A method for returning crosslinked sites of crosslinked elastomers to an uncrosslinked state

Abstract

To provide a method by which a crosslinked site of a crosslinked elastomer can easily be restored to an uncrosslinked sate.SOLUTION: Provided is a method for restoring a crosslinked site of a crosslinked elastomer to an uncrosslinked state, in which the crosslinked elastomer has a definite diboronic acid ester skeleton unit represented by general formula (1) at the crosslinked site, and which comprises a step for, in the presence of an organic solvent having an SP value (solubility parameter) of 7 (cal / cm3)1 / 2 to 10 (cal / cm3)1 / 2 inclusive, bringing the crosslinked elastomer into contact with a definite monoboronic acid compound represented by general formula (3), characterized in that, in the step, the abundance molar ratio of the monoboronic acid compound to the diboronic acid ester skeleton unit (monoboronic acid compound / diboronic acid ester skeleton unit) is controlled to more than 2 and not more than 5.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for returning the crosslinked sites of a crosslinked elastomer to an uncrosslinked state. [Background technology]

[0002] Recently, from the viewpoints of environmental issues and resource conservation, recycling of vulcanized rubber (crosslinked rubber) used in tires, etc. after use has been promoted. As a recycling technology for vulcanized rubber, a method of devulcanizing vulcanized rubber has been known, but the devulcanization reaction requires very high temperatures, which has the drawback of being energy inefficient.

[0003] Therefore, techniques for decomposing vulcanized rubber by methods other than devulcanization have also been investigated. For example, Patent Document 1 discloses a method for decomposing and recovering vulcanized rubber, in which vulcanized rubber is decomposed by a lipid peroxidation reaction, and the lipids are removed in alcohol to which an alkali has been added to recover the rubber component. This method is disclosed to have a low decomposition reaction temperature, excellent energy efficiency, a fast decomposition rate, and to produce decomposition products that are easy to recycle and recover. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2011-153272 Summary of the Invention [Problem to be solved by the invention]

[0005] However, the technology of Patent Document 1 above targets vulcanized rubber in general as the decomposition target, and does not consider at all rubber (crosslinked elastomer) crosslinked by a method other than vulcanization (sulfur crosslinking).

[0006] Furthermore, focusing on the crosslinked elastomer itself, the development of a crosslinked elastomer that can be easily decrosslinked and subsequently recrosslinked (having so-called reversible crosslinking properties) would be extremely useful from the viewpoint of further effective utilization and versatility of materials.

[0007] Therefore, an object of the present invention is to provide a method that can easily return the crosslinked sites of a crosslinked elastomer to an uncrosslinked state. [Means for solving the problem]

[0008] As a result of extensive research to solve the above problems, the present inventors have found that, if a crosslinked elastomer has a predetermined skeletal unit at the crosslinked site, the crosslinked site can be easily returned to an uncrosslinked state under predetermined conditions, and have thus completed the present invention. That is, the gist of the present invention that solves the above problems is as follows.

[0009] The method of the present invention is a method for returning the crosslinked site of a crosslinked elastomer to an uncrosslinked state, comprising the steps of: The crosslinked elastomer has a crosslinking site represented by the following general formula (1): [ka] [In the formula, X 1 is a single bond or any divalent group, The crosslinked elastomer was dissolved in a solution having an SP value (solubility parameter) of 7 (cal / cm 3 ) 1 / 2 More than 10(cal / cm 3 ) 1 / 2 In the presence of an organic solvent, [ka] [In the formula, X 2 is hydrogen or any monovalent group; In the step, the molar abundance ratio of the monoboronic acid compound to the diboronic acid ester skeleton unit (monoboronic acid compound / diboronic acid ester skeleton unit) is set to more than 2 and 5 or less. This method makes it possible to easily return the crosslinked sites of the crosslinked elastomer to an uncrosslinked state.

[0010] In the method of the present invention, the crosslinked elastomer preferably has a proportion of the diboronic acid ester skeleton unit of 17% by mass or less, which allows the crosslinked elastomer to sufficiently maintain its elastomeric properties.

[0011] In the method of the present invention, the crosslinked elastomer is preferably obtained by crosslinking a diene-based elastomer having a vinyl bond content of 30% by mass or less and a weight-average molecular weight of 1000 or more. In this case, gelation can be effectively suppressed when the crosslinked elastomer is heated and molded, and sufficient entanglement occurs in the crosslinked elastomer, allowing it to exhibit sufficient mechanical strength when used in rubber articles such as tires. [Effects of the Invention]

[0012] According to the present invention, a method can be provided that allows the crosslinked sites of a crosslinked elastomer to be easily returned to an uncrosslinked state. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, embodiments of the present invention will be described. However, these descriptions are for the purpose of illustrating the present invention and are not intended to limit the present invention in any way.

