Flexible long-chain chain extender as well as preparation method and application thereof

Abstract

The invention relates to the technical field of natural rubber modification, in particular to a flexible long-chain chain extender as well as a preparation method and application thereof. The structural general formula of the flexible long-chain chain extender is A-[flexible chain segment]-A, wherein: the [flexible segment] is a polyether polyol chain or a polyester polyol chain having a number average molecular weight of 500 to 5000 g / mol; a is an isocyanate group or an anhydride group. The invention also provides a natural rubber flexible long chain extension modification method based on end group functionalization, which comprises the following steps: pretreating an alpha end group and / or an omega end group of natural rubber, and introducing a hydroxyl or carboxyl active group; and reacting the functionalized natural rubber with a flexible long-chain chain extender at 60-80 DEG C to realize extension and crosslinking of a molecular chain. According to the invention, the flexible long-chain chain extender is introduced, and a flexible buffer section is embedded between natural rubber molecular chains, so that the dynamic stress is effectively dispersed, and the inherent defects of the traditional rigid small-molecule chain extender are overcome.

Description

Technical Field

[0001] This invention relates to the field of natural rubber modification technology, and in particular to a flexible long-chain chain extender, its preparation method, and its application. Background Technology

[0002] Chain extension of natural rubber is a core technology for achieving its high performance and has irreplaceable industrial significance for optimizing material structure and properties. During processing and storage, natural rubber is prone to molecular chain breakage, leading to a decrease in molecular weight and a widening of its distribution. This results in problems such as reduced strength, deterioration of aging resistance, and insufficient deformation recovery, which directly restricts its high-end applications under long-term dynamic stress and complex environmental conditions.

[0003] In particular, natural latex weather balloons, widely used in meteorological observation and scientific payload carrying, have extremely stringent performance requirements. During ascent, the balloon must continuously withstand low temperatures (down to -90°C), low pressure, intense ultraviolet radiation, and expansion deformation tens of times its own volume. Traditional natural latex balloons rely on their inherent strain-induced crystallization properties for strength, but the molecular chain network is still prone to breakage under long-term dynamic stress, leading to key bottlenecks such as significant low-temperature brittleness, limited fatigue resistance, and decreased airtightness with increasing flight time. These shortcomings directly affect the continuity of observation data, payload safety, and cost per mission.

[0004] The α- and ω-terminal structures of natural rubber enable controllable reinforcement of its molecular network. Functionalization allows for the introduction of active sites at the chain ends, creating conditions for precise chain extension and repair. However, existing chain extension technologies mostly employ small-molecule rigid crosslinking agents such as diepoxides and diisocyanates. While these agents can increase molecular weight, the short-chain rigid segments introduced into the rubber network exacerbate stress concentration at crosslinking points, hindering uniform stress distribution during dynamic expansion. This results in limited contribution to improving the material's low-temperature toughness, tear resistance, and resistance to repeated deformation, failing to meet the extreme requirements of weather balloons for high elasticity, high fatigue resistance, and adaptability to ultra-low temperature environments. Therefore, developing a novel chain extender capable of efficiently bonding with natural rubber end groups while simultaneously introducing long-range flexible segments to optimize the network structure is crucial for overcoming the technological bottlenecks in high-performance natural latex balloon materials and meeting the national demand for self-sufficiency in high-end meteorological observation and scientific research equipment. Summary of the Invention

[0005] To address the problems of high crosslinking network brittleness, stress concentration, and poor dynamic mechanical properties caused by the use of rigid small molecule chain extenders in existing natural rubber chain extension technologies, this invention provides a novel type of flexible long-chain chain extender, its preparation method, and a natural rubber chain extension modification method based on end-group functionalization.

[0006] To achieve the above objectives, the present invention provides the following solution: One of the technical solutions of this invention is a flexible long-chain chain extender with the general structural formula A-[flexible segment]-A; wherein: [flexible segment] is a polyether polyol chain or polyester polyol chain with a number average molecular weight of 500 to 5000 g / mol; A is an isocyanate group or an anhydride group (A is a capping group, and the A at both ends of the molecule is the same group).

