CN109810213B - Fluorine-containing polymer in water/supercritical carbon dioxide mixed system and preparation method thereof - Google Patents
Fluorine-containing polymer in water/supercritical carbon dioxide mixed system and preparation method thereof Download PDFInfo
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Abstract
The invention relates to a fluorine-containing polymer under a water/supercritical carbon dioxide mixed system and a preparation method thereof. The preparation method comprises the following steps: and carrying out polymerization reaction on the fluorine-containing vinyl comonomer and an initiator under a water/supercritical carbon dioxide mixed system to obtain the fluorine-containing polymer. The invention uses the water/supercritical carbon dioxide lower mixing system as a medium, can control the problems of heat and viscosity existing in bulk and solution polymerization, improves the production efficiency, and greatly improves the molecular weight compared with the simple polymerization of the supercritical carbon dioxide; at the same time, the use of fluorine-containing emulsifier and a large amount of chlorofluorocarbon solvent medium can be avoided. In addition, the fluorine-containing polymer prepared by the method can be separated from an aqueous phase, the product purity is high, the post-treatment is simple, the yellowing is small, the melt index is low, the processing is easy, and the molecular weight distribution coefficient (PDI) of the fluorine-containing polymer is usually between 1.5 and 2.5 and is smaller than that of various reported methods.
Description
Technical Field
The invention belongs to the field of polyvinylidene fluoride synthesis, and particularly relates to a fluorine-containing polymer in a water/supercritical carbon dioxide mixed system and a preparation method thereof.
Background
The preparation of fluoropolymers such as polyvinylidene fluoride and the like includes bulk and solution polymerization, suspension polymerization, emulsion polymerization, among which suspension polymerization and emulsion polymerization are commonly used in industrial production. In the bulk and solution polymerization preparation of the fluorine-containing polymer, local overheating is easy to occur, the product is adhered to the wall of a kettle and a stirring paddle, the problems of heat and viscosity exist, and a large amount of organic solvent is needed to influence the performance of the product; in the suspension polymerization of fluorine-containing polymers, in order to avoid the problems of slow mass transfer, slow reaction and low production efficiency under the traditional suspension polymerization conditions, a large amount of chlorofluoroalkane is often added to promote the mass transfer and dispersion of monomers, which brings trouble to the post-treatment, or high pressure is adopted to improve the speed of the monomers entering a reaction system; in emulsion polymerization of fluoropolymers, a large amount of fluorinated surfactant is used, which likewise brings about trouble in the work-up of the product. There is therefore a need for a process for preparing high performance fluoropolymers which avoids the use of expensive or environmentally unfriendly solvents and fluorosurfactants, and in addition there is a great need for polymerization processes, particularly for the polymerization of fluorinated monomers, which can be carried out industrially using conventional polymerization equipment.
Supercritical CO2(carbon dioxide fluid with temperature and pressure above critical point (31.1 ℃, 7.3 MPa)) is nontoxic, non-inflammable and low in price, and becomes an ideal substitute of organic solvent, fluorinated monomer can be usually dissolved in carbon dioxide, the purification and recovery of products after reaction are relatively simple, the products are easily separated from the products after being converted into gas through decompression, the complex post-treatment process of the organic solvent is completely omitted, and CO2Can be reused after being collected. But pure supercritical CO2As a reaction medium, the temperature is not easy to control, a sticking kettle and a stirring paddle are easy to generate, and the prepared fluorine-containing polymer has low molecular weight.
Therefore, the development of a new method with simple post-treatment to prepare the fluoropolymer with higher molecular weight and better performance has important research significance and economic value.
Disclosure of Invention
The invention aims to overcome the defects or shortcomings of complex post-treatment, low molecular weight and poor performance of the obtained fluorine-containing polymer in the prior art, and provides a preparation method of the fluorine-containing polymer in a water/supercritical carbon dioxide mixed system. According to the invention, a water/supercritical carbon dioxide lower mixing system is used as a medium, on one hand, the existence of a water phase is beneficial to heat transfer, on the other hand, the mass transfer effect of fluorine-containing olefin is enhanced under supercritical carbon dioxide, the problems of heat and viscosity in bulk and solution polymerization can be controlled, the production efficiency is improved, and meanwhile, compared with the simple polymerization of supercritical carbon dioxide, the molecular weight is greatly improved; at the same time, the use of fluorine-containing emulsifier and a large amount of chlorofluorocarbon solvent medium can be avoided. In addition, the fluorine-containing polymer prepared by the method can be separated from an aqueous phase, the fluorine-containing polymer can be simply collected by physical separation, the product purity is high, the post-treatment is simple, the yellowing is small, the melt index is low, the processing is easy, and the molecular weight distribution coefficient (PDI) of the fluorine-containing polymer is usually between 1.5 and 2.5 and is smaller than that of the fluorine-containing polymer prepared by various reported methods.
