A low viscosity polyborosiloxane and a method for preparing the same

Abstract

The application provides a low-viscosity polyborosiloxane and a preparation method thereof. The preparation method comprises the following steps: a) reacting end double-hydroxyl-terminated polydimethylsiloxane with a reactant 1, and removing water during the reaction to obtain a polyborosiloxane; and b) removing unreacted reactant 1 from the polyborosiloxane to obtain a low-viscosity polyborosiloxane; wherein the reactant 1 is diphenyl boronic anhydride and / or phenyl boronic acid. The polyborosiloxane prepared by the application does not contain a three-membered borate structure, has low viscosity, and is easy to be blended with other systems to prepare functional materials. Meanwhile, the polyborosiloxane has high boron content, large dynamic coordination bond density of B:O, and can match the relaxation time with the impact rate at low viscosity, and the Weissenberg number is less than 1, so that the energy can be dissipated more effectively.

Description

Technical Field

[0001] This invention relates to the field of organosilicon materials, and in particular to a low-viscosity polyborosiloxane and its preparation method. Background Technology

[0002] New-generation impact-resistant protective materials are mostly developed using supramolecular polymers as the matrix. These materials utilize the non-covalent interactions within the supramolecular polymers to impart velocity responsiveness while simultaneously dissipating energy. Among these, polyborosiloxane (PBS) is a research hotspot. As a typical solid-liquid transition material, PBS is soft and easily deformable at low strain rates, but rapidly hardens at high strain rates, exhibiting impact hardening behavior. Furthermore, many studies suggest that the impact resistance mechanism of PBS primarily involves the dissipation of energy through the dissociation of the dynamic B:O coordination bonds in its structure upon impact.

[0003] The non-covalent structure of PBS makes it prone to creep; therefore, PBS is currently often added as an active ingredient to other systems (such as polyurethane, thermoplastic elastomers, and fibers). For example, patent application number ZL02825922.X uses polyurethane foam as the matrix material and mixes it with PBS to prepare a self-supporting energy-absorbing composite material; patent application number CN202211329388.0 uses polyolefin and other elastomers as the matrix material and mixes them with PBS to prepare an ultra-lightweight elastic composite foam material for use in sports shoe soles. However, the PBS currently used has a high viscosity, making it difficult to mix evenly during the mixing process. In addition, high-viscosity PBS has a long relaxation time (~1s), which can lead to problems when subjected to impacts (strain rate ~10) in applications such as sports protective gear. 3 s -1 When the Weissenberg number (the product of the system's relaxation time and the external deformation rate) is much greater than 1, the B:O dynamic coordination bonds in PBS do not have enough time to dissipate the impact energy through dissociation. Therefore, it is necessary to prepare a PBS with low viscosity and short relaxation time, which can solve the problem of difficulty in uniform stirring and match the relaxation time of the PBS system with the external impact rate (Weissenberg number close to 1), thereby achieving efficient dissipation of impact energy. Summary of the Invention

[0004] In view of this, the present invention provides a low-viscosity polyborosiloxane and its preparation method. The polyborosiloxane prepared by the present invention does not contain a ternary borate ester structure, has low viscosity, and is easy to blend with other systems to prepare functional materials. Simultaneously, the polyborosiloxane has a high boron content and a high B:O dynamic coordination bond density. At low viscosity, the relaxation time can match the impact rate, and the Weissenberg number is less than 1, which can more effectively dissipate energy.

[0005] This invention provides a method for preparing low-viscosity polyborosiloxane, comprising the following steps:

[0006] a) React the terminally dihydroxyl-capped polydimethyl silicone oil with reactant 1, and remove water during the reaction to obtain polyborosiloxane.

[0007] b) Remove unreacted reactant 1 from the polyborosiloxane to obtain a low-viscosity polyborosiloxane;

[0008] Wherein, reactant 1 is diphenylboronic anhydride and / or phenylboronic acid.

[0009] Preferably, in step a), the number-average molecular weight of the terminally dihydroxyl-capped polydimethyl silicone oil is 700–20000 g / mol.

[0010] Preferably, in step a), the molar ratio of hydroxyl groups in reactant 1 to the polydimethyl silicone oil with terminal dihydroxyl end-capped hydroxyl groups is (0.5-1):1.

[0011] Preferably, in step a), the reaction temperature is 60–150°C and the time is 48–300 h.