[0007] In a preferred embodiment of the present invention, the polyether polyol is a polypropylene oxide diol or a polytetrahydrofuran diol; the polyester polyol is a polyethylene adipate diol or a polybutylene adipate diol.

[0008] The second technical solution of the present invention is a method for preparing the above-mentioned flexible long-chain chain extender. When A is an isocyanate group, the method for preparing the flexible long-chain chain extender includes the following steps: In the presence of a catalyst, polyether polyol or polyester polyol is reacted with diisocyanate monomer at 60-80℃, and the molar ratio of isocyanate group to hydroxyl group is controlled to be (2.1-2.5):1. After reacting for 3-5 hours, the mixture is purified by vacuum distillation to obtain a flexible long-chain chain extender with isocyanate group at the end. When A is an anhydride group, the preparation method of the flexible long-chain chain extender includes the following steps: First, react polyether polyol or polyester polyol with thionyl chloride to obtain a terminal acyl chloride intermediate; mix the terminal acyl chloride intermediate with a dicarboxylic acid anhydride at 0-5℃, then heat to room temperature and react for 4-8 hours, and after precipitation, washing and drying, obtain a flexible long-chain chain extender with an anhydride group at the end.

[0009] In a preferred embodiment of the present invention, the diisocyanate monomer is toluene diisocyanate (TDI), diphenylmethane diisocyanate (MDI), or isophorone diisocyanate (IPDI).

[0010] In a preferred embodiment of the present invention, the catalyst is dibutyltin dilaurate; the amount of the catalyst is 0.01%-0.05% of the total mass of the reactants (polyether polyol and diisocyanate monomer, or polyester polyol and diisocyanate monomer).

[0011] In a preferred embodiment of the present invention, the dicarboxylic anhydride is succinic anhydride, phthalic anhydride, or pyromellitic dianhydride.

[0012] The third technical solution of this invention is a method for modifying the flexible long-chain extension of natural rubber based on end-group functionalization, comprising the following steps: (1) Functionalization of natural rubber end groups: Pretreatment of the α-end groups and / or ω-end groups of natural rubber to introduce hydroxyl or carboxyl active groups; (2) Chain extension reaction: The natural rubber functionalized in step (1) is reacted with the above-mentioned flexible long chain extender at 60-80℃ to achieve the extension and cross-linking of molecular chains.

[0013] In a preferred embodiment of the present invention, in step (1), the method for pretreating the ω-terminal group is to convert the ω-terminal group into a carboxyl group by oxidation with trichloroisocyanuric acid.

[0014] In a preferred embodiment of the present invention, in step (1), the method for pretreating the α-terminal group is: by catalytic hydrolysis by phospholipase D, the pyrophosphate group at the α-terminus is converted into a group containing a phosphate hydroxyl group.

[0015] The pretreatment of the α- and / or ω-terminal groups of natural rubber also includes precipitation, washing, and vacuum drying to constant weight.

[0016] In a preferred embodiment of the present invention, in step (2), when the end group of the functionalized natural rubber is mainly carboxyl, a flexible long chain extender with A as an anhydride group is used; when the end group of the functionalized natural rubber is mainly hydroxyl, a flexible long chain extender with A as an isocyanate group is used.

[0017] After the reaction in step (2) is completed, the steps of precipitation, washing and vacuum drying are also included.

[0018] Compared with the prior art, the present invention has the following beneficial effects: This invention introduces a custom-designed flexible long-chain chain extender, embedding flexible buffer segments between natural rubber molecular chains to effectively disperse dynamic stress and overcome the inherent defects of traditional rigid small-molecule chain extenders. The resulting chain-extended modified natural rubber not only has a significantly increased molecular weight and a narrower molecular weight distribution, but also possesses excellent tensile strength, outstanding fatigue resistance, and good elastic recovery, making it particularly suitable for high-end rubber products with stringent dynamic performance requirements. Attached Figure Description

[0019] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0020] Figure 1 The graph shows the molecular weight distribution curves of natural rubber before and after DPNR modification in Example 1, as determined by GPC testing.

[0021] Figure 2 The stress-strain curve of the chain-extended modified natural rubber NR-g-IC-PE in Example 1 is shown.

