A pH-responsive star-shaped block copolymer heavy oil viscosity reducer and its preparation method and application

Abstract

The present invention provides a pH-responsive star-shaped block copolymer heavy oil viscosity reducer, its preparation method, and application. The pH-responsive star-shaped block copolymer heavy oil viscosity reducer is prepared by sequentially introducing a hydrophilic monomer, N-hydroxyethyl acrylamide (HEAm), and a pH-responsive monomer, dimethylaminoethyl methacrylate (DMAEMA), into a four-arm chain transfer agent via a reversible addition-fragmentation chain transfer (RAFT) polymerization method. The prepared pH-responsive star-shaped block copolymer heavy oil viscosity reducer can significantly reduce heavy oil viscosity in a deprotonated state and rapidly demulsify under acidic conditions.

Description

Technical Field

[0001] The invention relates to a pH-responsive star-shaped block copolymer heavy oil viscosity reducer and a preparation method and application thereof, belonging to the field of petrochemical industry. Background Art

[0002] With the continuous development of oil and gas resources, the efficient development and utilization of heavy oil fields has become increasingly important. At present, chemical viscosity reduction methods are widely used in the exploitation of heavy oil fields due to their low cost and good recovery effect.

[0003] Heavy oil is rich in natural active substances such as colloids and asphaltenes, which can easily form oil-in-water emulsions with higher viscosity. By adding suitable surfactant aqueous additives, the oil-in-water emulsion can be converted into a water-in-oil emulsion, which can greatly reduce the viscosity of the heavy oil and achieve the purpose of reducing the viscosity of the heavy oil. However, the heavy oil produced by emulsification and viscosity reduction is difficult to demulsify, the cost of emulsion processing is high, and the components of heavy oil vary greatly from place to place, and the viscosity reduction formula has poor versatility. Compared with ordinary emulsifiers, stimulus-responsive emulsifiers can switch between surface active and surface inactive forms according to changes in the environment. Under appropriate conditions, they can be used to adjust the stability of the emulsion, achieve emulsification of heavy oil during transportation, and demulsification after transportation. This can well make up for the shortcomings of traditional surfactants with a single function.

[0004] Star polymers are a unique macromolecular structure composed of three or more molecular chains radiating from a central core. They are the simplest branched polymers. Due to their unique structure, star polymers exhibit smaller dynamic mechanical dimensions in solution than linear polymers of the same molecular weight, resulting in lower solution and bulk viscosities, which are crucial for polymer processing. Furthermore, their compact structure allows for high density of functional groups within the polymer, giving them promising applications.

[0005] Therefore, how to apply star polymers to heavy oil viscosity reduction and prepare star polymer structured heavy oil viscosity reducers with stimulus responsiveness and excellent heavy oil viscosity reduction effect has important research value in the field of heavy oil viscosity reduction. Summary of the Invention

[0006] In response to the shortcomings of the prior art, the present invention provides a pH-responsive star-shaped block copolymer heavy oil viscosity reducer, as well as its preparation method and application. The pH-responsive star-shaped block copolymer heavy oil viscosity reducer of the present invention is prepared by sequentially introducing a hydrophilic monomer, N-hydroxyethyl acrylamide (HEAm), and a pH-responsive monomer, dimethylaminoethyl methacrylate (DMAEMA), into a four-arm chain transfer agent via a reversible addition-fragmentation chain transfer (RAFT) polymerization method. The prepared pH-responsive star-shaped block copolymer heavy oil viscosity reducer can significantly reduce the viscosity of heavy oil in a deprotonated state and can rapidly demulsify under acidic conditions.

[0007] The present invention is achieved through the following technical solutions:

[0008] A pH-responsive star-shaped block copolymer heavy oil viscosity reducer having a structure shown in the following formula I:

[0009]

[0010] Here, m is an integer from 190 to 400, and n is an integer from 40 to 400.

[0011] According to the present invention, preferably, the ratio of m to n is 1 to 5:1; more preferably, it is 2.5:1.

[0012] The preparation method of the pH-responsive star-shaped block copolymer heavy oil viscosity reducer comprises the following steps:

[0013] (1) In solvent A, 2-[[(butylthio)thiooxymethyl]thio]propionic acid and pentaerythritol are reacted in the presence of dimethylaminopyridine and N,N'-dicyclohexylcarbodiimide to obtain a four-arm chain transfer agent II;

[0014]

[0015] (2) In solvent B, N-hydroxyethyl acrylamide (HEAm) monomer and four-arm chain transfer agent II are polymerized under the initiation of a thermal initiator to obtain a four-arm poly-N-hydroxyethyl acrylamide macromolecular chain transfer agent III;

[0016]

[0017] Wherein, m is an integer from 190 to 400;

[0018] (3) In solvent C, dimethylaminoethyl methacrylate (DMAEMA) hydrochloride and four-arm poly (N-hydroxyethyl) acrylamide macromolecular chain transfer agent III were polymerized under the initiation of a thermal initiator to obtain a pH-responsive star-shaped block copolymer heavy oil viscosity reducer.

[0019] According to the preferred embodiment of the present invention, in step (1), the solvent A is anhydrous dichloromethane; the volume ratio of the molar number of 2-[[(butylthio)thiooxymethyl]thio]propionic acid to the solvent A is (5-10) mmol: (30-60) mL, preferably (8-9) mmol: (35-50) mL.

[0020] According to the preferred embodiment of the present invention, in step (1), the molar ratio of 2-[[(butylthio)thiooxymethyl]thio]propionic acid, pentaerythritol, dimethylaminopyridine and N,N'-dicyclohexylcarbodiimide is (5-20):(1-2):(0.2-0.5):(5-10), preferably (8-9):1.75:(0.3-0.4):(7-8).

[0021] According to a preferred embodiment of the present invention, in step (1), the reaction temperature of the 2-[[(butylthio)thiooxymethyl]thio]propionic acid and pentaerythritol is 20-30° C., the reaction time is 48-72 hours, and the reaction is carried out under nitrogen protection.

[0022] Preferably, according to the present invention, in step (1), N,N'-dicyclohexylcarbodiimide is added dropwise to a reaction system containing solvent A, dimethylaminopyridine, 2-[[(butylthio)thiooxymethyl]thio]propionic acid and pentaerythritol; the dropping temperature is 0 to 10°C, the dropping rate is 0.3 to 1.5 drops / min, and the dropping is carried out under nitrogen protection.

[0023] Preferably, N,N'-dicyclohexylcarbodiimide is added dropwise in the form of a dichloromethane solution of N,N'-dicyclohexylcarbodiimide to a reaction system containing solvent A, dimethylaminopyridine, 2-[[(butylthio)thiooxymethyl]thio]propionic acid and pentaerythritol; further preferably, in the dichloromethane solution of N,N'-dicyclohexylcarbodiimide, the concentration of N,N'-dicyclohexylcarbodiimide is (4-16):(15-30) mmol / mL, further preferably (7-8):15 mmol / mL.

