Metal complex, catalytic system, application of metal complex, polypropylene carbonate and preparation method of polypropylene carbonate

Abstract

The invention belongs to the field of polymer chemistry, and relates to a metal complex, a catalytic system and application thereof, and polypropylene carbonate and a preparation method thereof. The structure of the metal complex is as shown in formula (I): # imgabs0 #, wherein M is a trivalent metal ion; y is a monovalent acid radical ion; r1 is a substituted or unsubstituted cycloalkylene group or arylene group of C5-C12; r2 is H or alkyl of C1-C6; and R3 is an alkylene group of C1 to C10. Compared with the prior art, the method has the following advantages that the Salen metal complex with a novel phenothiazine structure is adopted as a main catalyst to efficiently catalyze copolymerization of carbon dioxide and epoxide to prepare the polypropylene carbonate, and the obtained polypropylene carbonate has the characteristics of high molecular weight and narrow molecular weight distribution.

Description

Technical Field

[0001] The invention belongs to the field of polymer chemistry, and in particular relates to a metal complex, a catalyst system, and applications thereof, a method for preparing polypropylene carbonate, and polypropylene carbonate prepared by the preparation method. Background Art

[0002] Polypropylene carbonate (PPC) is a completely degradable and environmentally friendly plastic synthesized from carbon dioxide and propylene oxide. It is completely biodegradable and will not pollute the environment. Due to its unique properties, PPC can be used in elastomers, coatings, synthetic leather, safety glass, surface coatings (resins, leather, etc.), soles, photoresists, adhesives, magnetic adhesives, conveyor belts, etc. High molecular weight PPC has a very broad application prospect in the field of environmentally friendly materials such as medical packaging and food packaging. The carbon dioxide content is 31% to 50%, which is the most in-depth and most promising carbon dioxide copolyester. The full utilization of carbon dioxide not only greatly reduces the consumption of upstream raw materials-petroleum, but also plays a positive role in alleviating the most serious problem of environmental pollution-the "greenhouse effect" caused by the emission of carbon dioxide.

[0003] At present, heterogeneous catalysts have been used in the industrial production of polycarbonate, such as rare earth ternary and zinc glutarate catalytic systems, but they have problems such as low catalytic activity and poor selectivity. After continuous development and improvement, the catalytic activity and selectivity of homogeneous catalysts have been significantly improved. For example, Salen metal complexes are the most concerned catalytic system in the field of copolymerization of carbon dioxide and epoxides. They have the advantages of high catalytic activity and high selectivity and have become a new research hotspot in recent years.

[0004] With the promotion of PPC, it is of great significance to develop new catalysts with high catalytic activity for the synthesis of polycarbonate lactone (PPC). Summary of the invention

[0005] In order to solve the above problems existing in the prior art, the present invention provides a metal complex, which is used as a main catalyst to solve the problems of low selectivity and small polymerization molecular weight existing in the prior art. When used for catalyzing the copolymerization of carbon dioxide and epoxide to prepare polypropylene carbonate, it has the characteristics of high catalytic activity, and the obtained polypropylene carbonate has the characteristics of high molecular weight and narrow molecular weight distribution.

[0006] In order to achieve the purpose of the present invention, the first aspect of the present invention provides a metal complex, the structure of the metal complex is shown in formula (I):

[0007]

[0008] Wherein: M is a trivalent metal ion; Y is a monovalent acid ion; R1 is a substituted or unsubstituted C5-C 12 R2 is H or a C1-C6 alkyl group; R3 is a C1-C 10 of alkylene.

[0009] The second aspect of the present invention provides a catalyst system, which comprises the metal complex as a main catalyst.

[0010] The third aspect of the present invention provides use of the metal complex or the catalyst system in the preparation of polypropylene carbonate.

[0011] A fourth aspect of the present invention provides a method for preparing polypropylene carbonate, comprising the following steps:

[0012] Under polymerization reaction conditions, epoxide, carbon dioxide and the catalyst system are contacted and reacted to obtain the polypropylene carbonate.

[0013] A fifth aspect of the present invention provides polypropylene carbonate, which is prepared by the above-mentioned preparation method.

