Metal complex, catalytic system and application and polypropylene carbonate and preparation method thereof

Abstract

The application belongs to the field of polymer chemistry, and relates to a metal complex, a catalytic system and application and polypropylene carbonate and a preparation method thereof. 12 The structure of the metal complex is shown in formula (I): in the formula, M is a trivalent metal ion; Y is a monovalent acid radical ion; R1 is a substituted or unsubstituted C5-C 10 Alkylene or arylene; R2 is H or C1-C6 alkyl; and R3 is C1-C 10 alkylene. Compared with the prior art, the application has the following advantages: the Salen metal complex with a new phenothiazine structure is used as a main catalyst to efficiently catalyze the copolymerization of carbon dioxide and an epoxide to prepare polypropylene carbonate, and the obtained polypropylene carbonate has the characteristics of high molecular weight and narrow molecular weight distribution.

Description

Technical Field

[0001] This invention belongs to the field of polymer chemistry, specifically relating to a metal complex, a catalyst system and their applications, a method for preparing polypropylene carbonate, and the polypropylene carbonate obtained by the method. Background Technology

[0002] Polypropylene carbonate (PPC) is a fully biodegradable and environmentally friendly plastic synthesized from carbon dioxide and propylene oxide. It is completely biodegradable and therefore does 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.), shoe soles, photoresists, adhesives, magnetic adhesives, conveyor belts, and other fields. High molecular weight PPC has broad application prospects in environmentally friendly materials such as medical packaging and food packaging. With a carbon dioxide content of 31%~50%, it is currently the most extensively researched and most promising carbon dioxide copolyester for industrialization. The full utilization of carbon dioxide not only significantly reduces the consumption of upstream raw material—petroleum—but also plays a positive role in mitigating the most serious environmental pollution problem—the "greenhouse effect" caused by carbon dioxide emissions.

[0003] Heterogeneous catalysts have been applied in the industrial production of polycarbonate, such as rare-earth ternary catalysts and zinc glutarate catalyst systems. However, they suffer from low catalytic activity and poor selectivity. Homogeneous catalysts, through continuous development and improvement, have achieved significant improvements in catalytic activity and selectivity. For example, Salen metal complexes are currently the most promising catalytic system in the field of carbon dioxide and epoxide copolymerization, possessing 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, the development of novel catalysts with high catalytic activity for the synthesis of polycarbonate lactone (PPC) is of great significance. Summary of the Invention

[0005] To address the aforementioned problems in the prior art, this invention provides a metal complex that serves as the main catalyst, thereby solving the issues of low selectivity and small polymer molecular weight in the prior art. When used to catalyze the copolymerization of carbon dioxide and epoxides to prepare polypropylene carbonate, it exhibits high catalytic activity, and the resulting polypropylene carbonate has a high molecular weight and a narrow molecular weight distribution.

[0006] To achieve the objectives of this invention, a first aspect of this invention provides a metal complex, the structure of which is shown in formula (I):

[0007] (I)

[0008] In the formula: M is a trivalent metal ion; Y is a monovalent acid radical ion; R1 is a substituted or unsubstituted C5-C ion. 12 R1 is a cycloalkylene or arylene group; R2 is H or a C1-C6 alkyl group; R3 is C1-C6. 10 Alkylene.

[0009] A second aspect of the present invention provides a catalyst system comprising the aforementioned metal complex as a main catalyst.

[0010] A third aspect of the invention provides the use of the metal complex or the catalyst system described herein 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: Under polymerization conditions, the epoxide, carbon dioxide, and the catalyst system are brought into contact and reacted to obtain the polypropylene carbonate.

[0012] A fifth aspect of the present invention provides a polypropylene carbonate, which is prepared by the above-described preparation method.

[0013] Compared with the prior art, the present invention has the following advantages: This invention uses a novel Salen metal complex with a phenothiazine structure as the main catalyst to efficiently catalyze the copolymerization of carbon dioxide and epoxide to prepare polypropylene carbonate. The resulting polypropylene carbonate has the characteristics of high molecular weight and narrow molecular weight distribution.

[0014] Other features and advantages of the present invention will be described in detail in the following detailed description section. Attached Figure Description

[0015] Exemplary embodiments of the present invention will be described in more detail with reference to the accompanying drawings.