[0024] According to the present invention, in step (1), the post-treatment method for the reaction solution obtained by the reaction of 2-[[(butylthio)thiooxymethyl]thio]propionic acid and pentaerythritol can be carried out according to the prior art; preferably, the post-treatment method comprises the steps of: filtering the reaction solution under reduced pressure to remove insoluble matter, rotary evaporating the filtrate to obtain a crude product, and then purifying the filtrate by column chromatography to obtain the four-arm chain transfer agent II. Preferably, the eluent used for column chromatography purification is a mixed solvent of ethyl acetate and petroleum ether, and the volume ratio of ethyl acetate to petroleum ether is (1-10):(10-50).

[0025] According to the present invention, preferably, in step (2), the thermal initiator is 2,2'-azabis(2-imidazoline) dihydrochloride (AIBI).

[0026] According to a preferred embodiment of the present invention, in step (2), solvent B is a mixed solvent of water and dioxane, and the volume ratio of water to dioxane in the mixed solvent is (1-1.5): (1-4).

[0027] According to the preferred embodiment of the present invention, in step (2), the molar ratio of N-hydroxyethyl acrylamide (HEAm) monomer, four-arm chain transfer agent II and thermal initiator is (800-1600):1:(0.1-0.5).

[0028] According to the preferred embodiment of the present invention, in step (2), the ratio of the molar number of N-hydroxyethyl acrylamide (HEAm) to the volume of solvent B is (4-10) mmol: (1-2) mL.

[0029] According to the preferred embodiment of the present invention, in step (2), the reaction is carried out under a N2 atmosphere; the reaction temperature is 50-80°C, and the reaction time is 1-8 hours. Preferably, the reaction temperature is 70°C, and the reaction time is 2-3 hours.

[0030] According to the present invention, in step (2), the post-treatment method for the reaction solution obtained by the reaction of N-hydroxyethyl acrylamide (HEAm) monomer with the four-arm chain transfer agent II can be carried out according to the prior art. Preferably, the post-treatment method comprises the steps of: adding the obtained reaction solution dropwise to tetrahydrofuran, resulting in a light yellow precipitate, and centrifuging to obtain a solid; dissolving the obtained solid in ethanol, and then adding the solution dropwise to tetrahydrofuran, resulting in a light yellow precipitate, and centrifuging to obtain a solid; repeating the above dissolution-precipitation steps 1-3 times; and vacuum drying the obtained solid to obtain the four-arm poly N-hydroxyethyl acrylamide macromolecular chain transfer agent III.

[0031] According to the preferred embodiment of the present invention, in step (3), the preparation method of dimethylaminoethyl methacrylate hydrochloride comprises the steps of: adding 10-12 mol·L dimethylaminoethyl methacrylate dropwise to dimethylaminoethyl methacrylate under stirring at 0-10°C; -1 The mixture is then subjected to rotary evaporation, recrystallization from tetrahydrofuran, filtration, washing, and drying to obtain dimethylaminoethyl methacrylate hydrochloride. Preferably, the ratio of the molar number of dimethylaminoethyl methacrylate to the volume of the hydrochloric acid solution is (20-50) mmol: (8-30) mL.

[0032] According to the present invention, preferably, in step (3), the thermal initiator is 2,2'-azabis(2-imidazoline) dihydrochloride (AIBI).

[0033] Preferably according to the present invention, in step (3), solvent C is water.

[0034] According to the preferred embodiment of the present invention, in step (3), the molar ratio of dimethylaminoethyl methacrylate hydrochloride, four-arm poly N-hydroxyethyl acrylamide macromolecular chain transfer agent III and thermal initiator is (200-1600):1:(0.4-0.7).

[0035] Preferably, according to the present invention, in step (3), the ratio of the molar number of dimethylaminoethyl methacrylate hydrochloride to the volume of solvent C is 0.5 to 5 mmol / mL.

[0036] According to the preferred embodiment of the present invention, in step (3), the reaction is carried out under a N2 atmosphere; the reaction temperature is 50-80°C, and the reaction time is 3-18 hours. Preferably, the reaction temperature is 70°C, and the reaction time is 4-6 hours.

[0037] According to the present invention, in step (3), the post-treatment method of the reaction solution obtained by the reaction of dimethylaminoethyl methacrylate hydrochloride and four-arm poly N-hydroxyethyl acrylamide macromolecular chain transfer agent III can be based on the existing technology. Preferably, the post-treatment method comprises the steps of: using 0.5 to 2 mol·L -1 The pH of the reaction solution is adjusted to 11 with an aqueous sodium hydroxide solution, the resulting reaction mixture is placed in a dialysis bag, and then dialyzed in water at room temperature for 2 to 4 days; the liquid in the dialysis bag is freeze-dried to obtain a pH-responsive star-shaped block copolymer heavy oil viscosity reducer; preferably, the cut-off molecular weight of the dialysis bag is 10,000 Da.

[0038] The pH-responsive star-shaped block copolymer heavy oil viscosity reducer is used as a heavy oil viscosity reducer to reduce the viscosity of heavy oil.

[0039] The preparation route of the pH-responsive star-shaped block copolymer heavy oil viscosity reducer of the present invention is as follows:

[0040]

[0041] Here, m is an integer from 190 to 400, and n is an integer from 40 to 400.

[0042] The technical features and beneficial effects of the present invention are as follows:

[0043] 1. This invention designed and synthesized a RAFT agent (II) containing four effective reaction sites, introducing N-hydroxyethyl acrylamide (HEAm) as the first block adjacent to the stellate core, and dimethylaminoethyl methacrylate (DMAEMA) containing a tertiary amine structure as the outer second block. Using the independently prepared multi-reaction site chain transfer agent as the stellate core, monomers were sequentially polymerized via aqueous reversible addition-fragmentation chain transfer (RAFT) polymerization, resulting in a pH-responsive stellate block copolymer thickening agent for heavy oil viscosity reduction, constructed in a core-first, arm-latter fashion.

