Fluorosilicone rubber with controllable molecular weight and method for preparing the same

Abstract

The application provides a fluorosilicone rubber with controllable molecular weight and a preparation method thereof, and belongs to the field of catalysis technology for preparing fluorosilicone rubber. First, 1,3,5-trimethyl-1,3,5-tris(3,3,3-trifluoropropyl)cyclotrisiloxane and a cyclosiloxane monomer are added into a polymerization reactor, heated, vacuumized and subjected to dehydration treatment, and the addition amount of the cyclosiloxane monomer accounts for 0.5%-20% of the total mass of the monomers. Then, a boron-containing alkaline composition is added into the polymerization reactor, and polymerization is carried out at 60-120 DEG C for 10-600 minutes to obtain the fluorosilicone rubber. The boron-containing alkaline composition is used as a catalyst for ring-opening polymerization of the cyclosiloxane, and the flowability of the catalyst is greatly improved; the catalyst can effectively inhibit side reactions in the polymerization process; and the prepared fluorosilicone rubber can meet the stability for use in high and low temperature environments without special neutralization process.

Description

Technical Field

[0001] This invention belongs to the field of catalytic technology for the preparation of fluorosilicone rubber, and relates to a molecular weight controllable fluorosilicone rubber raw material suitable for industrial production and its preparation method. Technical Background

[0002] The main chain structure of fluorosilicone rubber is composed primarily of repeating -Si-O- units with high flexibility and high bond energy, which endows it with excellent high and low temperature resistance. The side chains consist of trifluoropropyl (-CH2-CH2-CF3) and methyl (-CH3) groups, with fluorinated substituents significantly enhancing the material's resistance to oils and solvents. Therefore, fluorosilicone rubber remains stable under extreme temperature conditions (-55℃ to 230℃) and in oily environments. Its applications cover defense, aerospace, deep-sea exploration, automotive, and petrochemical industries, making it one of the world's best-performing synthetic rubbers.

[0003] In the production of fluorosilicone rubber, molecular weight control is crucial, directly affecting the mechanical properties of the rubber and the quality of the final product. Commonly used catalysts in the preparation process, such as alkali metal hydroxides, quaternary ammonium hydroxides, and quaternary phosphorus hydroxides, are all strongly alkaline. The strong electron attraction of the fluorine side groups increases the electrophilicity of the ester bonds, which triggers intramolecular "biting back" and intermolecular chain transfer side reactions during polymerization, leading to a decrease in molecular weight and uneven distribution, thus generating a large amount of low molecular weight polymers. For example, when using potassium hydroxide for bulk ring-opening polymerization of 1,3,5-trimethyl-1,3,5-tris(3,3,3-trifluoropropyl)cyclotrisiloxane (D3F), the average cyclic content at polymerization equilibrium can reach as high as 86.5% (Journal of Polymer Science Polymer Letter Edition, 1965, 3, 473-482).

[0004] To address these issues, two main strategies are currently employed. The first involves altering the catalytic system, such as using butyllithium with lower activity as the catalyst and dimethyl sulfoxide as a promoter, which effectively suppresses the "biting-back" reaction, thereby reducing the cyclic content at polymerization equilibrium (Journal of Polymer Science Part A, 1989, 27(4), 1241-1258). Although adjusting the additives and catalysts can reduce side reactions to some extent, it does not completely suppress them, potentially affecting the quality of fluorosilicone rubber. The second strategy involves controlling the polymerization time and terminating the reaction when the yield of the linear polymer is highest (CN 114381001 A). This requires greater control over the polymerization process and equipment, and may also lead to a decrease in batch stability of the product. Furthermore, the neutralization of the catalyst must be addressed at the end of polymerization. Residual catalyst may attack the silicone bonds in the polymer chain, causing chain breakage. Given the high viscosity and excellent oil and chemical resistance of high molecular weight fluorosilicone rubber, the addition and effective dispersion of neutralizing agents are particularly challenging. Therefore, special measures must be taken during the preparation process to ensure the high performance and safety of the product. Summary of the Invention

[0005] This invention aims to address the shortcomings of existing anionic ring-opening polymerization catalytic technologies for fluorinated cyclosiloxanes, including difficulties in controlling molecular weight, high cyclic content in the polymerization product, and poor storage stability of the polymer due to metal residues. By selecting boron-containing organic bases as catalysts, this invention provides a method for preparing fluorosilicone rubber with controllable molecular weight.

