Preparation method of bio-based recyclable high-performance hydrolysis-resistant polyurethane

Abstract

The invention provides a novel method for preparing poly (alkyl-delta-lactone) type high-strength and high-hydrolysis-resistance polyurethane by using a catalyst conversion strategy through a one-pot method, and compared with a traditional method, the method has the following beneficial effects: 1) the method for preparing polyurethane by using the catalyst conversion strategy belongs to one-pot synthesis, the process is simple, the cost is low, and the method is suitable for industrial production; the polyester polyol does not need to be separated and purified independently; 2) the prepared poly (alkyl-delta-lactone) type polyurethane has high molecular weight and good mechanical properties; 3) the poly (alkyl-delta-lactone) type polyurethane is endowed with excellent hydrolysis resistance due to good hydrophobicity and weak nucleophilicity of secondary hydroxyl, and the service life is prolonged; and 4) the poly (alkyl-delta-lactone) type polyurethane can be depolymerized and recovered at high yield under the bulk condition to obtain the alkyl-delta-lactone monomer.

Description

Technical Field

[0001] The present invention relates to the fields of polymer materials and chemical engineering, and in particular to a bio-based recyclable high-performance hydrolysis-resistant polyurethane and a preparation method thereof. Background Art

[0002] Polyurethane has rapidly become an indispensable key material in many industries due to its outstanding physical and chemical properties, such as excellent elasticity, wear resistance, and fatigue resistance. From daily clothing and furniture filling materials to implants in the medical field, the comfort layer of car seats, to thermal insulation materials and precision parts in the aerospace field, polyurethane has a wide range of applications, making it the sixth largest polymer material in the world. Compared with polyurethane prepared from polyether polyols, polyester polyurethane generally exhibits better thermal stability and better mechanical properties, but due to its rich ester bond structure, it is easily hydrolyzed and will be completely degraded in a relatively short time under the catalysis of acids and bases. This degradation characteristic greatly shortens the service life of polyester polyurethane materials. For example, polyurethanes prepared from polyols such as poly(γ-butyrolactone), polydioxanone and polyglycolic acid can be completely degraded in a short period of time (Polym.Degrad.Stab.2022,204,110116; Eur.Polym.J.2023,199,112447; Green Chem.2020,22,4055–4081). The hydrophilicity and hydrophobicity of the polyester component have an important influence on the hydrolytic stability of polyester polyurethane. It is speculated that the introduction of a hydrophobic structure of a methylene side chain can significantly improve the hydrolytic degradation stability of polyurethane materials.

[0003] Alkyl-δ-lactones are a class of six-membered ring lactones with various alkyl groups at the δ position. They are widely found in nature and can be easily prepared through efficient conversion of biomass platform molecules. The poly(alkyl-δ-lactones) obtained by ring-opening polymerization can be quantitatively depolymerized under appropriate conditions to recover the original monomers, thereby realizing a closed-loop life cycle of "monomer-polymer-monomer". Poly(alkyl-δ-caprolactones) have good flexibility and excellent biocompatibility, which make this type of polymer an ideal precursor for the preparation of high-performance polyurethanes and have great application potential in the field of constructing chemically recycled polyurethane materials. And due to the hydrophobic alkyl group and the weak nucleophilicity of the secondary hydroxyl group, the polyurethane prepared from poly(alkyl-δ-lactones) is expected to show good hydrolysis resistance while maintaining excellent performance, thereby significantly extending the service life.

[0004] In view of this, the present invention provides a novel method for preparing poly(alkyl-δ-lactone) type polyurethane with high strength and high hydrolysis resistance by one-pot method using catalyst conversion strategy. Compared with the traditional method, the method provided by the present invention has the following advantages: 1) the use of catalyst conversion strategy to prepare polyurethane is a one-pot synthesis, the process is simple, and there is no need to separate and purify polyester polyols separately; 2) the prepared poly(alkyl-δ-lactone) type polyurethane has a high molecular weight and good mechanical properties. 3) Due to its good hydrophobicity and weak nucleophilicity of secondary hydroxyl groups, poly(alkyl-δ-lactone) type polyurethane gives polyurethane excellent hydrolysis resistance and prolongs its service life; 4) poly(alkyl-δ-lactone) type polyurethane can be depolymerized and recovered at a high yield under bulk conditions to obtain alkyl-δ-lactone monomers. Summary of the invention