[0022] Figure 3 The elastic recovery behavior of natural rubber DPNR before and after modification under 50% strain in Example 1 is shown.

[0023] Figure 4 The elastic recovery behavior of natural rubber DPNR before and after modification under 100% strain in Example 1 is shown.

[0024] Figure 5 The graph shows the molecular weight distribution curves of the saponified natural rubber SPNR before and after GPC testing in Example 2.

[0025] Figure 6 The stress-strain curve of NR-g-AH-PES in Example 2 is shown. Detailed Implementation

[0026] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0027] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Any stated value or intermediate value within a stated range, as well as each smaller range between any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0028] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0029] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be apparent to those skilled in the art. This specification and embodiments are merely exemplary.

[0030] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0031] Unless otherwise specified, "room temperature" in this invention refers to 15-30°C.

[0032] This invention aims to innovatively propose a specific class of flexible long-chain chain extenders based on existing research on the functionalization of natural rubber end groups. These chain extenders use commercially available polyether polyols or polyester polyols as a flexible backbone, and through chemical synthesis, precisely replace their terminal hydroxyl groups with highly reactive benzene ring isocyanate groups or anhydride groups. This design allows the chain extender itself to possess both long-chain flexibility and high end-group reactivity. When it reacts with the functionalized end groups of natural rubber, it can embed a flexible buffer segment between the molecular chains, thereby effectively dispersing stress. This results in a significant increase in molecular weight while endowing natural rubber with superior overall dynamic mechanical properties.

[0033] The flexible long-chain chain extender provided by this invention is characterized by the following core structure: It uses polyester polyols or polyether polyols with a number-average molecular weight between 500 and 5000 g / mol as the flexible chain segment backbone, and chemically synthesizes the hydroxyl groups at both ends to convert them into highly reactive end-capping groups. The end-capping groups are of two distinct types: the first is a benzene ring-containing isocyanate group derived from toluene diisocyanate (TDI), diphenylmethane diisocyanate (MDI), or isophorone diisocyanate (IPDI); the second is an anhydride group derived from succinic anhydride (SA, also known as succinic anhydride), phthalic anhydride (PA), or pyromellitic dianhydride (PMDA). A specific chain extender molecule has the same type of end-capping groups at both ends, i.e., both ends of the molecule are isocyanate groups, or both ends of the molecule are anhydride groups. In other words, the flexible long-chain chain extender is a polyether polyol or polyester polyol whose end groups have been replaced with isocyanate groups or anhydride groups.

[0034] The preparation methods of the above-mentioned flexible long-chain chain extenders follow two independent synthetic routes: For chain extenders with isocyanate end groups, the preparation method includes: first, vacuum dehydrating polyether or polyester polyol, and then reacting it with excess diisocyanate monomer (such as TDI or MDI) at 60-80℃ under nitrogen protection and in the presence of organotin catalyst, controlling the molar ratio of isocyanate group to hydroxyl group to be (2.1-2.5):1, and after reacting for 3-5 hours, removing the residual monomer by vacuum distillation to obtain the target product.

[0035] For chain extenders with anhydride end groups, the preparation method includes: firstly, using thionyl chloride to convert the terminal hydroxyl group of a polyol into an acyl chloride group to obtain a terminal acyl chloride intermediate; then, under low temperature and alkaline conditions, reacting the terminal acyl chloride intermediate with a stoichiometric amount of dicarboxylic acid anhydride (such as pyromellitic dianhydride), and finally obtaining the anhydride-terminated chain extender by precipitation, washing and drying.

[0036] The method for modifying natural rubber using the aforementioned chain extenders is based on the functionalization of the end groups of natural rubber. This method includes: First, the α- and ω-terminal groups of natural rubber are identified and pretreated. Oxidation converts the ω-terminal group to a carboxyl group, while hydrolysis and phospholipase D catalysis convert the α-terminal group to a phosphate hydroxyl group, thus introducing active sites at the molecular chain ends. Subsequently, a matching chain extender is selected based on the properties of the functionalized end groups: for carboxyl-terminated natural rubber, anhydride-type chain extenders are preferred; for hydroxyl-terminated natural rubber, isocyanate-type chain extenders are preferred. The chain extension reaction is carried out in a solution system under inert gas protection, with the reaction temperature controlled at 60-80℃. By adjusting the molar ratio of the chain extender to the end functional groups, efficient and directional chain connection and extension are achieved.