[0006] To achieve the above objectives, the technical solution of the present invention is as follows:

[0007] First, the present invention provides a method for preparing fluorosilicone rubber, comprising the following steps:

[0008] The first step involves adding 1,3,5-trimethyl-1,3,5-tris(3,3,3-trifluoropropyl)cyclotrisiloxane and cyclosiloxane monomers into a polymerization reactor, heating and vacuuming for dehydration; wherein the amount of cyclosiloxane added accounts for 0.5%–20% of the total mass of the monomers.

[0009] The second step involves adding a boron-containing basic composition to a polymerization reactor. The amount of boron-containing basic composition (calculated as the corresponding alkali metal hydroxide) is 8–500 ppm. The polymerization reaction is started by heating to 60–120 degrees Celsius and polymerizing for 10–600 minutes to obtain fluorosilicone rubber.

[0010] Furthermore, in the second step, the boron-containing basic composition is prepared by reacting an alkali metal compound with a borane compound. The specific method is as follows: Under inert gas protection, an alkali metal compound is first added to a preparation vessel, followed by a borane compound slowly added at -40–60 degrees Celsius. The mixture is aged for 10–120 minutes to obtain a boron-containing basic composition with an alkali metal concentration of 0.1–1.0 M, which is a transparent solution. The reaction principle of this step is as follows: borane compounds with Lewis acidity can form complexes with organic base compounds with Lewis basicity. These complexes have good solubility in both nonpolar and polar solvents, facilitating the addition of materials. Furthermore, by selecting borane compounds with different substituents, the steric hindrance around the active oxygen anion can be effectively controlled. This helps reduce the attack of the active oxygen anion center on the ester bonds in the larger polymer chain, thereby effectively suppressing side reactions during polymerization.

[0011] Furthermore, in the second step, the molar ratio of the alkali metal compound to the borane compound in the boron-containing basic composition is 1:(0.2–10), preferably 1:(1–3).

[0012] Furthermore, in the second step, the alkali metal compound is selected from lithium hydride, sodium hydride, potassium hydride, lithium hydroxide, sodium hydroxide, potassium sodium hydroxide, lithium tert-butoxide, sodium tert-butoxide, potassium tert-butoxide, n-butyllithium, sec-butyllithium, tert-butyllithium, phenyllithium, lithium silicate, sodium silicate, potassium silicate, preferably n-butyllithium.

[0013] Furthermore, in the second step, the borane compound is selected from trimethylboron, triethylboron, tri-n-butylboron, tri-sec-butylboron, trioctylboron, triphenylboron, tri(pentafluorophenyl)boron, preferably triethylboron or triphenylboron.

[0014] Furthermore, in the second step, the borane compound needs to be prepared into a solution for easy operation before use. The solvent can be at least one selected from acetonitrile, dimethyl sulfoxide, N,N-dimethylformamide, N,N-dimethylacetamide, pyridine, acetone, 1,4-dioxane, methylpyrrolidone, phosphazene base P4-t-Bu, tetrahydrofuran, methyl formate, ethyl acetate, toluene, diethylene glycol dimethyl ether, and ethylene glycol dimethyl ether, to prepare a 0.1–1.0 M solution.

[0015] Furthermore, the cyclosiloxane monomer is one or more of hexamethylcyclotrisiloxane, octamethylcyclotetrasiloxane, 2,4,6-trivinyl-2,4,6-trimethylcyclotrisiloxane, and 2,4,6-triphenyl-2,4,6-trimethylcyclotrisiloxane, preferably 2,4,6-trivinyl-2,4,6-trimethylcyclotrisiloxane.

