Low-refractive-index transparent copolymer as well as preparation method and application thereof

Abstract

The invention discloses a low-refractive-index transparent copolymer. The structural general formula of the transparent copolymer is shown as a formula (a). The copolymer material prepared by the invention has the characteristics of high light transmittance, low refractive index, easiness in processing, easiness in recovery and the like, and is a novel optical polymer material.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of polymer synthesis, and particularly relates to a low-refractive transparent copolymer, a preparation method and application thereof. BACKGROUND

[0002] In modern optics and electronics industry, low-refractive transparent polymers have become the core material basis for the development of cutting-edge technology. From the anti-reflection coating of augmented reality (AR) lenses to flexible display substrates, to low dielectric constant films for high-speed communication, these applications all require materials to simultaneously achieve ultra-low refractive index (usually <1.40), high visible light transmittance (>90%), excellent thermal stability, and controllable dielectric properties. As a key characterization parameter of the speed of light propagation in a medium, the refractive index directly affects the reflection loss and imaging quality of optical devices. Taking AR devices as an example, when the refractive index decreases from 1.50 to 1.38, the interface reflectivity can be reduced by more than 60%, significantly improving the image contrast and brightness. However, the existing material systems have significant limitations in performance balance, processing cost, and environmental adaptability, which restricts the commercialization process of the next generation of optical devices.

[0003] For example, as described in patent US3978030A, traditional perfluorinated polymer Teflon AF can achieve ultra-low refractive index (1.29-1.34), but its industrialization faces three major bottlenecks: (1) Low interfacial adhesion. The extremely low surface energy of fluorine atoms leads to poor compatibility with common optical substrates, which restricts its application in AR lens multilayer films. (2) Cost and environmental risk of tetrafluoroethylene monomer. Tetrafluoroethylene (TFE) monomer needs to be prepared by high-temperature cracking of fluorochlorohydrocarbon, with a purity requirement of >99.99% (impurities can cause polymerization to run out of control). According to Solvay's 2024 annual report, the cost of TFE accounts for 68% of the total raw materials of perfluorinated resin. (3) Solvent toxicity and recycling cost. Solving perfluorinated polymers requires the use of fluorine-containing solvents such as perfluoro-2-butyl tetrahydrofuran (FC-75), perfluorotributylamine (FC-40), perfluorodecalin, and perfluoromethylcyclohexane, which have high bioaccumulation, resulting in high recycling costs. SUMMARY

[0004] In view of the technical problems existing in the prior art, the purpose of the present application is to provide a low-refractive transparent copolymer. Through innovative design of the molecular structure, the copolymer main chain is partially replaced by alternative monomers to replace tetrafluoroethylene, thereby reducing the raw material cost by 77% and reducing environmental risks. At the same time, based on the molecular polarity control, the polymer can be polymerized and dissolved in conventional hydrocarbon solvents, meeting the requirements of green processing, and the introduction of flexible ethylene segments and ether bonds improves the interfacial adhesion compared to the original fluoropolymer.

[0005] Another purpose of the present application is to provide a preparation method of the low-refractive transparent copolymer.

[0006] Another object of the present application is to provide the application of the low-refractive transparent copolymer.

[0007] To achieve the above objects, the present application adopts the following technical solutions:

[0008] A low-refractive transparent copolymer, the structural general formula of the transparent copolymer is shown as formula (a):

[0009]

[0010] wherein, R1-R4 are each independently one atom or group of H, F, CH3, CH2F, CHF2, CF3, and at least one of the atoms or groups is F, CH2F, CHF2 or CF3; R5 is F, C1-C 30 fluoroalkyl; x, y, z are integers greater than 0 and less than 1000; and x / (x+y+z)≤0.5. 30

[0011] In a specific embodiment, the low-refractive transparent copolymer satisfies at least one of the following properties:

[0012] (1) the weight average molecular weight ranges from 10000 to 200000, preferably from 30000 to 90000;

[0013] (2) the refractive index is less than 1.4;

[0014] (3) the glass transition temperature is between 20℃ and 180℃, preferably between 50℃ and 90℃.

