A low molecular weight polycarboxylic acid water reducing agent, its preparation method and application

Abstract

The present application relates to the technical field of concrete admixtures, and particularly relates to a low-molecular-weight polycarboxylic water-reducing agent, a preparation method and application thereof, wherein the adding amount of each raw material component of the low-molecular-weight polycarboxylic water-reducing agent is 30-45% of unsaturated polyether monomer, 3-5% of unsaturated organic acid, 1-2% of initiator, 1-3% of catalytic chain transfer agent, 1-2% of reducing agent, and the rest is water, wherein the unsaturated polyether monomer comprises one or both of vinyl alcohol polyether monomer and vinyl ester polyether monomer, and the catalytic chain transfer agent is a cobalt metal complex. The present application adds CoBF as the catalytic chain transfer agent in the raw material of the polycarboxylic water-reducing agent, and selects the unsaturated polyether monomer and the unsaturated organic acid with good copolymerization effect, so that the low-molecular-weight polycarboxylic water-reducing agent prepared therefrom has good adaptability and slump retention performance, and can ensure the construction performance of the concrete and the strength of the hardened concrete.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of concrete admixtures, and particularly relates to a low-molecular-weight polycarboxylate superplasticizer, a preparation method and application thereof. BACKGROUND

[0002] Since the polycarboxylate superplasticizer was invented, the polycarboxylate superplasticizer has become the most widely used concrete admixture. The polycarboxylate superplasticizer can effectively improve the fluidity, strength and durability of concrete, or reduce the cement consumption and increase the use proportion of industrial waste admixtures under the same performance, which is of great significance to realize energy saving and emission reduction and green environmental protection of the concrete industry. Due to the strong adjustability of the molecular structure of the polycarboxylate superplasticizer, the great potential of high performance, and the outstanding advantages that no formaldehyde is used in the production process, the polycarboxylate superplasticizer is the development direction of high-performance concrete admixtures and the research focus in the field of superplasticizers in the world.

[0003] From the molecular structure, the polycarboxylate is a comb-shaped polymer synthesized by free radical copolymerization of unsaturated carboxylic acid monomers and macromolecular polyether monomers. In the synthesis process, adjusting the types of raw material monomers, molecular mass, polyether side chain length and density, the amount of initiator and chain transfer agent of the polycarboxylate will directly affect its application performance. Among them, the molecular mass is the key factor affecting the performance of the polycarboxylate, and each type of polycarboxylate product has an optimal molecular mass to exert its maximum dispersion efficiency. In the existing research, the molecular mass of the polycarboxylate in the polycarboxylate superplasticizer is generally concentrated in 15000-60000. This is because when the molecular mass of the polycarboxylate is too large, the water-reducing performance of the polycarboxylate is not ideal, so there are few researches; and the case of too small molecular mass is rarely involved in the research, because it is difficult to obtain a polymer with an average molecular mass of less than 10000 by using ordinary free radical copolymerization. In ordinary free radical polymerization, the chain propagation reaction guided by free radicals is extremely fast, and a product with extremely high molecular mass will be obtained at a very low conversion rate. In order to slow down the rapid chain propagation reaction of free radicals, a method of adding mercaptan compounds (such as mercaptoethanol, mercaptopropyl alcohol, etc.) as chain transfer agents to the copolymerization reaction system is generally used to control the molecular mass of the free radical polymerization product. However, the chain transfer constant of mercaptan is generally 0.1-10, which is not enough to selectively transfer a large number of growing free radicals to the mercaptan group, so it is difficult to control the molecular mass of the polycarboxylate to be less than 10000 by using mercaptan as the chain transfer agent in the preparation of the polycarboxylate superplasticizer.

