Preparation of poly (1, 5-hexadiene)-g-polylactic acid (PLA) grafted copolymer and application of poly (1, 5-hexadiene)-g-polylactic acid (PLA) grafted copolymer in commercialized PLA toughening

Abstract

According to the invention, polyolefin containing double bonds in a side chain is successfully synthesized, the molecular weight of the obtained polyolefin containing double bonds is 3 * 10 < 3 >-1 * 10 < 5 > g / mol, the molecular weight distribution is 1.1-4.0, the glass transition temperature is-20-30 DEG C, the double bond content is 2-50 mol%, and the polyolefin containing double bonds can be used for chemically synthesizing polyolefin containing hydroxyl functional groups through thiol-ene click. A series of graft copolymers consisting of polyolefin main chains and polylactic acid side chains are successfully synthesized by taking the double-bond-containing polyolefin as a reaction intermediate, and the obtained graft copolymers with polyolefin as the main chains and polylactic acid PLA as the side chains have the molecular weight of 6 * 10 < 3 >-1 * 10 < 6 > g / mol, the molecular weight distribution of 1.2-4.0, the glass-transition temperature of-5-50 DEG C and the PLA content of 20-90 wt%. These graft copolymers can be used as a toughening agent for commercialized polylactic acid (PLA). The glass transition temperature of a blend of the graft copolymer and commercial polylactic acid (PLA) is 40-55 DEG C, the tensile strength is 30-60 MPa, the elongation at break is 8-900%, and excellent transparency and rigidity are kept. Therefore, the invention has original innovation, and provides a new direction for the development of the field of functionalized polyolefin materials.

Description

Technical Field

[0001] The present invention relates to a method for preparing a poly(1,5-hexadiene)-g-poly(lactic acid) (PLA) graft copolymer and the toughening application of the polymer prepared by this method to commercial poly(lactic acid) (PLA). Background Art

[0002] Polyolefin materials have the advantages of high cost performance, good mechanical properties, stable thermal properties, etc., and are widely used in various fields of industrial production and daily life, and have now become the largest class of synthetic polymer materials. However, the inherent non-polarity and inertness of polyolefin materials limit their applicability in fields such as mixing, adhesion, and dyeing. Therefore, synthesizing functionalized polyolefins to improve their surface properties and compatibility with other materials has important scientific significance and industrial application value. The two main strategies for synthesizing functionalized polyolefins include direct copolymerization of olefins with functional monomers and post-polymerization modification. Direct copolymerization includes free radical copolymerization and coordination copolymerization. However, free radical polymerization has strict conditions and low controllability, and coordination polymerization is prone to catalyst poisoning and a sharp decline in activity. In addition, post-polymerization modification is limited to specific chemical reactions, such as free radical grafting. In recent years, introducing reactive groups for controlled polymerization has become a promising third method. Polyolefin reaction intermediates are formed by introducing C═C side groups into polyolefins and can be further converted into functionalized polyolefins. Therefore, the polymerization of non-conjugated dienes provides a convenient and direct method for synthesizing these intermediates.

[0003] Waymouth et al. reported a series of metallocene catalysts that catalyze the homopolymerization of 1,5-hexadiene with a selectivity of up to 99% for the cyclization reaction, and successfully synthesized optically active poly(methylene-1,3-cyclopentane) (J. Am. Chem. Soc. 1993, 115, 91-98). Li et al. reported that the CNN tridentate coordinated Hf catalyst catalyzes the homopolymerization of 1,5-hexadiene or 1,7-octadiene, showing significant cyclization selectivity, activity, and monomer conversion (Macromolecules 2011, 44, 1062-1065). However, these polymers cannot be further functionalized due to the lack of double bond functional groups.