[0015] On the other hand, the preparation method of the aforementioned low-refractive transparent copolymer comprises the following steps:

[0016] (1) a synthesis step: adding ethylene, fluorine-containing monomer shown as structural general formula (b), monomer shown as structural general formula (c) and solvent into a reaction kettle, mixing uniformly under stirring condition; adding initiator and performing polymerization reaction to obtain the polymer solution of the transparent copolymer;

[0017]

[0018] wherein, the selection of R1-R5 is the same as in formula (a);

[0019] (2) a separation step: separating the mixture containing the dissolved transparent copolymer and organic solvent to obtain the transparent copolymer.

[0020] In a specific embodiment, the polymerization initiation system of step (1) is initiated by a free radical initiator. ​

[0021] Preferably, the free radical initiator is selected from at least one of thermally decomposable initiators and redox initiators;

[0022] More preferably, the thermally decomposable initiator is an azo initiator and / or a peroxide initiator;

[0023] More preferably, the azo initiator is azobisisobutyronitrile (AIBN) and / or azobisisoheptanenitrile; the peroxide initiator is at least one of benzoyl peroxide (BPO), lauroyl peroxide, and tert-butyl hydroperoxide.

[0024] In one specific implementation, the polymerization initiation system in step (1) is a coordination polymerization catalyst system;

[0025] Preferably, the coordination polymerization catalyst system comprises a main catalyst and a co-catalyst;

[0026] More preferably, the main catalyst is at least one of a Ziegler-Natta catalyst, a metallocene catalyst, or a non-metallocene catalyst;

[0027] More preferably, the main catalyst is a compound containing titanium, zirconium, vanadium or chromium; and the co-catalyst is an alkylaluminum compound and / or methylaluminoxane (MAO).

[0028] In a preferred embodiment, the Ziegler-Natta catalyst includes, but is not limited to, titanium-based catalysts, chromium-based catalysts, and vanadium-based catalysts; the metallocene catalyst includes, but is not limited to, monometallocene catalysts, dimetallocene catalysts, bridged metallocene catalysts, and CGC catalysts; and the non-metallocene catalyst is a post-transition metal organocatalyst or a pre-transition metal organocatalyst.

[0029] In this invention, the low-refractive-index transparent copolymer is polymerized from a fluorinated cyclic ether monomer (corresponding to structural formula (b)), an ethylene monomer, and a vinyl ether (corresponding to structural formula (c)). The fluorinated cyclic ether monomer and fluorinated substituted olefin polymers such as tetrafluoroethylene are difficult to dissolve in common organic solvents. This invention uses an addition copolymerization of the fluorinated cyclic ether monomer and ethylene to enhance the copolymer's solubility in common organic solvents. The introduction of a third monomer, the vinyl ether, further disrupts the regularity of the polymer chain, reducing the polymer's crystallinity. Furthermore, the long branches linked to the ether bonds further enhance the polymer's solubility in common solvents.

[0030] In this invention, the low-refractive-index transparent copolymer has a refractive index below 1.4, a fluorinated cyclic ether monomer content that is not zero and is below 50%, and a glass transition temperature between 20℃ and 180℃. The low-refractive-index transparent copolymer is a random or block organic polymer with a number-average molecular weight less than or equal to 100,000 and a weight-average molecular weight less than or equal to 200,000; wherein the molecular structure of the low-refractive-index transparent copolymer contains CF bonds, ether bonds, polyethylene chains, or other polyolefin chains.

[0031] In the low refractive index transparent copolymer of the present invention, the fluorinated cyclic ether monomer is a five-membered ring with carbon-carbon double bonds. The carbon-carbon double bonds can be opened for addition polymerization. At least one of the substituents on the five-membered ring contains a fluorine atom, a fluoromethyl group, a difluoromethylene group, or a trifluoromethyl group. The fluorine atom has the characteristic of low polarizability, which can effectively reduce the refractive index of the material.

[0032] The low-refractive-index transparent copolymer of the present invention has the characteristics of low refractive index, amorphous shape, and easy solubility. The refractive index is below 1.4, and no crystallization peak appears in its DSC characterization. The copolymer is soluble in aliphatic hydrocarbon solvents, aromatic hydrocarbon solvents, and halogenated hydrocarbon solvents, wherein the aliphatic hydrocarbon solvents are selected from C5-C4 solvents. 12 Straight-chain alkanes, C5-C 12 Branched alkanes, cycloalkanes, or mixtures thereof; the aromatic hydrocarbon solvent is selected from benzene, toluene, xylene, ethylbenzene, mesitylene, cumene, or mixtures thereof; the halogenated hydrocarbon solvent is selected from chloromethane, dichloromethane, chloroform, carbon tetrachloride, 1,2-dichloroethane, chlorobenzene, dichlorobenzene, trichloroethylene, tetrachloroethylene, or mixtures thereof.