[0012] Preferably, in step a), the water removal method is to use a water separator or a vacuum reaction to remove water.

[0013] Preferably, step b) specifically includes:

[0014] The polyborosiloxane was dissolved in an organic solvent, and then unreacted reactant 1 was removed by solid-liquid separation. The organic solvent was then removed from the resulting separated liquid to obtain a low-viscosity polyborosiloxane.

[0015] Preferably, the organic solvent is an anhydrous solvent;

[0016] The anhydrous solvent is selected from at least one of anhydrous tetrahydrofuran, anhydrous chloroform, anhydrous acetone, and anhydrous n-hexane.

[0017] Preferably, the solid-liquid separation method is filtration;

[0018] The filtration membrane used in the filtration process has a pore size of less than 0.25 μm.

[0019] Preferably, the low-viscosity polyborosiloxane obtained in step b) has the following specifications: boron content 500-2000 ppm, ternary borate ester unit content 0 wt%, diborary borate ester unit content 0 wt%-95 wt%, and monoborary borate ester unit content 5 wt%-95 wt%.

[0020] The viscosity of the low-viscosity polyborosiloxane is 200–10000 mPa·s.

[0021] The present invention also provides a low-viscosity polyborosiloxane prepared by the preparation method described in the above technical solution.

[0022] The preparation method provided by this invention involves reacting terminally dihydroxyl-capped polydimethyl silicone oil with a specific reactant 1 (diphenylboronic anhydride and / or phenylboronic acid), followed by purification and drying to obtain polyborosiloxane. The polyborosiloxane obtained by this invention has a high boron content and does not contain a ternary borate ester structure, exhibiting low viscosity and ease of blending with other systems to prepare functional materials. Furthermore, the high boron content and high B:O dynamic coordination bond density of the polyborosiloxane allow for relaxation time matching the impact rate at low viscosity, and a Weissenberg number less than 1, enabling more efficient energy dissipation.

[0023] Experimental results show that the viscosity of the polyborosiloxane provided by this invention is below 16830 mPa·s, and the relaxation time is 8.4 × 10⁻⁶. -5 Below s, the relaxation time can be matched with the impact rate at low viscosity, and the Weissenberg number is less than 1, thereby achieving the purpose of dissipating energy through the dissociation of B:O dynamic coordination bonds. Attached Figure Description

[0024] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0025] Figure 1 This is the master curve of the rheological properties of the polyboronsiloxane obtained in Example 1 of the present invention;

[0026] Figure 2 The rheological properties curve of the polyborosiloxane obtained in Comparative Example 1 of this invention is shown.

[0027] Figure 3 The viscosity curves are of the polyborosiloxanes obtained in Examples 1-3 and Comparative Examples 1-3 of this invention. Detailed Implementation

[0028] 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 application belongs. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application.

[0029] In this article, the technical features described in an open-ended manner include both closed technical solutions composed of the listed features and open technical solutions that include the listed features.

[0030] The term “and / or” as used herein includes any and all combinations of one or more of the related listed items.

[0031] In this document, numerical ranges are referred to as continuous unless otherwise specified, and include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Furthermore, when a range refers to an integer, it includes every integer between the minimum and maximum values ​​of the range. Additionally, when multiple ranges are provided to describe a feature or characteristic, the ranges may be combined. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are incorporated.

[0032] In this article, when referring to units for data ranges, if the unit is only followed by the right endpoint, it indicates that the units for the left and right endpoints are the same. For example, 60~150℃ means that the units for the left endpoint "60" and the right endpoint "150" are both in degrees Celsius (℃).

[0033] This document only specifically discloses some numerical ranges. However, any lower limit can be combined with any upper limit to form an unspecified range; and any lower limit can be combined with other lower limits to form an unspecified range, just as any upper limit can be combined with any other upper limit to form an unspecified range. Furthermore, each individually disclosed point or single value can itself serve as a lower or upper limit and be combined with any other point or single value or with other lower or upper limits to form an unspecified range.

[0034] This invention provides a method for preparing low-viscosity polyborosiloxane, comprising the following steps:

[0035] a) React the terminally dihydroxyl-capped polydimethyl silicone oil with reactant 1, and remove water during the reaction to obtain polyborosiloxane.

[0036] b) Remove unreacted reactant 1 from the polyborosiloxane to obtain a low-viscosity polyborosiloxane;

[0037] Wherein, reactant 1 is diphenylboronic anhydride and / or phenylboronic acid.