[0004] In the prior art, due to the difficulty in controlling the molecular weight of water-reducing agents to below 10,000, there is limited research on low molecular weight water-reducing agents, and even less research on low molecular weight water-reducing agents related to polycarboxylate superplasticizers. The existing research focuses on preparing low molecular weight modified polycarboxylate superplasticizers by introducing other groups to modify them, such as water-reducing agents with phosphate or phosphorous acid groups. Patent application number 201811651507.8 discloses a low molecular weight phosphate-based water-reducing agent and its preparation method. Phosphoric acid-based water-reducing agents are obtained by ring-opening epichlorohydrin through phosphorylation monomers, followed by further phosphorylation esterification and reaction with amino polyethers. These agents exhibit good compatibility and slump retention with cement. Low molecular weight non-polycarboxylic acid water-reducing agents, such as the low molecular weight sulfonated starch water-reducing agent and its preparation method disclosed in patent application number 202010000430.9, show good dispersibility and water reduction rate, but their compatibility and slump retention with cement are relatively weaker compared to polycarboxylic acid water-reducing agents. Both the low molecular weight modified polycarboxylic acid water-reducing agents and non-polycarboxylic acid water-reducing agents described above demonstrate certain advantages in improving concrete workability compared to their medium- and high molecular weight counterparts. Therefore, the preparation of high-performance low molecular weight unmodified polycarboxylic acid water-reducing agents has high research value and promising prospects. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art and provide a low molecular weight polycarboxylate superplasticizer and its preparation method. The prepared low molecular weight polycarboxylate superplasticizer is then applied to improve the workability of concrete, thereby overcoming the problem that it is difficult to prepare non-modified low molecular weight polycarboxylate superplasticizers with good slump retention in the prior art.

[0006] To achieve the above-mentioned objectives, the present invention is implemented through the following technical solution:

[0007] The first objective of this invention is to provide a method for preparing a low molecular weight polycarboxylate superplasticizer, comprising the following steps:

[0008] (1) Add unsaturated polyether monomer and catalytic chain transfer agent to water, mix well to obtain bottom solution;

[0009] (2) Add unsaturated organic acid and catalytic chain transfer agent to water, mix well, and obtain liquid A;

[0010] (3) Add the reducing agent to the water and mix well to obtain liquid B;

[0011] (4) Add the initiator to the base liquid, stir, and then add liquid A and liquid B to continue the reaction;

[0012] (5) After the reaction is complete, adjust the pH of the solution and cool it to room temperature to obtain a low molecular weight polycarboxylic acid water-reducing agent;

[0013] The unsaturated polyether monomer includes one or two of vinyl alcohol polyether monomers and vinyl ester polyether monomers, and the catalytic chain transfer agent is a cobalt metal complex.

[0014] Polycarboxylate superplasticizers possess excellent dispersing and slump-retaining properties. They are classified into ester-based and ether-based monomer superplasticizers. Ether-based monomer superplasticizers utilize raw materials including one or more of unsaturated alcohol polyethers, unsaturated ester polyethers, and unsaturated ether polyethers. The double bonds of these polyether molecules exhibit free radical copolymerization activity, allowing them to directly undergo free radical copolymerization with carboxylic acid compounds containing carbon-carbon double bonds. This produces comb-like polycarboxylate superplasticizer molecules with alternating carboxyl groups and polyether monomers. The carboxylic acid groups primarily adsorb onto the surface of cement particles, while the polyether side chain structure provides a retarding and slump-retaining effect through steric hindrance.

[0015] Depending on the polymerization method, the water-reducing and slump-retaining properties of polycarboxylate superplasticizers vary. The compatibility of ordinary medium-to-high molecular weight polycarboxylate superplasticizers obtained by conventional free radical polymerization with concrete raw materials has been a significant factor limiting their widespread use. Existing solutions aim to control the degree of polymerization of the main chain, the density of the side chains, and the types of functional groups to achieve compatibility between polycarboxylate superplasticizers and different types of concrete. However, this often involves introducing non-hydrophilic groups such as ester, amide, and anhydride groups to adjust the superplasticizer molecule. Since the degree of polymerization in free radical copolymerization is difficult to control at a low level, it is necessary to research improving the performance of polycarboxylate superplasticizers by adjusting their molecular weight.