[0005] The object of the present invention is to provide a bio-based, high-strength, recyclable, hydrolysis-resistant polyurethane having a chemical structure as shown in formula (I):

[0006]

[0007] x is a natural number greater than or equal to 0, m is a natural number greater than or equal to 2, y is a natural number greater than or equal to 1, and z is a natural number greater than or equal to 1;

[0008] Wherein R1 and R3 have the following structure:

[0009]

[0010] R2 has the following structure:

[0011]

[0012] The present invention also provides a method for preparing the polyurethane, comprising the following steps:

[0013] (1) mixing alkyl-δ-lactone, polyol initiator, strong base and urea uniformly, reacting at 10-50° C. for 1-30 min, and then adding an acidic substance to the above system to terminate the polymerization;

[0014] (2) adding isocyanate and a catalyst to the above system, reacting at 30-100° C. for 0.1-24 hours to obtain an isocyanate-terminated prepolymer;

[0015] (3) Finally, a measured amount of chain extender is added to the prepolymer, and the reaction is carried out at 30 to 120° C. for 0.1 to 24 hours to obtain polyurethane.

[0016] In the above preparation method, the alkyl-δ-lactone in step (1) is δ-caprolactone, δ-heptanolactone, δ-octanolactone, δ-nonanolactone, δ-decanolactone, δ-undecalactone, δ-dodecalactone, δ-tridecalactone, δ-tetradecanolactone, δ-pentadecalactone, δ-hexadecanolactone; the polyol initiator is ethylene glycol, propylene glycol, methyl propanediol, neopentyl glycol, butanediol, 1,4-cyclohexanediol, 1,6-hexanediol, diethylene glycol, hydroquinone-bis(β-hydroxyethyl) ether, glycerol α-allyl ether, dihydroxyethyl terephthalate, ethylene oxide-terminated bisphenol A , propylene glycol, trihydroxypropane, pentaerythritol; the strong base is sodium, potassium, potassium hydride, sodium hydride, hexa[tri(dimethylamine)phosphazene]tripolyphosphazene ({[(NMe2)3P=N]2P=N}3), phosphazene ligand P4-tert-butyl ([(NMe2)3P=N]3P=NtBu, tert-Bu-P4), phosphazene ligand P2-tert-butyl ([(NMe2)3P=N](NMe2)2P=NtBu, tert-Bu-P2); the acidic substance is acetic acid, benzoic acid, hydrochloric acid, sulfuric acid, phosphoric acid, diphenyl phosphate.

[0017] In the above preparation method, the urea in step (1) is:

[0018]

[0019] In the above preparation method, the molar ratio of the strong base to the polyol initiator in step (1) is 1 / 3 to 20 / 1; the molar ratio of the strong base to urea is 1 / 0.5 to 1 / 10; the molar ratio of the polyol initiator to alkyl-δ-caprolactone is 1 / 5 to 1 / 300; and the molar ratio of the acidic substance to the strong base is 1 / 1 to 10 / 1.

[0020] In the above preparation method, the polyurethane catalyst in step (2) is specifically N,N-dimethylcyclohexylamine, bis(2-dimethylaminoethyl) ether, triethylamine, triethylenediamine, N,N-dimethylbenzylamine, N-ethylmorpholine, N-methylmorpholine, N,N'-diethylpiperazine, pyridine, 4-dimethylaminopyridine, dibutyltin dilaurate, and stannous octoate. The isocyanate is toluene diisocyanate, methylcyclohexyl diisocyanate, isophorone diisocyanate, p-phenylene diisocyanate, 4,4'-diisocyanato-3,3'-dimethyl-1,1'-biphenyl, cyclohexyl dimethylene diisocyanate, diphenylmethane diisocyanate, polymethylene polyphenyl polyisocyanate, dicyclohexylmethane diisocyanate, hexamethylene diisocyanate, and 1,5-naphthalene diisocyanate.

[0021] In the above preparation method, the molar ratio of the isocyanate to the polyol initiator in step (2) is 1 / 1 to 11 / 1; the mass percentage of the catalyst (relative to the isocyanate) is 0.02 to 10 wt%.

[0022] In the above preparation method, the molar ratio of the chain extender to the polyol initiator in step (3) is 0.1 / 1 to 10 / 1. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is the GPC spectrum of the polyurethane prepared in Comparative Example 2 and Example 1-2.