[0044] 2. The pH-responsive star-shaped block copolymer heavy oil viscosity reducer of the present invention contains a poly (N-hydroxyethyl) acrylamide (PHEAm) segment and a poly (dimethylaminoethyl methacrylate) segment in its structure. Poly (N-hydroxyethyl) acrylamide (PHEAm) is an electrically neutral polymer with good thermal stability, and its properties are less affected by changes in pH. In addition, poly (N-hydroxyethyl) acrylamide (PHEAm) has been shown to have stronger hydrophilicity than common hydrophilic polymers (such as polyacrylamide); at the same time, the solution properties of the poly (N-hydroxyethyl) acrylamide segment are not affected by pH changes. Using it as a hydrophilic segment can make the polymer have a stronger affinity for the aqueous phase, which is conducive to forming an oil-in-water emulsion with heavy oil, significantly reducing the viscosity of the heavy oil and achieving the purpose of heavy oil viscosity reduction. The polydimethylaminoethyl methacrylate segment in the non-protonated state is hydrophobic, which is conducive to emulsifying heavy oil to form an oil-in-water emulsion and reduce the viscosity of heavy oil; its side chain contains a weak base group, which can accept protons under low pH conditions to form a hydrophilic quaternary ammonium salt polyelectrolyte, making the polymer hydrophilic, thereby giving the viscosity reducer pH-responsive demulsification ability; at the same time, the quaternary ammonium cation helps to destroy the double electrical layer at the emulsion interface, which is conducive to further oil-water separation. The core-shell structure of the star-shaped block makes the critical micelle concentration of the polymer aqueous solution lower, saving the amount of polymer product; at the same time, the functional group density in the star-shaped polymer is high, making the hydrophilic-hydrophobic transition of the polymer under pH changes easier to achieve and more sensitive. The star-shaped polymer viscosity reducer structure of the present invention is a whole, and the polydimethylaminoethyl methacrylate segment and the poly(N-hydroxyethyl acrylamide) segment are matched in an appropriate ratio. The various groups have complex interactions and work together to achieve the excellent effects of the present invention. If only poly (N-hydroxyethyl acrylamide) segments or poly (dimethylaminoethyl methacrylate) segments are introduced, the amphiphilic properties of the star polymer will be lost, making it difficult to achieve the purpose of emulsifying heavy oil and reducing the viscosity of heavy oil.

[0045] 3. The pH-responsive star-shaped block copolymer heavy oil viscosity reducer of the present invention is for the extra-heavy oil with a viscosity of 22430 mPa·s at 50°C. When the viscosity reducer of the present invention is used in an amount of 1000 mg·L -1 When the deprotonated polymer is added, the viscosity reduction rate can reach more than 99%, which has an excellent heavy oil viscosity reduction effect.

[0046] 4. The pH-responsive star-shaped block copolymer heavy oil viscosity reducer of the present invention has good pH responsiveness. In the deprotonated state, the polymer can form a stable oil-in-water emulsion of heavy oil in water, significantly reducing the viscosity of the heavy oil. When the pH of the emulsion is adjusted to acidic, the dehydration rate of the emulsion can reach more than 80% in 1 hour, and the emulsion is easy to break and the emulsion breaks quickly. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure 1 : is the nuclear magnetic spectrum of the four-arm chain transfer agent II in the embodiment; wherein the abscissa is the chemical shift.

[0048] Figure 2 Comparative NMR spectra of (a) four-arm poly (N-hydroxyethyl) acrylamide macromolecular chain transfer agent III and (b) star-shaped block copolymer I in Example 1; wherein the abscissa represents chemical shift. DETAILED DESCRIPTION

[0049] The present invention will be further described below with reference to specific embodiments, but is not limited thereto.

[0050] Meanwhile, the experimental methods described in the following examples, unless otherwise specified, are conventional methods; the reagents, materials and equipment, unless otherwise specified, can be obtained from commercial channels.

[0051] The four-arm chain transfer agent II used in the examples was prepared according to the following method:

[0052] 2-[[(Butylthio)thiooxymethyl]thio]propionic acid (2 g, 8.4 mmol), pentaerythritol (0.24 g, 1.75 mmol), and dimethylaminopyridine (0.085 mg, 0.35 mmol) were weighed into a 100 mL two-necked flask and dissolved in 35 mL of anhydrous dichloromethane. The mixture was placed in an ice-water bath (0°C) and, under nitrogen bubbling, 15 mL of an anhydrous dichloromethane solution containing N,N'-dicyclohexylcarbodiimide (1.585 g, 7.7 mmol) was added dropwise at a rate of 20 drops / min. After the addition was complete, the mixture was allowed to react at room temperature (25°C) for 48 h. After the reaction was complete, the solid residue was filtered off under reduced pressure, and the filtrate was rotary evaporated to remove the solvent. The mixture was then separated by column chromatography using petroleum ether / ethyl acetate (30 / 1, v / v) as the eluent to obtain four-arm chain transfer agent II as a yellow oil in a 51% yield. The obtained four-arm chain transfer agent II was dissolved in DMSO-d6 and 1 H NMR spectroscopy confirmed its successful synthesis. Figure 1 shown.

[0053] N-hydroxyethylacrylamide (HEAm) used in the examples was purchased from Shanghai Bid Pharmaceutical Technology Co., Ltd. and was purified by passing through a basic alumina column before use.

[0054] The dimethylaminoethyl methacrylate (DMAEMA) hydrochloride used in the examples was prepared according to the following method:

[0055] In a 0℃ ice-water bath, 15mL of 12mol·L-1 was added dropwise to 6g (38.17mmol) of dimethylaminoethyl methacrylate (available from Beijing Bailingwei Technology Co., Ltd.) under stirring. -1The mixture was added with a hydrochloric acid solution for 30 minutes. The excess solvent was then removed by rotary evaporation. The mixture was recrystallized in excess tetrahydrofuran, filtered, washed, and dried to obtain the target product, dimethylaminoethyl methacrylate hydrochloride, as a white powder with a yield of 85%.

[0056] Example 1

[0057] A method for preparing a pH-responsive star-shaped block copolymer heavy oil viscosity reducer comprises the following steps:

[0058] a. Preparation of Four-arm Poly (N-hydroxyethyl) Acrylamide Macromolecular Chain Transfer Agent (III)

[0059] Four-arm chain transfer agent II (3.6 mg, 0.0035 mmol) and N-hydroxyethyl acrylamide (326 mg, 2.83 mmol) were weighed into a 4 mL reaction vial and dissolved in 350 μL of dioxane and 150 μL of water. Three freeze-thaw cycles were performed under a nitrogen atmosphere to remove oxygen. The reaction vial was placed in a 70°C constant temperature stirring apparatus and polymerization was initiated by adding AIBI (0.3 mg, 0.0010 mmol). After 3 h, the reaction vial was immersed in liquid nitrogen and exposed to air to quench the reaction. After the reaction flask returned to room temperature, the reaction solution was added dropwise to an excess of tetrahydrofuran, resulting in the formation of a light yellow precipitate, which was then centrifuged to obtain a solid. The resulting solid was dissolved in a small amount of ethanol and then added dropwise to an excess of tetrahydrofuran, resulting in the formation of a light yellow precipitate, which was then centrifuged to obtain a solid. The above dissolution-precipitation steps were repeated twice to remove residual monomers. The resulting solid was vacuum dried at 40°C for 1 day to obtain a four-arm poly (N-hydroxyethyl) acrylamide macromolecular chain transfer agent III with a yield of 98.0%.

[0060] The polymer structure was calculated to be 4arm-PHEAm based on the NMR integration results. 198 (i.e., in the viscosity reducing agent structural formula, m is 198); the number average molecular weight of the obtained polymer was measured by aqueous size exclusion chromatography (SEC) method to be 40390Da (mobile phase: 0.5mol·L -1 NaCl aqueous solution, standard substance: polyethylene glycol).