[0016] A fluorosilicone rubber, prepared by the above-described method, has the following structural formula:

[0017]

[0018] Wherein, R is one or more of methyl, phenyl and vinyl, preferably methyl and vinyl;

[0019] Furthermore, the molecular weight of the fluorosilicone rubber is 0.2–3 million, with a molecular weight distribution of less than 1.50.

[0020] Beneficial effects of this invention:

[0021] (1) In the preparation of fluorosilicone rubber, this invention innovatively uses a boron-containing basic composition as a catalyst for the ring-opening polymerization of cyclosiloxanes. This catalyst is a transparent solution, and the boron compound with Lewis acidity can form complexes with organic base compounds with Lewis basicity. These complexes exhibit good solubility in both non-polar and polar solvents. Compared to traditional cyclosiloxane catalysts such as alkali gels, its fluidity is significantly improved, facilitating feeding operations in industrial production.

[0022] (2) Based on the method provided by this invention, by selecting borane compounds with different substituents, the steric hindrance around the oxyanion active center can be effectively controlled. This helps to reduce the attack of the oxyanion active center on the ester bonds in the bulky polymer chain, thereby effectively suppressing side reactions during the polymerization process.

[0023] (3) Using this catalytic system, no additional neutralization process is required to maintain the good storage stability of fluorosilicone rubber. This is mainly due to the introduction of borane compounds, where boron can form a buffer system with alkali metals, significantly reducing the catalytic effect of alkali metals in the rubber matrix. Therefore, fluorosilicone rubber prepared using this catalytic method can meet the stability requirements for use in high and low temperature environments without special neutralization processes. Attached Figure Description

[0024] Figure 1 The thermogravimetric curves of the fluorosilicone rubber prepared in Example 18 and Comparative Example 15 of this invention are shown. Detailed Implementation

[0025] The technical solutions of the present invention will be further described below with reference to the embodiments.

[0026] Examples 1–14 describe the preparation methods of boron-containing basic compositions, with the main preparation parameters shown in Table 1. The alkali metal concentration was determined using the ICP-OES method (determination parameters: the metered catalyst sample was sintered at 450°C for 5 hours, cooled, and then acidified; the treated sample was dissolved in water for ICP testing).

[0027] Table 1

[0028]

[0029]

[0030] The specific implementation methods of Examples 1–14 are as follows:

[0031] Example 1

[0032] 9.5 mL of tetrahydrofuran was added to a 100 mL reactor. Then, 0.01 g of lithium hydride (1.0 mmol) and 0.5 mL of triethylboron (1.0 M, 0.5 mmol) were added sequentially at room temperature, and the mixture was stirred thoroughly. The mixture was then gradually cooled to -40°C and maintained at this temperature for 120 minutes. A boron-containing alkali metal composition with an alkali metal concentration of 0.100 M was prepared.

[0033] Example 2

[0034] 9.0 mL of acetonitrile was added to a 100 mL reactor. Then, 0.02 g of sodium hydride (1.0 mmol) and 1.0 mL of tri-n-butylboron (1.0 M, 1.0 mmol) were added sequentially at room temperature, and the mixture was stirred thoroughly. The mixture was then gradually cooled to -20°C and maintained at this temperature for 100 minutes. A boron-containing alkali metal composition with an alkali metal concentration of 0.100 M was prepared.

[0035] Example 3

[0036] 2.2 mL of dimethyl sulfoxide was added to a 100 mL reactor. Then, 0.05 g of potassium hydride (1.2 mmol) and 7.8 mL of trioctylboron (1.0 M, 7.8 mmol) were added sequentially at room temperature, and the mixture was stirred thoroughly. The mixture was then gradually cooled to 0°C and maintained at this temperature for 80 minutes. A boron-containing alkali metal composition with an alkali metal concentration of 0.120 M was prepared.

[0037] Example 4

[0038] 6.5 mL of N,N-dimethylformamide was added to a 100 mL reactor. Then, 0.07 g of sodium hydroxide (1.7 mmol) and 3.5 g of triphenylborone (14.3 mmol) were added sequentially at room temperature, and the mixture was stirred thoroughly. The temperature was then gradually increased / decreased to 20°C and maintained for 60 minutes. A boron-containing alkali metal composition with an alkali metal concentration of 0.168 M was prepared.