[0038] Regarding step a) :

[0039] a) React the terminally dihydroxyl-capped polydimethyl silicone oil with reactant 1 to obtain polyborosiloxane.

[0040] In this invention, the terminally dihydroxyl-terminated polydimethyl silicone oil (i.e., terminally dihydroxyl-terminated polydimethylsiloxane) is a polydimethyl silicone oil with hydroxyl-terminated ends, preferably with a number-average molecular weight of 700–20000 g / mol. Specifically, it is preferred to use terminally dihydroxyl-terminated polydimethyl silicone oils within the above molecular weight range, and more preferably one or more of the following: 700 g / mol, 2000 g / mol, 3000 g / mol, 5000 g / mol, and 20000 g / mol. This invention does not impose any special restrictions on the source of the terminally dihydroxyl-terminated polydimethyl silicone oil; it can be a commercially available product or prepared using conventional methods in the art.

[0041] In this invention, reactant 1 is diphenylboronic anhydride and / or phenylboronic acid. The structure of diphenylboronic anhydride is shown in formula (1) below, and the structure of phenylboronic acid is shown in formula (2) below. This invention does not impose any special restrictions on the source of reactant 1; it can be a commercially available product. Using reactant 1 as a reaction raw material in this invention is beneficial for achieving the purpose of this invention. Specifically, the structure of low-viscosity polyborosiloxanes is mainly linear molecular chains. Multifunctional (greater than 2) boric acid compounds easily generate multi-component borate ester structures during the reaction, forming chemical crosslinking points centered on B, leading to increased viscosity. If multifunctional boric acid compounds are used to react low-reactivity polyborosiloxanes to achieve low viscosity, the boron content in the system will be too low, the B:O dynamic coordination bond density will be too low, and the energy consumption effect will be unsatisfactory.

[0042]

[0043] In this invention, the molar ratio of hydroxyl groups in reactant 1 to polydimethyl silicone oil with terminal dihydroxyl end-capped hydroxyl groups is preferably (0.5-1):1, specifically 0.5:1, 0.6:1, 0.7:1, 0.8:1, 0.9:1, 1:1, and more preferably 1:1.

[0044] In this invention, the reaction temperature is preferably 60–150°C, specifically 60°C, 70°C, 80°C, 90°C, 100°C, 110°C, 120°C, 130°C, 140°C, and 150°C, more preferably 120°C. The reaction time is preferably 48–300 hours, specifically 48 hours, 96 hours, 100 hours, 120 hours, 140 hours, 160 hours, 180 hours, 200 hours, 220 hours, 240 hours, 260 hours, 280 hours, and 300 hours. This invention controls the reaction under the above conditions. If the temperature is too low, the reaction efficiency is low; if the reaction temperature is too high, the boric acid compound will cause random breakage of the silicone oil, affecting the product structure; if the reaction time is too short, the polyborosiloxane reaction degree is low, the boron content is low, and the product effect is affected.

[0045] In this invention, dehydration is preferably performed during the reaction process. The reaction of terminally dihydroxyl-terminated polydimethyl silicone oil with the aforementioned specific reactant 1 produces water. The presence of water hinders the forward reaction, slowing down the reaction rate. Furthermore, the benzene-containing reactant 1 decomposes in the presence of high-temperature water to produce boric acid, leading to a side reaction (boric acid reacts with hydroxyl silicone oil to form a borate ter ester structure). The final product is difficult to remove as a byproduct. Therefore, this invention performs dehydration during the reaction process. In this invention, the dehydration method preferably includes using a water separator or a vacuum reaction, more preferably a vacuum reaction. The vacuum reaction refers to the reaction under vacuum conditions. Following the reaction in step a), polyborosiloxane is generated in the system.

[0046] Regarding step b) :

[0047] b) Remove unreacted reactant 1 from the polyborosiloxane to obtain a low-viscosity polyborosiloxane.

[0048] In this invention, step b) preferably includes: dissolving the polyborosiloxane in an organic solvent, then performing solid-liquid separation to remove unreacted reactant 1, and then removing the organic solvent from the resulting separated liquid to obtain a low-viscosity polyborosiloxane.

[0049] In this invention, the organic solvent is preferably an anhydrous solvent, more preferably at least one selected from anhydrous tetrahydrofuran, anhydrous chloroform, anhydrous acetone, and anhydrous n-hexane. The polyborosiloxane prepared by this invention does not contain ternary borate esters and is prone to hydrolysis. Purification with an anhydrous solvent can prevent the polyborosiloxane from being hydrolyzed by water in the solvent.