[0033] In this invention, ethylene, a fluorinated monomer of general formula (b), and a monomer of general formula (c) undergo free radical polymerization or coordination copolymerization in the presence of a solvent and an initiator to obtain a transparent copolymer with a low refractive index, which is dissolved in the solvent.

[0034] In a preferred embodiment, the polymerization reaction pressure is from 0.1 MPa to 50 MPa, the reaction temperature is from 10°C to 300°C, and the reaction time is from 0.5 hours to 20 hours.

[0035] In one specific implementation, the solvent in step (1) is selected from at least one of aliphatic hydrocarbon solvents, aromatic hydrocarbon solvents, and halogenated hydrocarbon solvents;

[0036] In a preferred embodiment, the aliphatic hydrocarbon solvent is selected from C5-C64. 12 Straight-chain alkanes, C5-C 12 Branched alkanes, cycloalkanes, or mixtures thereof;

[0037] In a preferred embodiment, the aromatic hydrocarbon solvent is selected from benzene, toluene, xylene, ethylbenzene, mesitylene, cumene, or mixtures thereof;

[0038] In a preferred embodiment, the halogenated hydrocarbon solvent is selected from chloroalkanes and fluorinated hydrocarbons, preferably chloromethane, dichloromethane, chloroform, carbon tetrachloride, 1,2-dichloroethane, chlorobenzene, dichlorobenzene, trichloroethylene, tetrachloroethylene, or mixtures thereof.

[0039] In one specific implementation, the molar ratio of ethylene, the fluorinated monomer represented by general formula (b), and the monomer represented by general formula (c) in step (1) is (A+B):C = (1:99) to (99:1), and the molar ratio of ethylene to the fluorinated monomer represented by general formula (b) is 1:99 to 99:1.

[0040] In one specific implementation, the mass ratio of the total mass of ethylene, the fluorinated monomer represented by general formula (b), and the monomer represented by general formula (c) to the solvent in step (1) is 1:99 to 99:1.

[0041] In one specific implementation, the separation method in step (2) includes flash evaporation or precipitation;

[0042] In a preferred embodiment, the flash evaporation method involves introducing the mixture into a flash tank and flash evaporating it within a temperature range of 150°C to 250°C and a pressure range of -0.1 MPa to 0 MPa, thereby vaporizing the solvent and separating it from the molten polymer.

[0043] Optionally, the process may also include a step of devolatilizing the resulting molten polymer.

[0044] In this invention, the core of the flash evaporation method lies in the synergistic effect of rapid pressure reduction and heating, which causes the solvent to vaporize instantaneously and separate from the molten polymer. This step can be performed in any flash evaporation equipment capable of achieving the aforementioned pressure reduction and heating functions, including but not limited to standard-sized flash tanks, stirred flash kettles, multi-stage flash evaporation systems, or flash evaporation units integrated into the front section of a devolatilization extruder; there are no particular limitations. The separation effect of this invention mainly depends on the control of process parameters (such as temperature, pressure drop, and residence time), and is independent of the specific internal structure of the flash evaporation equipment (such as baffle design, feed distributor type, and vapor-liquid separation space size). Those skilled in the art can determine the process parameters and design a suitable flash tank structure to implement this invention based on the production scale and material characteristics.

[0045] In one specific implementation, the precipitation method involves adding a precipitant to the mixture to precipitate the dissolved transparent copolymer; then performing solid-liquid separation, washing, and drying on the precipitated polymer.

[0046] In one specific embodiment, the precipitant is a liquid that is miscible with the polymerization solvent and can reduce the solubility of the polymer to the level of precipitation. Its selection does not affect the achievement of the technical effect of the present invention. Specifically, the precipitant is selected from alcohols or mixtures of alcohols and their acids, ketones and their mixtures; preferably, it is ethanol or a mixture of ethanol and hydrochloric acid.

[0047] On the other hand, the aforementioned low-refractive-index transparent copolymer or the low-refractive-index transparent copolymer prepared by the aforementioned method is preferably used in the manufacture of transparent optical elements that require control of light transmission behavior in scenarios where interface reflection loss is suppressed or light transmittance is enhanced, and more preferably in optical fiber cladding, anti-reflection coating, display screen, Raman spectroscopy sensor, fluorescence spectroscopy sensor, and long optical path measurement.

