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Detailed Description In order that those skilled in the art will better understand the technical solutions of the present invention, the present invention will be further described in detail with reference to the following embodiments. The synthetic reaction path of the R- (+) -2- (4-hydroxyphenoxy) propionic acid provided by the invention is as follows: the above route comprises the following steps: the first step is as follows: mixing phenol and S-2-chloropropionic acid or salts thereof in a closed environment, adding water and a catalyst, and heating for alkylation reaction under the protection of inert gas to prepare 2-phenoxypropionic acid; the second step is that: dissolving 2-phenoxy propionic acid in water, adding a catalyst, adding a halogenating reagent, and reacting at 20-30 ℃ for 8h to prepare 2- (4-halogenophenoxy) propionic acid; the third step: putting the catalyst load into a copper chloride aqueous solution (mass fraction is 20-30%), heating to 50-100 ℃, reacting for 3-10 h under heat preservation, cooling and filtering. Drying the obtained solid, placing the dried solid in a muffle furnace, and activating for 2-3h at the temperature of 150-; the fourth step: adding 2- (4-halogenated phenoxy) propionic acid, 30% liquid alkali and a supported copper chloride catalyst into an autoclave, carrying out a micro-pressure reaction at the temperature of 150 ℃ and 160 ℃ in a sealed manner, and preparing the R-2- (4-hydroxyphenoxy) propionic acid by filtering and acidifying after the reaction is finished. Preferably, the first alkylation reaction specifically comprises: mixing phenol, S-2-chloropropionic acid or salt thereof and a catalyst according to an equivalent ratio of 1 (0.8-1.2) to (0.5-3.0), preferably 1 (0.9-1.0) to (1-1.5); the reaction time is about 2-10 hours, and more preferably 4-6 hours; the heat preservation temperature is 30-100 ℃, and more preferably 50-70 ℃; the inert gas is nitrogen; the first catalyst is selected from: DMF, triethylamine, pyridine, sodium hydroxide, potassium carbonate, sodium bicarbonate, more preferablySodium hydroxide is selected; the reaction is finished when the conversion rate of the chloropropionic acid is more than 99 percent, and the reaction is finished when the conversion rate of the chloropropionic acid is more than 99.5 percent; the method for the central control analysis of the S-2-chloropropionic acid comprises the following steps: high performance liquid HPLC, Agilent C18 column (5 μm, 250X 4.6 mm); mobile phase: 0.05% H3PO4Methanol (90: 10); detection wavelength: 210 nm; flow rate: 1.0 ml/min; column temperature: 40 ℃; sample introduction amount: 10 μ l. According to the invention, the second step reaction is specifically: 2-phenoxypropionic acid, a halogenating agent and a catalyst are mixed according to an equivalent ratio of 1 (1.0-1.5) to 0.005-0.05, and more preferably 1 (1.1-1.2) to 0.01-0.02; the reaction time is about 4-8 hours, preferably 5-6 hours; the heat preservation temperature is 0-50 ℃, and more preferably 20-30 ℃; the reaction is ended when the conversion rate of the 2-phenoxypropionic acid is more than 95 percent, and the reaction is ended when the conversion rate of the 2-phenoxypropionic acid is more than 98 percent; the 2-phenoxypropionic acid central control analysis method comprises the following steps: HPLC, Agilent C18 column (5 μm, 250X 4.6 mm); mobile phase: 0.05% H3PO4Methanol (90: 10); detection wavelength: 210 nm; flow rate: 1.0 ml/min; column temperature: 40 ℃; sample introduction amount: 10 μ l. The second catalyst is selected from: DMF, triethylamine, pyridine, sodium hydroxide, potassium carbonate, sodium bicarbonate, more preferably DMF; preferably, the halogenating agent is selected from: NBS, NCS, bromine, phosphorus trichloride, phosphorus pentachloride, phosphorus oxychloride, thionyl chloride, phosgene, diphosgene, triphosgene, more preferably NBS. According to the invention, the third step reaction is specifically as follows: mixing the copper chloride aqueous solution and the molecular sieve according to the mass ratio of 1 (5-10), and preferably 1 (7-8); the reaction time is about 3-10 hours, preferably 5-6 hours; the heat preservation temperature is 50-100 ℃, and more preferably 70-80 ℃; the third catalyst is a supported catalyst, and the supported matter is selected from a 3A (potassium A type) molecular sieve, a 4A (sodium A type) molecular sieve, a 5A (calcium A type) molecular sieve, a 10Z (calcium Z type) molecular sieve, a 13Z (sodium Z type) molecular sieve, a Y (sodium Y type) molecular sieve, a zeolite type