In order to achieve the purpose, the invention adopts the following technical scheme:
a preparation method of a fluorine-containing polymer under a water/supercritical carbon dioxide mixed system comprises the following steps: and carrying out polymerization reaction on the fluorine-containing vinyl monomer and an initiator under a water/supercritical carbon dioxide mixed system to obtain the fluorine-containing polymer.
It should be understood that a water/supercritical carbon dioxide hybrid system refers to water being present in a non-vapor state and carbon dioxide being present in a supercritical state.
According to the invention, a water/supercritical carbon dioxide lower mixing system is used as a medium, on one hand, the existence of a water phase is beneficial to heat transfer, on the other hand, the mass transfer effect of fluorine-containing olefin is enhanced under supercritical carbon dioxide, the problems of heat and viscosity in bulk and solution polymerization can be controlled, the production efficiency is improved, and meanwhile, compared with the polymerization which only uses supercritical carbon dioxide as a solvent, the molecular weight is greatly improved; at the same time, the use of fluorine-containing emulsifier and a large amount of chlorofluorocarbon solvent medium can be avoided. In addition, the fluorine-containing polymer prepared by the method can be separated from an aqueous phase, the fluorine-containing polymer can be simply collected by physical separation, the product purity is high, the post-treatment is simple, the yellowing is small, the melt index is low, the processing is easy, and the molecular weight distribution coefficient (PDI) of the fluorine-containing polymer is usually between 1.5 and 2.5 and is smaller than that of the fluorine-containing polymer prepared by various reported methods.
Preferably, the mass ratio of water to carbon dioxide in the water/supercritical carbon dioxide mixed system is 1-25: 1.
More preferably, the mass ratio of water to carbon dioxide in the water/supercritical carbon dioxide mixed system is 10: 1.
Preferably, the pressure of the polymerization reaction is 7.39-15 MPa, and the temperature of the polymerization reaction is 31.5-95 ℃.
Under these pressure and temperature conditions, carbon dioxide is in the supercritical state and water is in the non-vapor state.
Fluorine-containing vinyl monomers and initiators conventional in the art may be used in the present invention.
Preferably, the fluorine-containing vinyl monomer is one or more of a fluoro olefin monomer, a fluoro acrylate monomer or a fluoro vinyl ether monomer.
More preferably, the fluoroolefin monomer is one or more of vinylidene fluoride, tetrafluoroethylene, hexafluoropropylene, vinyl fluoride or chlorotrifluoroethylene.
Most preferably, the fluoroolefin monomer is vinylidene fluoride.
More preferably, the fluoroacrylate monomer is one or more of trifluoroethyl methacrylate, trifluoroethyl acrylate, hexafluorobutyl methacrylate, hexafluorobutyl acrylate, dodecafluoroheptyl methacrylate or dodecafluoroheptyl acrylate.
More preferably, the fluorinated vinyl ether monomer is one or more of perfluoromethyl vinyl ether, perfluoroethyl vinyl ether or perfluoropropyl vinyl ether.
Preferably, the initiator is a water-soluble initiator.
More preferably, the water-soluble initiator is one or more of inorganic peroxide, an oxidation-reduction initiation system, a water-soluble azo initiator or a fluorine-containing initiator.
Preferably, the inorganic peroxide is one or more of hydrogen peroxide, persulfate, potassium permanganate or disuccinic acid peroxide.
Preferably, the oxidizing agent in the redox initiation system is a persulfate and the reducing agent is a sulfite or a bisulfite.
More preferably, the persulfate is one or more of sodium sulfite, ammonium sulfite, potassium sulfite, sodium bisulfite, sodium metabisulfite or potassium metabisulfite.
Preferably, the water-soluble azo initiator is one or more of 2,2 ' -azo (2-amidinopropane) dihydrochloride, 2 ' -azo [2- (2-imidazoline-2-yl) propane ] dihydrochloride or 2,2 ' -azo [ 2-methyl-N- (2-methacrylic acid) propionamide.