[0016] This invention discovered in experiments that using a cobalt metal complex—cobalt II oxime boron fluoride complex (CoBF)—as a catalytic chain transfer agent in the synthesis of polycarboxylate superplasticizers, instead of conventional thiol compound chain transfer agents, and simultaneously selecting one or two of the unsaturated polyether monomers, such as vinyl alcohol polyether monomers and vinyl ester polyether monomers, allows for very low molecular weight of the free radical polymerization product even with extremely low CoBF dosage, while ensuring that the monomer conversion rate is not significantly affected. During the reaction, the addition of the catalytic chain transfer agent CoBF is divided into two parts: one part is added directly to the base liquid, and the other part is mixed in liquid A. Then, as liquid A is added, it is gradually added dropwise to the reaction system. This avoids the reaction becoming too violent if the catalytic chain transfer agent is added entirely to the reaction system, and also further ensures that the molecular weight distribution of the prepared low molecular weight polycarboxylate superplasticizer is more concentrated, resulting in better water-reducing agent performance.

[0017] This invention uses CoBF as a raw material for preparing low molecular weight polycarboxylate superplasticizers. The main principle is that, under the action of CoBF, hydrogen from the free radical chains is transferred to another olefin, forming a macromolecule with unsaturated terminal bonds. These terminal double bonds have sufficient activity to react with acrylic (ester) monomers through a fracture-addition mechanism or a free radical polymerization mechanism to obtain grafted or block polymers. The low molecular weight polycarboxylate superplasticizer prepared in this invention, due to its small molecular weight and linear structure, can insert into the lamellar structure of soil particles, thereby hindering the adsorption and consumption of the polycarboxylate superplasticizer by the strongly adsorbing soil. This ensures that the polycarboxylate superplasticizer maintains an effective adsorption capacity for cement particles, guaranteeing the workability of fresh concrete and the strength of hardened concrete.

[0018] Preferably, the catalytic chain transfer agent is a cobalt II oxime boron fluoride complex.

[0019] Preferably, the unsaturated polyether monomer in step (1) includes one or more of the following: polyethylene glycol monomethyl methacrylate, methyl allyl alcohol polyoxyethylene ether, isopentenyl alcohol polyethylene ether, and allyl alcohol polyoxyethylene ether, with a weight average molecular weight of 1000-2000.

[0020] Preferably, the unsaturated organic acid in step (2) includes one or both of methacrylic acid and acrylic acid.

[0021] In experiments, this invention found that when CoBF is used as a catalytic chain transfer agent, and the unsaturated polyether monomer is preferably one or more of methacrylate polyethylene glycol monomethyl ether, methyl allyl alcohol polyoxyethylene ether, isopentenyl alcohol polyethylene ether, and allyl alcohol polyoxyethylene ether with a weight average molecular weight of 1000-2000, and the unsaturated organic acid is preferably one or two of methacrylic acid and acrylic acid, a low molecular weight polycarboxylic acid water-reducing agent with terminal double bond functionalization can be obtained in a one-step reaction under relatively mild conditions. Furthermore, tests have shown that the obtained water-reducing agent has good adaptability and slump retention performance when applied to concrete.

[0022] Preferably, the reducing agent in step (3) is one or more of vitamin C, ferrous sulfate, oxalic acid, and potassium / sodium borohydride.

[0023] Preferably, the initiator in step (4) is one or two of azobisisobutyronitrile, 2,2'-azobisisobutyronitrile, and ammonium persulfate;

[0024] In step (4), liquid A and liquid B are added by dripping at a temperature of 30-35°C, and the temperature is maintained for 1-2 hours after the dripping is completed.

[0025] Preferably, in step (5), the pH of the solution is adjusted to 6-8.

[0026] Preferably, the addition amounts of each raw material component, by mass percentage, are: 30-45% unsaturated polyether monomer, 3-5% unsaturated organic acid, 1-2% initiator, 1-3% catalytic chain transfer agent, 1-2% reducing agent, and the balance being water.

[0027] A second objective of this invention is to provide a low molecular weight polycarboxylate superplasticizer, which is prepared by the above-described preparation method.