[0024] Figure 2 The tensile curves of the polyurethanes prepared in Comparative Example 2 and Example 1-2 are shown in FIG. 1 , and the tensile rate is 50 mm / min.

[0025] Figure 3 is the infrared spectrum of the polyurethane prepared in Example 1.

[0026] Figure 4 The polyurethane obtained in Example 5 1 H NMR spectrum.

[0027] Figure 5 The polyurethane obtained in Example 5 13 C NMR spectrum.

[0028] Figure 6 is the water contact angle of the four polyurethanes (Examples 1, 4, 5, and 6) in Example 7.

[0029] Figure 7 The four polyurethanes (Examples 1, 4, 5, and 6) in Example 7 were prepared in an alkaline solution (1 mol L -1 NaOH solution)

[0030] Histogram of mass loss in .

[0031] Figure 8 The four polyurethanes (Examples 1, 4, 5, and 6) in Example 7 were prepared in an acidic solution (1 mol L -1 HCl solution)

[0032] Histogram of mass loss in . DETAILED DESCRIPTION

[0033] The present invention is described in detail with reference to the following embodiments, but the present invention is not limited to these embodiments.

[0034] Unless otherwise specified, the materials and reagents used in the following examples can be obtained from commercial sources.

[0035] Comparative Example 1

[0036] (0.02mmol, 7.4mg) phosphazene ligand P2-tert-butyl, cocatalyst U2 (0.04mmol, 8.5mg), (1.5mmol, 135mg) 1,4-butanediol were added to a reaction tube, and then (30mmol, 3.3mL) δ-caprolactone was added to the reaction tube and stirred. The reaction was carried out under nitrogen protection for 20min, and then (0.022mmol, 5.5mg) diphenyl phosphate was added and stirred for 10min to terminate. Subsequently, the reaction tube was placed in a 70°C oil bath and stirred for 10min, and (1.575mmol, 413mg) dicyclohexylmethane diisocyanate and (0.1wt%, 4mg) dibutyltin dilaurate were added, and the reaction was carried out at 70°C for 2h, then the temperature was raised to 90°C for 2h, and finally cured at 100°C for 2h to obtain polyurethane. GPC measured a weight average molecular weight of 165.1 kg / mol, a number average molecular weight of 102.6 kg / mol, and a molecular weight distribution of 1.61. Since no chain extender was added, the hard segment content was only 11 wt%, and the obtained polyurethane exhibited creep properties.

[0037] Comparative Example 2

[0038] ((0.02mmol, 7.4mg) phosphazene ligand P2-tert-butyl, cocatalyst U2 (0.04mmol, 8.5mg), (6mmol, 540mg) 1,4-butanediol were added to the reaction tube, and then (30mmol, 3.3mL) δ-caprolactone was added to the reaction tube and stirred. The reaction was carried out under nitrogen protection for 20min, and then (0.022mmol, 5.5mg) diphenyl phosphate was added and stirred for 10min to terminate. The reaction tube was then placed at 7 The mixture was stirred in an oil bath at 0°C for 10 min, and dicyclohexylmethane diisocyanate (6.3 mmol, 1652 mg) and dibutyltin dilaurate (0.1 wt%, 16 mg) were added. The mixture was first reacted at 70°C for 2 h, then heated to 90°C for 2 h, and finally cured at 100°C for 2 h to obtain polyurethane. The weight average molecular weight measured by GPC was 186.5 kg / mol, the number average molecular weight was 104.8 kg / mol, and the molecular weight distribution was 1.78. The GPC spectrum is shown in the figure below. Figure 1 The hard segment content of the prepared polyurethane is as high as 32wt%, but because no chain extender is added, the lengths of the soft and hard segments are short, and the mechanical properties of the obtained polyurethane are poor, with a breaking strength of 8.7MPa and a breaking elongation of 980%. The tensile curve is shown in Figure 2 shown.