[0039] Example 5

[0040] 4.3 mL of pyridine was added to a 100 mL reactor, followed by the addition of 0.12 g of lithium tert-butoxide (1.8 mmol) and 5.6 g of tris(pentafluorophenyl)boron (10.9 mmol) at room temperature, followed by thorough mixing. The mixture was then gradually heated to 40 °C and maintained at that temperature for 40 minutes. This yielded a boron-containing alkali metal composition with an alkali metal concentration of 0.182 M.

[0041] Example 6

[0042] 6.4 mL of acetone was added to a 100 mL reactor. Then, 0.10 g of sodium tert-butoxide (1.0 mmol) and 3.5 mL of trimethylboron (1.0 M, 3.5 mmol) were added sequentially at room temperature, and the mixture was stirred thoroughly. The temperature was then gradually increased to 60°C and maintained for 10 minutes. A boron-containing alkali metal composition with an alkali metal concentration of 0.101 M was prepared.

[0043] Example 7

[0044] 2.3 mL of 1,4-dioxane was added to a 100 mL reactor. Then, 0.17 mL of potassium tert-butoxide (1.5 mmol) and 7.5 mL of tri-n-butylborone (1.0 M, 7.5 mmol) were added sequentially at room temperature, and the mixture was stirred thoroughly. The mixture was then gradually cooled to -40°C and maintained at this temperature for 120 minutes. A boron-containing alkali metal composition with an alkali metal concentration of 0.150 M was prepared.

[0045] Example 8

[0046] 1.2 mL of methylpyrrolidone was added to a 100 mL reactor. Then, 0.17 mL of n-butyllithium (0.01 M, 1.7 mmol) and 8.7 mL of trioctylboron (1.0 M, 8.7 mmol) were added sequentially at room temperature, and the mixture was stirred thoroughly. The mixture was then gradually cooled to -20°C and maintained at this temperature for 100 minutes. A boron-containing alkali metal composition with an alkali metal concentration of 0.173 M was prepared.

[0047] Example 9

[0048] 9.4 mL of phosphazene base was added to a 100 mL reactor, followed by the sequential addition of 0.11 mL of sec-butyllithium (0.01 M, 1.1 mmol) and 0.5 g of triphenylboron (2.2 mmol) at room temperature, followed by thorough mixing. The mixture was then gradually cooled to 0°C and maintained at this temperature for 80 minutes. A boron-containing alkali metal composition with an alkali metal concentration of 0.111 M was obtained.

[0049] Example 10

[0050] 7.8 mL of methyl formate was added to a 100 mL reactor. Then, 0.13 mL of tert-butyllithium (0.01 M, 1.3 mmol) and 2.0 g of tris(pentafluorophenyl)boron (4.0 mmol) were added sequentially at room temperature, and the mixture was stirred thoroughly. The temperature was then gradually lowered to 20°C and maintained for 60 minutes. A boron-containing alkali metal composition with an alkali metal concentration of 0.132 M was prepared.

[0051] Example 11

[0052] 7.4 mL of ethyl acetate was added to a 100 mL reactor. Then, 0.16 mL of phenyllithium (0.01 M, 1.6 mmol) and 2.4 mL of triethylboron (1.0 M, 2.4 mmol) were added sequentially at room temperature, and the mixture was stirred thoroughly. The temperature was then gradually increased to 40°C and maintained for 40 minutes. A boron-containing alkali metal composition with an alkali metal concentration of 0.160 M was prepared.

[0053] Example 12

[0054] 0.6 mL of toluene was added to a 100 mL reactor. Then, 0.29 mL of lithium silicate (0.031 M, 9.1 mmol) and 9.1 mL of trisec-butylboron (1.0 M, 9.1 mmol) were added sequentially at room temperature, and the mixture was stirred thoroughly. The temperature was then gradually increased to 60 °C and maintained for 10 minutes. A boron-containing alkali metal composition with an alkali metal concentration of 0.908 M was prepared.