[0004] In the present invention, a highly active and high copolymerization performance hafnium metal catalyst is selected to catalyze the homopolymerization of 1,5-hexadiene to prepare poly(1,5-hexadiene), and the molecular weight of the obtained polyolefin containing double bonds is 3×10 3 -1×10 5g / mol, with a molecular weight distribution of 1.1 to 4.0, a glass transition temperature of -20 to 30 °C, and a double bond content of 2 - 50 mol%. This polymerization reaction avoids the cross-linking of the polymer and introduces hydroxyl functional groups through thiol-ene chemistry. Using the hydroxyl-containing polyolefin as a macromolecular initiator, the ring-opening polymerization of L-lactide (L-LA) is catalyzed by an organic catalyst to prepare a graft copolymer with a polyolefin main chain and a polylactic acid (PLA) side chain; further, the toughness of commercial polylactic acid PLA is enhanced by the prepared graft copolymer. Therefore, the present invention has original innovation and provides a new direction for the development of the polyolefin field. Summary of the Invention

[0005] The object of the present invention is to provide a method for preparing a polyolefin containing a certain content of double bonds by using a hafnium metal catalyst, and a polymerization method for introducing a hydroxyl group to initiate ring-opening polymerization to obtain a graft polymer. The polymer obtained by this method can be used as an effective toughening agent to toughen commercial polylactic acid PLA. To achieve the above object, on the one hand, the present invention provides a method for preparing a polyolefin containing double bonds, comprising the following steps: (1) dissolving a hafnium metal catalyst and a cocatalyst in an organic solvent and stirring at room temperature for 2 - 10 minutes; (2) adding 1,5-hexadiene to the above mixed solution, reacting at a certain temperature for a period of time, adding an acidic substance to terminate the reaction, and adding the reaction mixture to ethanol for precipitation to obtain poly(1,5-hexadiene); in the above preparation method, the organic solvent in step (1) is one or more of n-hexane, heptane, pentane, and toluene; in the above preparation method, the hafnium metal catalyst includes the hafnium metal catalyst shown in formula (I); the cocatalyst includes one or more of an aluminum compound and a boron compound; the aluminum compound is one or more of methylaluminoxane, ethylaluminoxane, isobutylaluminoxane, trimethylaluminum, triethylaluminum, triisobutylaluminum, tri-n-hexylaluminum, chloro-diethylaluminum, sesqui-chloro-diethylaluminum, and diethylaluminum dichloride; the boron compound is one or more of tris(pentafluorophenyl)borane and triphenylcarbenium tetrakis(pentafluorophenyl)borate; the content molar ratio of the hafnium metal catalyst based on the Hf element to the boron compound based on the boron element is 1:0.5 - 100, preferably 1:0.5 - 10, and the content molar ratio of the hafnium metal catalyst based on the Hf element to the aluminum compound based on the aluminum element is 1:10 - 1000, preferably 1:10 - 100; the chemical structural formula of the poly(1,5-hexadiene) is shown in (II), where the molar ratio of x / y is 98:2 to 50:50; in the above preparation method, the reaction temperature in step (2) is 0 - 50 °C; the reaction time is 0.1 - 24 h; in the above preparation method, the acidic substance is glacial acetic acid, benzoic acid, hydrochloric acid, sulfuric acid, phosphoric acid, and the molar ratio of the acidic substance to the hafnium metal catalyst is 1 / 1 - 10 / 1.

[0006]

[0007]

[0008] In a second aspect, the present invention further provides a method for introducing hydroxyl groups into a polymer containing externally pendant double bonds prepared by the above method, wherein the hydroxyl group content in the polymer is 2-50 mol%; the method includes the following steps: (1) dissolving the polyolefin containing double bonds prepared in claim 1 in an organic solvent and placing it in an inert gas atmosphere; (2) adding 2,2'-azobisisobutyronitrile (AIBN) and β-mercaptoethanol to the above mixed solution, reacting for a period of time at a certain temperature, and adding the reaction mixture to ethanol for precipitation to obtain a polymer containing hydroxyl groups; in the above preparation method, the organic solvent in step (1) is one or more of dichloromethane, chloroform, chlorobenzene, toluene, xylene, and mesitylene; the reaction temperature in step (2) is 50-120 °C; the reaction time is 0.5-24 h.