[0061] b. Preparation of star-shaped block copolymers

[0062] The four-arm poly N-hydroxyethyl acrylamide macromolecular chain transfer agent III (330 mg, 0.0035 mmol) and dimethylaminoethyl methacrylate hydrochloride (136 mg, 0.71 mmol) prepared by the above method were weighed into a 4 mL reaction bottle and fully dissolved by adding 0.8 mL of water. Freeze and thaw three times under N2 atmosphere to remove oxygen, place the reaction bottle in a constant temperature stirring device at 70 ° C, and add AIBI (0.5 mg, 0.0017 mmol) to initiate polymerization. After 5 h, the reaction bottle was immersed in liquid nitrogen and exposed to air to quench the reaction. After the reaction bottle returned to room temperature, it was heated with 1 mol·L -1 The reaction mixture was adjusted to pH 11 with an aqueous sodium hydroxide solution to deprotonate the polymer. The resulting reaction mixture was placed in a dialysis bag (with a molecular weight cutoff of 10,000 Da) and dialyzed against water at room temperature for three days to remove unreacted monomers and excess sodium hydroxide. The dialysate in the dialysis bag was freeze-dried to obtain a pale yellow four-arm poly(N-hydroxyethylacrylamide-block-dimethylaminoethyl methacrylate) star-shaped block polymer (I) in a 94.0% yield.

[0063] The NMR comparison spectra of the four-arm poly (N-hydroxyethyl) acrylamide macromolecular chain transfer agent III and the star-shaped block copolymer I in this embodiment are shown in FIG. Figure 2 The star-shaped block copolymer retains the characteristic peak of poly(N-hydroxyethylacrylamide) and shows two -C H 3, indicating the successful copolymerization of the two monomers.

[0064] The polymer structure was calculated to be 4arm-PHEAm based on the NMR integration results. 198 -b-PDMAEMA 42 (i.e., in the viscosity reducing agent structure, m is 198 and n is 42); the number average molecular weight of the obtained polymer was measured by aqueous size exclusion chromatography (SEC) method and was 43418 Da (mobile phase: 0.5 mol·L -1 NaCl aqueous solution, standard substance: polyethylene glycol).

[0065] Example 2

[0066] A method for preparing a pH-responsive star-shaped block copolymer heavy oil viscosity reducer comprises the following steps:

[0067] a. Preparation of four-arm poly N-hydroxyethyl acrylamide macromolecular chain transfer agent (III) was the same as in step a of Example 1.

[0068] b. Preparation of star-shaped block copolymers

[0069] The four-arm poly N-hydroxyethyl acrylamide macromolecular chain transfer agent III (330 mg, 0.0035 mmol) and dimethylaminoethyl methacrylate hydrochloride (272 mg, 1.42 mmol) prepared by the above method were weighed into a 4 mL reaction bottle and fully dissolved by adding 1.0 mL of water. Freeze and thaw three times under N2 atmosphere to remove oxygen, place the reaction bottle in a constant temperature stirring device at 70 ° C, and add AIBI (0.5 mg, 0.0017 mmol) to initiate polymerization. After 5 h, the reaction bottle was immersed in liquid nitrogen and exposed to air to quench the reaction. After the reaction bottle returned to room temperature, it was heated with 1 mol·L -1 The reaction mixture was adjusted to pH 11 with an aqueous sodium hydroxide solution to deprotonate the polymer. The resulting reaction mixture was placed in a dialysis bag (with a molecular weight cutoff of 10,000 Da) and dialyzed against water at room temperature for three days to remove unreacted monomer and excess sodium hydroxide. The dialysate in the dialysis bag was freeze-dried to obtain a pale yellow four-arm poly(N-hydroxyethylacrylamide-block-dimethylaminoethyl methacrylate) star-shaped block polymer (compound of Formula I) in a 95.5% yield.

[0070] The polymer structure was calculated to be 4arm-PHEAm based on the NMR integration results. 198 -b-PDMAEMA 80 (i.e., in the viscosity reducing agent structure, m is 198 and n is 80); the number average molecular weight of the obtained polymer was measured by aqueous size exclusion chromatography (SEC) method and was 47264Da (mobile phase: 0.5mol·L -1 NaCl aqueous solution, standard substance: polyethylene glycol).

[0071] Example 3

[0072] A method for preparing a pH-responsive star-shaped block copolymer heavy oil viscosity reducer comprises the following steps:

[0073] a. Preparation of four-arm poly N-hydroxyethyl acrylamide macromolecular chain transfer agent (III) was the same as in step a of Example 1.

[0074] b. Preparation of star-shaped block copolymers

[0075] The four-arm poly N-hydroxyethyl acrylamide macromolecular chain transfer agent III (330 mg, 0.0035 mmol) and dimethylaminoethyl methacrylate hydrochloride (408 mg, 2.13 mmol) prepared by the above method were weighed into a 4 mL reaction bottle and fully dissolved by adding 1.2 mL of water. Freeze and thaw three times under N2 atmosphere to remove oxygen, place the reaction bottle in a constant temperature stirring device at 70 ° C, and add AIBI (0.5 mg, 0.0017 mmol) to initiate polymerization. After 5 h, the reaction bottle was immersed in liquid nitrogen and exposed to air to quench the reaction. After the reaction bottle returned to room temperature, it was heated with 1 mol·L -1 The reaction mixture was adjusted to pH 11 with an aqueous sodium hydroxide solution to deprotonate the polymer. The resulting reaction mixture was placed in a dialysis bag (with a molecular weight cutoff of 10,000 Da) and dialyzed against water at room temperature for three days to remove unreacted monomer and excess sodium hydroxide. The dialysate in the dialysis bag was freeze-dried to obtain a pale yellow four-arm poly(N-hydroxyethylacrylamide-block-dimethylaminoethyl methacrylate) star-shaped block polymer (compound of Formula I) in an 86.3% yield.

[0076] The polymer structure was calculated to be 4arm-PHEAm based on the NMR integration results. 198 -b-PDMAEMA 113 (i.e., in the viscosity reducing agent structure, m is 198 and n is 113); the number average molecular weight of the obtained polymer was measured by aqueous size exclusion chromatography (SEC) method and was 53301Da (mobile phase: 0.5mol·L -1 NaCl aqueous solution, standard substance: polyethylene glycol).

[0077] Example 4

[0078] A method for preparing a pH-responsive star-shaped block copolymer heavy oil viscosity reducer comprises the following steps:

[0079] a. Preparation of four-arm poly N-hydroxyethyl acrylamide macromolecular chain transfer agent (III) was the same as in step a of Example 1.