[0039] Example 1

[0040] (0.02mmol, 7.4mg) phosphazene ligand P2-tert-butyl, cocatalyst U2 (0.04mmol, 8.5mg), (1.5mmol, 135mg) 1,4-butanediol were added to a reaction tube, and then (30mmol, 3.3mL) δ-caprolactone was added to the reaction tube and stirred. The reaction was carried out under nitrogen protection for 20min, and then (0.022mmol, 5.5mg) diphenyl phosphate was added and stirred for 10min to terminate. Subsequently, the reaction tube was placed in an oil bath at 70°C and stirred for 10min, and then (3.15mmol, 826mg) dicyclohexylmethane diisocyanate and (0.1wt%, 8mg) dibutyltin dilaurate were added and reacted for 40min to obtain an isocyanate-terminated prepolymer. Finally, 1,4-butanediol (1.5 mmol, 135 mg) was added under nitrogen protection, and the reaction was carried out at 70°C for 2 h, then at 90°C for 2 h, and finally at 100°C for 2 h to obtain polyurethane. The infrared spectrum shows that the characteristic peak of isocyanate disappears, indicating that the polyurethane has reacted completely. Figure 3 As shown. GPC measured the weight average molecular weight to be 210.7 kg / mol, the number average molecular weight to be 102.7 kg / mol, and the molecular weight distribution to be 2.05. The GPC spectrum is shown as Figure 1 Due to the addition of chain extender, the hard segment content of the obtained polyurethane is increased to 23wt%, showing good mechanical properties, with a breaking strength of 32MPa and a breaking elongation of 1520%. The tensile curve is as follows Figure 2 shown.

[0041] Example 2

[0042] (0.02mmol, 7.4mg) phosphazene ligand P2-tert-butyl, cocatalyst U2 (0.04mmol, 8.5mg), (1.5mmol, 135mg) 1,4-butanediol were added to a reaction tube, and then (30mmol, 3.3mL) δ-caprolactone was added to the reaction tube and stirred. The reaction was carried out under nitrogen protection for 20min, and then (0.022mmol, 5.5mg) diphenyl phosphate was added and stirred for 10min to terminate. Subsequently, the reaction tube was placed in an oil bath at 70°C and stirred for 10min, and then (4.725mmol, 1240mg) dicyclohexylmethane diisocyanate and (0.1wt%, 12mg) dibutyltin dilaurate were added and reacted for 40min to obtain an isocyanate-terminated prepolymer. Finally, 1,4-butanediol (3 mmol, 270 mg) was added under nitrogen protection, and the reaction was carried out at 70°C for 2 h, then at 90°C for 2 h, and finally cured at 100°C for 2 h to obtain polyurethane. The weight average molecular weight measured by GPC was 206.8 kg / mol, the number average molecular weight was 99.4 kg / mol, and the molecular weight distribution was 2.08. The GPC spectrum is shown in Figure 1 The hard segment content of the obtained polyurethane is 32wt%, which is similar to the hard segment content of Comparative Example 2, but the mechanical properties are significantly improved, the breaking strength is as high as 60MPa, and the tensile curve is as follows Figure 2 shown.

[0043] Example 3

[0044] (0.02mmol, 7.4mg) phosphazene ligand P2-tert-butyl, cocatalyst U2 (0.04mmol, 8.5mg), (1.5mmol, 135mg) 1,4-butanediol were added to a reaction tube, and then (30mmol, 3.3mL) δ-caprolactone was added to the reaction tube and stirred. The reaction was carried out under nitrogen protection for 20min, and then (0.022mmol, 5.5mg) diphenyl phosphate was added and stirred for 10min to terminate. Subsequently, the reaction tube was placed in an oil bath at 70°C and stirred for 10min, and then (4.725mmol, 1240mg) dicyclohexylmethane diisocyanate and (0.1wt%, 12mg) dibutyltin dilaurate were added and reacted for 40min to obtain an isocyanate-terminated prepolymer. Finally, trimethylolpropane (1 mmol, 143 mg) was added under nitrogen protection, followed by reaction at 70° C. for 4 h, and finally cured at 100° C. for 2 h to obtain a thermosetting polyurethane elastomer.