[0016] Figure 1 The 1H NMR spectrum of the catalyst prepared according to the present invention is shown. Detailed Implementation

[0017] The following provides a detailed description of specific embodiments of the present invention. It should be understood that the specific embodiments described herein are for illustrative and explanatory purposes only and are not intended to limit the scope of the invention.

[0018] To achieve the objectives of this invention, a first aspect of this invention provides a metal complex, the structure of which is shown in formula (I):

[0019] (I)

[0020] In the formula: M is a trivalent metal ion; Y is a monovalent acid radical ion; R1 is a substituted or unsubstituted C5-C ion. 12 R1 is a cycloalkylene or arylene group; R2 is H or a C1-C6 alkyl group; R3 is C1-C6. 10 Alkylene.

[0021] 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 them.

[0022] According to the present invention, preferably, Y is NO3. - Cl - and Br - At least one of them.

[0023] According to the present invention, preferably, R1 is a substituted or unsubstituted cyclohexylene, phenylene, or naphthylene; the substituted group is a halogen, cyano, C1-C4 alkoxy, or halogen-substituted C1-C4 alkyl.

[0024] Further, preferably, R1 is selected from the following groups: .

[0025] According to the present invention, preferably, R2 is H or a C1-C6 alkyl group, wherein 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 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.

[0026] According to the present invention, preferably, R3 is a C1-C6 alkylene group.

[0027] According to the present invention, preferably, the method for preparing the metal complex includes the following steps: a) After mixing salicylaldehyde and / or its derivatives, paraformaldehyde, hydrobromic acid, and a small amount of concentrated sulfuric acid, the mixture is heated to 30-100℃ and allowed to react completely. The mixture is then separated and purified to obtain the first compound. The molar ratio of salicylaldehyde and / or its derivatives, paraformaldehyde, and hydrobromic acid is 1:(1~2):(7~9). b) Dissolve the first compound obtained in step a), sodium hydride, and phenothiazine in a first organic solvent, heat to 0-80℃, and after complete reaction, separate and purify to obtain the second compound; the molar ratio of the first compound, sodium hydride, and phenothiazine is 1:(5~8):(1~1.5). c) Dissolve the second compound obtained in step b), salicylaldehyde or a derivative of salicylaldehyde, and the diamine compound in a second organic solvent, heat to 30-100℃, and after complete reaction, separate and purify to obtain the 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) Dissolve the third compound obtained in step c), sodium methoxide, and cobalt acetate in a third organic solvent. After the reaction is complete, add a solution of the metal salt in the third organic solvent. Heat to 40-100°C. After the reaction is complete, separate and purify 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.

[0028] A second aspect of the present invention provides a catalyst system comprising the aforementioned metal complex as a main catalyst.

[0029] According to the present invention, preferably, the catalyst system further includes a co-catalyst, which is an ammonium salt, preferably selected from at least one of ammonium bromide, ammonium chloride, ammonium azide, and ammonium monoxide.

[0030] According to the present invention, preferably, the molar ratio of the main catalyst to the co-catalyst is 1:(0.1~10), more preferably 1:(0.6~3).

[0031] A third aspect of the invention provides the use of the metal complex or the catalyst system described herein in the preparation of polypropylene carbonate.

[0032] A fourth aspect of the present invention provides a method for preparing polypropylene carbonate, comprising the following steps: Under polymerization conditions, the epoxide, carbon dioxide, and the catalyst system are brought into contact and reacted to obtain the polypropylene carbonate.

[0033] 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-epoxybutane, 2,3-epoxybutane, cyclopentane oxide, cyclohexane oxide, 3,4-epoxy-1-cyclohexene, and cyclooctane oxide.

[0034] According to the present invention, preferably, the system of the contact reaction also includes a solvent.

[0035] Further, preferably, the solvent is selected from at least one of dichloromethane, tetrahydrofuran, toluene, N,N-dimethylformamide, N,N-dimethylacetamide and dimethyl sulfoxide.

[0036] According to the present invention, preferably, the preparation method includes the following steps: (1) Mix the epoxide, main catalyst, co-catalyst and optional solvent; (2) Add carbon dioxide, adjust to polymerization reaction conditions, and carry out contact reaction to obtain the polypropylene carbonate.

[0037] According to the present invention, preferably, the molar ratio of the epoxide to the main catalyst is 100~60000:1, more preferably 1000~50000:1, and the molar ratio of the epoxide to the co-catalyst is 20~0.05:1.