[0080] b. Preparation of star-shaped block copolymers

[0081] The four-arm poly N-hydroxyethyl acrylamide macromolecular chain transfer agent III (330 mg, 0.0035 mmol) and dimethylaminoethyl methacrylate hydrochloride (544 mg, 2.83 mmol) prepared by the above method were weighed into a 4 mL reaction bottle and fully dissolved by adding 1.5 mL of water. Freeze and thaw three times under N2 atmosphere to remove oxygen, place the reaction bottle in a constant temperature stirring device at 70 ° C, and add AIBI (0.5 mg, 0.0017 mmol) to initiate polymerization. After 5 h, the reaction bottle was immersed in liquid nitrogen and exposed to air to quench the reaction. After the reaction bottle returned to room temperature, it was heated with 1 mol·L -1 The reaction mixture was adjusted to pH 11 with an aqueous sodium hydroxide solution to deprotonate the polymer. The resulting reaction mixture was placed in a dialysis bag (with a molecular weight cutoff of 10,000 Da) and dialyzed against water at room temperature for three days to remove unreacted monomers and excess sodium hydroxide. The dialysate in the dialysis bag was freeze-dried to obtain a pale yellow four-arm poly(N-hydroxyethylacrylamide-block-dimethylaminoethyl methacrylate) star-shaped block polymer (compound of Formula I) in an 87.5% yield.

[0082] The polymer structure was calculated to be 4arm-PHEAm based on the NMR integration results. 198 -b-PDMAEMA 147 (i.e., in the viscosity reducing agent structure, m is 198 and n is 147); the number average molecular weight of the obtained polymer was measured by aqueous size exclusion chromatography (SEC) method and was 58384Da (mobile phase: 0.5mol·L -1 NaCl aqueous solution, standard substance: polyethylene glycol).

[0083] Example 5

[0084] A method for preparing a pH-responsive star-shaped block copolymer heavy oil viscosity reducer comprises the following steps:

[0085] a. Preparation of Four-arm Poly (N-hydroxyethyl) Acrylamide Macromolecular Chain Transfer Agent (III)

[0086] Four-arm chain transfer agent II (1.8 mg, 0.0018 mmol) and N-hydroxyethyl acrylamide (326 mg, 2.83 mmol) were weighed into a 4 mL reaction vial and dissolved in 350 μL of dioxane and 150 μL of water. Three freeze-thaw cycles were performed under a nitrogen atmosphere to remove oxygen. The reaction vial was placed in a 70°C constant temperature stirring apparatus and polymerization was initiated by adding AIBI (0.2 mg, 0.0007 mmol). After 3 h, the reaction vial was immersed in liquid nitrogen and exposed to air to quench the reaction. After the reaction flask returned to room temperature, the reaction solution was added dropwise to an excess of tetrahydrofuran, resulting in the formation of a light yellow precipitate, which was then centrifuged to obtain a solid. The resulting solid was dissolved in a small amount of ethanol and then added dropwise to an excess of tetrahydrofuran, resulting in the formation of a light yellow precipitate, which was then centrifuged to obtain a solid. The above dissolution-precipitation steps were repeated twice to remove residual monomers. The resulting solid was vacuum dried at 40°C for 1 day to obtain a four-arm poly (N-hydroxyethyl) acrylamide macromolecular chain transfer agent III with a yield of 95.1%.

[0087] The polymer structure was calculated to be 4arm-PHEAm based on the NMR integration results. 395 (i.e., in the viscosity reducing agent structure, m is 395); the number average molecular weight of the obtained polymer was measured by aqueous size exclusion chromatography (SEC) method to be 93571Da (mobile phase: 0.5mol·L -1 NaCl aqueous solution, standard substance: polyethylene glycol).

[0088] b. Preparation of star-shaped block copolymers

[0089] The four-arm poly N-hydroxyethyl acrylamide macromolecular chain transfer agent III (328 mg, 0.0018 mmol) and dimethylaminoethyl methacrylate hydrochloride (136 mg, 0.71 mmol) prepared by the above method were weighed into a 4 mL reaction bottle and fully dissolved by adding 0.8 mL of water. Freeze and thaw three times under N2 atmosphere to remove oxygen, place the reaction bottle in a constant temperature stirring device at 70 ° C, and add AIBI (0.35 mg, 0.0012 mmol) to initiate polymerization. After 5 h, the reaction bottle was immersed in liquid nitrogen and exposed to air to quench the reaction. After the reaction bottle returned to room temperature, it was heated with 1 mol·L -1 The reaction mixture was adjusted to pH 11 with an aqueous sodium hydroxide solution to deprotonate the polymer. The resulting reaction mixture was placed in a dialysis bag (with a molecular weight cutoff of 10,000 Da) and dialyzed against water at room temperature for three days to remove unreacted monomers and excess sodium hydroxide. The dialysate in the dialysis bag was freeze-dried to obtain a pale yellow four-arm poly(N-hydroxyethylacrylamide-block-dimethylaminoethyl methacrylate) star-shaped block polymer (Formula I) in a 94.9% yield.

[0090] The polymer structure was calculated to be 4arm-PHEAm based on the NMR integration results.395 -b-PDMAEMA 82 (i.e., in the viscosity reducing agent structure, m is 395 and n is 82); the number average molecular weight of the obtained polymer was measured by aqueous size exclusion chromatography (SEC) method and was 96505Da (mobile phase: 0.5mol·L -1 NaCl aqueous solution, standard substance: polyethylene glycol).

[0091] Example 6

[0092] A method for preparing a pH-responsive star-shaped block copolymer heavy oil viscosity reducer comprises the following steps:

[0093] a. Preparation of four-arm poly N-hydroxyethyl acrylamide macromolecular chain transfer agent (III) was the same as in Example 5 step a.

[0094] b. Preparation of star-shaped block copolymers

[0095] The four-arm poly N-hydroxyethyl acrylamide macromolecular chain transfer agent III (328 mg, 0.0018 mmol) and dimethylaminoethyl methacrylate hydrochloride (272 mg, 1.42 mmol) prepared by the above method were weighed into a 4 mL reaction bottle and fully dissolved by adding 1.0 mL of water. Freeze and thaw three times under N2 atmosphere to remove oxygen, place the reaction bottle in a constant temperature stirring device at 70 ° C, and add AIBI (0.35 mg, 0.0012 mmol) to initiate polymerization. After 5 h, the reaction bottle was immersed in liquid nitrogen and exposed to air to quench the reaction. After the reaction bottle returned to room temperature, it was heated with 1 mol·L -1 The reaction mixture was adjusted to pH 11 with aqueous sodium hydroxide solution to deprotonate the polymer. The resulting reaction mixture was placed in a dialysis bag (with a molecular weight cutoff of 10,000 Da) and dialyzed against water at room temperature for three days to remove unreacted monomers and excess sodium hydroxide. The dialysate in the dialysis bag was freeze-dried to obtain a pale yellow four-arm poly(N-hydroxyethylacrylamide-block-dimethylaminoethyl methacrylate) star-shaped block polymer (Formula I) in a 91.2% yield.