[0014] Compared with the prior art, the present invention has the following advantages:

[0015] The invention adopts a novel Salen metal complex with a phenothiazine structure as a main catalyst to efficiently catalyze the copolymerization of carbon dioxide and epoxide to prepare polypropylene carbonate. The obtained polypropylene carbonate has the characteristics of high molecular weight and narrow molecular weight distribution.

[0016] Other features and advantages of the present invention will be described in detail in the following detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Exemplary embodiments of the present invention will be described in more detail with reference to the accompanying drawings.

[0018] Figure 1 The hydrogen nuclear magnetic spectrum of the catalyst prepared in Example 1 of the present invention is shown. DETAILED DESCRIPTION

[0019] The specific embodiments of the present invention are described in detail below. It should be understood that the specific embodiments described herein are only used to illustrate and explain the present invention, and are not used to limit the present invention.

[0020] In order to achieve the purpose of the present invention, the first aspect of the present invention provides a metal complex, the structure of the metal complex is shown in formula (I):

[0021]

[0022] Wherein: M is a trivalent metal ion; Y is a monovalent acid ion; R1 is a substituted or unsubstituted C5-C 12 R2 is H or a C1-C6 alkyl group; R3 is a C1-C 10 of alkylene.

[0023] According to the present invention, preferably, M is Co 3+ , Fe 3+ 、Al 3+ 、Ni 3+ , Mn 3+ Cr 3+ And Ru 3+ At least one of .

[0024] According to the present invention, preferably, Y is at least one of NO3-, Cl- and Br-.

[0025] According to the present invention, preferably, R1 is a substituted or unsubstituted cyclohexylene, phenylene or naphthylene; the substituted group is a halogen, a cyano group, a C1-C4 alkoxy group or a halogen-substituted C1-C4 alkyl group.

[0026] Further, preferably, R1 is selected from the following groups:

[0027]

[0028] ---: Indicates the connection position.

[0029] According to the present invention, preferably, R2 is H or a C1-C6 alkyl group, and the C1-C6 alkyl group includes but is not limited to methyl, ethyl, n-propyl (n-Pr, -CH2CH2CH3), isopropyl (i-Pr, -CH(CH3)2), n-butyl (n-Bu, -CH2CH2CH2CH3), isobutyl (i-Bu, -CH2CH(CH3)2), sec-butyl (s-Bu, -CH(CH3)CH2CH3), tert-butyl (t-Bu, -C(CH3)3), n-pentyl (-CH2CH2CH2CH2CH3), 2-pentyl (-CH(CH3)CH2CH2CH3), 3-pentyl (- At least one of 1-butyl (-CH(CH2CH3)2), 2-methyl-2-butyl (-C(CH3)2CH2CH3), 3-methyl-2-butyl (-CH-(CH3)CH(CH3)2), 3-methyl-1-butyl (-CH2CH2CH(CH3)2), 2-methyl-1-butyl (-CH2CH(CH3)CH2CH3), n-hexyl (-CH2CH2CH2CH2CH2CH3), 2-hexyl (-CH(CH3)CH2CH2CH2CH3) and 3-hexyl (-CH(CH2CH3)(CH2CH2CH3)); further, preferably, R2 is H or t-Bu.

[0030] According to the present invention, preferably, R3 is a C1-C6 alkylene group.

[0031] According to the present invention, preferably, the method for preparing the metal complex comprises the following steps:

[0032] a) mixing salicylaldehyde and / or a derivative of salicylaldehyde, polyformaldehyde, hydrobromic acid and a small amount of concentrated sulfuric acid, heating to 30-100° C., reacting sufficiently, and then separating and purifying to obtain a first compound; the molar ratio of salicylaldehyde and / or a derivative of salicylaldehyde, polyformaldehyde and hydrobromic acid is 1:(1-2):(7-9);

[0033] b) dissolving the first compound obtained in step a), sodium hydride and phenothiazine in a first organic solvent, heating to 0-80° C., reacting sufficiently, separating and purifying to obtain a second compound; the molar ratio of the first compound, sodium hydride and phenothiazine is 1:(5-8):(1-1.5);

[0034] c) dissolving the second compound, salicylaldehyde or a derivative of salicylaldehyde and a diamine compound obtained in step b) in a second organic solvent, heating to 30-100° C., and separating and purifying after sufficient reaction to obtain a third compound; the molar ratio of the second compound, salicylaldehyde and / or a derivative of salicylaldehyde and the diamine compound is 1:(0.8-1.2):(0.8-1.2);