[0028] A third objective of this invention is to provide the application of the aforementioned low molecular weight polycarboxylate superplasticizer in concrete.

[0029] The present invention has the following beneficial effects:

[0030] (1) In this invention, CoBF is added to the raw materials of polycarboxylate superplasticizer as a catalytic chain transfer agent to replace the conventional thiol compound chain transfer agent. At the same time, unsaturated polyether monomers and unsaturated organic acids with good copolymerization effect under the condition of CoBF as catalytic chain transfer agent are screened. Through the above screening of raw materials in copolymerization reaction and the control of catalytic chain transfer agent in addition step, the reaction is carried out under milder conditions to obtain a low molecular weight polycarboxylate superplasticizer with good performance in the next step.

[0031] (2) The low molecular weight polycarboxylate superplasticizer prepared by the present invention has good adaptability and slump retention properties, which can ensure the construction performance of concrete and the strength of hardened concrete.

[0032] (3) The preparation method of the low molecular weight polycarboxylate superplasticizer in this invention is simple, the synthesis process is pollution-free, and the reaction conditions are mild, which helps to reduce economic costs and save energy. Detailed Implementation

[0033] The present invention will be further described below with reference to specific embodiments. Those skilled in the art will be able to implement the present invention based on these descriptions. Furthermore, the embodiments of the present invention described below are generally only some, not all, of the embodiments of the present invention. Therefore, all other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort should fall within the scope of protection of the present invention.

[0034] The reaction mechanism of CoBF as a catalytic chain transfer agent in this invention is as follows: First, the double bond at the end of the monomer is initiated by the initiator to form a monomer radical. The monomer radical undergoes chain growth, and then Co(II) abstracts hydrogen atoms from the polymer radical to form a Co(III)-H intermediate. The polymer radical, having lost hydrogen atoms, forms a dead polymer with a terminal double bond. This Co(III)-H intermediate then initiates the formation of a new radical from the terminal double bond of the monomer, thus completing a cyclic process. Therefore, guided by this mechanism, those skilled in the art will readily recognize that, in addition to CoBF, other cobalt metal complexes (II or III valence) can also play a catalytic chain transfer role in radical copolymerization.

[0035] Example 1

[0036] (1) 360.0g of polyethylene glycol monomethyl methacrylate with a molecular weight of 1000, 4.2g of catalyst chain transfer agent CoBF and 330.5g of water were respectively put into a four-necked flask and stirred at 30°C until completely dissolved to obtain the reaction base liquid;

[0037] (2) Mix 40.5g of methacrylic acid, 8.4g of catalytic chain transfer agent CoBF with 73g of water to obtain liquid A;

[0038] (3) Dissolve 11.7g of vitamin C and 4.0g of ferrous sulfate in 137.3g of water to obtain liquid B;

[0039] (4) Five minutes before the start of the reaction, add 10.8 g of initiator ammonium persulfate (APS) to the substrate, add liquids A and B dropwise at a constant rate, maintain a constant temperature of 32°C and stir, control the dropwise addition time to 2.5 hours, keep the temperature for 90 minutes after the dropwise addition is completed, adjust the pH of the solution to 6-8, and obtain the low molecular weight polycarboxylate superplasticizer LJ-01 with a molecular weight of 8250 (tested using Agilent 1260Ⅱ-GPC).

[0040] Example 2

[0041] (1) 325.0g of polyethylene glycol monomethyl methacrylate with a molecular weight of 1000, 9.0g of catalyst chain transfer agent CoBF and 330.5g of water were respectively put into a four-necked flask and stirred at 30°C until completely dissolved to obtain the reaction base liquid;

[0042] (2) Mix 49.3g of methacrylic acid, 18.0g of catalytic chain transfer agent CoBF with 73g of water until homogeneous to obtain liquid A;

[0043] (3) Dissolve 13.5g of vitamin C and 4.5g of ferrous sulfate in 137.0g of water to obtain liquid B;

[0044] (4) Five minutes before the start of the reaction, add 18.0g of initiator ammonium persulfate (APS) to the base material, add liquid A and B dropwise at a constant rate, keep the temperature at 30°C and stir, control the dropwise addition time to 2.5 hours, keep the temperature for 60 minutes after the dropwise addition is completed, adjust the pH of the solution to 6-8, and obtain the low molecular weight polycarboxylate superplasticizer LJ-02 with a molecular weight of 8130.