[0050] In this invention, the solid-liquid separation method is preferably filtration. In this invention, the pore size of the filter membrane used for filtration is preferably below 0.25 μm, so as to filter out as many unreacted diboronic acid compounds as possible from the crude reaction product.

[0051] In this invention, after the above solid-liquid separation, the organic solvent is removed from the obtained separated liquid. The preferred method for removing the organic solvent is rotary evaporation and drying. The drying is preferably vacuum drying. The preferred temperature for vacuum drying is 30–70°C. After the above treatment, purified low-viscosity polyborosiloxane is obtained.

[0052] The polyborosiloxane obtained by this invention has a high boron content and does not contain ternary borate ester structures. Its product specifications are as follows: boron content 500–2000 ppm, ternary borate ester unit content 0 wt%, diborate ester unit content 0 wt%–95 wt%, and monoborate ester unit content 5 wt%–95 wt%. The viscosity of the polyborosiloxane obtained by this invention is 200–10000 mPa·s.

[0053] The preparation method provided by this invention involves reacting terminally dihydroxyl-capped polydimethyl silicone oil with reactant 1, followed by purification and drying to obtain polyborosiloxane. The polyborosiloxane obtained by this invention has a high boron content and does not contain a ternary borate ester structure, exhibiting low viscosity and ease of blending with other systems to prepare functional materials. Furthermore, the high boron content and high B:O dynamic coordination bond density of the polyborosiloxane allow for relaxation time matching the impact rate at low viscosity, and a Weissenberg number less than 1, enabling more efficient energy dissipation.

[0054] Experimental results show that the viscosity of the polyborosiloxane provided by this invention is below 16830 mPa·s, and the relaxation time is 8.4 × 10⁻⁶. -5 Below s, the relaxation time can be matched with the impact rate at low viscosity, and the Weissenberg number is less than 1, thereby achieving the purpose of dissipating energy through the dissociation of B:O dynamic coordination bonds.

[0055] To further understand the present invention, preferred embodiments of the present invention are described below in conjunction with examples. However, it should be understood that these descriptions are only for further illustrating the features and advantages of the present invention, and not for limiting the scope of the claims of the present invention.

[0056] The raw materials used in the following examples were both self-made and commercially available. The dihydroxyl-terminated polydimethylsiloxanes were purchased from Sigma Aldrich, USA, with number average molecular weights of 700 g / mol, 2000 g / mol, 3000 g / mol, 5000 g / mol, and 20000 g / mol. Reactant 1 was also purchased from Sigma Aldrich, USA.

[0057] Example 1

[0058] Dihydroxyl-terminated polydimethylsiloxane (molecular weight 700 g / mol) and phenylboronic acid were blended at a hydroxyl molar ratio of 1:1 (phenylboronic acid:silicone oil) and reacted under vacuum at 120 °C for 200 h. The resulting reaction product was then rapidly dissolved in anhydrous n-hexane, unreacted phenylboronic acid was removed by filtration, the solvent was removed by rotary evaporation, and the product was dried under vacuum at 80 °C for 24 h to obtain the polyborosiloxane product.

[0059] Example 2

[0060] Dihydroxyl-terminated polydimethylsiloxane (molecular weight 700 g / mol) and diphenylboronic anhydride were blended at a hydroxyl molar ratio of 1:1 (diphenylboronic anhydride:silicone oil) and reacted under vacuum at 120 °C for 200 h. The resulting reaction product was then rapidly dissolved in anhydrous n-hexane, unreacted diphenylboronic anhydride was removed by filtration, the solvent was removed by rotary evaporation, and the product was dried under vacuum at 80 °C for 24 h to obtain the polyborosiloxane product.

[0061] Example 3

[0062] Dihydroxyl-terminated polydimethylsiloxane (molecular weight 5000 g / mol) and phenylboronic acid were blended at a hydroxyl molar ratio of 1:1 (phenylboronic acid:silicone oil) and reacted under vacuum at 120 °C for 260 h. The resulting reaction product was then rapidly dissolved in anhydrous n-hexane, unreacted phenylboronic acid was removed by filtration, the solvent was removed by rotary evaporation, and the product was dried under vacuum at 80 °C for 24 h to obtain the polyborosiloxane product.