[0037] Unless otherwise specified, the technical solutions described in this invention are all conventional solutions in the field, and the reagents or raw materials used are all purchased from commercial channels or are publicly available unless otherwise specified.

[0038] The deproteinized natural rubber (DPNR) used in this embodiment of the invention was purchased from Hainan Xiangyuan Industrial Co., Ltd., with a protein content of <0.2wt%.

[0039] The saponified natural rubber (SPNR) used in this embodiment of the invention was prepared according to the method in Polymers 2021, 13(8), 1319, with the acid value reduced to less than 0.5 mg KOH / g.

[0040] The technical solutions provided by the present invention will be described in detail below with reference to the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0041] Example 1: Chain extension modification of natural rubber using isocyanate-terminated polyether-type flexible chain extender Synthesis of the flexible chain extender (IC-PE): 100 g (0.05 mol) of polypropylene oxide diol (PPG-2000) with a number-average molecular weight (Mn) of 2000 g / mol was added to a dry four-necked flask equipped with a stirrer, thermometer, condenser, and nitrogen inlet. The mixture was stirred and dehydrated for 2 hours at 120 °C and a vacuum of -0.098 MPa until the moisture content was below 0.03% (determined by Karl Fischer method). Subsequently, the system was cooled to 70 °C, and under nitrogen protection, 20.2 g (0.116 mol) of toluene diisocyanate (TDI-80) dried through a 4A molecular sieve was slowly added dropwise, controlling the molar ratio of isocyanate groups (-NCO) to hydroxyl groups (-OH) at 2.3:1. After the addition was complete, 0.03 g of dibutyltin dilaurate (DBTDL) was added as a catalyst. The temperature was slowly increased to 80±2℃ and maintained for 4 hours. During the reaction, the -NCO content was tracked using di-n-butylamine titration. The reaction endpoint was considered reached when the measured value was close to the theoretical value (3.8%). After the reaction, unreacted TDI monomers were removed under vacuum at 80℃ and -0.095 MPa for 2 hours to obtain an isocyanate-terminated polyether (IC-PE) type flexible chain extender with -NCO-terminated groups. This was a slightly yellow, viscous liquid, and its -NCO content was measured to be 3.75%.

[0042] Pretreatment of natural rubber for end-group functionalization (ω-terminal carboxylation): 100g of deproteinized natural rubber (DPNR) was dissolved in 500mL of toluene to prepare a solution with a mass concentration of approximately 20%. 2.0g of trichloroisocyanuric acid (TCCA) was added to this solution as an oxidant, and the reaction was carried out at 50℃ for 6 hours. This process aims to selectively oxidize the ω-terminal methyl groups of the natural rubber molecular chain, converting them to carboxyl groups (-COOH). After the reaction, the solution was poured into a large volume of anhydrous ethanol to precipitate the rubber. The precipitate was washed several times to remove residual oxidant and byproducts, and then vacuum dried at 40℃ to constant weight to obtain ω-carboxylated natural rubber (NR-COOH). The carboxyl content was determined to be 0.12 mmol / 100g rubber by potentiometric titration.

[0043] Chain extension reaction: 100g of the prepared NR-COOH (based on dry rubber) was dissolved in 400mL of anhydrous DMF to form a homogeneous solution. Under nitrogen protection, the system was heated to 75℃. Using a constant pressure dropping funnel, 18.5g of the IC-PE chain extender synthesized in step (1) (based on a molar ratio of -NCO to -COOH of 1.05:1) was slowly added dropwise to the rubber solution over approximately 30 minutes. After the addition was complete, the reaction was continued at 75±2℃ for 3 hours. After the reaction was completed, the product was poured into a large amount of ethanol / water (1:1, v / v) mixture to precipitate. The resulting rubber block was repeatedly washed and then vacuum dried at 50℃ for 24 hours to obtain the chain-extended modified natural rubber NR-g-IC-PE.