[0045] Example 3

[0046] (1) 365.0g of polyethylene glycol monomethyl methacrylate with a molecular weight of 1000, 6.0g of catalyst chain transfer agent CoBF and 330.5g of water were respectively put into a four-necked flask and stirred at 30°C until completely dissolved to obtain the reaction base liquid;

[0047] (2) Mix 39.4g of methacrylic acid, 12.0g of catalytic chain transfer agent CoBF with 73g of water until homogeneous to obtain liquid A;

[0048] (3) Dissolve 10.5g of vitamin C and 3.5g of ferrous sulfate in 147.0g of water to obtain liquid B;

[0049] (4) Five minutes before the start of the reaction, add 14.0 g of initiator ammonium persulfate (APS) to the substrate, add liquids A and B dropwise at a constant rate, keep the temperature at 35°C and stir, control the dropwise addition time to 2.5 hours, keep the temperature for 120 minutes after the dropwise addition is completed, adjust the pH of the solution to 6-8, and obtain the low molecular weight polycarboxylate superplasticizer LJ-03 with a molecular weight of 8320.

[0050] Example 4

[0051] (1) 435.0g of polyethylene glycol monomethyl methacrylate with a molecular weight of 1000, 3.3g of catalyst chain transfer agent CoBF and 330.5g of water were respectively put into a four-necked flask and stirred at 30°C until completely dissolved to obtain the reaction base liquid;

[0052] (2) Mix 29.5g of methacrylic acid, 6.5g of catalytic chain transfer agent CoBF with 73g of water to obtain liquid A;

[0053] (3) Dissolve 7.3g of vitamin C and 2.5g of ferrous sulfate in 147.0g of water to obtain liquid B;

[0054] (4) Five minutes before the start of the reaction, add 9.7g of initiator ammonium persulfate (APS) to the base material, add liquid A and B dropwise at a constant rate, keep the temperature at 32°C and stir, control the dropwise addition time to 2.5 hours, keep the temperature for 90 minutes after the dropwise addition is completed, adjust the pH of the solution to 6-8, and obtain the low molecular weight polycarboxylate superplasticizer LJ-04 with a molecular weight of 8450.

[0055] Comparative Example 1

[0056] (1) 360.0g of isopentenyl alcohol polyoxyethylene ether (TPEG) with a molecular weight of 2400, 4.2g of catalyst chain transfer agent CoBF and 330.5g of water were respectively put into a four-necked flask and stirred at 30°C until completely dissolved to obtain the reaction base liquid.

[0057] (2) Mix 40.5g of methacrylic acid, 8.4g of catalytic chain transfer agent CoBF with 73g of water to obtain liquid A;

[0058] (3) Dissolve 11.7g of vitamin C and 4.0g of ferrous sulfate in 137.3g of water to obtain liquid B;

[0059] (4) Five minutes before the start of the reaction, add 10.8g of initiator ammonium persulfate (APS) to the base material, add liquid A and B dropwise at a constant rate, keep the temperature at 32°C and stir, control the dropwise addition time to 2.5 hours, keep the temperature for 90 minutes after the dropwise addition ends, adjust the pH of the solution to 6-8, and obtain the low molecular weight polycarboxylate superplasticizer DB-01 with a molecular weight of 24500.