Preferably, the fluorine-containing initiator is (Z (CF2)p COO)2Or one or more perfluoroalkyl diacyl peroxides; wherein Z is a hydrogen atom, a fluorine atom or a chlorine atom, and p is an integer of 1 to 10.
More preferably, the perfluoroalkyl diacyl peroxide is one or both of perfluoropropyl diacyl peroxide and perfluoro-tert-butyl peroxide.
Preferably, the polymerization process further comprises the step of adding a chain transfer agent.
The addition of a chain transfer agent can regulate the degree of polymerization of the copolymer.
Chain transfer agents conventional in the art may be used in the present invention.
Preferably, the chain transfer agent is one or more of trichlorofluoromethane, 1, 2-difluoro-1, 1,2, 2-tetrachloroethane, 1, 2-trifluoro-1, 2, 2-trichloroethane or trichlorofluoroethane.
The amount of the chain transfer agent can be adjusted and optimized according to the polymerization degree of the copolymer.
Preferably, the addition amount of the chain transfer agent is 0.05-5% of the mass of the fluorine-containing vinyl monomer.
More preferably, the amount of the chain transfer agent added is 0.5% by mass of the fluorine-containing vinyl monomer.
Preferably, the chain transfer agent is added in multiple portions.
Compared with the prior art, the invention has the following beneficial effects:
according to the invention, a water/supercritical carbon dioxide lower mixing system is used as a medium, on one hand, the existence of a water phase is beneficial to heat transfer, on the other hand, the mass transfer effect of fluorine-containing olefin is enhanced under supercritical carbon dioxide, the problems of heat and viscosity in bulk and solution polymerization can be controlled, the production efficiency is improved, and meanwhile, compared with the simple polymerization of supercritical carbon dioxide, the molecular weight is greatly improved; at the same time, the use of fluorine-containing emulsifier and a large amount of chlorofluorocarbon solvent medium can be avoided. In addition, the fluorine-containing polymer prepared by the method can be separated from an aqueous phase, the fluorine-containing polymer can be simply collected by physical separation, the product purity is high, the post-treatment is simple, the yellowing is small, the melt index is low, the processing is easy, and the molecular weight distribution coefficient (PDI) of the fluorine-containing polymer is usually between 1.5 and 2.5 and is smaller than that of the fluorine-containing polymer prepared by various reported methods.
Detailed Description
The present invention will be further described with reference to the following examples. These examples are merely representative descriptions of the present invention, but the present invention is not limited thereto. The test methods used in the following examples are, unless otherwise specified, all conventional methods, and the raw materials, reagents and the like used are, unless otherwise specified, all commercially available raw materials and reagents from conventional markets and the like.
Example 1
The embodiment provides a preparation method of polyvinylidene fluoride in a water/supercritical carbon dioxide mixed system. The specific process is as follows.
Adding 3000g of water into a 5L stainless steel belt jacket and a stirred polymerization reaction kettle, vacuumizing for replacing nitrogen for 5 times, detecting that the oxygen content in the kettle is lower than 20ppm, and injecting carbon dioxide and vinylidene fluoride monomer into the kettle by a compressor for reactionControlling the addition speed of the reactor system at 8.0MPa, the monomer amount is 35 wt% of the total monomer amount, heating to 65 deg.C, stirring at 800r/min, and adding 5.0g K2S2O8Aqueous solution (20 wt%). After the polymerization reaction starts, continuously replenishing the vinylidene fluoride monomer to maintain the system pressure at 8.0MPa (the mass ratio of water to carbon dioxide is 10: 1). When the polymerization reaction is carried out for 30min, adding 1,1, 2-trifluoro-1, 2, 2-trichloroethane serving as a chain transfer agent accounting for about 0.5wt% of the total monomer mass by using a syringe pump; and when the polymerization reaction is carried out for 3.5 hours, adding a chain transfer agent 1,1, 2-trifluoro-1, 2, 2-trichloroethane accounting for about 0.5wt% of the total monomer mass, stopping adding when all monomers are completely added and 600g is totally added, stopping stirring when the system pressure is reduced to 7.5 MPa, wherein the polymerization time is 7.0 hours, cooling the temperature of a reaction kettle to normal temperature, discharging the pressure in the reactor, opening the reaction kettle, taking out a product in the reaction kettle, centrifuging, washing and drying to obtain 560g of powdered PVDF, and the yield is 93.90%.