[0063] Comparative Example 1

[0064] The procedure was carried out as described in Example 1, except that the raw material phenylboronic acid was replaced with an equimolar amount of boric acid, and the reaction was a conventional reaction (not a vacuum reaction) with a conventional reaction time. Details are as follows:

[0065] Dihydroxyl-terminated polydimethylsiloxane (molecular weight 700 g / mol) and boric acid were blended at a hydroxyl molar ratio (boric acid:silicone oil) of 1:1 and reacted at 120 °C for 24 h. The resulting reaction product was then rapidly dissolved in anhydrous n-hexane, unreacted boric acid was removed by filtration, the solvent was removed by rotary evaporation, and the product was then vacuum dried at 80 °C for 24 h to obtain the polyborosiloxane product.

[0066] Comparative Example 2

[0067] The procedure was carried out as described in Example 1, except that the reaction was a conventional reaction (not a vacuum reaction), and the reaction time was the conventional time. Details are as follows:

[0068] Dihydroxyl-terminated polydimethylsiloxane (molecular weight 700 g / mol) and phenylboronic acid were blended at a hydroxyl molar ratio (phenylboronic acid:silicone oil) of 1:1 and reacted at 120 °C for 24 h. The resulting reaction product was then rapidly dissolved in anhydrous n-hexane, unreacted phenylboronic acid was removed by filtration, the solvent was removed by rotary evaporation, and the product was then vacuum dried at 80 °C for 24 h to obtain the polyborosiloxane product.

[0069] Comparative Example 3

[0070] The procedure was carried out as described in Example 1, except that the raw material phenylboronic acid was replaced with an equimolar amount of boric acid, and the reaction conditions were changed. Specifically:

[0071] Dihydroxyl-terminated polydimethylsiloxane (molecular weight 700 g / mol) and boric acid were blended at a hydroxyl molar ratio (boric acid:silicone oil) of 1:1 and reacted at 20 °C for 3 h. The resulting reaction product was then rapidly dissolved in anhydrous n-hexane, unreacted boric acid was removed by filtration, the solvent was removed by rotary evaporation, and the product was vacuum dried at 80 °C for 24 h to obtain the polyborosiloxane product.

[0072] Product Testing :

[0073] The polyborosiloxane products obtained in each embodiment and comparative example were characterized in terms of structure and rheology.

[0074] The viscosity and relaxation time of polyborosiloxanes are highly dependent on the boron content and the content of chemically cross-linked structures. When chemically cross-linked structures are present in polyborosiloxanes, the chemical network increases the viscosity of the system, and the boron at the cross-linking points also forms dynamic B:O coordination bonds, increasing the relaxation time. The test results of polyborosiloxanes in the various examples and comparative examples are shown in Table 1.

[0075] Table 1: Structural and rheological property parameters of the products obtained in each embodiment and comparative example

[0076]

[0077] As can be seen from the test results in Table 1, the polyborosiloxanes obtained in Examples 1-3 of this invention have low viscosity and short relaxation time, which can match the impact rate (taking a 1.02m falling ball impact as an example, the impact velocity is 4.47m / s, the sample thickness is 6mm, and the impact strain rate is 745s). -1 The Weissenberg number is less than 1, which can effectively dissipate energy. Comparative Example 1, using boric acid as a raw material, prepared a polyborosiloxane with a high content of ternary borate ester, resulting in high system viscosity, long relaxation time, and a Weissenberg number far greater than 1, making it unsuitable as a filler for preparing impact-resistant functional materials. Comparative Example 2, using phenylboronic acid as a raw material, failed to remove the product water in time during the reaction, leading to side reactions and the final formation of ternary borate ester in the system, which increased the system viscosity. Comparative Example 3, using boric acid as a raw material, prepared a polyborosiloxane with a low degree of reaction at a low temperature and in a short time, resulting in low system viscosity and an even shorter relaxation time that could not be measured by instruments. However, the boron content was too low, and the B:O dynamic coordination bond density was low, making it ineffective for impact resistance.