[0060] Comparative Example 2

[0061] (1) 360.0g of polyethylene glycol monomethyl methacrylate with a molecular weight of 1000, 12.6g of catalyst chain transfer agent CoBF and 330.5g of water were respectively put into a four-necked flask and stirred at 30°C until completely dissolved to obtain the reaction base liquid;

[0062] (2) Mix 40.5g of methacrylic acid with 73g of water until homogeneous to obtain liquid A;

[0063] (3) Dissolve 11.7g of vitamin C and 4.0g of ferrous sulfate in 137.3g of water to obtain liquid B;

[0064] (4) Five minutes before the start of the reaction, add 10.8g of initiator ammonium persulfate (APS) to the base material, add liquid A and B dropwise at a constant rate, keep the temperature at 32°C and stir, control the dropwise addition time to 2.5 hours, keep the temperature for 90 minutes after the dropwise addition is completed, adjust the pH of the solution to 6-8, and obtain the low molecular weight polycarboxylate superplasticizer DB-02 with a molecular weight of 9000.

[0065] Comparative Example 3

[0066] (1) 360.0g of polyethylene glycol monomethyl methacrylate with a molecular weight of 1000, 4.2g of chain transfer agent mercaptoethanol, and 330.5g of water were respectively placed into a four-necked flask and stirred at 30°C until completely dissolved to obtain the reaction base liquid.

[0067] (2) Mix 40.5g of methacrylic acid, 8.4g of chain transfer agent mercaptoethanol and 73g of water evenly to obtain liquid A;

[0068] (3) Dissolve 11.7g of vitamin C and 4.0g of ferrous sulfate in 137.3g of water to obtain liquid B;

[0069] (4) Five minutes before the start of the reaction, add 10.8g of initiator ammonium persulfate (APS) to the base material, add liquid A and B dropwise at a constant rate, keep the temperature at 32°C and stir, control the dropwise addition time to 2.5 hours, keep the temperature for 90 minutes after the dropwise addition is completed, adjust the pH of the solution to 6-8, and obtain the low molecular weight polycarboxylate superplasticizer DB-03 with a molecular weight of 14650.

[0070] Comparative Example 4

[0071] Commercially available water-reducing agent: Fuzhou Xulong—PC613, DB-04.

[0072] By comparing the preparation results of Examples 1-4 and Comparative Examples 1-3, it can be seen that when vinyl alcohol polyether monomers or vinyl ester polyether monomers are selected as unsaturated polyether monomers in the raw materials, and CoBF is selected as the chain transfer catalyst, and CoBF is added in steps, the molecular weight of the free radical polymerized polycarboxylate superplasticizer can be made very low. When the type of unsaturated polyether monomer or the chain transfer catalyst is changed, it is impossible to achieve the effect of preparing low molecular weight polycarboxylate superplasticizers below 10,000. Moreover, the effect of adding the chain transfer catalyst CoBF in steps is better than adding it all at once to the bottom liquid.

[0073] Performance Evaluation 1

[0074] The water-reducing agents LJ-01~LJ-04 and DB-01~DB-04 obtained in the experiment were used to test the flowability of cement paste. The flowability of cement paste was tested according to GB / T 8077-2012 "Test Method for Homogeneity of Concrete Admixtures". The initial flowability of cement paste with different water-reducing agents was measured at 0.5h, 1h and 1.5h. The water-cement ratio was 0.29 and the dosage of low molecular weight polycarboxylate water-reducing agent was 0.5%. The results of the cement paste flowability test are shown in Table 1.

[0075] Table 1. Results of Cement Paste Flowability Test

[0076]

[0077] As shown in Table 1, compared with the initial fluidity data of the paste, the dispersion performance of polycarboxylate superplasticizer decreases slightly when the molecular weight decreases. However, comparing the fluidity loss over time, it can be found that superplasticizers with high molecular weight show a rapid decreasing trend in the later stage, while the loss of superplasticizers with low molecular weight is relatively gradual. This is beneficial to the practical application of superplasticizers.

[0078] Performance Evaluation 2

[0079] The apparent viscosity of mortar slurries obtained from the experiments using water-reducing agents LJ-01~LJ-04 and DB-01~DB-04 was tested. A rheometer was used to measure the apparent viscosity of the slurry, keeping the water-cement ratio constant. The apparent viscosity (in Pa·s) was measured at a flowability of 300 mm. The results are shown in Table 2.

[0080] Table 2. Results of apparent viscosity test of slurry (flowability 300 mm).