[0009] In a third aspect, the present invention further provides a method for initiating the ring-opening polymerization of L-lactide (L-LA) using the polymer containing hydroxyl groups prepared by the above method as a macroinitiator; the method includes the following steps:

[0010] (1) Dissolve the polyolefin containing hydroxyl groups prepared in claim 3 in an organic solvent, add a catalyst, and place it in an inert gas atmosphere; (2) add L-lactide (L-LA) to the above mixed solution, react for a period of time at a certain temperature, add an acidic substance to terminate the reaction, and add the reaction mixture to ethanol for precipitation to obtain a copolymer; in the above preparation method, the organic solvent in step (1) is one or more of dichloromethane, chloroform, chlorobenzene, toluene, xylene, and mesitylene; the catalyst in step (1) is an organophosphazene base selected from one or more of cyclotriphosphazene base (CTPB, chemical structural formula as shown in (III)), phosphazene P4 (t-Bu-P4, chemical structural formula as shown in (IV)), and phosphazene P2 (t-Bu-P2, chemical structural formula as shown in (V)); the reaction temperature in step (2) is 0-120 °C; the reaction time is 0.1-24 h; the molecular weight of the obtained graft copolymer with a polyolefin main chain and a poly(lactic acid) (PLA) side chain is 6×10 3 -1×10 6 g / mol, the molecular weight distribution is 1.2-4.0, the glass transition temperature is -5-50 °C, and the PLA content is 20-90 wt%;

[0011]

[0012] Fourthly, the present invention also provides a method for toughening commercial poly(lactic acid) (PLA) using a graft copolymer with a polyolefin main chain and poly(lactic acid) (PLA) side chains prepared by the above method; the method comprises the following steps: (1) Before processing, all graft polymers are dried in a vacuum oven at 50 °C for 12 hours to remove moisture and organic solvents; (2) Subsequently, a blend of commercial poly(lactic acid) (PLA) and the graft copolymer with a composition range of 99 / 1 to 50 / 50 (weight ratio: w / w) is mixed in a toluene solution at 105 °C; (3) Thereafter, the blend is precipitated in ethanol and thoroughly dried in a vacuum oven; the glass transition temperature of the toughened commercial poly(lactic acid) (PLA) is 40 - 55 °C, the tensile strength is 30 - 60 MPa, and the elongation at break is 8 - 900%. Detailed implementation manners

[0013] The present invention will be further illustrated by examples, but the present invention is not limited thereto. The examples of the present invention can enable those skilled in the art to understand the present invention more comprehensively.

[0014] The experimental methods used in the following examples are all conventional methods unless otherwise specified.

[0015] The hafnium metal catalysts in the present invention, Hf1 (Organometallics 2021, 40, 242 - 252), Hf2 (Organometallics 2024, 43, 2472–2479) are synthesized with reference to the literature methods. The organic catalyst CTPB (Angew.Chem., Int.Ed. 2017, 56, 12987 - 12990) is synthesized with reference to the literature. Commercial PLA (grade: PLA3001D) is purchased from NatureWorks.

[0016] Example 1

[0017] Dissolve 5 μmol of catalyst Hf1, 6 μmol of tris(pentafluorophenyl)borate, and 2.5 mmol of methylaluminoxane MAO in 18 mL of ultra-dry toluene, place it at 25 °C and stir for 5 min, and add 2 mL of 1,5-hexadiene to the reaction tube with a syringe. The reaction is carried out under nitrogen protection for 60 min, and 2 drops of hydrochloric acid are added to terminate the reaction. Pour the reaction mixture into 50 mL of ethanol, and centrifuge to separate the precipitate to obtain the polymer. The number-average molecular weight measured by GPC is 0.5 kg / mol, and the molecular weight distribution is 2.12.

[0018] Example 2

[0019] Dissolve 5 μmol of catalyst Hf1, 6 μmol of tris(pentafluorophenyl)borate, and 2.5 mmol of methylaluminoxane (MAO) in 14 mL of ultra-dry toluene, place it in a 50 °C bath and stir for 5 min, then add 6 mL of 1,5-hexadiene to the reaction tube using a syringe. The reaction is carried out under nitrogen protection for 24 h, and then terminated by adding 2 drops of hydrochloric acid. Pour the reaction mixture into 50 mL of ethanol, and centrifuge to separate the precipitate to obtain the polymer. The number-average molecular weight measured by GPC is 100 kg / mol, and the molecular weight distribution is 4.00.