[0078] The rheological properties of Example 1 are as follows: Figure 1 As shown. The testing process is as follows: A Peltier plate temperature control system was selected on the ARES-G2 rotational rheometer, and a parallel plate with a diameter of 50 mm was used for testing. Frequency sweep tests were performed at -80℃, -70℃, -60℃, -50℃, -40℃, -30℃, -20℃, -10℃, 0℃, 10℃, and 20℃, with a strain of 1% and a frequency range of 400 rad / s to 0.4 rad / s. After time-temperature superposition, the master curve at 20℃ was obtained. Figure 1 The test results show that the intersection of the storage modulus G' and the loss modulus G” is at 11904 rad / s. The reciprocal of the intersection frequency of G' and G” (1 / 11904 s) is the relaxation time of the sample, which is 8.4 × 10⁻⁶. -5At this point, the relaxation time of the polyborosiloxane is relatively short, which can match the impact rate (taking a 1.02m falling ball impact as an example, the impact velocity is 4.47m / s, the sample thickness is 6mm, and the impact strain rate is 745s). -1 The Weissenberg number is less than 1, which can effectively dissipate energy. The rheological formation of Comparative Example 1 is as follows: Figure 2 As shown, the intersection of G' and G” is at 0.159 rad / s. The reciprocal of this frequency (1 / 0.159 s) is the relaxation time of the sample, which is 6.3 s. At this time, the relaxation time of polyborosiloxane is long, and the Weissenberg number is much greater than 1, making it an unideal filler for preparing impact-resistant functional materials. The viscosities of Examples 1-3 and Comparative Examples 1-3 are as follows: Figure 3 As shown.

[0079] This document uses specific examples to illustrate the principles and implementation methods of the present invention. The descriptions of these embodiments are merely to aid in understanding the method and core ideas of the present invention, including the best mode, and to enable any person skilled in the art to practice the present invention, including manufacturing and using any device or system, and implementing any combined method. It should be noted that those skilled in the art can make various improvements and modifications to the present invention without departing from its principles, and these improvements and modifications also fall within the scope of protection of the claims. The scope of protection of this patent is defined by the claims and may include other embodiments that can be conceived by those skilled in the art. If these other embodiments have structural elements similar to those expressed in the claims, or if they include equivalent structural elements that are not substantially different from those expressed in the claims, then these other embodiments should also be included within the scope of the claims.

Claims

1. A method for preparing a low-viscosity polyborosiloxane, characterized in that, Includes the following steps: a) The polydimethyl silicone oil with terminal dihydroxyl end capping is reacted with reactant 1 under vacuum to remove water, and polyborosiloxane is obtained. The reaction temperature is 60~150℃ and the time is 48~300h; b) Remove unreacted reactant 1 from the polyborosiloxane to obtain a low-viscosity polyborosiloxane; Wherein, reactant 1 is diphenylboronic anhydride and / or phenylboronic acid; The viscosity of the low-viscosity polyborosiloxane is below 16830 mPa·s; The low-viscosity polyborosiloxane has a ternary borate ester unit content of 0 wt%, a diborate ester unit content of 0 wt% to 95 wt%, and a monoborate ester unit content of 5 wt% to 95 wt%.

2. The preparation method according to claim 1, characterized in that, In step a), the number-average molecular weight of the terminally dihydroxyl-capped polydimethyl silicone oil is 700~20000 g / mol.

3. The preparation method according to claim 1, characterized in that, In step a), the molar ratio of hydroxyl groups in reactant 1 to the polydimethyl silicone oil with terminal dihydroxyl end-capped hydroxyl groups is (0.5~1):

1.

4. The preparation method according to claim 1, characterized in that, Step b) specifically includes: The polyborosiloxane was dissolved in an organic solvent, and then unreacted reactant 1 was removed by solid-liquid separation. The organic solvent was then removed from the resulting separated liquid to obtain a low-viscosity polyborosiloxane.

5. The preparation method according to claim 4, characterized in that, The organic solvent is an anhydrous solvent; The anhydrous solvent is selected from at least one of anhydrous tetrahydrofuran, anhydrous chloroform, anhydrous acetone, and anhydrous n-hexane.

6. The preparation method according to claim 4, characterized in that, The solid-liquid separation method is filtration; The filtration membrane used in the filtration process has a pore size of less than 0.25 μm.

7. The preparation method according to claim 1, characterized in that, The low-viscosity polyborosiloxane obtained in step b) has the following specifications: boron content 500~2000ppm, ternary borate ester unit content 0wt%, diborary borate ester unit content 0wt%~95wt%, and monoborary borate ester unit content 5wt%~95wt%. The viscosity of the low-viscosity polyborosiloxane is 200~10000 mPa·s.

8. A low-viscosity polyborosiloxane prepared by any one of claims 1 to 7.

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