[0081]

[0082] The results above show that the larger the molecular weight of polycarboxylate superplasticizer, the greater the apparent viscosity, indicating that low molecular weight polycarboxylate superplasticizer has the effect of reducing apparent viscosity and is a viscosity-reducing superplasticizer.

[0083] Performance Evaluation 3

[0084] Concrete performance testing was conducted according to GB / T 50080-2016 "Standard for Test Methods of Performance of Ordinary Concrete Mixtures". The admixture dosage was 0.18%, and the concrete mix proportion was: m(cement):m(sand):m(aggregate):m(water) = 360:748:1124:1750. The concrete performance test results are shown in the table below.

[0085] Table 3. Concrete performance test results

[0086]

[0087] The concrete performance test results show that low molecular weight polycarboxylate superplasticizers reduce the initial slump of concrete, but the slump loss is relatively small in a short period of time. This is consistent with the results of cement paste fluidity tests, indicating that reducing the molecular weight of polycarboxylate superplasticizers decreases its dispersion performance, but its slump retention performance is better than that of conventional polycarboxylate superplasticizers. Concrete strength test results show that reducing the molecular weight of the superplasticizer does not affect the strength.

Claims

1. A method for preparing a low molecular weight polycarboxylic acid water reducer, characterized by, The method comprises the following steps: (1) adding unsaturated polyether monomer and catalytic chain transfer agent into water, mixing uniformly to obtain a bottom solution; (2) adding unsaturated organic acid and catalytic chain transfer agent into water, mixing uniformly to obtain liquid A; (3) adding reducing agent into water, mixing uniformly to obtain liquid B; (4) adding initiator into the bottom solution, stirring, then adding liquid A and liquid B to continue the reaction; (5) after the reaction is completed, adjusting the pH of the solution, and cooling to room temperature to obtain low-molecular-weight polycarboxylic acid water reducer; The catalytic chain transfer agent is a cobalt metal complex. The unsaturated polyether monomer in step (1) is one or more of methacrylate polyethylene glycol monomethyl ether, methallyl alcohol polyoxyethylene ether, iso-pentenyl alcohol polyoxyethylene ether, and allyl alcohol polyoxyethylene ether with a weight average molecular weight of 1000-2000. In step (4), liquid A and liquid B are added dropwise, the dropwise adding temperature is 30-35℃, and the temperature is kept for 1-2 hours after the dropwise adding is completed.

2. The method for preparing a low molecular weight polycarboxylic acid water reducer according to claim 1, characterized by, The catalytic chain transfer agent is a cobalt II oxime boron fluoride complex.

3. The method for preparing a low molecular weight polycarboxylic acid water reducer according to claim 1 or 2, characterized by, The unsaturated organic acid in step (2) includes one or both of methacrylic acid and acrylic acid.

4. The method for preparing a low molecular weight polycarboxylate superplasticizer as described in claim 1, characterized in that, The reducing agent in step (3) is one or more of vitamin C, ferrous sulfate, oxalic acid, and potassium / sodium borohydride.

5. The method for preparing a low molecular weight polycarboxylic acid water reducer according to claim 1 or 4, characterized by, The initiator in step (4) is one or both of azobisimidozoline hydrochloride and 2,2'-azobis isobutyronitrile.

6. The method for preparing a low molecular weight polycarboxylate superplasticizer as described in claim 5, characterized in that, In step (5), the pH of the solution is adjusted to 6-8.

7. The method for preparing a low molecular weight polycarboxylate superplasticizer as described in claim 1, characterized in that, According to the mass percentage, the adding amount of each raw material component is: unsaturated polyether monomer 30-45%, unsaturated organic acid 3-5%, initiator 1-2%, catalytic chain transfer agent 1-3%, reducing agent 1-2%, and the balance is water.

8. A low molecular weight polycarboxylic acid water reducing agent, characterized by, Prepared by the preparation method in any one of claims 1-7.

9. Application of the low-molecular-weight polycarboxylic acid water reducer in claim 8 in concrete.

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