[0014] (Method of returning the crosslinked portion of a crosslinked elastomer to an uncrosslinked state) The method of returning the crosslinked site of a crosslinked elastomer of one embodiment of the present invention to an uncrosslinked state (hereinafter, may be referred to as the "method of the present embodiment") comprises adding a compound represented by the following general formula (1) to the crosslinked site: [ka] [In the formula, X 1 is a single bond or any divalent group], and the crosslinked site of the crosslinked elastomer having a diboronic acid ester skeleton unit represented by the formula [I] is returned to an uncrosslinked state, and the crosslinked elastomer is 3 ) 1 / 2 More than 10(cal / cm 3 ) 1 / 2 In the presence of an organic solvent, [ka] [In the formula, X 2 is hydrogen or any monovalent group (monoboronic acid contacting step), In the step, the molar abundance ratio of the monoboronic acid compound to the diboronic acid ester skeleton unit (monoboronic acid compound / diboronic acid ester skeleton unit) is set to more than 2 and 5 or less.

[0015] The crosslinking via the diboronic acid ester skeleton unit in the crosslinked elastomer described above is not a sulfur-based bond as in typical vulcanized rubber. Furthermore, when the crosslinked elastomer described above is contacted with a monoboronic acid compound in the presence of a specific organic solvent, an exchange reaction occurs at the crosslinking sites of the crosslinked elastomer, and the crosslinking via the diboronic acid ester skeleton unit replaces the bond of the monoboronic acid compound. In this regard, since the monoboronic acid compound has only one boronic acid that functions as a bond to the elastomer, the linkage between the elastomers is eliminated. Therefore, the method of this embodiment makes it possible to easily return the crosslinking sites of the crosslinked elastomer to an uncrosslinked state (in other words, to decrosslink). Furthermore, the elastomer (decrosslinked elastomer) treated by the method of this embodiment can be further crosslinked again, so it can be expected to be widely used in various rubber articles.

[0016] <Crosslinked elastomer> The crosslinked elastomer used in the method of the present embodiment has a crosslinking site containing a compound represented by the following general formula (1): [ka] [In the formula, X 1 is a single bond or any divalent group. This diboronic acid ester skeleton unit is typically derived from the structure of a crosslinking agent used in producing a crosslinked elastomer.

[0017] X in general formula (1) 1 Examples of divalent groups that may be mentioned include hydrocarbon groups, and specific examples include linear or branched aliphatic groups having 1 to 10 carbon atoms (alkylene groups, alkenylene groups, alkynylene groups); aromatic-containing groups having 6 to 20 carbon atoms (1,2-phenylene groups, 1,3-phenylene groups, 1,4-phenylene groups, 1,4-naphthylene groups, 1,5-naphthylene groups, 2,6-naphthylene groups, 4,4'-biphenylene groups, etc.). In addition, the aliphatic groups and aromatic-containing groups include -O-, -S-, -OC(=O)-, -C(=O)-O-, -OC(=O)-O-, -NR 2 -C(=O)-, -C(=O)-NR 2 -, -NR 2 - or -C(=O)- may be present. 2 represents a hydrogen atom or an alkyl group having 1 to 6 carbon atoms.

[0018] The crosslinked elastomer used in the method of this embodiment preferably has a proportion of the diboronic acid ester skeleton unit described above of 17% by mass or less. In this case, the elastomeric properties of the crosslinked elastomer can be sufficiently maintained. From the same viewpoint, the proportion of the diboronic acid ester skeleton unit in the crosslinked elastomer is more preferably 14% by mass or less, and even more preferably 10% by mass or less. On the other hand, the lower limit of the proportion of the diboronic acid ester skeleton unit in the crosslinked elastomer is not particularly limited as long as it is more than 0% by mass, but from the viewpoint of increasing the strength to the same level as that of sulfur crosslinking, it is preferably 1% by mass or more, and more preferably 2% by mass or more. The proportion of the diboronic acid ester skeleton unit in the crosslinked elastomer can be calculated based on the blend amounts of the elastomer and diboronic acid ester compound used in the production.