[0020] Example 3

[0021] Dissolve 5 μmol of catalyst Hf2, 6 μmol of tris(pentafluorophenyl)borate, and 2.5 mmol of methylaluminoxane (MAO) in 18 mL of ultra-dry toluene, place it in a 25 °C bath and stir for 5 min, then add 2 mL of 1,5-hexadiene to the reaction tube using a syringe. The reaction is carried out under nitrogen protection for 60 min, and then terminated by adding 2 drops of hydrochloric acid. Pour the reaction mixture into 50 mL of ethanol, and centrifuge to separate the precipitate to obtain the polymer. The number-average molecular weight measured by GPC is 0.5 kg / mol, and the molecular weight distribution is 1.56.

[0022] Example 4

[0023] Dissolve 5 μmol of catalyst Hf2, 6 μmol of tris(pentafluorophenyl)borate, and 2.5 mmol of methylaluminoxane (MAO) in 14 mL of ultra-dry toluene, place it in a 50 °C bath and stir for 5 min, then use a syringe to add 6 mL of 1,5-hexadiene to the reaction tube. The reaction is carried out under nitrogen protection for 24 h, and then terminated by adding 2 drops of hydrochloric acid. Pour the reaction mixture into 50 mL of ethanol, and centrifuge to separate the precipitate to obtain the polymer. The number-average molecular weight measured by GPC is 80 kg / mol, and the molecular weight distribution is 3.60.

[0024] The collected polymer is subjected to solvent removal to obtain a solid polymer, which is dried in vacuo and weighed to obtain the polymer yield data, and the corresponding monomer conversion rates are listed in Table 1. The MCP (cyclic structure) content and VTM (externally pendant double bond) content of the prepared polymer are calculated according to 1 the 1H NMR spectrum and listed in Table 1. The glass transition temperature and melting temperature of the polymer are measured using a differential scanning calorimeter and listed in Table 1.

[0025] Table 1

[0026]

[0027] Example 5

[0028] Take 0.3 g of the polymer obtained in Example 3 and place it in a Schlenk flask. Dry it under vacuum at 40 °C for 1 h. Inject 5 mL of toluene under a nitrogen atmosphere. After stirring to dissolve it thoroughly, add 0.18 mL of β-mercaptoethanol and 0.06 g of azobisisobutyronitrile. React overnight at 80 °C, precipitate with a large amount of absolute ethanol, wash the polymer, filter it, and dry it under vacuum at 60 °C for 12 h. The hydroxyl content is determined by 1 1H NMR spectrum and is 10.4 mol%.

[0029] Example 6

[0030] The polymerization reaction process and reaction conditions are the same as those in Example 5, but take 0.3 g of the polymer obtained in Example 4 and place it in a Schlenk flask. The hydroxyl content is determined by 1 1H NMR spectrum and is 50.0 mol%.

[0031] Example 7

[0032] Dissolve the polymer (0.02 mmol, 86 mg) obtained in Example 5, the CTPB catalyst (0.02 mmol, 2.78 mg), and L-lactide (1.00 mmol, 144 mg) in 2.0 mL of toluene. Place it in an 80 °C oil bath, and carry out the reaction under nitrogen protection for 90 min. Add 2 drops of acetic acid to terminate the reaction. Pour the reaction mixture into 10 mL of ice-cold ethanol, and centrifuge to separate the precipitate to obtain the polymer. The number-average molecular weight measured by GPC is 0.6 kg / mol, the molecular weight distribution is 1.2, T g =-5.6 °C, and the PLA content in the copolymer is 20.0 wt%.