[0038] According to the present invention, preferably, the polymerization reaction conditions include: a pressure of 0.01~10MPa, more preferably 0.1~6MPa, a temperature of 10~200℃, more preferably 25~100℃, and a reaction time of 0.5~48h, more preferably 1~5h.

[0039] 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 and drying.

[0040] A fifth aspect of the present invention provides a polypropylene carbonate, which is prepared by the above-described preparation method.

[0041] According to the present invention, preferably, the polypropylene carbonate has a number-average molecular weight of 10,000 to 250,000 g / mol and a molecular weight distribution index of 1.12 to 1.30.

[0042] According to a specific embodiment of the present invention, preferably, the polypropylene carbonate has a number-average molecular weight of 54,000 to 130,000 g / mol and a molecular weight distribution index of 1.12 to 1.27.

[0043] The present invention will be further described below with reference to the embodiments, but the scope of the present invention is not limited to these embodiments.

[0044] The number-average molecular weight (Mn) of polymers was determined using gel permeation chromatography (GPC). n The molecular weight distribution index (PDI) was determined using a Shimadzu LC-20AD gel permeation chromatograph with tetrahydrofuran as the mobile phase and narrow-distribution polystyrene as the standard. The flow rate of the mobile phase was 1.0 mL / min.

[0045] Preparation Example 1

[0046] Preparation of Salen metal catalyst Cat-1, in formula (I) R1 is o-phenylene, R2 is hydrogen, R3 is -CH2-CH2-CH2-CH2-, and M is Co. 3+ Y is NO3 - .

[0047]

[0048] (I)

[0049] The process includes 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, reacting completely, and then separating and purifying; b) Dissolving the compound obtained in step a) (0.01 mol), sodium hydride (0.06 mol), and phenothiazine (0.012 mol) in 30 mL of tetrahydrofuran, heating to 30°C, reacting completely, and then separating and purifying; c) Dissolving the compound obtained in step b) (0.01 mol)... d) The compound obtained in step c (10 mmol), 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 the reaction was complete; 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 the reaction was complete, 10 mL of methanol solution containing 25 mmol of silver nitrate was added, heated to 70 °C, and separated and purified after the reaction was complete to obtain the above metal complex.

[0050] Preparation Example 2

[0051] Preparation of Salen metal catalysts Cat-2 / Cat-3, in formula (I) R1 is o-phenylene, R2 is hydrogen, R3 is -CH2-CH2-CH2-CH2-, and M is Co. 3+ Y is Cl - .

[0052] The preparation method is the same as in Example 1, except that silver nitrate is replaced with lithium chloride.

[0053] Preparation Example 3

[0054] Preparation of Salen metal catalysts Cat-2 / Cat-3, in formula (I) R1 is o-phenylene, R2 is hydrogen, R3 is -CH2-CH2-CH2-CH2-, and M is Co. 3+ Y is Br - .

[0055] The preparation method is the same as in Example 2, except that silver nitrate is replaced with lithium bromide.

[0056] Preparation Example 4

[0057] Preparation of Salen metal catalyst Cat-4, in formula (I) R1 is o-cyclohexylene, R2 is hydrogen, R3 is -CH2-CH2-CH2-CH2-, and M is Co. 3+ Y is NO3 - .

[0058] The preparation method is the same as in Example 1, except that o-phenylenediamine is replaced with cyclohexanediamine.

[0059] Preparation Example 5

[0060] Preparation of Salen metal catalyst Cat-5, in formula (I) R1 is 4-methoxy-o-phenylene, R2 is hydrogen, R3 is -CH2-CH2-CH2-CH2-, and M is Co 3+ Y is NO3 - .

[0061] The preparation method is the same as in Example 1, except that o-phenylenediamine is replaced with 4-methoxy-o-phenylenediamine.

[0062] Example 1

[0063] Propylene oxide (0.4 mol, 23.232 g), Salen metal catalyst Cat-1 (0.0004 mol, 0.3909 g), and tetrabutylammonium bromide (0.04 mol, 12.8948 g) were added to a 250 mL autoclave. The reaction apparatus was sealed, and carbon dioxide was introduced and the pressure was adjusted to 2 MPa. The reaction temperature was adjusted to 25 °C. o C. After stirring the reaction for 2 hours, the pressure relief valve was opened to release unreacted carbon dioxide and recover unreacted propylene oxide. Then, tetrahydrofuran was added to dissolve the polymerization product, followed by the addition of methanol to precipitate the product. The mixture was then filtered, washed, and dried to obtain a pure PPC product. The number-average molecular weight of the PPC was tested to be 105 kg / mol, the molecular weight distribution was 1.15, and the polycarbonate structural unit content was 94%.