[0019] The crosslinked elastomer used in the method of this embodiment is preferably obtained by crosslinking a diene-based elastomer. That is, the crosslinked elastomer of this embodiment is preferably a crosslinked product of a diene-based elastomer. Here, the diene-based elastomer refers to an elastomer containing at least a diene unit, and examples thereof include natural rubber (NR), butadiene rubber (BR), styrene-butadiene rubber (SBR), isoprene rubber (IR), and chloroprene rubber (CR). The diene-based elastomer may be used alone or in combination of two or more. Among these, butadiene rubber (BR) and styrene-butadiene rubber (SBR) are preferred as the diene-based elastomer from the viewpoint of ensuring sufficient mechanical strength of rubber articles such as tires.

[0020] The diene elastomer preferably has a vinyl bond content of 30% by mass or less and a weight-average molecular weight (Mw) of 1,000 or more. In other words, the crosslinked elastomer used in the method of this embodiment is preferably obtained by crosslinking a diene elastomer having a vinyl bond content of 30% by mass or less and a weight-average molecular weight of 1,000 or more. When the vinyl bond content of the diene elastomer is 30% by mass or less, gelation can be effectively suppressed when the crosslinked elastomer is heated and molded. When the weight-average molecular weight (Mw) of the diene elastomer is 1,000 or more, sufficient entanglement occurs within the crosslinked elastomer, allowing it to exhibit sufficient mechanical strength when used in rubber articles such as tires. From the same viewpoint, the vinyl bond content of the diene elastomer is preferably 28% by mass or less, more preferably 26% by mass or less. On the other hand, the lower limit of the vinyl bond content of the diene elastomer is preferably 3% by mass or more, more preferably 5% by mass or more, even more preferably 8% by mass or more, and particularly preferably 10% by mass or more. In this specification, the vinyl bond content of a diene elastomer refers to the mass proportion of diene units that are vinyl-bonded to the entire diene elastomer, not the proportion of vinyl-bonded units to the diene units. The weight average molecular weight (Mw) can be determined by gel permeation chromatography (GPC) using polystyrene as a standard substance.

[0021] Furthermore, when the crosslinked elastomer is obtained by crosslinking a diene-based elastomer, it is preferable that the diboronic acid ester skeleton unit in the crosslinked elastomer be bonded to a carbon atom constituting a vinyl group remaining in a vinyl-bonded diene unit in the diene-based elastomer. The bond between the diboronic acid ester skeleton unit and the diene-based elastomer may be a direct bond or a bond via an arbitrary group (e.g., an alkylene group having 1 to 3 carbon atoms).

[0022] The crosslinked elastomer used in the method of this embodiment may have a filler such as carbon black dispersed therein by blending it during the production thereof. When the filler is dispersed in the crosslinked elastomer, the blending amount of the filler per 100 parts by mass of the crosslinked elastomer may be 10 parts by mass or more, and may be 100 parts by mass or less.

[0023] The crosslinked elastomer used in the method of the present embodiment can be prepared by, for example, reacting a diene-based elastomer with a compound represented by the following general formula (2): [ka] [In the formula, X 1 is a single bond or any divalent group, and Y 1 and Y 2are each independently a single bond or a divalent hydrocarbon group. According to this production method, by utilizing a radical reaction, crosslinks are formed between diene-based elastomers via a predetermined skeleton, and the above-mentioned crosslinked elastomer can be easily produced. Note that X in general formula (2) 1 represents X in general formula (1). 1 Corresponds to.

[0024] Specifically, in the above-mentioned production method, a crosslinked elastomer can be obtained by blending the diene elastomer with a radical initiator and a diboronic acid ester compound, mixing them in a mixer or the like, and heating the mixture. The heating temperature and time are preferably adjusted appropriately taking into consideration the vinyl bond content of the diene elastomer used, the type and amount of the radical initiator used, etc.

[0025] In the above-described production method, in addition to the diene elastomer, radical initiator, and diboronic acid ester compound, an appropriate amount of other components may be further blended, such as a filler such as carbon black.

[0026] <Organic solvents> The organic solvent used in the method of this embodiment has an SP value (solubility parameter) of 7 (cal / cm 3 ) 1 / 2 More than 10(cal / cm 3 ) 1 / 2 If the SP value of the organic solvent is outside the above range, the compatibility with the crosslinked elastomer may be reduced, and the exchange reaction with the monoboronic acid compound at the crosslinked site may not occur sufficiently. In addition, the SP value of the organic solvent should be 8 (cal / cm) or less, from the viewpoint of ensuring the above exchange reaction. 3 ) 1 / 2 More than 9.5 (cal / cm 3 ) 1 / 2 The following is preferred: The SP value can be calculated according to the Hansen method. The organic solvent may be a single solvent or a mixed solvent.

