionized water to rapidly cool the polymerization system, filter using a funnel to obtain the polymerization product, wash the product once with 10g of deionized water, filter again, and remove volatiles in a vacuum drying oven at 75℃ to obtain 13.2g of white powdered polyacrylonitrile with a conversion rate of 70.8%. The viscosity-average molecular weight was determined to be 10623 g / mol using the viscosity method. -1 .

[0050] Comparative Example 1

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

[0052] Add 90g of deionized water to the polymerization container, start stirring, and maintain the polymerization system temperature at 35℃. Then add 0.3g of ammonium persulfate and 8g of acrylonitrile, continuously purge with nitrogen for 20min, then stop purging and seal the polymerization container. Raise the polymerization system temperature to 70℃ and maintain the temperature for 120min. After polymerization, add 50g of deionized water to rapidly lower the polymerization system temperature. Filter the product through a funnel, wash the product once with 100g of deionized water, filter again, and vacuum dry at 65℃ in a vacuum drying oven to remove volatiles, yielding 4.31g of white powdered polyacrylonitrile with a conversion rate of 53.9%. The viscosity-average molecular weight was determined to be 56042 g / mol using the viscosity method. -1 .

[0053] Comparative Example 2

[0054] This comparative example provides a method for preparing low molecular weight polyacrylonitrile, which employs a first-order regulation method by introducing a free radical quencher. The specific steps are as follows:

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

[0056] The beneficial effects of the present invention will be described in detail below with reference to the performance tests of polyacrylonitrile in Examples 1-5 and Comparative Examples 1-2.

[0057] like Figure 1 The figures show scanning electron microscope (SEM) images of polyacrylonitrile in Examples 1, 2, and Comparative Example 1. As can be seen from the figures, molecular weight determines the particle size of polyacrylonitrile. Example 3 synthesized the polyacrylonitrile with the lowest molecular weight and the smallest particle size. Figure 2 The figure shows the Fourier transform infrared absorption spectra (FT-IR) of polyacrylonitrile in Examples 1, 2, and 3. As can be seen from the figure, the three samples show peaks at the same position. Analysis of the results indicates that the peak at 1452 cm⁻¹ is the most significant. -1 The peaks are caused by carbon-hydrogen single bonds, specifically the in-plane bending vibration peaks of CH; 2244 cm⁻¹ -1 There is a distinct sharp peak at 1075 cm⁻¹, which is the characteristic C≡N stretching vibration peak of the cyano group. -1 The absorption peak at that location corresponds to the stretching vibration peak of the C-CN pair; therefore, analysis of the spectrum indicates that the product is a polyacrylonitrile homopolymer. For example... Figure 3 The image shown is the 1H NMR spectrum of polyacrylonitrile in Example 1. 1 As shown in the figure (H-NMR), there are two obvious signal peaks in the spectrum, δ2.04 and δ3.15, which represent hydrogen atoms with different chemical shifts, denoted as 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, respectively. Figure 4The figure shows a comparison of the state changes of polyacrylonitrile in Examples 1, 2, and Comparative Example 1 in a visual melting point apparatus. As can be seen from the figure, all three samples began to turn yellow in the range of 170℃ to 190℃. This may be due to partial oxidation of PAN and the beginning of polymer chain cyclization. It is clearly evident that the example with the lowest molecular weight showed the fastest color change, indicating that lower molecular weight results in higher polymer activity. Figure 5 The figure shows a comparison of the solubility of polyacrylonitrile in N,N-dimethylformamide (DMF) in Examples 1, 2, and Comparative Example 1 (a: 20 mg / mL). -1 b: 10mg / mL -1 As can be seen from the figure, low molecular weight polyacrylonitrile exhibits good solubility in DMF. The molecular weight difference between Example 1 and Example 2 is not significant, and the solubility difference is also not significant. 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 1-5 and Comparative Examples 1-2 was tested by viscosity method, and the conversion rate was calculated by weighing method. The viscosity-average molecular weight and conversion rate test results of the polymers are 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 1-5 successfully prepared low molecular weight polyacrylonitrile through graded control using free radical quenchers. Compared with the traditional aqueous precipitation polymerization method in Comparative Example 1, the molecular weight of the polyacrylonitrile obtained by this invention can be lower than 8000 gmol. -1 Compared with the first-level control in Comparative Example 2, the conversion rate of the present invention is significantly improved, making it more suitable for industrial production. Furthermore, as can be seen from the comparison of Examples 1 to 3, the hierarchical control approach proposed in the present invention starts from the most basic principle of polymerization, adjusting the molecular weight by adjusting the polymerization time. The present invention introduces a free radical quencher, a small amount of which will preferentially attack the less active long-chain active species, terminating their growth. At the same time, the addition of the monomer acrylonitrile can not only activate the initiator to generate more active free radicals, but also promote the growth of short-chain active species. Finally, the present invention adds the free radical quencher in batches during the polymerization process, hierarchically controlling the reaction, thereby keeping the overall molecular weight of the polymerization system within the expected range.

[0062] In summary, this invention, based on the traditional aqueous precipitation polymerization method, employs a free radical quencher for hierarchical control to successfully prepare low molecular weight polyacrylonitrile, effectively controlling the molecular weight of polyacrylonitrile and improving the conversion rate, which is beneficial for industrial production.

[0063] The above description is merely a specific embodiment of the present invention. Any feature disclosed in this specification may be replaced by other equivalent or similar features unless otherwise specified. All disclosed features, or steps in all methods or processes, may be combined in any way except for mutually exclusive features and / or steps.

Claims

1. A method for preparing low molecular weight polyacrylonitrile through hierarchical control of free radical quenchers, characterized in that, Includes the following steps: Step 1. Add the solvent to the polymerization container, heat to 25~40 ℃, then add acrylonitrile monomer and initiator, purge with inert gas for 15~30 min, seal the polymerization container, then heat to 65~75 ℃, polymerize for 20~40 min to complete the first-stage polymerization; Step 2. Add the free radical quencher and acrylonitrile monomer to the polymerization container, seal the polymerization container and continue polymerization for 30-50 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 30-50 minutes to complete the tertiary polymerization; Step 4. After polymerization, add deionized water to cool the polymerization system, and then filter to obtain the polymerization product; The polymerization product was washed with deionized water, filtered again, and dried to obtain low molecular weight polyacrylonitrile. In step 1, the amount of acrylonitrile monomer added is 10% to 25% of the solvent mass, and the amount of initiator added is 1% to 5% of the acrylonitrile monomer mass in the same step; In steps 2 and 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; the free radical quencher is one or a mixture of several of butylated hydroxytoluene, butylated hydroxyanisole, tannic acid, and gossypol.

2. The method for preparing low molecular weight polyacrylonitrile by hierarchical control of free radical quenchers 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 one or a mixture of several of dimethyl sulfoxide, isopropanol, and tert-butanol.

3. The method for preparing low molecular weight polyacrylonitrile by hierarchical control of free radical quenchers according to claim 1, characterized in that, In step 1, the initiator is one or a mixture of several of the following: ammonium persulfate, potassium persulfate, hydrogen peroxide, and azobisisobutyronitrile.

4. The method for preparing low molecular weight polyacrylonitrile by hierarchical control of free radical quenchers according to claim 1, characterized in that, In step 1, the inert gas is nitrogen.

5. The method for preparing low molecular weight polyacrylonitrile by hierarchical control of free radical quenchers according to claim 1, characterized in that, In step 4, drying is completed in a vacuum drying oven at 50 ℃ to 70 ℃.

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