[0064] Example 2

[0065] Glycidyl ether (0.4 mol, 104.112 g) and Salen metal catalyst Cat-1 (0.0004 mol, 0.3909 g) were added to a 250 mL autoclave. Co-catalysts (0.04 mol, consisting of bis(triphenylphosphine)ammonium chloride / tetrabutylammonium azide / bis(triphenylphosphine)borate imine, with corresponding amounts of 22.9616 g / 11.3792 g / 22.1368 g respectively) were added. The reaction apparatus was sealed, carbon dioxide was introduced, and the pressure was adjusted to 2 MPa. The reaction temperature was adjusted to 25 °C. oC. After stirring the reaction for 2 hours, the pressure relief valve was opened to release unreacted carbon dioxide and recover unreacted propylene oxide. Then, tetrahydrofuran was added to dissolve the polymerization product, followed by the addition of methanol to precipitate the product. The mixture was then filtered, washed, and dried to obtain pure PPC product. The number-average molecular weights of the PPC molecules were tested to be 117 kg / mol, 95 kg / mol, and 102 kg / mol, respectively, with molecular weight distributions of 1.27, 1.16, and 1.15, respectively. The polycarbonate structural unit contents were 96%, 98%, and 97%, respectively.

[0066] Example 3

[0067] Propylene oxide (0.4 mol, 23.232 g), 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 to a 250 mL autoclave. The reaction apparatus was sealed, and carbon dioxide was introduced and the pressure was adjusted to 2 MPa. The reaction temperature was adjusted to 25 °C. o C. After stirring the reaction for 2 hours, the pressure relief valve was opened to release unreacted carbon dioxide and recover unreacted propylene oxide. Then, tetrahydrofuran was added to dissolve the polymerization product, followed by the addition of methanol to precipitate the product. The mixture was then filtered, washed, and dried to obtain a pure PPC product. The number-average molecular weight of the PPC was tested to be 87 kg / mol, the molecular weight distribution was 1.19, and the polycarbonate structural unit content was 98%.

[0068] Example 4

[0069] Propylene oxide (0.4 mol, 23.232 g), Salen metal catalyst Cat-1 (0.01 mmol, 0.0391 g), and tetrabutylammonium bromide (0.02 mol, 6.4474 g) were added to a 250 mL autoclave. The reaction apparatus was sealed, and carbon dioxide was introduced and the pressures were adjusted to 0.1 MPa and 6 MPa, respectively. The reaction temperature was adjusted to 25 °C. o C. After stirring the reaction for 2 hours, the pressure relief valve was opened to release unreacted carbon dioxide and recover unreacted propylene oxide. Then, tetrahydrofuran was added to dissolve the polymerization product, followed by the addition of methanol to precipitate the product. The mixture was then filtered, washed, and dried to obtain pure PPC product. The number-average molecular weights of the PPC molecules were tested to be 54 kg / mol and 132 kg / mol, respectively, with molecular weight distributions of 1.17 and 1.19, and polycarbonate structural unit contents of 95% and 98%, respectively.

[0070] Example 5

[0071] Propylene oxide (0.4 mol, 23.232 g), Salen metal catalyst Cat-1 (0.0004 mol, 0.3909 g), and tetrabutylammonium bromide (0.04 mol, 12.8948 g) were added to a 250 mL autoclave. The reaction apparatus was sealed, and carbon dioxide was introduced and the pressure was adjusted to 2 MPa. The reaction temperature was then adjusted to 100 °C. o C. After stirring the reaction for 48 hours, the pressure relief valve was opened to release unreacted carbon dioxide and recover unreacted propylene oxide. Then, tetrahydrofuran was added to dissolve the polymerization product, followed by the addition of methanol to precipitate the product. The mixture was then filtered, washed, and dried to obtain a pure PPC product. The number-average molecular weight of the PPC was tested to be 117 kg / mol, the molecular weight distribution was 1.23, and the polycarbonate structural unit content was 90%.