Example 2
The embodiment provides a preparation method of polyvinylidene fluoride in a water/supercritical carbon dioxide mixed system. The specific process is as follows.
Adding 3000g of water into a 5L stainless steel belt jacket and a stirred polymerization reaction kettle, vacuumizing for replacing nitrogen for 5 times, detecting that the oxygen content in the kettle is lower than 20ppm, simultaneously injecting carbon dioxide and vinylidene fluoride monomers by using a compressor, controlling the adding speed of the carbon dioxide and the vinylidene fluoride monomers, wherein the injected monomers account for 35 wt% of the total monomers, raising the temperature to 75 ℃ of the polymerization temperature, starting stirring, controlling the stirring speed to be 800r/min, and adding 10g of 2, 2' -azo (2-amidinopropane) dihydrochloride by using an injection pump after the pressure temperature is stable. After the polymerization reaction begins, continuously replenishing the vinylidene fluoride monomer to maintain the system pressure at 10.0 MPa. When the polymerization reaction is carried out for 30min, adding 1,1, 2-trifluoro-1, 2, 2-trichloroethane serving as a chain transfer agent accounting for about 0.05wt% of the total monomer mass by using a syringe pump; and when the polymerization reaction is carried out for 3.5 hours, adding a chain transfer agent 1,1, 2-trifluoro-1, 2, 2-trichloroethane accounting for about 0.05wt% of the total monomer mass, stopping adding when all monomers are completely added and 600g is totally added, stopping stirring when the system pressure is reduced to 7.5 MPa, wherein the polymerization time is 5.5 hours, cooling the temperature of a reaction kettle to normal temperature, discharging the pressure in the reactor, opening the reaction kettle, taking out a product in the reaction kettle, centrifuging, washing and drying to obtain 540 g of powdered PVDF, and the yield is 90%.
Example 3
The embodiment provides a preparation method of polyvinylidene fluoride in a water/supercritical carbon dioxide mixed system. The specific process is as follows.
Adding 3000g of water into a 5L stainless steel belt jacket and a stirred polymerization reaction kettle, vacuumizing for 5 times to replace nitrogen, detecting that the oxygen content in the kettle is lower than 20ppm, simultaneously injecting carbon dioxide and vinylidene fluoride monomers by a compressor to ensure that the pressure of the reaction kettle system is 15MPa, controlling the adding speed of the carbon dioxide and the vinylidene fluoride monomers, wherein the injected monomers account for 35 wt% of the total monomers, heating to the polymerization temperature of 90 ℃, starting stirring, controlling the stirring speed to be 800r/min, adding 5.0g K by using an injection pump after the pressure and the temperature are stable2S2O8Aqueous solution (20 wt%). After the polymerization reaction starts, continuously replenishing the vinylidene fluoride monomer to maintain the system pressure at 15.0 MPa. When the polymerization reaction is carried out for 30min, adding 1,1, 2-trifluoro-1, 2, 2-trichloroethane serving as a chain transfer agent accounting for about 5wt% of the total monomer mass by using a syringe pump; and when the polymerization reaction is carried out for 3.5 hours, adding a chain transfer agent 1,1, 2-trifluoro-1, 2, 2-trichloroethane accounting for about 5wt% of the total monomer mass by using an injection pump, stopping adding when all monomers are completely added and 600g is totally added, stopping stirring when the system pressure is reduced to 7.5 MPa, wherein the polymerization time is 4.5 hours, discharging the pressure in the reactor when the temperature of the reaction kettle is cooled to normal temperature, opening the reaction kettle, taking out a product in the reaction kettle, centrifuging, washing and drying to obtain 525g of powdered PVDF, and the yield is 87.5%.
Example 4
The embodiment provides a preparation method of polyvinylidene fluoride-trifluoroethyl methacrylate copolymer resin in a water/supercritical carbon dioxide mixed system. The specific process is as follows.