[0096] The polymer structure was calculated to be 4arm-PHEAm based on the NMR integration results. 395 -b-PDMAEMA 161 (i.e., in the viscosity reducing agent structure, m is 395 and n is 161); the number average molecular weight of the obtained polymer was measured by aqueous size exclusion chromatography (SEC) method and was 99274Da (mobile phase: 0.5mol·L -1 NaCl aqueous solution, standard substance: polyethylene glycol).

[0097] Example 7

[0098] A method for preparing a pH-responsive star-shaped block copolymer heavy oil viscosity reducer comprises the following steps:

[0099] a. Preparation of four-arm poly N-hydroxyethyl acrylamide macromolecular chain transfer agent (III) was the same as in Example 5 step a.

[0100] b. Preparation of star-shaped block copolymers

[0101] The four-arm poly N-hydroxyethyl acrylamide macromolecular chain transfer agent III (328 mg, 0.0018 mmol) and dimethylaminoethyl methacrylate hydrochloride (408 mg, 2.13 mmol) prepared by the above method were weighed into a 4 mL reaction bottle and fully dissolved by adding 1.2 mL of water. Freeze and thaw three times under N2 atmosphere to remove oxygen, place the reaction bottle in a constant temperature stirring device at 70 ° C, and add AIBI (0.35 mg, 0.0012 mmol) to initiate polymerization. After 5 h, the reaction bottle was immersed in liquid nitrogen and exposed to air to quench the reaction. After the reaction bottle returned to room temperature, it was heated with 1 mol·L -1 The reaction mixture was adjusted to pH 11 with an aqueous sodium hydroxide solution to deprotonate the polymer. The resulting reaction mixture was placed in a dialysis bag (with a molecular weight cutoff of 10,000 Da) and dialyzed against water at room temperature for three days to remove unreacted monomers and excess sodium hydroxide. The dialysate in the dialysis bag was freeze-dried to obtain a pale yellow four-arm poly(N-hydroxyethylacrylamide-block-dimethylaminoethyl methacrylate) star-shaped block polymer (compound of Formula I) in an 87.4% yield.

[0102] The polymer structure was calculated to be 4arm-PHEAm based on the NMR integration results. 395 -b-PDMAEMA 226 (i.e., in the viscosity reducing agent structure, m is 395 and n is 226); the number average molecular weight of the obtained polymer was measured by aqueous size exclusion chromatography (SEC) method and was 102617 Da (mobile phase: 0.5 mol·L -1 NaCl aqueous solution, standard substance: polyethylene glycol).

[0103] Example 8

[0104] A method for preparing a pH-responsive star-shaped block copolymer heavy oil viscosity reducer comprises the following steps:

[0105] a. Preparation of four-arm poly N-hydroxyethyl acrylamide macromolecular chain transfer agent (III) was the same as in Example 5 step a.

[0106] b. Preparation of star-shaped block copolymers

[0107] Weigh the four-arm poly N-hydroxyethyl acrylamide macromolecular chain transfer agent III (328 mg, 0.0018 mmol) and dimethylaminoethyl methacrylate hydrochloride (544 mg, 2.83 mmol) prepared by the above method into a 4 mL reaction bottle, and add 1.5 mL of water to fully dissolve. Freeze and thaw three times under N2 atmosphere to remove oxygen, place the reaction bottle in a constant temperature stirring device at 70 ° C, and add AIBI (0.35 mg, 0.0012 mmol) to initiate polymerization. After 5 h, immerse the reaction bottle in liquid nitrogen and expose it to air to quench the reaction. After the reaction bottle returns to room temperature, use 1 mol·L -1 The reaction mixture was adjusted to pH 11 with an aqueous sodium hydroxide solution to deprotonate the polymer. The resulting reaction mixture was placed in a dialysis bag (with a molecular weight cutoff of 10,000 Da) and dialyzed against water at room temperature for three days to remove unreacted monomers and excess sodium hydroxide. The dialysate in the dialysis bag was freeze-dried to obtain a pale yellow four-arm poly(N-hydroxyethylacrylamide-block-dimethylaminoethyl methacrylate) star-shaped block polymer (Formula I) in an 82.3% yield.

[0108] The polymer structure was calculated to be 4arm-PHEAm based on the NMR integration results. 395 -b-PDMAEMA 293 (i.e., in the viscosity reducing agent structure, m is 395 and n is 293); the number average molecular weight of the obtained polymer was measured by aqueous size exclusion chromatography (SEC) method and was 106162Da (mobile phase: 0.5mol·L -1 NaCl aqueous solution, standard substance: polyethylene glycol).

[0109] Comparative Example 1

[0110] A method for preparing a star-shaped homopolymer comprises the following steps:

[0111] Preparation of four-arm poly N-hydroxyethyl acrylamide macromolecular chain transfer agent (III): same as step a of Example 1.

[0112] Comparative Example 2

[0113] A method for preparing a linear block copolymer heavy oil viscosity reducer comprises the following steps:

[0114] a. Preparation of Linear Poly (N-Hydroxyethyl) Acrylamide Macromolecular Chain Transfer Agent

[0115] The linear chain transfer agent 2-[[(butylthio)thiooxymethyl]thio]propionic acid (0.84 mg, 0.0035 mmol) and N-hydroxyethyl acrylamide (326 mg, 2.83 mmol) were weighed into a 4 mL reaction vial. 20 μL of dioxane and 480 μL of water were added to dissolve the mixture. Three freeze-thaw cycles were performed under a nitrogen atmosphere to remove oxygen. The reaction vial was placed in a 70°C thermostatic stirring apparatus and polymerization was initiated by adding AIBI (0.1 mg, 0.0003 mmol). After 3 h, the reaction was quenched by immersing the reaction vial in liquid nitrogen and exposing it to air. After the reaction flask returned to room temperature, the reaction solution was added dropwise to an excess of tetrahydrofuran, resulting in a precipitate, which was then centrifuged to obtain a solid. The resulting solid was dissolved in a small amount of ethanol and then added dropwise to an excess of tetrahydrofuran, resulting in a precipitate, which was then centrifuged to obtain a solid. The above dissolution-precipitation steps were repeated twice to remove residual monomers. The resulting solid was vacuum dried at 40°C for 1 day to obtain a linear poly (N-hydroxyethyl acrylamide) macromolecular chain transfer agent with a yield of 98.7%.

[0116] The polymer structure was calculated to be PHEAm based on the NMR integration results. 803 (i.e., in the polymer structure, the number of N-hydroxyethyl acrylamide chain segments is 803); the number average molecular weight of the obtained polymer was measured by aqueous size exclusion chromatography (SEC) method to be 41913 Da (mobile phase: 0.5 mol·L -1 NaCl aqueous solution, standard substance: polyethylene glycol).