[0035] d) dissolving the third compound, sodium methoxide and cobalt acetate obtained in step c) in a third organic solvent, adding a solution of a metal salt in the third organic solvent after sufficient reaction, heating to 40-100° C., separating and purifying after sufficient reaction to obtain the metal complex; the molar ratio of the third compound, sodium methoxide and cobalt acetate is 1:(1-2):(1-3);

[0036] Preferably, the metal salt is selected from at least one of silver nitrate, lithium chloride and lithium bromide;

[0037] The first organic solvent is selected from at least one of tetrahydrofuran, toluene, dichloromethane, N,N-dimethylformamide, N,N-dimethylacetamide and xylene;

[0038] The second organic solvent is ethanol;

[0039] The third organic solvent is methanol.

[0040] The second aspect of the present invention provides a catalyst system, which comprises the metal complex as a main catalyst.

[0041] According to the present invention, preferably, the catalyst system further comprises a co-catalyst, and the co-catalyst is an ammonium salt, preferably at least one selected from ammonium bromide salts, ammonium chloride salts, ammonium azide salts and iminium salts.

[0042] According to the present invention, preferably, the molar ratio of the main catalyst to the co-catalyst is 1:(0.1-10), preferably 1:(0.6-3).

[0043] The third aspect of the present invention provides use of the metal complex or the catalyst system in the preparation of polypropylene carbonate.

[0044] A fourth aspect of the present invention provides a method for preparing polypropylene carbonate, comprising the following steps:

[0045] Under polymerization reaction conditions, epoxide, carbon dioxide and the catalyst system are contacted and reacted to obtain the polypropylene carbonate.

[0046] According to the present invention, preferably, the epoxide is selected from at least one of propylene oxide, ethylene oxide, epichlorohydrin, glycidyl ether, glycidyl ester, 1,2-butylene oxide, 2,3-butylene oxide, cyclopentane oxide, cyclohexene oxide, 3,4-epoxy-1-cyclohexene and cyclooctane oxide.

[0047] According to the present invention, preferably, the contact reaction system also includes a solvent.

[0048] Further, preferably, the solvent is selected from at least one of dichloromethane, tetrahydrofuran, toluene, N,N-dimethylformamide, N,N-dimethylacetamide and dimethyl sulfoxide.

[0049] According to the present invention, preferably, the preparation method comprises the following steps:

[0050] (1) mixing epoxide, main catalyst, co-catalyst and optional solvent;

[0051] (2) adding carbon dioxide, adjusting to polymerization reaction conditions, and performing a contact reaction to obtain the polypropylene carbonate.

[0052] According to the present invention, preferably, the molar ratio of the epoxide to the main catalyst is 100 to 60,000:1, preferably 1,000 to 50,000:1, and the molar ratio of the epoxide to the co-catalyst is 20 to 0.05:1.

[0053] According to the present invention, preferably, the polymerization reaction conditions include: pressure of 0.01-10 MPa, preferably 0.1-6 MPa, temperature of 10-200° C., preferably 25-100° C., reaction time of 0.5-48 h, preferably 1-5 h.

[0054] According to the present invention, preferably, the reaction further includes a separation and purification step, which includes at least one step of vacuuming to remove unreacted raw materials, heating to remove unreacted raw materials, dissolving the product with excess solvent, adding methanol to precipitate and centrifuging to dry.

[0055] A fifth aspect of the present invention provides polypropylene carbonate, which is prepared by the above-mentioned preparation method.

[0056] According to the present invention, preferably, the number average molecular weight of the polypropylene carbonate is 10000-250000 g / mol, and the molecular weight distribution index is 1.12-1.30.

[0057] According to a specific embodiment of the present invention, preferably, the number average molecular weight of the polypropylene carbonate is 54,000 to 130,000 g / mol, and the molecular weight distribution index is 1.12 to 1.27.

[0058] The present invention will be further described below in conjunction with embodiments, but the scope of the present invention is not limited to these embodiments.

[0059] The number average molecular weight (M) of the polymers was determined by gel permeation chromatography (GPC). n) and molecular weight distribution index (PDI) were determined by Shimadzu LC-20AD gel permeation chromatograph, with tetrahydrofuran as the mobile phase, narrow distribution polystyrene as the standard sample, and the mobile phase flow rate was 1.0 mL / min.