Adding 3000g of water into a 5L stainless steel belt jacket and a stirred polymerization reaction kettle, vacuumizing to replace nitrogen for 5 times, detecting that the oxygen content in the kettle is lower than 20ppm,simultaneously injecting carbon dioxide and vinylidene fluoride monomer into the reactor system by a compressor at the pressure of 10.0MPa, controlling the adding speed of the carbon dioxide and the vinylidene fluoride monomer, wherein the injected monomer accounts for 35 wt% of the total monomer, heating to the polymerization temperature of 75 ℃, starting stirring at the stirring speed of 800r/min, and adding 10g K by a syringe pump after the pressure and the temperature are stable2S2O8Aqueous solution (20 wt%). After the polymerization reaction begins, continuously replenishing the vinylidene fluoride monomer to maintain the system pressure at 10.0 MPa. When the polymerization reaction proceeded for 30min, a chain transfer agent 1,1, 2-trifluoro-1, 2, 2-trichloroethane and 6g trifluoroethyl methacrylate comonomer, which account for about 0.5wt% of the total monomer mass, were added by a syringe pump; when the polymerization reaction is carried out for 3.5h, adding a chain transfer agent 1,1, 2-trifluoro-1, 2, 2-trichloroethane and 6g of trifluoroethyl methacrylate comonomer which accounts for about 0.5wt% of the total monomer mass by using an injection pump, stopping adding when all monomers are added and the total amount is 600g, and stopping stirring when the system pressure is reduced to 7.5 MPa, wherein the polymerization time is 8.5h, the temperature of the reaction kettle is cooled to the normal temperature, the pressure in the reactor is discharged, the reaction kettle is opened, the product in the reaction kettle is taken out, and 530 g of PVDF powder is obtained by centrifuging, washing and drying, and the yield is 88.3%.
Comparative example 1
The comparative example provides a method for preparing polyvinylidene fluoride in a system with water as a medium. The specific process is as follows.
Adding 3000g of water into a 5L stainless steel belt jacket and a stirred polymerization reaction kettle, vacuumizing for replacing nitrogen for 5 times, detecting that the oxygen content in the kettle is lower than 20ppm, simultaneously injecting vinylidene fluoride monomer by a compressor to enable the system pressure of the reaction kettle to be 8.00MPa, heating to the polymerization temperature of 65 ℃, starting stirring, controlling the stirring speed to be 800r/min, adding 5.0g K by a syringe pump2S2O8Aqueous solution (20 wt%). After the polymerization reaction starts, continuously replenishing the vinylidene fluoride monomer to maintain the system pressure at 8.00 MPa. When the polymerization reaction is carried out for 30min, a chain transfer agent 1,1, 2-trifluoro-1, 2, 2-trichloroethane accounting for about 0.5wt% of the total monomer mass is added by a syringe pump, when the polymerization reaction is carried out for 3.5h, the chain transfer agent 1,1, 2-trifluoro-1, 2, 2-trichloroethane accounting for about 0.5wt% of the total monomer mass is added, and all the monomers are completely reactedAdding a total of 600g, stopping adding, stopping the reaction when the system pressure is reduced to 7.5 MPa, stopping the reaction, keeping the polymerization time for 14.5h, cooling the temperature of the reaction kettle to the normal temperature, discharging the pressure in the reactor, opening the reaction kettle, taking out a product in the reaction kettle, centrifuging, washing and drying to obtain PVDF powder, wherein the yield is 9.90%.
Comparative example 2
The comparative example provides a preparation method of polyvinylidene fluoride in a system with supercritical carbon dioxide as a medium. The specific process is as follows.
Vacuumizing and replacing nitrogen for 5 times in a 5L polymerization reaction kettle with a stainless steel belt jacket and stirring, detecting that the oxygen content in the kettle is lower than 20ppm, simultaneously injecting carbon dioxide and vinylidene fluoride monomers by a compressor to ensure that the pressure of the reaction kettle system is 8.0MPa, controlling the adding speed of the carbon dioxide and the vinylidene fluoride monomers, wherein the injected monomers account for 35 wt% of the total monomers, heating to the polymerization temperature of 65 ℃, starting stirring, controlling the stirring speed to be 800r/min, and adding 5.0g K by using an injection pump after the pressure and the temperature are stable2S2O8Aqueous solution (20 wt%). After the polymerization reaction begins, continuously replenishing the vinylidene fluoride monomer to maintain the system pressure at 8.0 MPa. When the polymerization reaction is carried out for 30min, adding 1,1, 2-trifluoro-1, 2, 2-trichloroethane serving as a chain transfer agent accounting for about 0.5wt% of the total monomer mass by using a syringe pump; and when the polymerization reaction is carried out for 3.5 hours, adding a chain transfer agent 1,1, 2-trifluoro-1, 2, 2-trichloroethane accounting for about 0.5wt% of the total monomer mass, stopping adding when all monomers are completely added and 600g is totally added, stopping stirring when the system pressure is reduced to 7.5 MPa, wherein the polymerization time is 7.0 hours, cooling the temperature of a reaction kettle to normal temperature, discharging the pressure in the reactor, opening the reaction kettle, taking out a product in the reaction kettle, centrifuging, washing and drying to obtain 420g of powdered PVDF, and the yield is 70%. The temperature of the reaction kettle is difficult to control in the experimental process, the materials are loose and powdery after the experiment is finished, but the materials are seriously adhered to the kettle wall and the stirring paddle, and the kettle is difficult to clean.