molecular sieve, α -alumina, β -alumina, gamma-alumina, ion exchange resin, and more preferably gamma-alumina. The supported catalyst is selected from the group consisting essentially of: copper chloride, ferric chloride, magnesium chloride, cuprous chloride, ferrous chloride, cupric oxide, cuprous oxide, copper powder, and more preferably cupric chloride. According to the invention, the fourth reaction step is specifically: 2- (4-halogenated phenoxy) propionic acid, 30% liquid alkali and a supported copper chloride catalyst are mixed according to an equivalent ratio of 1 (2-10) to (0.01-0.2), and more preferably 1 (3-5) to (0.05-0.1); the heat preservation temperature is 100-200 ℃, and more preferably 150-160 ℃; the reaction pressure is controlled between 0.05 and 1bar, and more preferably between 0.1 and 0.2 bar; the reaction is ended when the conversion rate of the 2- (4-halogenated phenoxy) propionic acid is more than 98 percent, and the reaction is ended when the conversion rate of the 2- (4-halogenated phenoxy) propionic acid is more than 99.5 percent; the central control analysis method of the 2- (4-halogenated phenoxy) propionic acid comprises the following steps: high performance liquid HPLC, Agilent C18 column (5 μm, 250X 4.6 mm); mobile phase: 0.05% H3PO4Methanol (90: 10); detection wavelength: 210 nm; flow rate: 1.0 ml/min; column temperature: 40 ℃; sample introduction amount: 10 μ l. The following are specific examples, which illustrate specific technical solutions of the present invention in detail: the invention relates to a preparation method of R- (+) -2- (4-hydroxyphenoxy) propionic acid, which comprises the following steps: the first step is as follows: adding phenol (1.0eq), S-2-chloropropionic acid (0.9-1.0 eq), sodium hydroxide (1.0-1.5 eq) and water (5.0eq) into a reaction kettle, replacing with nitrogen for three times, heating to 50-70 ℃, keeping the temperature for reaction for 4-6 hours, and monitoring the conversion rate of the S-2-chloropropionic acid by HPLC (high performance liquid chromatography) to be more than 99.5 percent to finish the reaction. Cooling, adjusting pH to less than 1 with 31% hydrochloric acid, cooling to below 10 deg.C, stirring for 30 min for crystallization, filtering to obtain 2-phenoxypropionic acid crude product, and recrystallizing with water to obtain product with purity of not less than 99%. The second step is that: dissolving 2-phenoxypropionic acid (1.0eq) in water, adding a catalyst (0.01-0.02 eq), adding a halogenating agent (1.1-1.2 eq) at 20-30 ℃, reacting for 8 hours at 20-30 ℃ after the addition is finished, and monitoring the conversion rate of the 2-phenoxypropionic acid by HPLC to be more than 98 percent to finish the reaction. Cooling, adjusting pH to less than 1 with 31% hydrochloric acid, layering, cooling water phase to below 10 deg.C, stirring for 30 min, crystallizing, and vacuum filtering to obtain crude 2- (4-halogenated phenoxy) propionic acid product. The third step: adding gamma-alumina (7-8 eq) into copper chloride aqueous solution (20-25 wt%, 1.0eq), heating to 70-80 ℃ under stirring, and reacting for 5-6 hours under heat preservation. After the reaction is finished, cooling to room temperature, performing suction filtration, rinsing the filter cake with a small amount of water, airing, placing the filter cake in a muffle furnace, and activating at the temperature of 150-200 ℃ for 2-3h to obtain the gamma-alumina supported copper chloride catalyst. The fourth step: putting 1.0eq of 2- (4-halogenated phenoxy) propionic acid, 3-5 eq of 30% liquid alkali and 0.05-0.1 eq of molecular sieve-loaded copper chloride catalyst into a pressure kettle, replacing with nitrogen for three times, and sealing the equipment. Heating to 150-160 ℃ under stirring, controlling the reaction pressure at 0.1-0.2 bar, and finishing the reaction when the conversion rate of the 2- (4-halogenated phenoxy) propionic acid is monitored by HPLC to be more than 99.5%. Cooling, replacing with nitrogen for three times, transferring the reaction solution to an acidification kettle, adjusting the pH to be less than 1 by using 31% hydrochloric acid, cooling to be below 10 ℃, stirring for 30 minutes for crystallization, performing suction filtration to obtain a crude product of R- (+) -2- (4-hydroxyphenoxy) propionic acid, and recrystallizing by using water, wherein the purity is more than or equal to 99.5%. The above reaction equation is: main reaction: side reaction: treatment of impurities a and B in side reactions: the impurity A and the impurity B are basically insoluble in water, when the pH is less than 1, the reaction solution is