[0048] Compared with the prior art, the present invention has the following beneficial effects:

[0049] The low-refractive-index transparent copolymer prepared by this invention is easy to process and recycle. This copolymer is suitable for manufacturing transparent optical elements requiring controlled light transmission behavior, especially for applications such as suppressing interface reflection loss or enhancing light transmittance. Applications include, but are not limited to, fiber optic cladding, anti-reflection coatings, display screens, Raman spectroscopy sensors, fluorescence spectroscopy sensors, and long-path measurement. Detailed Implementation

[0050] To facilitate understanding of the present invention, the following description, in conjunction with embodiments, will further illustrate the invention. It should be understood that the following embodiments are merely for a better understanding of the invention and do not imply that the invention is limited to these embodiments.

[0051] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items. Locational terms such as top and bottom, mentioned or possibly used in this specification, are relative concepts and may therefore vary depending on their location and usage.

[0052] The main raw materials for the following embodiments are sourced from the following sources:

[0053] All monomers involved in the embodiments of this invention are commercially available polymerization-grade chemical products and are used without further purification. The catalysts Me2Si(Ind)2ZrCl2 (CAS No. 121009-93-6) and Et(IndH4)2ZrCl2 (CAS No. 100163-29-9) involved in the embodiments of this invention were purchased from Maclean's; the co-catalyst MAO was purchased from Bailingwei Company.

[0054] The specific information is as follows:

[0055] 1. Structural general formula (b) type monomer

[0056] Monomer B1: C5H2F6O2, purchased from Inocare, CAS number 89444-43-9.

[0057]

[0058] Monomer B2: C5H6F2O2, purchased from Inocare, CAS number 2674027-60-0.

[0059]

[0060] Monomer B3: C5F8O2, purchased from Bailingwei Company, CAS No. 37697-64-6.

[0061]

[0062] 2. Structural general formula (c) type monomer

[0063] The monomer C1:C3H6O was purchased from Inocare, CAS number 107-25-5.

[0064]

[0065] Monomer C2:C3H3F3O was purchased from Inocare, CAS number 1645-89-2.

[0066]

[0067] The monomer C3:C4H3F5O was purchased from Inocare, CAS number 134282-35-2.

[0068]

[0069] Ethylene, solvents, initiators, etc., were purchased from open commercial channels.

[0070] The test methods involved in the following embodiments are as follows:

[0071] Weight-average molecular weight (Mw) and molecular weight distribution (PDI) were determined by high-temperature gel permeation chromatography, using 1,2,4-trichlorobenzene as the mobile phase and narrow-distribution polystyrene as the standard (a standard for high-temperature GPC) at 150℃.

[0072] The glass transition temperature (Tg) was determined by DSC at a heating rate of 10 °C / min and a temperature range of 25–300 °C.

[0073] Optical performance testing: The refractive index of the cyclic olefin copolymer material was tested according to ASTM D542, and the transmittance was measured according to GB / T 2410-2008.

[0074] Example 1-1

[0075] In a reactor equipped with a stirrer, 150 g of cyclohexane solvent was added. Nitrogen gas was purged to purge the reactor, and then 11.2 g (0.40 mol) of ethylene, 72.82 g (0.35 mol) of monomer B1, and 14.52 g (0.25 mol) of monomer C1 were added under a pressure of 3 MPa. The initiator was then added.

[0076] 2.0 g of AIBN was reacted at 200 °C for 8 hours. A 30 wt% hydrochloric acid-containing ethanol solution was added to the polymerization solution to precipitate the product, followed by separation and drying.

[0077] Examples 1-2

[0078] In a reactor equipped with a stirrer, 80 g of hexane solvent was added. Nitrogen gas was then purged, and under a pressure of 4 MPa, 5.6 g (0.20 mol) of ethylene, 104.0 g (0.5 mol) of monomer B1, 33.62 g (0.3 mol) of monomer C2, and an initiator were added.

[0079] 2.0 g of AIBN was reacted at 240 °C for 6 hours. A 30 wt% hydrochloric acid-containing ethanol solution was added to the polymerization solution to precipitate the product, followed by separation and drying.