[0033] Example 8

[0034] Dissolve the polymer (0.02 mmol, 86 mg) obtained in Example 5, the CTPB catalyst (0.02 mmol, 2.78 mg), and L-lactide (3.00 mmol, 432 mg) in 2.0 mL of toluene. Place it in an 80 °C oil bath, and carry out the reaction under nitrogen protection for 90 min. Add 2 drops of acetic acid to terminate the reaction. Pour the reaction mixture into 10 mL of ice-cold ethanol, and centrifuge to separate the precipitate to obtain the polymer. The number-average molecular weight measured by GPC is 16 kg / mol, the molecular weight distribution is 2.79, T g =35.4 °C, and the PLA content in the copolymer is 68.3 wt%.

[0035] Example 9

[0036] The polymer obtained in Example 6 (0.04 mmol, 320 mg), CTPB catalyst (0.04 mmol, 5.56 mg), and L-lactide (5.00 mmol, 1440 mg) were dissolved in 6.0 mL of toluene and placed in an 80 °C oil bath. The reaction was carried out under nitrogen protection for 90 min, and 2 drops of acetic acid were added to terminate the reaction. The reaction mixture was poured into 10 mL of ice-cold ethanol, and the precipitate was separated by centrifugation to obtain the polymer. The number-average molecular weight measured by GPC was 110 kg / mol, the molecular weight distribution was 2.97, T g = 29.7 °C, and the PLA content in the copolymer was 53.2 wt%.

[0037] Example 10

[0038] The polymer obtained in Example 6 (0.06 mmol, 480 mg), CTPB catalyst (0.06 mmol, 8.34 mg), and L-lactide (10.00 mmol, 2880 mg) were dissolved in 6.0 mL of toluene and placed in an 80 °C oil bath. The reaction was carried out under nitrogen protection for 90 min, and 2 drops of acetic acid were added to terminate the reaction. The reaction mixture was poured into 10 mL of ice-cold ethanol, and the precipitate was separated by centrifugation to obtain the polymer. The number-average molecular weight measured by GPC was 800 kg / mol, the molecular weight distribution was 3.86, T g = 40.1 °C, and the PLA content in the copolymer was 65.0 wt%.

[0039] Example 11

[0040] The polymer obtained in Example 6 (0.1 mmol, 800 mg), CTPB catalyst (0.1 mmol, 13.9 mg), and L-lactide (20.00 mmol, 5760 mg) were dissolved in 10.0 mL of toluene and placed in an 80 °C oil bath. The reaction was carried out under nitrogen protection for 90 min, and 2 drops of acetic acid were added to terminate the reaction. The reaction mixture was poured into 10 mL of ice-cold ethanol, and the precipitate was separated by centrifugation to obtain the polymer. The number-average molecular weight measured by GPC was 1000 kg / mol, the molecular weight distribution was 3.98, T g = 50.4 °C, and the PLA content in the copolymer was 90.3 wt%.

[0041] Example 12

[0042] The polymer of Example 7 was dried in a vacuum oven at 50 °C for 12 hours. Subsequently, a blend of 1 wt% graft polymer and 99 wt% commercial PLA was thoroughly mixed in a toluene solution at 105 °C. After that, the blend was precipitated in ethanol and thoroughly dried in a vacuum oven. Finally, they were made into suitable samples according to the test requirements. Tensile stress: 36.2 MPa, elongation at break: 85%.

[0043] Example 13

[0044] The polymer of Example 7 was dried in a vacuum oven at 50 °C for 12 hours. Subsequently, 5 wt% of the graft polymer was thoroughly mixed with 95 wt% of a commercial blend of PLA and the graft polymer in a toluene solution at 105 °C. Thereafter, the blend was precipitated in ethanol and thoroughly dried in a vacuum oven. Finally, suitable samples were prepared according to the test requirements. Tensile stress: 49.5 MPa, Elongation at break: 176%.

[0045] Example 14

[0046] The polymer of Example 7 was dried in a vacuum oven at 50 °C for 12 hours. Subsequently, 50 wt% of the graft polymer was thoroughly mixed with 50 wt% of a commercial blend of PLA and the graft polymer in a toluene solution at 105 °C. Thereafter, the blend was precipitated in ethanol and thoroughly dried in a vacuum oven. Finally, suitable samples were prepared according to the test requirements. Tensile stress: 30.5 MPa, Elongation at break: 500%.