[0117] b. Preparation of linear block copolymers

[0118] The linear poly (N-hydroxyethyl acrylamide) macromolecular chain transfer agent (324 mg, 0.0035 mmol) and dimethylaminoethyl methacrylate hydrochloride (136 mg, 0.71 mmol) prepared by the above method were weighed into a 4 mL reaction bottle and fully dissolved by adding 0.8 mL of water. Freeze and thaw three times under N2 atmosphere to remove oxygen, place the reaction bottle in a constant temperature stirring device at 70 ° C, and add AIBI (0.15 mg, 0.0005 mmol) to initiate polymerization. After 5 h, the reaction bottle was immersed in liquid nitrogen and exposed to air to quench the reaction. After the reaction bottle returned to room temperature, it was heated with 1 mol·L -1 The reaction mixture was adjusted to pH 11 with an aqueous sodium hydroxide solution to deprotonate the polymer. The resulting reaction mixture was placed in a dialysis bag (with a molecular weight cutoff of 10,000 Da) and dialyzed against water at room temperature for three days to remove unreacted monomer and excess sodium hydroxide. The dialysate in the dialysis bag was freeze-dried to obtain a linear poly(N-hydroxyethylacrylamide-block-dimethylaminoethyl methacrylate) linear block polymer in a yield of 96.1%.

[0119] The polymer structure was calculated to be PHEAm based on the NMR integration results.803 -b-PDMAEMA 177 (i.e., in the polymer structure, the number of N-hydroxyethyl acrylamide chain segments is 803, and the number of dimethylaminoethyl methacrylate chain segments is 177). The number average molecular weight of the obtained polymer was measured by aqueous size exclusion chromatography (SEC) method and was 45128 Da (mobile phase: 0.5 mol·L - 1 NaCl aqueous solution, standard substance: polyethylene glycol).

[0120] Test example

[0121] Viscosity reduction test

[0122] The viscosity reducers prepared in the examples and comparative examples were prepared to a mass concentration of 1000 mg·L -1 The polymer aqueous solution was added to the heavy oil at a mass ratio of 7:3. The mixture was preheated at 50°C for 15 minutes and stirred to thoroughly mix the oil and water, yielding an oil-water mixture. The viscosity of the mixture at 50°C was measured using a rotational viscometer. The viscosity of the crude oil was 22,430 mPa·s. The test results are shown in Table 1.

[0123] Table 1 Apparent viscosity and viscosity reduction rate of polymer aqueous solution / heavy oil mixture at 150℃

[0124]

[0125] As can be seen, the star-shaped block copolymers prepared in the examples of the present invention can effectively reduce the viscosity of heavy oil and can be used as efficient heavy oil viscosity reducers. The heavy oil viscosity reducers obtained in the examples are all in a deprotonated form. In this case, the dimethylaminoethyl methacrylate (DMAEMA) block is in a hydrophobic state, and the star-shaped block copolymers can exhibit amphiphilic properties and emulsify heavy oil. In contrast, the star-shaped homopolymer of Comparative Example 1, which contains only poly (N-hydroxyethyl acrylamide) segments, cannot form an emulsion with heavy oil and has no viscosity reduction effect.

[0126] Further analysis of the data in the table reveals that the pH-responsive block copolymers in their deprotonated state exhibit the best viscosity reduction performance in Examples 2 and 6, namely, the star-shaped block copolymers in which the ratio of N-hydroxyethylacrylamide structural units to dimethylaminoethyl methacrylate structural units is approximately 2.5:1. Furthermore, the linear polymer of Comparative Example 2, which has the same monomer composition as the star-shaped polymer of Example 1, exhibits a lower viscosity reduction than the star-shaped polymer.

[0127] Dehydration rate test

[0128] The viscosity reducers prepared in the examples and comparative examples were prepared to a mass concentration of 1000 mg·L -1The polymer aqueous solution was prepared in parallel with two groups of oil-water mixtures according to the above method; 0.1 mol·L -1 The pH of one oil-water mixture was adjusted to 5.0 using aqueous HCl (i.e., acidic oil-water mixture). The other oil-water mixture served as a control (i.e., oil-water mixture). The prepared oil-water mixture was poured into a graduated centrifuge tube and maintained at 50°C. The amount of water removed from the mixture after standing for 1 hour in both the deprotonated and protonated polymer states was recorded.

[0129] Table 2 Dehydration of viscosity reducers in deprotonated and protonated states at 50°C

[0130]

[0131]

[0132] As can be seen from Table 2, the pH-responsive star-shaped block copolymer has a low dehydration rate and a low degree of oil-water separation in the deprotonated form. After 1 hour, a large number of oil-in-water droplets are still observed on the upper part of the centrifuge tube. However, when the emulsion is adjusted to acidic and the polymer is in a protonated state, the oil droplets on the upper part of the centrifuge tube quickly coalesce, the oil-water separation is fast, and the dehydration rate is high. This shows that the pH-responsive star-shaped block copolymer has pH-controlled demulsification properties.

[0133] Further analysis of the data in the table, combined with the examples, reveals that the pH-responsive star-shaped block copolymers of Examples 2 and 6 exhibit low dehydration rates and stable emulsions in their oil-water mixtures before pH adjustment. After pH adjustment, the dehydration rates change significantly, with the pH-responsive emulsions exhibiting optimal stability. This is achieved when the ratio of N-hydroxyethyl acrylamide structural units to dimethylaminoethyl methacrylate structural units is approximately 2.5:1. These star-shaped block copolymers can form highly stable emulsions in the deprotonated state, and when the emulsion is adjusted to an acidic state, the emulsion breaks rapidly. Furthermore, compared to the star-shaped polymer, the dehydration rate of the heavy oil emulsion formed by the linear polymer of Comparative Example 2 in the deprotonated state is higher than that of Example 1, indicating that, under the same monomer composition, the linear polymer forms an emulsion with lower stability.

[0134] The above embodiments are preferred implementations of the present invention, but the implementation of the present invention is not limited to the above embodiments. Any other substitutions, modifications, combinations, changes, simplifications, etc. that do not deviate from the spirit and principles of the present invention should be considered equivalent replacement methods and are included in the scope of protection of the present invention.

Claims

1. A pH-responsive star-shaped block copolymer heavy oil viscosity reducer, characterized in that: It has the structure shown in the following formula I: Here, m is an integer from 190 to 400, and n is an integer from 40 to 400.

2. The pH-responsive star-shaped block copolymer heavy oil viscosity reducer according to claim 1, characterized in that: The ratio of m to n is 1 to 5:

1.