[0080] Examples 1-3

[0081] In a reactor equipped with a stirrer, 50 g of toluene solvent was added. Nitrogen gas was then purged, and under a pressure of 2.5 MPa, 16.8 g (0.6 mol) of ethylene, 34.03 g (0.25 mol) of monomer B2, 8.71 g (0.15 mol) of monomer C1, and an initiator were added.

[0082] 2.0 g of AIBN was reacted at 180 °C for 9 hours. A 30 wt% hydrochloric acid-containing ethanol solution was added to the polymerization solution to precipitate the product, followed by separation and drying.

[0083] Examples 1-4

[0084] In a reactor equipped with a stirrer, 200 g of cyclohexane solvent was added. Nitrogen gas was purged to purge the reactor, and then 4.2 g (0.15 mol) of ethylene, 20.42 g (0.15 mol) of monomer B2, 113.44 g (0.7 mol) of monomer C3, and initiator were added under a pressure of 3 MPa.

[0085] BPO 2.0 g was reacted at 190 °C for 5 hours. A 30 wt% hydrochloric acid-containing ethanol solution was added to the polymerization solution to precipitate the product, followed by separation and drying.

[0086] Examples 1-5

[0087] In a reactor equipped with a stirrer, 100 g of hexane solvent was added. Nitrogen gas was purged to purge the reactor, and then 11.2 g (0.40 mol) of ethylene, 97.62 g (0.4 mol) of monomer B3, 22.41 g (0.2 mol) of monomer C2, and initiator were added under a pressure of 5 MPa.

[0088] 2.0 g of BPO was reacted at 230 °C for 4 hours. A 30 wt% hydrochloric acid-containing ethanol solution was added to the polymerization solution to precipitate the product, followed by separation and drying.

[0089] Examples 1-6

[0090] In a reactor equipped with a stirrer, 90 g of cyclohexane solvent was added. Nitrogen gas was purged to purge the reactor, and then ethylene (2.8 g, 0.1 mol), monomer B3 (122.0 g, 0.5 mol), monomer C3 (64.82 g, 0.4 mol), and initiator were added under a pressure of 4 MPa.

[0091] 2.0 g of BPO was reacted at 260 °C for 8 hours. A 30 wt% hydrochloric acid-containing ethanol solution was added to the polymerization solution to precipitate the product, followed by separation and drying.

[0092] Example 2-1

[0093] In an anhydrous and oxygen-free stirred reactor, 150 g of cyclohexane solvent was added. Ethylene: 12.6 g (0.45 mol), monomer B1: 64.42 g (0.3 mol), monomer C2: 28.01 g (0.25 mol), metallocene catalyst Me₂Si(Ind)₂ZrCl₂ (5 mg), and MAO co-catalyst were added at an aluminum-to-titanium ratio of 200. The reaction was carried out at 90 °C for 0.5 hours. A 30 wt% hydrochloric acid-containing ethanol solution was added to the polymerization solution to precipitate the product, followed by separation and drying.

[0094] Example 2-2

[0095] In an anhydrous and oxygen-free stirred reactor, 150 g of hexane solvent was added. Ethylene: 15.4 g (0.55 mol), monomer B1: 41.61 g (0.2 mol), monomer C1: 14.52 g (0.25 mol), metallocene catalyst Me₂Si(Ind)₂ZrCl₂ (5 mg), and MAO co-catalyst were added at an aluminum-to-titanium ratio of 200. The reaction was carried out at 100 °C for 1 hour. A 30 wt% hydrochloric acid-containing ethanol solution was added to the polymerization solution to precipitate the product, followed by separation and drying.

[0096] Example 2-3

[0097] In an anhydrous and oxygen-free stirred reactor, 50 g of toluene solvent was added. Ethylene: 14 g (0.5 mol), monomer B2: 34.03 g (0.25 mol), monomer C2: 28.01 g (0.25 mol), metallocene catalyst Me2Si(Ind)2ZrCl2: 5 mg, and MAO co-catalyst were added at a ratio of Aluminum to Titanium of 200. The reaction was carried out at 60 °C for 1.5 hours. A 30 wt% hydrochloric acid-containing ethanol solution was added to the polymerization solution to precipitate the product, followed by separation and drying.

[0098] Examples 2-4

[0099] In an anhydrous and oxygen-free stirred reactor, 150 g of cyclohexane solvent was added. Ethylene: 5.6 g (0.2 mol), monomer B2: 40.83 g (0.3 mol), monomer C3: 81.03 g (0.5 mol), and metallocene catalyst Et(IndH4)2ZrCl2 were added under a pressure of 3 MPa.

