Method for producing 1,2-polybutadiene and catalyst composition

Abstract

The purpose of the present invention is to provide a method for producing 1,2-polybutadiene that is capable of producing 1,2-polybutadiene that has high solubility in a polymerization solvent and does not tend to precipitate, and to provide a catalyst composition used therefor.　This method for producing 1,2-polybutadiene is characterized by comprising a step for polymerizing 1,3-butadiene in the presence of a catalyst system containing (a) a carboxylic acid salt of a transition metal, (b) an aluminoxane compound, and (c) a phosphine compound represented by general formula (1). In general formula (1), R1 and R2 represent the same or different substituted or unsubstituted C6-30 aryl groups, and R3 represents a substituted or unsubstituted C4-20 branched alkyl group or a substituted C6-30 cycloalkyl group.

Description

Method for producing 1,2-polybutadiene and catalyst composition

[0001] The present invention relates to a method for producing 1,2-polybutadiene by polymerizing 1,3-butadiene in the presence of a specific catalyst system to produce syndiotactic 1,2-polybutadiene, and a catalyst composition used therein.

[0002] Syndiotactic 1,2-polybutadiene has a structure consisting of highly crystalline regions and amorphous portions, and therefore functions as a thermoplastic elastomer. Furthermore, because of the presence of highly chemically reactive carbon-carbon double bonds in the molecule, it also functions as a conventional vulcanized rubber or a thermosetting resin with an increased crosslink density. Furthermore, because of its excellent processability, 1,2-polybutadiene is also used as a modifier for other resins and thermoplastic elastomers (for example, in tires to improve strength) and as a medical polymer material.

[0003] Conventional methods for producing 1,2-polybutadiene include a method in which 1,3-butadiene is polymerized in the presence of a cis-polymerization catalyst to produce cis-1,4-polybutadiene, and then a 1,2-polymerization catalyst is added to this polymerization system to polymerize 1,3-butadiene to produce a final polybutadiene (see, for example, Patent Document 1), and a method in which butadiene or butadiene and a conjugated diene other than butadiene are polymerized in the presence of a catalyst containing a cobalt compound and an aluminoxane (see, for example, Patent Document 2).

[0004] However, in the above-mentioned method for producing 1,2-polybutadiene, a polymer may precipitate in the polymerization tank during the course of polymerization in the polymerization tank. When the polymer precipitates in the polymerization tank, it becomes necessary to stop the polymerization and clean up the polymerization tank, which results in a problem of reduced productivity in terms of the time required for production and the yield.

[0005] Japanese Patent Publication No. 49-017667 Japanese Patent Application Laid-Open No. 2006-089759

[0006] The present invention has been made in light of the above circumstances, and an object of the present invention is to provide a method for producing 1,2-polybutadiene that can produce 1,2-polybutadiene that has high solubility in a polymerization solvent and is less likely to precipitate, and a catalyst composition to be used therein.

[0007] As a result of intensive research to achieve the above object, the present inventors have found that by using a specific catalyst composition as a polymerization catalyst, it is possible to selectively synthesize 1,2-polybutadiene that has high solubility in a polymerization solvent and is less likely to precipitate, and have thus completed the present invention.

[0008] The method for producing 1,2-polybutadiene of the present invention is characterized by comprising a step of polymerizing 1,3-butadiene in the presence of a catalyst system containing (a) a carboxylate of a transition metal, (b) an aluminoxane compound, and (c) a phosphine compound represented by the following general formula (1):

[0009]

[0010] [In the above general formula (1), R 1 and R 2 represents a substituted or unsubstituted aryl group having 6 to 30 carbon atoms, and may be the same or different from each other. 3 represents a substituted or unsubstituted branched alkyl group having 4 to 20 carbon atoms, or a substituted cycloalkyl group having 6 to 30 carbon atoms.

[0011] In the method for producing 1,2-polybutadiene of the present invention, the transition metal constituting the (a) carboxylate of a transition metal is preferably cobalt.

[0012] In the method for producing 1,2-polybutadiene of the present invention, the 1,2-polybutadiene preferably has a 1,2-vinyl bond content of 30% by mass or more.