[0060] Preparation Example 1

[0061] Preparation of Salen metal catalyst Cat-1, wherein R1 is o-phenylene, R2 is hydrogen, R3 is -CH2-CH2-CH2-CH2-, and M is Co 3+ , Y is NO3 - .

[0062]

[0063] The method comprises the following steps: a) mixing salicylaldehyde (0.2 mol), paraformaldehyde (0.3 mol), hydrobromic acid (1.6 mol) and a small amount of concentrated sulfuric acid (3 drops), heating to 70° C., separating and purifying after sufficient reaction; b) dissolving the compound (0.01 mol) obtained in step a), sodium hydride (0.06 mol) and phenothiazine (0.012 mol) in 30 mL of tetrahydrofuran, heating to 30° C., separating and purifying after sufficient reaction; c) mixing the compound (0.01 mol) obtained in step b) Salicylaldehyde (0.01 mol) and o-phenylenediamine compound (0.01 mol) were dissolved in 50 mL of ethanol, heated to 70°C, and separated and purified after sufficient reaction; d) the compound obtained in step c (10 mmol), sodium methoxide (15 mmol) and cobalt acetate (20 mmol) were dissolved in 200 mL of methanol, and after sufficient reaction, 10 mL of methanol solution in which 25 mmol of silver nitrate was dissolved was added, and the mixture was heated to 70°C, separated and purified after sufficient reaction to obtain the above metal complex, and the nuclear magnetic hydrogen spectrum of the complex was as follows: Figure 1 as shown.

[0064] Preparation Example 2

[0065] Preparation of Salen metal catalyst Cat-2 / Cat-3, wherein R1 is o-phenylene, R2 is hydrogen, R3 is -CH2-CH2-CH2-CH2-, and M is Co 3+ , Y is Cl - .

[0066] The preparation method is the same as that of Example 1, except that silver nitrate is replaced by lithium chloride.

[0067] Preparation Example 3

[0068] Preparation of Salen metal catalyst Cat-2 / Cat-3, wherein R1 is o-phenylene, R2 is hydrogen, R3 is -CH2-CH2-CH2-CH2-, and M is Co3+ , Y is Br - .

[0069] The preparation method is the same as that of Example 2, except that silver nitrate is replaced by lithium bromide.

[0070] Preparation Example 4

[0071] Preparation of Salen metal catalyst Cat-4, wherein R1 is o-cyclohexylene, R2 is hydrogen, R3 is -CH2-CH2-CH2-CH2-, and M is Co 3+ , Y is NO3 - .

[0072] The preparation method is the same as that of Example 1, except that o-phenylenediamine is replaced by cyclohexylenediamine.

[0073] Preparation Example 5

[0074] Preparation of Salen metal catalyst Cat-5, wherein R1 is 4-methoxy-o-phenylene, R2 is hydrogen, R3 is -CH2-CH2-CH2-CH2-, and M is Co 3+ , Y is NO3 - .

[0075] The preparation method is the same as that of Example 1, except that o-phenylenediamine is replaced by 4-methoxy o-phenylenediamine.

[0076] Example 1

[0077] In a 250mL autoclave, add propylene oxide (0.4mol, 23.232g) and Salen metal catalyst Cat-1 (0.0004mol, 0.3909g), tetrabutylammonium bromide (0.04mol, 12.8948g), close the reaction device, then introduce carbon dioxide and adjust the pressure to 2MPa, adjust the reaction temperature to 25°C, stir and react for 2h, open the pressure relief valve, release the unreacted carbon dioxide and recover the unreacted propylene oxide, then add tetrahydrofuran to dissolve the polymerization product, add methanol to precipitate, filter, wash, and dry to obtain a pure PPC product. The number average molecular weight of the tested molecular PPC is 105kg / mol, the molecular weight distribution is 1.15, and the polycarbonate structural unit content is 94%.