[0100] 5 mg of MAO was used as the co-catalyst, with an aluminum-to-titanium ratio of 200. The reaction was carried out at 120°C for 1 hour. A 30 wt% hydrochloric acid-containing ethanol solution was added to the polymerization solution to precipitate the product, followed by separation and drying.

[0101] Examples 2-5

[0102] In an anhydrous and oxygen-free stirred reactor, add 150g of hexane as solvent. Add ethylene under a pressure of 2.5MPa.

[0103] 4.2 g (0.15 mol), monomer B3: 97.62 g (0.4 mol), monomer C1: 26.14 g (0.45 mol), metallocene catalyst

[0104] 5 mg of Et(IndH4)2ZrCl2 was used as the co-catalyst, with an aluminum-to-titanium ratio of 200. The reaction was carried out at 90°C for 1 hour. A 30 wt% hydrochloric acid-containing ethanol solution was added to the polymerization solution to precipitate the product, followed by separation and drying.

[0105] Examples 2-6

[0106] In an anhydrous and oxygen-free stirred reactor, 150 g of toluene solvent was added. Ethylene: 16.8 g (0.6 mol), monomer B3: 48.81 g (0.2 mol), monomer C3: 32.41 g (0.2 mol), and metallocene catalyst Et(IndH4)2ZrCl2 were added under a pressure of 3 MPa.

[0107] 5 mg of MAO was used as the co-catalyst, with an aluminum-to-titanium ratio of 200, and the reaction was carried out at 80°C for 0.5 hours. A 30 wt% hydrochloric acid-containing ethanol solution was added to the polymerization solution to precipitate the product, followed by separation and drying.

[0108] Comparative Example 1

[0109] In a reactor equipped with a stirrer, 90 g of cyclohexane solvent was added. Nitrogen gas was purged, and then 2.8 g (0.1 mol) of ethylene, 122.0 g (0.5 mol) of monomer B3, and 2.0 g of initiator BPO were added under a pressure of 4 MPa. The reaction was carried out at 260 °C for 8 hours. A 30 wt% hydrochloric acid-containing ethanol solution was added to the polymerization solution to precipitate the product, which was then separated and dried.

[0110] Comparative Example 2

[0111] In a reactor equipped with a stirrer, 80 g of hexane solvent was added. Nitrogen gas was purged, and then 5.6 g (0.20 mol) of ethylene, 33.62 g (0.3 mol) of monomer C2, and 2.0 g of initiator AIBN were added under a pressure of 4 MPa. The reaction was carried out at 240 °C for 6 hours. A 30 wt% hydrochloric acid-containing ethanol solution was added to the polymerization solution to precipitate the product, which was then separated and dried.

[0112]

[0113]

[0114] The samples prepared in the examples and comparative examples were analyzed and tested, and the test results are shown in the table below:

[0115]

[0116] Analysis of the comparative and example data shows that the introduction of monomers of structural formula (b) results in high light transmittance and low refractive index in the material. The refractive index is further reduced by introducing an appropriate amount of the third monomer. The introduction of monomers of structural formula (c) can effectively lower the glass transition temperature of the material, making it more processable.

[0117] It is readily understood that the above embodiments are merely illustrative examples for clear explanation and do not imply that the invention is limited thereto. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations. However, obvious variations or modifications derived therefrom remain within the scope of protection of this invention.

Claims

1. A low-refractive-index transparent copolymer, characterized in that, The general structural formula of the transparent copolymer is shown in formula (a): Wherein, R1 to R4 are each independently an atom or group selected from H, F, CH3, CH2F, CHF2, and CF3, and at least one of the atoms or groups is F, CH2F, CHF2, or CF3; R5 is F, C1-C 30 Alkyl, C1-C 30 The fluoroalkyl group; wherein x, y, z are integers greater than 0 and less than 1000; and x / (x+y+z)≤0.

5.

2. The low-refractive-index transparent copolymer according to claim 1, characterized in that, The low-refractive-index transparent copolymer satisfies at least one of the following properties: (1) The weight-average molecular weight ranges from 10,000 to 200,000, preferably from 30,000 to 90,000; (2) The refractive index is less than 1.4; (3) The glass transition temperature is between 20℃ and 180℃.