[0072] Example 6

[0073] Propylene oxide (0.4 mol, 23.232 g), Salen metal catalyst Cat-2 / Cat-3 (0.0004 mol, 0.3897 g / 0.4073 g), and tetrabutylammonium bromide (0.04 mol, 12.8948 g) were added to a 250 mL autoclave. The reaction apparatus was sealed, and carbon dioxide was introduced and the pressure was adjusted to 2 MPa. The reaction temperature was adjusted to 25 °C. o C. After stirring the reaction for 2 hours, the pressure relief valve was opened to release unreacted carbon dioxide and recover unreacted propylene oxide. Then, tetrahydrofuran was added to dissolve the polymerization product, followed by the addition of methanol to precipitate the product. The mixture was then filtered, washed, and dried to obtain a pure PPC product. The number-average molecular weights of the PPC molecules were tested to be 101 kg / mol and 126 kg / mol, respectively, with molecular weight distributions of 1.14 and 1.21, and polycarbonate structural unit contents of 95% and 97%, respectively.

[0074] Example 7

[0075] 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), tetrabutylammonium bromide (0.04 mol, 12.8948 g) were added to a 250 mL autoclave. The reaction apparatus was sealed, and carbon dioxide was introduced and the pressure was adjusted to 2 MPa. The reaction temperature was adjusted to 25 °C. oC. After stirring the reaction for 2 hours, the pressure relief valve was opened to release unreacted carbon dioxide and recover unreacted propylene oxide. Then, tetrahydrofuran was added to dissolve the polymerization product, followed by the addition of methanol to precipitate the product. The mixture was then filtered, washed, and dried to obtain pure PPC product. The number-average molecular weights of the PPC molecules were tested to be 107 kg / mol, 72 kg / mol, and 87 kg / mol, respectively, with molecular weight distributions of 1.13, 1.24, and 1.16, and polycarbonate structural unit contents of 95%, 97%, and 99%, respectively.

[0076] Example 8

[0077] Epichlorohydrin / epoxycyclooctane / 3,4-epoxy-1-cyclohexene (0.4 mol, 37.008 g / 50.48 g / 38.452 g), Salen metal catalyst Cat-1 (0.0004 mol, 0.3909 g), and tetrabutylammonium bromide (0.04 mol, 12.8948 g) were added to a 250 mL autoclave. The reaction apparatus was sealed, carbon dioxide was introduced, and the pressure was adjusted to 2 MPa. The reaction temperature was adjusted to 25 °C. o C. After stirring the reaction for 2 hours, the pressure relief valve was opened to release unreacted carbon dioxide and recover unreacted propylene oxide. Then, tetrahydrofuran was added to dissolve the polymerization product, followed by the addition of methanol to precipitate the product. The mixture was then filtered, washed, and dried to obtain pure PPC product. The number-average molecular weights of the PPC molecules were tested to be 77 kg / mol, 67 kg / mol, and 83 g / mol, respectively, with molecular weight distributions of 1.13, 1.18, and 1.16, and polycarbonate structural unit contents of 94%, 96%, and 97%, respectively.

[0078] The various embodiments of the present invention have been described above. These descriptions are exemplary and not exhaustive, nor are they limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments.

[0079] The endpoints and any values ​​of the ranges disclosed herein 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 the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

Claims

1. A metal complex, characterized in that, The structure of the metal complex is shown in formula (Ⅰ): （Ⅰ） In the formula: M is Co 3+ Y is NO3 - Cl - and Br - At least one of the following: R1 is a substituted or unsubstituted cyclohexylene, phenylene, or naphthylene; the substituted group is a halogen, cyano, C1-C4 alkoxy, or halogen-substituted C1-C4 alkyl group; R2 is H or a C1-C6 alkyl group; R3 is C1-C6 alkyl. 10 Alkylene.