[0078] Example 2

[0079] In an autoclave with a volume of 250 mL, glycidyl ether (0.4 mol, 104.112 g) and Salen metal catalyst Cat-1 (0.0004 mol, 0.3909 g) and a co-catalyst (0.04 mol, bis(triphenylphosphorane)ammonium chloride / tetrabutylammonium azide / bis(triphenylphosphine)boric acid imide, the corresponding addition amounts are 22.9616 g / 11.3792 g / 22.1368 g, respectively) were added, the reaction apparatus was closed, and then carbon dioxide was introduced and the pressure was adjusted to 2 MPa, and the reaction temperature was adjusted to 25°C. After stirring the reaction for 2 hours, the pressure relief valve was opened to release the unreacted carbon dioxide and recover the unreacted propylene oxide, and then tetrahydrofuran was added to dissolve the polymerization product, and then methanol was added for precipitation, filtered, washed, and dried to obtain a pure PPC product. The tested number average molecular weights of the PPC molecules were 117 kg / mol, 95 kg / mol, and 102 kg / mol, respectively; the molecular weight distributions were 1.27, 1.16, and 1.15, respectively; and the polycarbonate structural unit contents were 96%, 98%, and 97%, respectively.

[0080] Example 3

[0081] In a 250 mL autoclave, propylene oxide (0.4 mol, 23.232 g) and Salen metal catalyst Cat-1 ((0.0004 mol, 0.3909 g), tetrabutylammonium bromide (0.04 mol, 12.8948 g) and 28 mL of dichloromethane were added, the reaction apparatus was closed, carbon dioxide was then introduced and the pressure was adjusted to 2 MPa, the reaction temperature was adjusted to 25°C, and after stirring the reaction for 2 hours, the pressure relief valve was opened to release the unreacted carbon dioxide and recover the unreacted propylene oxide, and then tetrahydrofuran was added to dissolve the polymerization product, and methanol was added for precipitation, filtered, washed, and dried to obtain a pure PPC product. The number average molecular weight of the molecular PPC was tested to be 87 kg / mol, the molecular weight distribution was 1.19, and the content of the polycarbonate structural unit was 98%.

[0082] Example 4

[0083] In a 250mL autoclave, add propylene oxide (0.4mol, 23.232g) and Salen metal catalyst Cat-1 (0.01mmol, 0.0391g), tetrabutylammonium bromide (0.02mol, 6.4474g), close the reaction device, then introduce carbon dioxide and adjust the pressure to 0.1MPa / 6MPa respectively, adjust the reaction temperature to 25°C, stir and react for 2h, open the pressure relief valve, release the unreacted carbon dioxide and recover the unreacted propylene oxide, then add tetrahydrofuran to dissolve the polymer product, add methanol to precipitate, filter, wash, and dry to obtain a pure PPC product. The number average molecular weight of the tested molecular PPC is 54kg / mol and 132kg / mol respectively, the molecular weight distribution is 1.17 and 1.19 respectively, and the content of the polycarbonate structural unit is 95% and 98% respectively.

[0084] Example 5

[0085] In a 250mL autoclave, add propylene oxide (0.4mol, 23.232g) and Salen metal catalyst Cat-1 (0.0004mol, 0.3909g), tetrabutylammonium bromide (0.04mol, 12.8948g), close the reaction device, then introduce carbon dioxide and adjust the pressure to 2MPa, adjust the reaction temperature to 100°C, stir and react for 48h, open the pressure relief valve, release the unreacted carbon dioxide and recover the unreacted propylene oxide, then add tetrahydrofuran to dissolve the polymer product, add methanol for precipitation, filter, wash, and dry to obtain a pure PPC product. The number average molecular weight of the tested molecular PPC is 117kg / mol, the molecular weight distribution is 1.23, and the content of the polycarbonate structural unit is 90%.

[0086] Example 6

[0087] In a 250mL autoclave, add propylene oxide (0.4mol, 23.232g) and Salen metal catalyst Cat-2 / Cat-3 (0.0004mol, 0.3897g / 0.4073g), tetrabutylammonium bromide (0.04mol, 12.8948g), close the reaction device, then introduce carbon dioxide and adjust the pressure to 2MPa, adjust the reaction temperature to 25°C, stir and react for 2h, open the pressure relief valve, release the unreacted carbon dioxide and recover the unreacted propylene oxide, then add tetrahydrofuran to dissolve the polymerization product, add methanol to precipitate, filter, wash, and dry to obtain a pure PPC product. The number average molecular weight of the tested molecular PPC is 101kg / mol and 126kg / mol, respectively, the molecular weight distribution is 1.14 and 1.21, respectively, and the content of the polycarbonate structural unit is 95% and 97%, respectively.