3. A method for preparing the pH-responsive star-shaped block copolymer heavy oil viscosity reducer according to any one of claims 1 or 2, comprising the steps of: (1) In solvent A, 2-[[(butylthio)thiooxymethyl]thio]propionic acid and pentaerythritol are reacted in the presence of dimethylaminopyridine and N,N'-dicyclohexylcarbodiimide to obtain a four-arm chain transfer agent II; (2) In solvent B, N-hydroxyethyl acrylamide (HEAm) monomer and four-arm chain transfer agent II are polymerized under the initiation of a thermal initiator to obtain a four-arm poly-N-hydroxyethyl acrylamide macromolecular chain transfer agent III; in, m is an integer from 190 to 400; (3) In solvent C, dimethylaminoethyl methacrylate (DMAEMA) hydrochloride and four-arm poly (N-hydroxyethyl) acrylamide macromolecular chain transfer agent III were polymerized under the initiation of a thermal initiator to obtain a pH-responsive star-shaped block copolymer heavy oil viscosity reducer.

4. The method for preparing the pH-responsive star-shaped block copolymer heavy oil viscosity reducer according to claim 3, characterized in that: In step (1), one or more of the following conditions are included: i. The solvent A is anhydrous dichloromethane; the volume ratio of the molar number of 2-[[(butylthio)thiooxymethyl]thio]propionic acid to the solvent A is (5-10) mmol: (30-60) mL; ii. The molar ratio of the 2-[[(butylthio)thiooxymethyl]thio]propionic acid, pentaerythritol, dimethylaminopyridine and N,N'-dicyclohexylcarbodiimide is (5-20):(1-2):(0.2-0.5):(5-10); iii. the reaction temperature of the 2-[[(butylthio)thiooxymethyl]thio]propionic acid and pentaerythritol is 20-30° C.; the reaction time is 48-72 h; and the reaction is carried out under nitrogen protection; iv. A post-treatment method for a reaction solution obtained by the reaction of 2-[[(butylthio)thiooxymethyl]thio]propionic acid and pentaerythritol comprises the following steps: filtering the reaction solution under reduced pressure to remove insoluble matter, rotary evaporating the filtrate to obtain a crude product, and then purifying the crude product by column chromatography to obtain a four-arm chain transfer agent II; the eluent used for the column chromatography purification is a mixed solvent of ethyl acetate and petroleum ether, and the volume ratio of ethyl acetate to petroleum ether is (1-10):(10-50).

5. The method for preparing the pH-responsive star-shaped block copolymer heavy oil viscosity reducer according to claim 3, characterized in that: In step (1), N,N'-dicyclohexylcarbodiimide is added dropwise to a reaction system containing solvent A, dimethylaminopyridine, 2-[[(butylthio)thiooxymethyl]thio]propionic acid and pentaerythritol; the dropping temperature is 0 to 10°C, the dropping rate is 0.3 to 1.5 drops / min, and the dropping is carried out under nitrogen protection; N,N'-dicyclohexylcarbodiimide is added dropwise in the form of a dichloromethane solution of N,N'-dicyclohexylcarbodiimide to a reaction system containing solvent A, dimethylaminopyridine, 2-[[(butylthio)thiooxymethyl]thio]propionic acid and pentaerythritol; the concentration of N,N'-dicyclohexylcarbodiimide in the dichloromethane solution is (4-16):(15-30) mmol / mL.

6. The method for preparing the pH-responsive star-shaped block copolymer heavy oil viscosity reducer according to claim 3, characterized in that: In step (2), one or more of the following conditions are included: i. The thermal initiator is 2,2'-azabis(2-imidazoline) dihydrochloride (AIBI); ii. Solvent B is a mixed solvent of water and dioxane, wherein the volume ratio of water to dioxane in the mixed solvent is (1-1.5):(1-4); iii. the molar ratio of N-hydroxyethyl acrylamide (HEAm) monomer, four-arm chain transfer agent II and thermal initiator is (800-1600):1:(0.1-0.5); iv. The ratio of the molar number of N-hydroxyethyl acrylamide (HEAm) to the volume of solvent B is (4-10) mmol: (1-2) mL; v. The reaction is carried out under N2 atmosphere; the reaction temperature is 50-80°C, and the reaction time is 1-8h.

7. The method for preparing the pH-responsive star-shaped block copolymer heavy oil viscosity reducer according to claim 3, characterized in that: In step (2), the post-treatment method of the reaction solution obtained by the reaction of N-hydroxyethyl acrylamide (HEAm) monomer and four-arm chain transfer agent II comprises the following steps: adding the obtained reaction solution dropwise to tetrahydrofuran, a light yellow precipitate appears, and centrifuging to obtain a solid; dissolving the obtained solid in ethanol, and then adding the solution dropwise to tetrahydrofuran, a light yellow precipitate appears, and centrifuging to obtain a solid; repeating the above dissolution-precipitation steps 1-3 times; and vacuum drying the obtained solid to obtain a four-arm poly N-hydroxyethyl acrylamide macromolecular chain transfer agent III.

8. The method for preparing the pH-responsive star-shaped block copolymer heavy oil viscosity reducer according to claim 3, characterized in that: In step (3), the preparation method of dimethylaminoethyl methacrylate hydrochloride comprises the steps of: adding 10-12 mol·L dimethylaminoethyl methacrylate dropwise to dimethylaminoethyl methacrylate under stirring at 0-10°C. -1 The mixture was stirred for 2 hours and then subjected to rotary evaporation, recrystallization from tetrahydrofuran, filtration, washing and drying to obtain dimethylaminoethyl methacrylate hydrochloride; the ratio of the molar number of dimethylaminoethyl methacrylate to the volume of the hydrochloric acid aqueous solution was (20-50) mmol: (8-30) mL.

9. The method for preparing the pH-responsive star-shaped block copolymer heavy oil viscosity reducer according to claim 3, characterized in that: In step (3), one or more of the following conditions are included: i. The thermal initiator is 2,2'-azabis(2-imidazoline) dihydrochloride (AIBI); ii. Solvent C is water; iii. the molar ratio of dimethylaminoethyl methacrylate hydrochloride, four-arm poly N-hydroxyethyl acrylamide macromolecular chain transfer agent III and thermal initiator is (200-1600):1:(0.4-0.7); iv. a ratio of the moles of dimethylaminoethyl methacrylate hydrochloride to the volume of solvent C of 0.5 to 5 mmol / mL; v. The reaction is carried out under N2 atmosphere; the reaction temperature is 50-80°C and the reaction time is 3-18h; vi. A post-treatment method for the reaction solution obtained by reacting dimethylaminoethyl methacrylate hydrochloride with a four-arm poly N-hydroxyethyl acrylamide macromolecular chain transfer agent III comprises the following steps: -1 The pH of the reaction solution is adjusted to 11 with an aqueous sodium hydroxide solution, the resulting reaction mixture is placed in a dialysis bag, and then dialyzed in water at room temperature for 2 to 4 days; the liquid in the dialysis bag is freeze-dried to obtain a pH-responsive star-shaped block copolymer heavy oil viscosity reducer; the dialysis bag has a molecular weight cutoff of 10,000 Da.

10. Use of the pH-responsive star-shaped block copolymer as a heavy oil viscosity reducer according to any one of claims 1 or 2, as a heavy oil viscosity reducer for reducing the viscosity of heavy oil.

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