2. The metal complex according to claim 1, wherein, R1 is selected from the following groups: R2 is either 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 includes the following steps: a) After mixing salicylaldehyde or its derivatives, paraformaldehyde, hydrobromic acid, and a small amount of concentrated sulfuric acid, the mixture is heated to 30-100℃ and allowed to react completely. The mixture is then separated and purified to obtain the first compound. The molar ratio of salicylaldehyde or its derivatives, paraformaldehyde, and hydrobromic acid is 1:(1~2):(7~9). b) Dissolve the first compound obtained in step a), sodium hydride, and phenothiazine in a first organic solvent, heat to 0-80℃, and after complete reaction, separate and purify to obtain the second compound; the molar ratio of the first compound, sodium hydride, and phenothiazine is 1:(5~8):(1~1.5). c) Dissolve the second compound obtained in step b), salicylaldehyde or a derivative of salicylaldehyde, and the diamine compound in a second organic solvent, heat to 30-100℃, and after complete reaction, separate and purify to obtain the third compound; the molar ratio of the second compound, salicylaldehyde or a derivative of salicylaldehyde, and the diamine compound is 1:(0.8~1.2):(0.8~1.2). d) Dissolve the third compound obtained in step c), sodium methoxide and cobalt acetate in a third organic solvent. After the reaction is complete, add a solution of the metal salt in the third organic solvent. Heat to 40-100℃. After the reaction is complete, separate and purify to obtain the metal complex. The molar ratio of the third compound, sodium methoxide and cobalt acetate is 1:(1~2):(1~3).

4. The metal complex according to claim 3, wherein, 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.

5. A catalyst system, characterized in that, The catalyst system includes the metal complex described in any one of claims 1-4 as the main catalyst.

6. The catalyst system according to claim 5, wherein, The catalyst system also includes a co-catalyst, which is an ammonium salt; The molar ratio of the main catalyst to the co-catalyst is 1:(0.1~10).

7. The catalyst system according to claim 6, wherein, The co-catalyst is selected from at least one of ammonium bromide, ammonium chloride, ammonium azide, and ammonium monoxide.

8. The catalyst system according to claim 6, wherein, The molar ratio of the main catalyst to the co-catalyst is 1:(0.6~3).

9. The use of the metal complex according to any one of claims 1-4 or the catalyst system according to any one of claims 5-8 in the preparation of polypropylene carbonate.

10. A method for preparing polypropylene carbonate, characterized in that, Includes the following steps: Under polymerization conditions, the epoxide, carbon dioxide, and the catalyst system according to any one of claims 5-8 are subjected to a contact reaction to obtain the polypropylene carbonate.

11. The preparation method according to claim 10, wherein, The epoxide is selected from at least one of propylene oxide, ethylene oxide, epichlorohydrin, glycidyl ether, glycidyl ester, 1,2-epoxybutane, 2,3-epoxybutane, cyclopentane oxide, cyclohexane oxide, 3,4-epoxy-1-cyclohexene, and cyclooctane oxide.

12. The preparation method according to claim 10, wherein, The system of the contact reaction also includes a solvent.

13. The preparation method according to claim 12, wherein, The solvent is selected from at least one of dichloromethane, tetrahydrofuran, toluene, N,N-dimethylformamide, N,N-dimethylacetamide, and dimethyl sulfoxide.

14. The preparation method according to any one of claims 10-13, wherein, The preparation method includes the following steps: (1) Mix the epoxide, main catalyst, co-catalyst and optional solvent; (2) Add carbon dioxide, adjust to polymerization reaction conditions, and carry out contact reaction to obtain the polypropylene carbonate.

15. The preparation method according to claim 14, wherein, The molar ratio of the epoxide to the main catalyst is 100~40000:1, and the molar ratio of the epoxide to the co-catalyst is 20~0.05:

1.

16. The preparation method according to claim 15, wherein, The molar ratio of the epoxide to the main catalyst is 1000~50000:

1.

17. The preparation method according to claim 14, wherein, The polymerization reaction conditions include: pressure of 0.01~10MPa, temperature of 10~200℃, and reaction time of 0.5~48h.

18. The preparation method according to claim 17, wherein, The polymerization reaction conditions include: pressure of 0.1~6MPa, temperature of 25~100℃, and reaction time of 1~5h.

19. The preparation method according to claim 14, wherein, The reaction also includes a separation and purification step, which includes at least one of the following steps: vacuum removal of unreacted raw materials, heating removal of unreacted raw materials, dissolving the product with excess solvent, adding methanol to precipitate, and centrifuging and drying.

20. The preparation method according to claim 10, wherein, The polypropylene carbonate has a number-average molecular weight of 10,000 to 250,000 g / mol and a molecular weight distribution index of 1.12 to 1.30.

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