[0088] Example 7

[0089] In an autoclave with a volume of 250 mL, propylene oxide (0.4 mol, 23.232 g) and Salen metal catalyst Cat-4 / Cat-5 / Cat-6 (0.0004 mol, 0.3925 g / 0.4029 g / 0.4109 g) and tetrabutylammonium bromide (0.04 mol, 12.8948 g) were added, and the reaction device was closed. Then, carbon dioxide was introduced and the pressure was adjusted to 2 MPa. The reaction temperature was adjusted to 25°C. After stirring the reaction for 2 hours, the pressure relief valve was opened to release the unreacted carbon dioxide and recover the unreacted propylene oxide. Then, tetrahydrofuran was added to dissolve the polymerization product, and methanol was added for precipitation. The reaction was filtered, washed, and dried to obtain a pure PPC product. The tested number average molecular weights of the PPC molecules were 107 kg / mol, 72 kg / mol, and 87 kg / mol, respectively; the molecular weight distributions were 1.13, 1.24, and 1.16, respectively; and the contents of the polycarbonate structural units were 95%, 97%, and 99%, respectively.

[0090] Example 8

[0091] In an autoclave with a volume of 250 mL, epichlorohydrin / epoxycyclooctane / 3,4-epoxy-1-cyclohexene (0.4 mol, 37.008 g / 50.48 g / 38.452 g) and Salen metal catalyst Cat-1 (0.0004 mol, 0.3909 g), tetrabutylammonium bromide (0.04 mol, 12.8948 g) were added respectively, the reaction device was closed, and then carbon dioxide was introduced and the pressure was adjusted to 2 MPa, and the reaction temperature was adjusted to 25°C. After stirring the reaction for 2 hours, the pressure relief valve was opened to release the unreacted carbon dioxide and recover the unreacted propylene oxide, and then tetrahydrofuran was added to dissolve the polymerization product, and then methanol was added for precipitation, filtered, washed, and dried to obtain a pure PPC product. The tested number average molecular weights of the PPC molecules were 77 kg / mol, 67 kg / mol, and 83 g / mol, respectively; the molecular weight distributions were 1.13, 1.18, and 1.16, respectively; and the contents of the polycarbonate structural units were 94%, 96%, and 97%, respectively.

[0092] The embodiments of the present invention have been described above, and the above description is exemplary, not exhaustive, and is not limited to the disclosed embodiments. Many modifications and changes will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments.

[0093] The endpoints and any values ​​of the ranges disclosed in this article are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of each range, the endpoint values ​​of each range and the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this article.

Claims

1. A metal complex, characterized in that The structure of the metal complex is shown in formula (I): Wherein: M is a trivalent metal ion; Y is a monovalent acid ion; R1 is a substituted or unsubstituted C5-C 12 R2 is H or a C1-C6 alkyl group; R3 is a C1-C 10 of alkylene.

2. The metal complex according to claim 1, wherein M is Co 3+ , Fe 3+ 、Al 3+ 、Ni 3+ , Mn 3+ Cr 3+ And Ru 3+ At least one of; Y is NO3 - , Cl - and Br - At least one of; R1 is a substituted or unsubstituted cyclohexylene, phenylene or naphthylene; the substituted group is a halogen, a cyano group, a C1-C4 alkoxy group or a halogen-substituted C1-C4 alkyl group; Preferably, R1 is selected from the following groups: R2 is H or a C1-C6 alkyl group, preferably H or t-Bu; R3 is a C1-C6 alkylene group.