[0027] <Monoboronic acid compounds> The monoboronic acid compound used in the method of the present embodiment is represented by the following general formula (3): [ka] [In the formula, X 2 is hydrogen or any monovalent group. That is, the monoboronic acid compound has one boronic acid. X in general formula (3) 2 Examples of the monovalent group that can be mentioned include hydrocarbon groups, and specific examples include linear or branched aliphatic groups having 1 to 10 carbon atoms (alkyl groups, alkenyl groups, alkynyl groups); aromatic-containing groups having 6 to 20 carbon atoms (phenyl groups, naphthyl groups, biphenyl groups, etc.); heteroaromatic groups (2-furyl groups, etc.). The aliphatic groups and aromatic-containing groups include -O-, -S-, -OC(=O)-, -C(=O)-O-, -OC(=O)-O-, -NR 2 -C(=O)-, -C(=O)-NR 2 -, -NR 2 - or -C(=O)- may be present. 2 represents a hydrogen atom or an alkyl group having 1 to 6 carbon atoms.

[0028] X in general formula (3) 2 The monovalent group which can be represented by X in the above general formula (1) 1 It is preferable that the hydrogen atom is the same as a hydrogen atom bonded to one of the bonds of a divalent group that can be represented by the formula (1). 1 When a crosslinked elastomer in which X is a phenylene group is used, 2 It is preferable to use a monoboronic acid compound in which is a phenyl group, in which case the exchange reaction can be more effectively carried out.

[0029] <Monoboronic acid contact step> In the monoboronic acid contact step, the crosslinked elastomer is contacted with a monoboronic acid compound in the presence of the above-mentioned organic solvent. This contact causes an exchange reaction at the crosslinked sites of the elastomer. Typically, the crosslinked sites of the two elastomers to be linked are represented by the following general formula (4): [ka] At the same time, typically, the crosslinked portion is returned to an uncrosslinked state by being capped with a functional group represented by the following general formula (9): [ka] Thus, a diboronic acid compound represented by the following formula is produced.

[0030] In the monoboronic acid contacting step, the ratio of the diboronic acid ester skeleton units contained in the crosslinked elastomer to the monoboronic acid compound is crucial. Specifically, the molar ratio of the monoboronic acid compound to the diboronic acid ester skeleton units (monoboronic acid compound / diboronic acid ester skeleton units) must be greater than 2 and not greater than 5. If the molar ratio is 2 or less, there is a risk that the chemical equivalent of boronic acid required to break the linkages between elastomers may be insufficient. Furthermore, if the molar ratio is greater than 5, a large amount of the monoboronic acid compound remains, which can hinder further re-crosslinking after de-crosslinking.

[0031] The preferred range of the molar abundance ratio of the monoboronic acid compound to the diboronic acid ester skeleton unit (monoboronic acid compound / diboronic acid ester skeleton unit) varies depending on whether or not a filler such as carbon black is dispersed in the crosslinked elastomer. When a filler is not dispersed in the crosslinked elastomer, the molar abundance ratio is preferably 4 or more. When a filler is dispersed in the crosslinked elastomer, the molar abundance ratio is preferably 2.5 or more. In these cases, decrosslinking can be achieved more sufficiently and effectively.

[0032] In the monoboronic acid contact step, the concentration of the crosslinked elastomer (the mass ratio of the crosslinked elastomer to the total mass of the crosslinked elastomer and the organic solvent) is preferably 5% by mass or more and 65% by mass or less. If the concentration of the crosslinked elastomer is within the above range, decrosslinking can be performed more sufficiently and effectively. From the same viewpoint, the concentration of the crosslinked elastomer is more preferably 10% by mass or more. In particular, when a filler is dispersed in the crosslinked elastomer, the concentration of the crosslinked elastomer is more preferably 20% by mass or more. The lower the concentration of the crosslinked elastomer, the more the crosslinked elastomer is immersed in the organic solvent, while the higher the concentration of the crosslinked elastomer, the more the crosslinked elastomer is swollen with the organic solvent.