[0055] Example 13

[0056] 5.0 mL of diethylene glycol dimethyl ether was added to a 100 mL reactor. Then, 0.43 mL of sodium silicate (0.0215 M, 9.2 mmol) and 4.6 mL of trioctylboron (1.0 M, 4.6 mmol) were added sequentially at room temperature, and the mixture was stirred thoroughly. The temperature was then gradually lowered to -20°C and maintained for 100 minutes. A boron-containing alkali metal composition with an alkali metal concentration of 0.918 M was prepared.

[0057] Example 14

[0058] 7.6 mL of ethylene glycol dimethyl ether was added to a 100 mL reactor. Then, 0.41 mL of potassium silicate (0.024 M, 10.0 mmol) and 1.9 g of triphenylborone (8.0 mmol) were added sequentially at room temperature, and the mixture was stirred thoroughly. The mixture was then gradually cooled to 0°C and maintained at that temperature for 80 minutes. A boron-containing alkali metal composition with an alkali metal concentration of 1.000 M was prepared.

[0059] Comparative Example 15 is a comparative example of fluorosilicone rubber prepared using a conventional catalyst.

[0060] 250 g of 1,3,5-trimethyl-1,3,5-tris(3,3,3-trifluoropropyl)cyclotrisiloxane was added to a reactor. After maintaining the reactor at 60°C under vacuum for 60 minutes, 23 mg of lithium tert-butoxide was added. After thorough stirring and dispersion, 1.2 mL of dimethyl sulfoxide was added. Polymerization was carried out at 60°C for 120 minutes to obtain fluorosilicone rubber.

[0061] Examples 16–27 illustrate the use of the above catalyst in the preparation of fluorosilicone rubber.

[0062] The cyclosiloxane monomers corresponding to those in Table 2 were added to a 500 mL reactor and preheated to the specified polymerization temperature. The specified catalyst was added all at once and the polymerization time was maintained at a given time. The molecular weight, molecular weight distribution and small molecule cyclic content were determined by gel permeation chromatography and the corresponding results are listed in Table 2.

[0063] Table 2

[0064]

[0065] Note: a The numerical value for the catalyst corresponding to the catalyst prepared in the numbered examples indicates the amount of catalyst used. b For ease of calculation, the catalyst dosage is measured according to the corresponding alkali metal hydroxide. c D3F, D3, D3 Vi D3 Ph These correspond to 1,3,5-trimethyl-1,3,5-tris(3,3,3-trifluoropropyl)cyclotrisiloxane, hexamethylcyclotrisiloxane, 2,4,6-trivinyl-2,4,6-trimethylcyclotrisiloxane, and 2,4,6-triphenyl-2,4,6-trimethylcyclotrisiloxane, respectively.

[0066] The structures of the fluorosilicone rubber of formula (1) prepared in Examples 15–27, wherein the repeating units without R substituents in Comparative Examples 15 and Examples 16–22 are homopolymers of fluorosiloxanes, the R substituents in Examples 23 and 24 are vinyl, the R substituents in Examples 25 and 26 are phenyl, and the R substituents in Example 27 are methyl.

[0067] Compared to Comparative Example 15, Example 16 was prepared using the fluorosilicone rubber preparation method provided by this invention, while Comparative Example 15 used a conventional alkali metal catalytic system. Both examples maintained the same monomer feed ratio, alkali metal ratio, and polymerization conditions. The polymerization results showed that Comparative Example 15 produced a high percentage (12.3%) of low molecular weight cyclic structures. This was mainly due to the "biting back" side reaction of the polymer chains initiated by the basic active centers during polymerization, leading to a decrease in molecular weight and a wider molecular weight distribution. In contrast, the fluorosilicone rubber prepared in Example 16 had only 0.7% cyclic structures, and a high molecular weight polymer could be obtained under the same catalyst dosage. This indicates that the catalytic system used in Example 16 almost completely suppressed the "biting back" side reaction commonly seen in polymerization processes initiated by conventional catalysts.