[0047] Example 15

[0048] The polymer of Example 8 was dried in a vacuum oven at 50 °C for 12 hours. Subsequently, 1 wt% of the graft polymer was thoroughly mixed with 99 wt% of a commercial blend of PLA and the graft polymer in a toluene solution at 105 °C. Thereafter, the blend was precipitated in ethanol and thoroughly dried in a vacuum oven. Finally, suitable samples were prepared according to the test requirements. Tensile stress: 45.5 MPa, Elongation at break: 160%.

[0049] Example 16

[0050] The polymer of Example 8 was dried in a vacuum oven at 50 °C for 12 hours. Subsequently, 5 wt% of the graft polymer was thoroughly mixed with 95 wt% of a commercial blend of PLA and the graft polymer in a toluene solution at 105 °C. Thereafter, the blend was precipitated in ethanol and thoroughly dried in a vacuum oven. Finally, suitable samples were prepared according to the test requirements. Tensile stress: 53.0 MPa, Elongation at break: 350%.

[0051] Example 17

[0052] The polymer of Example 8 was dried in a vacuum oven at 50 °C for 12 hours. Subsequently, 50 wt% of the graft polymer was thoroughly mixed with 50 wt% of a commercial blend of PLA and the graft polymer in a toluene solution at 105 °C. Thereafter, the blend was precipitated in ethanol and thoroughly dried in a vacuum oven. Finally, suitable samples were prepared according to the test requirements. Tensile stress: 35.5 MPa, Elongation at break: 900%. Description of the Drawings

[0053] Figure 1 1H NMR spectrum of the poly(1,5 - hexadiene) prepared in Example 1 1 .

[0054] Figure 2 1H NMR spectrum of the poly(1,5 - hexadiene) prepared in Example 2 1 .

[0055] Figure 3 13C NMR spectrum of the poly(1,5 - hexadiene) prepared in Example 3 13 .

[0056] Figure 4 1H NMR spectrum of the poly(1,5 - hexadiene - hydroxy) prepared in Example 5 1 .

[0057] Figure 5 DSC curve of the poly(1,5 - hexadiene) prepared in Example 3

[0058] Figure 6 1H NMR spectrum of the poly(1,5 - hexadiene - g - PLA) prepared in Example 8 1 .

[0059] Figure 7 DSC spectrum of the poly(1,5 - hexadiene - g - PLA) prepared in Example 8

[0060] Figure 8 Stress - strain curve of the graft copolymer blended with commercial PLA in Example 13

[0061] Figure 9 Stress - strain curve of the graft copolymer blended with commercial PLA in Example 17

Claims

1. A method for preparing a polyolefin containing double bonds, comprising the following steps: (1) dissolving a hafnium metal catalyst and a cocatalyst in an organic solvent and stirring for 2-10 minutes at room temperature; (2) adding 1,5-hexadiene to the above mixed solution, reacting for a period of time at a certain temperature, adding an acidic substance to terminate the reaction, and adding the reaction mixture to ethanol for precipitation to obtain poly(1,5-hexadiene); wherein: In step (1), the organic solvent is one or more of n-hexane, heptane, pentane, and toluene; the molecular weight of the polyolefin containing double bonds obtained in the present invention is 3×10 3 -1×10 5 g / mol, the molecular weight distribution is 1.1 to 4.0, the glass transition temperature is -20 to 30°C, and the double bond content is 2-50 mol%.