[0045] Example 4

[0046] (0.1mmol, 37mg) phosphazene ligand P2-tert-butyl, cocatalyst U2 (0.2mmol, 42mg), (1mmol, 90mg) 1,4-butanediol were added to the reaction tube, and then (20mmol, 2.8mL) δ-octanolactone was added to the reaction tube and stirred. The reaction was carried out under nitrogen protection for 10min, and then (0.11mmol, 27.5mg) diphenyl phosphate was added and stirred for 10min to terminate. Subsequently, the reaction tube was placed in a 70°C oil bath and stirred for 10min, and (2.1mmol, 550mg) dicyclohexylmethane diisocyanate and (0.1wt%, 5mg) dibutyltin dilaurate were added and reacted for 40min to obtain an isocyanate-terminated prepolymer. Finally, (1mmol, 90mg) 1,4-butanediol was added under nitrogen protection, and the reaction was carried out at 100°C for 6h to obtain a polyurethane. GPC measured the weight average molecular weight to be 233.7 kg / mol, the number average molecular weight to be 115.4 kg / mol, and the molecular weight distribution to be 2.04.

[0047] Example 5

[0048] (0.1mmol, 37mg) phosphazene ligand P2-tert-butyl, cocatalyst U2 (0.2mmol, 42mg), (1mmol, 90mg) 1,4-butanediol were added to the reaction tube, and then (20mmol, 3.7mL) δ-undecalactone was added to the reaction tube and stirred. The reaction was carried out under nitrogen protection for 10min, and then (0.11mmol, 27.5mg) diphenyl phosphate was added and stirred for 10min to terminate. Subsequently, the reaction tube was placed in a 70°C oil bath and stirred for 10min, and (2.1mmol, 550mg) dicyclohexylmethane diisocyanate and (0.1wt%, 5mg) dibutyltin dilaurate were added and reacted for 40min to obtain an isocyanate-terminated prepolymer. Finally, (1mmol, 90mg) 1,4-butanediol was added under nitrogen protection, and the reaction was carried out at 100°C for 6h to obtain a polyurethane. The weight average molecular weight measured by GPC was 211.6 kg / mol, the number average molecular weight was 110.2 kg / mol, and the molecular weight distribution was 1.92. 1 HNMR spectrum Figure 4 As shown, the obtained polyurethane 13 C NMR spectrum Figure 5 shown.

[0049] Example 6

[0050] (0.1mmol, 37mg) phosphazene ligand P2-tert-butyl, cocatalyst U2 (0.2mmol, 42mg), (1mmol, 90mg) 1,4-butanediol were added to the reaction tube, and then (20mmol, 4.8mL) δ-tetradecaprolactone was added to the reaction tube and stirred. The reaction was carried out under nitrogen protection for 10min, and then (0.11mmol, 27.5mg) diphenyl phosphate was added and stirred for 10min to terminate. Subsequently, the reaction tube was placed in a 70°C oil bath and stirred for 10min, and (2.1mmol, 550mg) dicyclohexylmethane diisocyanate and (0.1wt%, 5mg) dibutyltin dilaurate were added and reacted for 40min to obtain an isocyanate-terminated prepolymer. Finally, (1mmol, 90mg) 1,4-butanediol was added under nitrogen protection, and the reaction was carried out at 100°C for 6h to obtain a polyurethane. GPC measured the weight average molecular weight to be 215.6 kg / mol, the number average molecular weight to be 114.7 kg / mol, and the molecular weight distribution to be 1.88.

[0051] Example 7

[0052] Polyurethane films with different side chain lengths (polyurethane samples in Examples 1, 4, 5, and 6) were prepared by solvent casting and then cut into square samples with a size of 4 mm long * 4 mm wide * 0.25 mm thick. Four samples were weighed and immersed in water containing acid (1 mol L-1 HCl) and alkaline (1 mol L -1 The specimens were taken out from the solution at regular intervals, washed with distilled water, and weighed after being completely dried in a vacuum. The specimens were then immersed in the above four solutions again and the process was repeated. The hydrophobic angle of the obtained polyurethane increased with the increase of the side chain length. The hydrophobic angle measured by WCA is shown in Figure 6 The hydrolytic degradation changes of the four polyurethanes within 160 days are shown in Figure 7 and 8 As shown, it indicates that the hydrophobic side chain can give polyurethane good hydrolysis resistance.

[0053] Example 8

[0054] The polyurethane samples in Examples 1, 4, 5, and 6 were cut into pieces (3 g) and (60 mg) of stannous octoate were added to a reaction bottle and the mixture was stirred. Using a vacuum distillation apparatus, the reaction bottle was heated to 170° C. for 4 h, and pure alkyl-δ-caprolactone (yield: ˜95%) was recovered.