[0033] The specific conditions (temperature, time, etc.) for the monoboronic acid contact step are not particularly limited. However, in order to increase the frequency of contact with the monoboronic acid compound, it is preferable to finely pulverize the crosslinked elastomer in advance. Furthermore, after the monoboronic acid contact step, a solid-liquid separation treatment may be performed by a known method. [Example]

[0034] The present invention will be described in more detail below with reference to examples. However, these examples are intended to illustrate the present invention and are not intended to limit the present invention in any way.

[0035] In the following, in addition to a method for returning the crosslinked sites of the crosslinked elastomer of the present invention to an uncrosslinked state (decrosslinking), examples of the production of the crosslinked elastomer and re-crosslinking after decrosslinking are also shown in a series.

[0036] (Preparation of Crosslinked Elastomer) Elastomer compositions were prepared according to the formulations shown in Table 1. Next, these elastomer compositions were heated under the conditions shown in Table 1 using a rubber processability analyzer (manufactured by Alpha Technologies), and the storage modulus G' was measured. The results are shown in Table 1. In all examples, the storage modulus G' was approximately the desired value, and therefore it was determined that crosslinking had occurred. Reference Example 1 is an example of sulfur crosslinking (vulcanization) without using carbon black as a filler, while Examples 1-1 to 1-4 are examples in which the carbon black was not used and the conditions were appropriately adjusted so that the storage modulus G' approximated that of Reference Example 1. Similarly, Reference Example 2 is an example of sulfur crosslinking (vulcanization) with carbon black as a filler, while Example 1-5 is an example in which the carbon black was used and the conditions were appropriately adjusted so that the storage modulus G' approximated that of Reference Example 2.

[0037] [Table 1]

[0038] *1 Diene elastomer: Asahi Kasei Corporation, "Tufden (registered trademark) 2000R", styrene-butadiene rubber, vinyl bond content: 10 to 26% by mass, weight average molecular weight: 373,000 (3 significant digits) *2 BDB: Diboronic acid ester compound (synthetic product) represented by the following formula (5) [ka] *3 CB: HAF grade carbon black *4 Radical initiator: azobisisobutyronitrile (AIBN) *5 Vulcanization accelerator DPG: 1,3-diphenylguanidine *6 Vulcanization accelerator MBTS: Di-2-benzothiazolyl disulfide *7 Vulcanization accelerator TBBS: N-(tert-butyl)-2-benzothiazole sulfenamide

[0039] The crosslinked elastomers obtained in Examples 1-1 to 1-5 were found to be a compound of the following formula (6) by appropriate use of solution NMR: [ka] Furthermore, in these crosslinked elastomers, the diboronic acid ester skeleton unit is directly bonded to the carbon atom constituting the vinyl group remaining in the vinyl-bonded butadiene unit in the diene-based elastomer, thereby forming a crosslink.

[0040] (Decrosslinking treatment of crosslinked elastomer) Here, the crosslinked elastomer obtained in Example 1-4 was selected as an elastomer not mixed with carbon black, and the crosslinked elastomer obtained in Example 1-5 was selected as an elastomer mixed with carbon black. This crosslinked elastomer was finely crushed and then dissolved in tetrahydrofuran (THF, SP value: 8.95 (cal / cm 3 ) 1 / 2 ), and then phenylboronic acid (PhB(OH)2) as the monoboronic acid compound (BA) was added and mixed thoroughly, and the mixture was left to stand at room temperature (approximately 25°C) for one day. At this time, the concentration of the crosslinked elastomer (mass ratio of the crosslinked elastomer to the total of the crosslinked elastomer and organic solvent) and the molar abundance ratio of the monoboronic acid compound to the diboronic acid ester skeleton unit in the crosslinked elastomer (BA / BDB skeleton unit) were adjusted to be as shown in Table 2. After standing, the mixture was cast and the solvent was evaporated to obtain a treated elastomer.

[0041] <Evaluation of decrosslinking> The resulting treated elastomer was subjected to roll molding. If roll molding was possible, it was evaluated as having been decrosslinked (the crosslinked sites had returned to an uncrosslinked state), thereby imparting softness. The results are shown in Table 2.

[0042] [Table 2]

[0043] As shown in Table 2, the evaluation results for decrosslinking were good in all of Examples 2-1 to 2-7. That is, the crosslinked elastomers obtained in Examples 1-4 and 1-5 could be easily decrosslinked by contacting them with a monoboronic acid compound in the presence of a predetermined organic solvent.