[0044] Tests were conducted on the natural rubber before and after modification: the number average molecular weight (Mn) of NR-g-IC-PE decreased from 8.5 × 10⁻⁶ for the raw material DPNR. 5 g / mol increased to 2.1 × 10 6 g / mol, the molecular weight distribution index (PDI) narrowed from 2.8 to 1.9 (e.g. Figure 1 (As shown in Table 1). The preparation method of the product represented by the TDI rigid chain extender in Table 1 is as follows: The only difference from Example 1 is that, in the chain extension reaction step, the IC-PE chain extender is replaced with TDI monomer. Dumbbell-shaped test pieces were prepared according to ASTM D412 standard, and their tensile strength was 26.5 MPa, and the elongation at break was 720% (as shown in Table 1). Figure 2 As shown), it possesses good elastic recovery capability and exhibits significant fatigue resistance after 1000 cycles (e.g. Figure 3 , Figure 4 (and Table 2).

[0045] Table 1 Molecular weight distribution data

[0046] Table 2 Cyclic Tensile Test Data

[0047] Example 2: Chain extension modification of natural rubber using an anhydride-terminated polyester flexible chain extender Synthesis of flexible chain extender (AH-PES): In a dry four-necked flask, 75 g (0.05 mol) of polyethylene adipate diol (PEA-1500) with a number-average molecular weight (Mn) of 1500 g / mol was added and dissolved in 150 mL of anhydrous dichloromethane. The mixture was cooled to 0-5 °C in an ice-water bath under nitrogen protection. 16.7 g (0.14 mol) of thionyl chloride (SOCl2) was slowly added dropwise. After the addition was complete, the ice bath was removed, and the mixture was allowed to react at room temperature for 4 hours. After the reaction was complete, excess SOCl2 and solvent were removed by rotary evaporation at 40 °C to obtain a terminal acyl chloride intermediate, which was a light brown viscous liquid. The terminal acyl chloride intermediate was redissolved in 200 mL of anhydrous tetrahydrofuran (THF) and transferred to a dropping funnel. In another dry four-necked flask, 27.2 g (0.125 mol) of pyromellitic dianhydride (PMDA) and 150 mL of anhydrous THF were added, along with 10 g of anhydrous pyridine as an acid absorbent. Under a 0-5°C ice-water bath and vigorous stirring, a THF solution of the terminal acyl chloride intermediate was slowly added dropwise. After the addition was complete, the mixture was allowed to rise naturally to room temperature, and the reaction continued for 6 hours. After the reaction was complete, the pyridine hydrochloride was removed by filtration. The filtrate was concentrated and precipitated in excess n-hexane. The resulting solid was washed three times with n-hexane and dried under vacuum at 40°C to obtain an anhydride-terminated polyester (AH-PES) type flexible chain extender, which was a white or off-white powder.

[0048] Pretreatment for end-group functionalization of natural rubber (α-hydroxylation): 100g of saponified natural rubber (SPNR) was placed in 500mL of phosphate buffer solution (pH 7.2) and swollen at 35℃ for 1 hour. Subsequently, 1500U of phospholipase D (PLD) was added, and enzymatic hydrolysis was carried out at 35±1℃ for 10 hours. This process aims to hydrolyze the pyrophosphate groups at the α-terminus of the natural rubber, exposing the phosphate hydroxyl groups (P-OH). After the reaction, the rubber block was repeatedly washed with deionized water until the washing solution was neutral, then dehydrated with acetone, and finally vacuum dried at 35℃ to constant weight to obtain α-hydroxylated natural rubber (NR-OH).

[0049] Chain extension reaction: 100g of NR-OH (based on dry rubber) and 300mL of anhydrous toluene were added to a reaction vessel and heated and stirred until completely dissolved. Under nitrogen protection, the system was heated to 80℃. 22.8g of the AH-PES chain extender synthesized in step (1) (based on a molar ratio of anhydride group to -OH group of 1.02:1) was slowly added, along with 0.5g of triethylamine as a catalyst. The reaction was carried out at 80±2℃ for 4 hours. After the reaction was completed, the product solution was poured into a large amount of methanol to precipitate the rubber. The rubber block was washed several times with methanol and then vacuum dried at 50℃ for 24 hours to obtain chain-extended modified natural rubber NR-g-AH-PES.