Claims

1. A bio-based, high-strength, recyclable, hydrolysis-resistant polyurethane, characterized in that: It has a chemical structure as shown in formula (I): x is a natural number greater than or equal to 0, m is a natural number greater than or equal to 2, y is a natural number greater than or equal to 1, and z is a natural number greater than or equal to 1.

2. The polyurethane according to claim 1, characterized in that R1 and R3 have the following structures: R2 has the following structure:

3. The method for preparing polyurethane according to claim 1, characterized in that: The steps include: (1) mixing alkyl-δ-lactone, polyol initiator, strong base and urea uniformly, reacting at 10-50° C. for 1-30 min, and then adding an acidic substance to the above system to terminate the polymerization; (2) adding isocyanate and a catalyst to the above system, reacting at 30-100° C. for 0.1-24 hours to obtain an isocyanate-terminated prepolymer; (3) Finally, a measured amount of chain extender is added to the prepolymer, and the reaction is carried out at 30 to 120° C. for 0.1 to 24 hours to obtain polyurethane.

4. The method according to claim 3, characterized in that In the above preparation method, the alkyl-δ-lactone in step (1) is δ-caprolactone, δ-heptanolactone, δ-octanolactone, δ-nonanolactone, δ-decanolactone, δ-undecalactone, δ-dodecalactone, δ-tridecalactone, δ-tetradecanolactone, δ-pentadecalactone, δ-hexadecanolactone; the polyol initiator is ethylene glycol, propylene glycol, methyl propanediol, neopentyl glycol, butanediol, 1,4-cyclohexanediol, 1,6-hexanediol, diethylene glycol, hydroquinone-bis(β-hydroxyethyl) ether, glycerol α-allyl ether, dihydroxyethyl terephthalate, ethylene oxide-terminated bisphenol A , propylene glycol, trihydroxypropane, pentaerythritol; the strong base is sodium, potassium, potassium hydride, sodium hydride, hexa[tri(dimethylamine)phosphazene]tripolyphosphazene ({[(NMe2)3P=N]2P=N}3), phosphazene ligand P4-tert-butyl ([(NMe2)3P=N]3P=NtBu, tert-Bu-P4), phosphazene ligand P2-tert-butyl ([(NMe2)3P=N](NMe2)2P=NtBu, tert-Bu-P2); the acidic substance is acetic acid, benzoic acid, hydrochloric acid, sulfuric acid, phosphoric acid, diphenyl phosphate.

5. The method according to claim 3, characterized in that: In the above preparation method, the urea in step (1) has one of the following structures:

6. The method according to claim 3, characterized in that In the above preparation method, the molar ratio of the strong base to the polyol initiator in step (1) is 1 / 3 to 20 / 1; the molar ratio of the strong base to urea is 1 / 0.5 to 1 / 10; the molar ratio of the polyol initiator to alkyl-δ-caprolactone is 1 / 5 to 1 / 300; and the molar ratio of the acidic substance to the strong base is 1 / 1 to 10 / 1.

7. The method according to claim 3, characterized in that In the above preparation method, the catalyst in step (2) is N,N-dimethylcyclohexylamine, bis(2-dimethylaminoethyl) ether, triethylamine, triethylenediamine, N,N-dimethylbenzylamine, N-ethylmorpholine, N-methylmorpholine, N,N'-diethylpiperazine, pyridine, 4-dimethylaminopyridine, dibutyltin dilaurate, and stannous octoate; and the isocyanate is toluene diisocyanate, methylcyclohexyl diisocyanate, isophorone diisocyanate, p-phenylene diisocyanate, 4,4'-diisocyanato-3,3'-dimethyl-1,1'-biphenyl, cyclohexyl dimethylene diisocyanate, diphenylmethane diisocyanate, polymethylene polyphenyl polyisocyanate, dicyclohexylmethane diisocyanate, hexamethylene diisocyanate, and 1,5-naphthalene diisocyanate.

8. The method according to claim 3, characterized in that In the above preparation method, the molar ratio of the isocyanate to the polyol initiator in step (2) is 1 / 1 to 11 / 1; the mass percentage of the catalyst (relative to the isocyanate) is 0.02 to 10 wt%.

9. The method according to claim 3, characterized in that: In the above preparation method, the molar ratio of the chain extender to the polyol initiator in step (3) is 0.1 / 1 to 10 / 1.

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