kept stand to separate an oil layer, and the impurities can be separated. The invention will now be further illustrated with reference to the following examples. Example 4 Synthesis of R- (+) -2- (4-hydroxyphenoxy) propionic acid 122.5g (0.5mol, 1.0eq) of 2- (4-bromophenoxy) propionic acid, 67g (4.0eq) of 30% liquid alkali, 6.2g (0.05eq) of a gamma-alumina supported copper chloride catalyst, and 200g of water were put into a pressure vessel, replaced with nitrogen three times, and the apparatus was sealed. Heating to 150-160 ℃ under stirring, controlling the reaction pressure at 0.1-0.2 bar, and finishing the reaction when the content of 2- (4-halogenated phenoxy) propionic acid is monitored by HPLC (high performance liquid chromatography) to be less than or equal to 0.5%. Cooling, replacing with nitrogen for three times, transferring the reaction solution to an acidification kettle, adjusting the pH to be less than 1 by using 31% hydrochloric acid, cooling to be below 10 ℃, stirring for 30 minutes for crystallization, and performing suction filtration and recrystallization by using water to obtain 89.2g of R- (+) -2- (4-hydroxyphenoxy) propionic acid, wherein the yield is 98%, and the purity is more than or equal to 99.5%. Comparative example 1 Preparation of 3-phenoxypropionic acid (refer to the preparation process of 2-phenoxypropionic acid in example 1), the specific data are shown in table 1: table 1: analytical data for preparing 2-phenoxypropionic acid at different feed ratios Comparative example 2 Preparation of 2- (4-bromophenoxy) propionic acid (refer to preparation of 2- (4-bromophenoxy) propionic acid in example 1), the specific data are shown in table 2: table 2: analytical data for preparing 2- (4-bromophenoxy) propionic acid at different feed ratios Comparative example 3 Preparation of R- (+) -2- (4-hydroxyphenoxy) propionic acid by catalysis of molecular sieve loaded copper chloride catalyst 122.5g (0.5mol, 1.0eq) of 2- (4-bromophenoxy) propionic acid, 67g (4.0eq) of 30% liquid alkali, 6.2g (0.05eq) of 4A molecular sieve-supported copper chloride catalyst and 200g of water are put into a pressure kettle, nitrogen is replaced for three times, and the equipment is sealed. Heating to 150-160 ℃ under stirring, controlling the reaction pressure at 0.1-0.2 bar, and finishing the reaction when the content of 2- (4-halogenated phenoxy) propionic acid is monitored by HPLC (high performance liquid chromatography) to be less than or equal to 0.5%. Cooling, replacing with nitrogen for three times, transferring the reaction solution to an acidification kettle, adjusting the pH to be less than 1 by using 31% hydrochloric acid, cooling to be below 10 ℃, stirring for 30 minutes for crystallization, and performing suction filtration and recrystallization by using water to obtain 81.9g of R- (+) -2- (4-hydroxyphenoxy) propionic acid, wherein the yield is 90%, and the purity is more than or equal to 97.5%. Comparative example 4 Preparation of R- (+) -2- (4-hydroxyphenoxy) propionic acid by catalysis of gamma-alumina supported copper oxide catalyst 122.5g (0.5mol, 1.0eq) of 2- (4-bromophenoxy) propionic acid, 67g (4.0eq) of 30% liquid alkali, 6.2g (0.05eq) of a gamma-alumina-supported copper oxide catalyst, and 200g of water were put into a pressure vessel, replaced with nitrogen three times, and the apparatus was sealed. Heating to 150-160 ℃ under stirring, controlling the reaction pressure at 0.1-0.2 bar, and finishing the reaction when the content of 2- (4-halogenated phenoxy) propionic acid is monitored by HPLC (high performance liquid chromatography) to be less than or equal to 0.5%. Cooling, replacing with nitrogen for three times, transferring the reaction solution to an acidification kettle, adjusting the pH to be less than 1 by using 31% hydrochloric acid, cooling to be below 10 ℃, stirring for 30 minutes for crystallization, and performing suction filtration and recrystallization by using water to obtain 68.3g of R- (+) -2- (4-hydroxyphenoxy) propionic acid, wherein the yield is 75%, and the purity is more than or equal to 92.3%. The above is only a preferred embodiment of the present invention, and it should be noted that the above preferred embodiment should not be considered as limiting the present invention, and the protection scope of the present invention should be subject to the scope defined by the claims. It will be apparent to those skilled in the art that various modifications and adaptations can be made without departing from the spirit and scope of the invention, and these modifications and adaptations should be considered within the scope of the invention.