[0050] Tests were conducted on the natural rubber before and after modification: the number average molecular weight (Mn) of NR-g-AH-PES decreased from 7.8 × 10⁻⁶ in the raw material SPNR. 5 g / mol significantly increased to 3.5 × 10 6 g / mol, molecular weight distribution index (PDI) is 2.1 (e.g. Figure 5 (As shown in Table 2). The preparation method of the product represented by the rigid chain extender of PA in Table 2 is as follows: The only difference from Example 2 is that in the chain extension reaction step, the AH-PES chain extender is replaced with PA monomer (phthalic anhydride). Its tensile strength reaches 30.2 MPa and its tear strength is 48 kN / m (as shown in Table 2). Figure 6 (As shown).

[0051] Table 3 Molecular weight distribution data

[0052] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A flexible long-chain extender, characterized by, The structural general formula is A-[flexible chain segment]-A; wherein: [flexible chain segment] is a polyether polyol chain or a polyester polyol chain with a number average molecular weight of 500-5000 g / mol; A is an isocyanate group or an anhydride group.

2. The flexible long-chain extender according to claim 1, characterized in that, The polyether polyol is a polypropylene glycol diol or a polytetrahydrofuran diol; and the polyester polyol is a polyethylene glycol adipate diol or a polybutylene glycol adipate diol.

3. A process for the preparation of the flexible long-chain extender of claim 1 or 2, characterized in that, When A is an isocyanate group, the preparation method of the flexible long-chain chain extender comprises the following steps: reacting the polyether polyol or the polyester polyol with a diisocyanate monomer at 60-80°C in the presence of a catalyst, controlling the molar ratio of isocyanate groups to hydroxyl groups to be (2.1-2.5):1, and after reacting for 3-5 hours, purifying by vacuum distillation to obtain the flexible long-chain chain extender with an isocyanate group at the end; When A is an anhydride group, the preparation method of the flexible long-chain chain extender comprises the following steps: first, reacting the polyether polyol or the polyester polyol with sulfoxide chloride to obtain an acyl chloride intermediate at the end; mixing the acyl chloride intermediate with a diacid anhydride at 0-5°C, and then reacting at room temperature for 4-8 hours, and after precipitation, washing and drying, the flexible long-chain chain extender with an anhydride group at the end is obtained.

4. The production method according to claim 3, characterized by, The diisocyanate monomer is toluene diisocyanate, diphenyl methane diisocyanate or isophorone diisocyanate.

5. The preparation method according to claim 3, characterized in that, The catalyst is dibutyl tin dilaurate; and the amount of the catalyst is 0.01%-0.05% of the total mass of the reactants.

6. The preparation method according to claim 3, characterized in that, The diacid anhydride is succinic anhydride, phthalic anhydride or pyromellitic dianhydride.

7. A method for flexible long chain chain extension modification of natural rubber based on end group functionalization, characterized by, Comprising the following steps: (1) natural rubber end group functionalization: pretreating the α end group and / or ω end group of natural rubber to introduce hydroxyl or carboxyl active groups; (2) chain extension reaction: reacting the natural rubber functionalized in step (1) with the flexible long-chain chain extender of claim 1 or 2 at 60-80°C to achieve chain extension and crosslinking.

8. The method of end group functionalized natural rubber flexible long chain chain extension modification according to claim 7, characterized in that, In step (1), the method for pretreating the ω end group is: converting the ω end group into a carboxyl group by oxidation with trichloroisocyanuric acid.

9. The method of end group functionalized natural rubber flexible long chain chain extension modification based on the method of claim 7, characterized in that, In step (1), the method for pretreating the α end group is: converting the α end pyrophosphoric acid ester group into a phosphorus-containing hydroxyl group by catalytic hydrolysis with phospholipase D.

10. The method of end group functionalized natural rubber flexible long chain chain extension modification based on the method of claim 7, characterized in that, In step (2), when the end groups of the functionalized natural rubber are mainly carboxyl groups, the flexible long-chain chain extender with A being an anhydride group is used; and when the end groups of the functionalized natural rubber are mainly hydroxyl groups, the flexible long-chain chain extender with A being an isocyanate group is used.

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