3. The method for preparing the low-refractive-index transparent copolymer according to claim 1 or 2, characterized in that, Includes the following steps: (1) Synthesis steps: Ethylene, fluorine-containing monomers of general structural formula (b), monomers of general structural formula (c) and solvent are added to a reaction vessel and mixed evenly under stirring conditions; an initiator is added and a polymerization reaction is carried out to obtain a polymer solution of the transparent copolymer; The selection of R1 to R5 is the same as in equation (a); (2) Separation step: Separate the mixture containing the dissolved transparent copolymer and the organic solvent to obtain the transparent copolymer.

4. The preparation method according to claim 3, characterized in that, The polymerization initiation system in step (1) is initiated by a free radical initiator; Preferably, the free radical initiator is selected from at least one of thermally decomposable initiators and redox initiators; More preferably, the thermally decomposable initiator is an azo initiator and / or a peroxide initiator; More preferably, the azo initiator is azobisisobutyronitrile (AIBN) and / or azobisisoheptanenitrile; the peroxide initiator is at least one of benzoyl peroxide (BPO), lauroyl peroxide, and tert-butyl hydroperoxide.

5. The preparation method according to claim 3, characterized in that, The polymerization initiation system in step (1) is a coordination polymerization catalyst system. Preferably, the coordination polymerization catalyst system comprises a main catalyst and a co-catalyst; More preferably, the main catalyst is at least one of a Ziegler-Natta catalyst, a metallocene catalyst, or a non-metallocene catalyst; More preferably, the main catalyst is a compound containing titanium, zirconium, vanadium or chromium; and the co-catalyst is an alkylaluminum compound and / or methylaluminoxane (MAO).

6. The preparation method according to claim 3, characterized in that, The solvent mentioned in step (1) is selected from at least one of aliphatic hydrocarbon solvents, aromatic hydrocarbon solvents, and halogenated hydrocarbon solvents; Preferably, the aliphatic hydrocarbon solvent is selected from C5-C64. 12 Straight-chain alkanes, C5-C 12 Branched alkanes, cycloalkanes, or mixtures thereof; and / or The aromatic hydrocarbon solvent is selected from benzene, toluene, xylene, ethylbenzene, mesitylene, cumene, or mixtures thereof; and / or The halogenated hydrocarbon solvent is selected from chloroalkanes and fluorinated hydrocarbons, preferably chloromethane, dichloromethane, chloroform, carbon tetrachloride, 1,2-dichloroethane, chlorobenzene, dichlorobenzene, trichloroethylene, tetrachloroethylene, or mixtures thereof.

7. The preparation method according to any one of claims 3-6, characterized in that, In step (1), the molar ratio of ethylene, the fluorinated monomer shown in general formula (b), and the monomer shown in general formula (c) is (A+B):C = (1:99) to (99:1), and the molar ratio of ethylene to the fluorinated monomer shown in general formula (b) is 1:99 to 99:

1.

8. The preparation method according to any one of claims 3-6, characterized in that, The mass ratio of the total mass of ethylene, the fluorinated monomer shown in general formula (b), and the monomer shown in general formula (c) to the solvent in step (1) is 1:99 to 99:

1.

9. The preparation method according to claim 3, characterized in that, The separation methods described in step (2) include flash evaporation and precipitation. Preferably, the flash evaporation method involves introducing the mixture into a flash tank and flash evaporating it at a temperature of 150°C to 250°C under reduced pressure, thereby vaporizing the solvent and separating it from the molten polymer. Optionally, the process may also include a step of devolatilizing the resulting molten polymer; Preferably, the precipitation method involves adding a precipitant to the mixture to precipitate the dissolved transparent copolymer; then performing solid-liquid separation, washing, and drying on the precipitated polymer. More preferably, the precipitant is selected from alcohols or mixtures of alcohols and their acids, ketones and their mixtures.

10. The application of the low-refractive-index transparent copolymer of claim 1 or 2 or the low-refractive-index transparent copolymer prepared by any one of claims 3-9 in the manufacture of transparent optical elements for which light transmission behavior needs to be regulated, preferably in scenarios where interface reflection loss is suppressed or light transmittance is enhanced, and more preferably in optical fiber cladding, anti-reflection coating, display screen, Raman spectroscopy sensor, fluorescence spectroscopy sensor, and long optical path measurement.

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