3. The metal complex according to claim 1 or 2, wherein The preparation method of the metal complex comprises the following steps: a) mixing salicylaldehyde and / or a derivative of salicylaldehyde, polyformaldehyde, hydrobromic acid and a small amount of concentrated sulfuric acid, heating to 30-100° C., reacting sufficiently, and then separating and purifying to obtain a first compound; the molar ratio of salicylaldehyde and / or a derivative of salicylaldehyde, polyformaldehyde and hydrobromic acid is 1:(1-2):(7-9); b) dissolving the first compound obtained in step a), sodium hydride and phenothiazine in a first organic solvent, heating to 0-80° C., reacting sufficiently, separating and purifying to obtain a second compound; the molar ratio of the first compound, sodium hydride and phenothiazine is 1:(5-8):(1-1.5); c) dissolving the second compound, salicylaldehyde or a derivative of salicylaldehyde and a diamine compound obtained in step b) in a second organic solvent, heating to 30-100° C., and separating and purifying after sufficient reaction to obtain a third compound; the molar ratio of the second compound, salicylaldehyde and / or a derivative of salicylaldehyde and the diamine compound is 1:(0.8-1.2):(0.8-1.2); d) dissolving the third compound, sodium methoxide and cobalt acetate obtained in step c) in a third organic solvent, adding a solution of a metal salt in the third organic solvent after sufficient reaction, heating to 40-100° C., separating and purifying after sufficient reaction to obtain the metal complex; the molar ratio of the third compound, sodium methoxide and cobalt acetate is 1:(1-2):(1-3); Preferably, the metal salt is selected from at least one of silver nitrate, lithium chloride and lithium bromide; The first organic solvent is selected from at least one of tetrahydrofuran, toluene, dichloromethane, N,N-dimethylformamide, N,N-dimethylacetamide and xylene; The second organic solvent is ethanol; The third organic solvent is methanol.

4. A catalyst system, characterized in that The catalyst system comprises the metal complex according to any one of claims 1 to 3 as a main catalyst.

5. The catalyst system according to claim 4, wherein The catalyst system further comprises a co-catalyst, wherein the co-catalyst is an ammonium salt, preferably at least one selected from ammonium bromide salt, ammonium chloride salt, ammonium azide salt and iminium salt; The molar ratio of the main catalyst to the co-catalyst is 1:(0.1-10), preferably 1-(0.6-3).

6. Use of the metal complex according to any one of claims 1 to 3 or the catalyst system according to claim 4 or 5 in the preparation of polypropylene carbonate.

7. A method for preparing polypropylene carbonate, characterized in that: The steps include: Under polymerization reaction conditions, epoxide, carbon dioxide and the catalyst system according to claim 4 or 5 are contacted and reacted to obtain the polypropylene carbonate.

8. The preparation method according to claim 7, wherein: The epoxide is selected from at least one of propylene oxide, ethylene oxide, epichlorohydrin, glycidyl ether, glycidyl ester, 1,2-butylene oxide, 2,3-butylene oxide, cyclopentane oxide, cyclohexene oxide, 3,4-epoxy-1-cyclohexene and cyclooctane oxide.

9. The preparation method according to claim 7, wherein: The contact reaction system also includes a solvent; Preferably, the solvent is selected from at least one of dichloromethane, tetrahydrofuran, toluene, N,N-dimethylformamide, N,N-dimethylacetamide and dimethyl sulfoxide.

10. The preparation method according to any one of claims 7 to 9, wherein: The preparation method comprises the following steps: (1) mixing epoxide, main catalyst, co-catalyst and optional solvent; (2) adding carbon dioxide, adjusting to polymerization reaction conditions, and performing a contact reaction to obtain the polypropylene carbonate.

11. The preparation method according to claim 10, wherein: The molar ratio of the epoxide to the main catalyst is 100 to 40,000:1, preferably 1,000 to 50,000:1, and the molar ratio of the epoxide to the co-catalyst is 20 to 0.05:

1.

12. The preparation method according to claim 10, wherein: The polymerization reaction conditions include: pressure of 0.01 to 10 MPa, preferably 0.1 to 6 MPa, temperature of 10 to 200° C., preferably 25 to 100° C., and reaction time of 0.5 to 48 h, preferably 1 to 5 h.

13. The preparation method according to claim 10, wherein: The method further comprises a separation and purification step after the reaction, wherein the separation and purification step comprises at least one of vacuuming to remove unreacted raw materials, heating to remove unreacted raw materials, dissolving the product with an excess solvent, adding methanol to precipitate, and centrifuging and drying.

14. A polypropylene carbonate, characterized in that: The polypropylene carbonate is prepared by the preparation method described in any one of claims 7 to 13; Preferably, the number average molecular weight of the polypropylene carbonate is 10,000 to 250,000 g / mol, and the molecular weight distribution index is 1.12 to 1.30.

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