[0044] In the treated elastomers (decrosslinked elastomers) obtained in Examples 2-1 to 2-7, solution NMR was appropriately used to confirm that the bonding site of the skeleton represented by the above formula (6) in the crosslinked elastomer before treatment was replaced by the bonding site of the skeleton represented by the following formula (7): [ka] It was confirmed that the crosslinking was eliminated (the crosslinked site returned to an uncrosslinked state). At the same time, by appropriately using solution NMR, it was possible to identify the functional group represented by the following formula (8): [ka] It was confirmed that a diboronic acid compound ((1,4-phenylene)diboronic acid) represented by the formula:

[0045] Furthermore, since the crosslinked elastomers obtained in Examples 1-1 to 1-3 also have the above-mentioned diboronic acid ester skeleton unit, it is believed that they can be easily decrosslinked by the same treatment as above.

[0046] (Recrosslinking of decrosslinked elastomer) Here, the decrosslinked elastomer (treated elastomer) obtained in Example 2-2 or Example 2-3 was selected as an elastomer not containing carbon black, and the decrosslinked elastomer (treated elastomer) obtained in Example 2-5 was selected as an elastomer containing carbon black. This decrosslinked elastomer was placed in a solvent shown in Table 3 together with the diboronic acid compound produced during the decrosslinking treatment, and a radical initiator was then added in the formulation shown in Table 3, followed by standing overnight at room temperature (approximately 25°C). At this time, the concentration of the decrosslinked elastomer (the mass proportion of the decrosslinked elastomer in the total of the decrosslinked elastomer and the solvent) was adjusted to be as shown in Table 3. After standing, the mixture was cast and the solvent was evaporated to obtain a treated elastomer.

[0047] <Evaluation of crosslinking (recrosslinking)> The resulting treated elastomer was heated at 120°C for 10 to 30 minutes (until the storage modulus G' value stabilized) using a rubber processability analyzer (Alpha Technologies), and the storage modulus G' was measured. In all cases, the measured storage modulus G' was 20% or more compared to the storage modulus G' of the crosslinked elastomer before the addition of the monoboronic acid compound, indicating that crosslinking had occurred. More specifically, if the measured storage modulus G' was 50% or more compared to the storage modulus G' of the crosslinked elastomer before the addition of the phenylboronic acid, the crosslinking was evaluated as having a sufficient amount of crosslinking, and was evaluated as "Good." On the other hand, if the measured storage modulus G' was less than 50%, the crosslinking was evaluated as "Fair." The results are shown in Table 3.

[0048] [Table 3]

[0049] As shown in Table 3, the uncrosslinked elastomers obtained in Examples 2-2, 2-3, and 2-5 could be easily crosslinked (recrosslinked) by contacting them with a diboronic acid compound in the presence of a specific solvent.

[0050] The treated elastomers (re-crosslinked elastomers) obtained in Examples 3-1 to 3-7 were analyzed by solution NMR as follows: [ka] It was confirmed that each of the treated elastomers (re-crosslinked elastomers) obtained in Examples 3-1 to 3-7 had a structure substantially equivalent to that of the crosslinked elastomers obtained in Examples 1-1 to 1-5. [Industrial Applicability]

[0051] According to the present invention, a method can be provided that allows the crosslinked sites of a crosslinked elastomer to be easily returned to an uncrosslinked state.

Claims

1. A method for returning a crosslinked site of a crosslinked elastomer to an uncrosslinked state, comprising: The crosslinked elastomer has a crosslinking site represented by the following general formula (1): 【Chemistry 1】 [In the formula, X １ is a single bond or any divalent group, the crosslinked elastomer is obtained by crosslinking a diene-based elastomer having a vinyl bond content of 30% by mass or less and a weight-average molecular weight of 1,000 or more; The crosslinked elastomer is dissolved in a solvent having an SP value (solubility parameter) of 7 (cal / cm ３ ) １／２ More than 10 (cal / cm ３ ) １／２ In the presence of an organic solvent, 【Chemistry 2】 [In the formula, X ２ is hydrogen or any monovalent group; The method for returning crosslinked sites of a crosslinked elastomer to an uncrosslinked state, wherein in the step, the molar abundance ratio of the monoboronic acid compound to the diboronic acid ester skeletal unit (monoboronic acid compound / diboronic acid ester skeletal unit) is greater than 2 and not greater than 5.

2. The method according to claim 1 , wherein the crosslinked elastomer has a proportion of the diboronic acid ester skeleton unit of 17% by mass or less.

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