2. The preparation method of the polyolefin containing double bonds according to claim 1, characterized in that: The hafnium metal catalyst includes the hafnium metal catalyst shown in formula (I); the cocatalyst includes one or more of an aluminum compound and a boron compound; the aluminum compound is one or more of methylaluminoxane, ethylaluminoxane, isobutylaluminoxane, trimethylaluminum, triethylaluminum, triisobutylaluminum, tri-n-hexylaluminum, monochlorodiethylaluminum, sesquialter monochlorodiethylaluminum, and dichloroethylaluminum; the boron compound is one or more of tris(pentafluorophenyl)borane and triphenylcarbenium tetrakis(pentafluorophenyl)borate; the content molar ratio of the hafnium metal catalyst in terms of Hf element to the boron compound in terms of boron element is 1:0.5 - 100, preferably 1:0.5 - 10, and the content molar ratio of the hafnium metal catalyst in terms of Hf element to the aluminum compound in terms of aluminum element is 1:10 - 1000, preferably 1:10 - 100; the chemical structural formula of the poly(1,5-hexadiene) is shown in (II), where the x / y molar ratio is 98:2 to 50:50; In step (2), the reaction temperature is 0 - 50 °C; the reaction time is 0.1 - 24 h; the acidic substance is glacial acetic acid, benzoic acid, hydrochloric acid, sulfuric acid, phosphoric acid, and the molar ratio of the acidic substance to the hafnium metal catalyst is 1 / 1 - 10 / 1.

3. A method for preparing a polyolefin containing hydroxyl groups, characterized in that: Introduce hydroxyl groups into the polyolefin containing double bonds prepared in claim 1, and the hydroxyl group content in the prepared polyolefin containing hydroxyl groups is 2 - 50 mol%; it includes the following steps: (1) Dissolve the polyolefin containing double bonds prepared in claim 1 in an organic solvent and place it in an inert gas atmosphere; (2) Add 2,2'-azobisisobutyronitrile (AIBN) and β-mercaptoethanol to the above mixed solution, react for a certain period of time at a certain temperature, and add the reaction mixture to ethanol for precipitation to obtain a polymer containing hydroxyl groups; in step (1), the organic solvent is one or more of dichloromethane, chloroform, chlorobenzene, toluene, xylene, and mesitylene; in step (2), the reaction temperature is 50 - 120 °C; the reaction time is 0.5 - 24 h.

4. A method for preparing a graft copolymer with a polyolefin main chain and a polylactic acid (PLA) side chain, characterized in that: Using the polyolefin containing hydroxyl groups prepared in claim 3 as a macroinitiator to initiate the ring-opening polymerization of L-lactide (L-LA); the method comprises the following steps: (1) dissolving the polyolefin containing hydroxyl groups prepared in claim 3 in an organic solvent, adding a catalyst, and placing it in an inert gas atmosphere; (2) adding L-lactide (L-LA) to the above mixed solution, reacting for a certain period of time at a certain temperature, adding an acidic substance to terminate the reaction, and adding the reaction mixture to ethanol for precipitation to obtain a copolymer; in the step (1), the organic solvent is one or more of dichloromethane, chloroform, chlorobenzene, toluene, xylene, and mesitylene; in the step (1), the catalyst is an organophosphazene base, selected from one or more of cyclotriphosphazene base (CTPB, chemical structural formula as shown in (III)), phosphazene P4 (t-Bu-P4, chemical structural formula as shown in (IV)), and phosphazene P2 (t-Bu-P2, chemical structural formula as shown in (V)); in the step (2), the reaction temperature is 0-120 °C; the reaction time is 0.1-24 h; the molecular weight of the graft copolymer with a polyolefin main chain and a polylactic acid (PLA) side chain is 6×10 3 -1×10 6 g / mol, the molecular weight distribution is 1.2-4.0, the glass transition temperature is -5-50 °C, and the PLA content is 20-90 wt%; 5. A method for toughening commercial poly(lactic acid) (PLA), characterized in that: Use the graft copolymer with a polyolefin main chain and a polylactic acid (PLA) side chain prepared in claim 4 as a toughening agent to toughen commercial polylactic acid (PLA); it includes the following steps: (1) Before processing, dry all the graft polymers in a vacuum oven at 50 °C for 12 hours to remove moisture and organic solvents; (2) Subsequently, mix a blend of commercial polylactic acid (PLA) and the graft copolymer with a composition range of 99 / 1 to 50 / 50 (weight ratio: w / w) in a toluene solution at 105 °C; (3) Then, precipitate the blend in ethanol and thoroughly dry it in a vacuum oven; the glass transition temperature of the obtained toughened commercial polylactic acid (PLA) is 40 - 55 °C, the tensile strength is 30 - 60 MPa, and the elongation at break is 8 - 900%.

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