[0068] Thermogravimetric analysis was further performed on the obtained fluorosilicone rubbers with similar molecular weights, including Comparative Example 15 and Comparative Example 18. The results showed that the fluorosilicone rubber prepared in Example 18 had a temperature of 383°C at 5% thermogravimetric analysis, while that of Comparative Example 15 was 297°C. Neither of them underwent a neutralization process. In Comparative Example 15, the residual conventional catalyst in the fluorosilicone rubber matrix led to catalytic degradation of the rubber during heat treatment; while Example 18 maintained high stability even at higher temperatures.

[0069] The embodiments described above merely illustrate specific implementations of the present invention and should not be considered as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention.

Claims

1. A method for preparing fluorosilicone rubber with controllable molecular weight, characterized in that, Includes the following steps: The first step involves adding 1,3,5-trimethyl-1,3,5-tris(3,3,3-trifluoropropyl)cyclotrisiloxane and cyclosiloxane monomers into a polymerization reactor, heating, and then subjecting the reactor to vacuum dehydration. The amount of cyclosiloxane monomer added is 0.5%–20% of the total monomer mass. The second step involves adding a boron-containing basic composition to a polymerization reactor. The amount of boron-containing basic composition is 8–500 ppm, calculated based on the corresponding alkali metal hydroxide. The polymerization reaction is initiated at 60–120 degrees Celsius and carried out for 10–600 minutes to prepare fluorosilicone rubber. The boron-containing basic composition is prepared by reacting an alkali metal compound with a borane compound. The borane compound is selected from trimethylboron, triethylboron, tri-n-butylboron, tri-sec-butylboron, trioctylboron, triphenylboron, and tri(pentafluorophenyl)boron. The molar ratio of the alkali metal compound to the borane compound is 1:(0.2–10). The boron-containing alkaline composition is prepared as follows: Under inert gas protection, an alkali metal compound is first added to a preparation vessel, and then a borane compound is slowly added at -40–60 degrees Celsius. After aging for 10–120 minutes, a boron-containing alkaline composition with an alkali metal concentration of 0.1–1.0 M is obtained. The alkali metal compound is selected from lithium hydride, sodium hydride, potassium hydride, lithium hydroxide, sodium hydroxide, potassium hydroxide, lithium tert-butoxide, sodium tert-butoxide, potassium tert-butoxide, n-butyllithium, sec-butyllithium, tert-butyllithium, phenyllithium, lithium silicate, sodium silicate, and potassium silicate.

2. The method for preparing a molecular weight controllable fluorosilicone rubber according to claim 1, characterized in that, In the first step, the cyclosiloxane monomer is one or more of hexamethylcyclotrisiloxane, octamethylcyclotetrasiloxane, 2,4,6-trivinyl-2,4,6-trimethylcyclotrisiloxane, and 2,4,6-triphenyl-2,4,6-trimethylcyclotrisiloxane.

3. The method for preparing a molecular weight controllable fluorosilicone rubber according to claim 1, characterized in that: The molar ratio of the alkali metal compound to the borane compound is 1:(1–3). The alkali metal compound is n-butyllithium; The borane compounds mentioned are triethylboron and triphenylboron.

4. The method for preparing a molecular weight controllable fluorosilicone rubber according to claim 1, characterized in that, Before use, the borane compound is mixed with a solvent to form a solution with a concentration of 0.1–1.0 M; the solvent includes at least one selected from acetonitrile, dimethyl sulfoxide, N,N-dimethylformamide, N,N-dimethylacetamide, pyridine, acetone, 1,4-dioxane, methylpyrrolidone, phosphazene base P4-t-Bu, tetrahydrofuran, methyl formate, ethyl acetate, toluene, diethylene glycol dimethyl ether, and ethylene glycol dimethyl ether.

5. A fluorosilicone rubber, characterized in that, The fluorosilicone rubber is prepared by any one of the preparation methods described in claims 1-4, and the structural formula of the fluorosilicone rubber is as follows: ; Wherein, R is one or more of methyl, phenyl, and vinyl.

6. The fluorosilicone rubber according to claim 5, characterized in that, The fluorosilicone rubber has a molecular weight of 0.2–3 million, with a molecular weight distribution of less than 1.50.

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