The PVDF resins prepared in examples 1-3 and comparative examples 1-2 and the polyvinylidene fluoride-trifluoroethyl methacrylate copolymer resin prepared in example 4 are subjected to corresponding test characterization:
and (3) determination of melt index: a PVDF resin of about 10g is taken, under a load of 5.0kg and at a test temperature of 230 ℃ and the mass of the melt passing through a standard die capillary per 10min is measured, expressed as MFR, in g/10 min.
Determination of molecular weight and distribution thereof: 0.2 wt% PVDF solution, solvent N, N Dimethylformamide (DMF), test temperature 55 ℃, mobile phase: DMF; the flow rate was 1 ml/min.
And (3) testing high-temperature yellowing resistance: the test powder was spread on aluminum foil paper and after heating at 230 ℃ for 3h, the color change of the sample was observed.
TABLE 1 Performance test results of PVDF resins obtained in examples 1 to 3 and comparative examples 1 to 2
Note: in comparative examples 1 and 2, no corresponding values were determined for the melt index under the test conditions, owing to the low molecular weight.
As can be seen from table 1, the PVDF resin and the vinylidene fluoride-trifluoroethyl methacrylate copolymer resin prepared in this example have excellent molecular weights and molecular weight distributions, and also have good melt index and yellowing performance, the PVDF resin is suitable for being used as a material for injection molding or an outer cladding layer of a wire and cable, and the vinylidene fluoride-trifluoroethyl methacrylate copolymer resin can be used as a lithium battery binder or a diaphragm material; in the comparative example 1, only water is selected as a medium, so that the reaction rate is slow and the molecular weight is low; comparative example 2 only selects supercritical carbon dioxide as the medium, the temperature of the reaction kettle is difficult to control, the kettle wall and the stirring paddle are seriously adhered, the kettle is difficult to clean, and the molecular weight is low.
The above-mentioned embodiments are intended to illustrate the objects, technical solutions and advantages of the present invention in further detail, and it should be understood that the above-mentioned embodiments are merely exemplary embodiments of the present invention, and are not intended to limit the scope of the present invention, and any modifications, equivalent substitutions, improvements and the like made within the spirit and principle of the present invention should be included in the scope of the present invention.
Claims (9)
1. A preparation method of a fluorine-containing polymer under a water/supercritical carbon dioxide mixed system is characterized by comprising the following steps: stirring the fluorine-containing vinyl monomer and the initiator under a water/supercritical carbon dioxide mixed system for polymerization reaction to obtain the fluorine-containing polymer; the mass ratio of water to carbon dioxide in the water/supercritical carbon dioxide mixed system is 1-25: 1; the monomer is vinylidene fluoride or the combination of vinylidene fluoride and trifluoroethyl methacrylate.
2. The method according to claim 1, wherein the pressure of the polymerization reaction is 7.39 to 15MPa, and the temperature of the polymerization reaction is 31.5 to 95 ℃.
3. The method of claim 1, wherein the initiator is a water-soluble initiator.
4. The method according to claim 3, wherein the water-soluble initiator is one or more selected from the group consisting of inorganic peroxides, redox initiator systems, water-soluble azo initiators and fluorine-containing initiators.
5. The method of claim 1, further comprising the step of adding a chain transfer agent during the polymerization reaction.
6. The preparation method according to claim 5, wherein the chain transfer agent is one or more of trichlorofluoromethane, 1, 2-difluoro-1, 1,2, 2-tetrachloroethane, 1, 2-trifluoro-1, 2, 2-trichloroethane or trichlorofluoroethane.
7. The method according to claim 5, wherein the amount of the chain transfer agent added is 0.05 to 5% by mass based on the amount of the fluorine-containing vinyl monomer added.
8. A fluoropolymer obtained by the production method according to any one of claims 1 to 7.
9. The fluoropolymer according to claim 8, wherein the fluoropolymer has a molecular weight distribution coefficient PDI of 1.5 to 2.5.
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