[0013] The catalyst composition of the present invention is characterized by containing (a) a carboxylate of a transition metal, (b) an aluminoxane compound, and (c) a phosphine compound represented by the following general formula (1):

[0014]

[0015] [In the above general formula (1), R 1 and R 2 represents a substituted or unsubstituted aryl group having 6 to 30 carbon atoms, and may be the same or different from each other. 3 represents a substituted or unsubstituted branched alkyl group having 4 to 20 carbon atoms, or a substituted cycloalkyl group having 6 to 30 carbon atoms.

[0016] In the catalyst composition of the present invention, the transition metal constituting the (a) carboxylate of a transition metal is preferably cobalt.

[0017] According to the method for producing 1,2-polybutadiene of the present invention, 1,2-polybutadiene has high solubility in the polymerization solvent at the desired polymerization temperature and is less likely to precipitate in the polymerization vessel during polymerization, thereby eliminating the need to stop the polymerization during the polymerization step to clean up the polymerization vessel and achieving high productivity.The catalyst composition of the present invention makes it possible to produce 1,2-polybutadiene that has high solubility in the above polymerization solvent, thereby ensuring high productivity.

[0018] The present invention provides a method for producing 1,2-polybutadiene, comprising the step of polymerizing 1,3-butadiene in the presence of a catalyst system comprising a catalyst composition (hereinafter also referred to as the "specific catalyst composition") containing (a) a carboxylate of a transition metal, (b) an aluminoxane compound, and (c) a phosphine compound represented by the general formula (1) (hereinafter also referred to as the "specific phosphine compound").

[0019] Examples of the transition metal constituting the (a) transition metal carboxylate of the specific catalyst composition include manganese, iron, cobalt, nickel, etc., and among these, cobalt is preferred. Specific examples of the (a) transition metal carboxylate include manganese(II) oxalate, manganese(II) formate, manganese(II) acetate, manganese(III), manganese(II) benzoate, manganese(II) 4-cyclohexylbutyrate, manganese(II) 2-ethylhexanoate, etc., and hydrates thereof; iron(II) oxalate, iron(II) acetate, iron(II) fumarate, iron(II) lactate, iron(II) gluconate, sodium iron(II) citrate, iron(III), iron(II) benzoate, iron(II) stearate, etc., and hydrates thereof; cobalt oxalate, Cobalt(II), cobalt(II) acetate, cobalt(II) fumarate, cobalt(II) lactate, cobalt(II) gluconate, cobalt(III) citrate, cobalt(II) benzoate, cobalt(II) stearate, cobalt(II) octoate, etc., and hydrates thereof; nickel(II) formate, nickel(II) oxalate, nickel(II) acetate, nickel(II) fumarate, nickel(II) lactate, nickel(II) gluconate, nickel(II) benzoate, nickel(II) stearate, etc., and hydrates thereof.

[0020] The aluminoxane compound (b) of the specific catalyst composition can include, for example, compounds represented by the following general formula (2) or (3).

[0021]

[0022]

[0023] In the aluminoxane compound represented by the general formula (2) or (3), R 4 ~R 9 are each independently a hydrocarbon group such as a methyl group, an ethyl group, a propyl group, or a butyl group, preferably a methyl group or an ethyl group, and particularly preferably a methyl group, and m is an integer of 2 or more, preferably 5 or more, and more preferably an integer of 10 to 100.

[0024] Specific examples of the aluminoxane compound (b) include methylaluminoxane, ethylaluminoxane, propylaluminoxane, and butylaluminoxane, with methylaluminoxane being particularly preferred.

[0025] In the above general formula (1) representing the specific phosphine compound (c) of the specific catalyst composition, R 1 and R 2 represents a substituted or unsubstituted aryl group having 6 to 30 carbon atoms, and may be the same or different.

[0026] R 1 and R 2 is preferably a substituted or unsubstituted aryl group having 6 to 15 carbon atoms. When the aryl group is substituted, examples of the substituent include an alkyl group having 1 to 12 carbon atoms and an alkoxyl group having 1 to 12 carbon atoms, and when the number of substituents is two or more, they may be the same or different. Specific examples of substituted or unsubstituted aryl groups having 6 to 30 carbon atoms include a phenyl group, a 2-methylphenyl group, a 4-methylphenyl group, a 2-tert-butylphenyl group, a 2,4,6-trimethylphenyl group, a 2-isopropylphenyl group, a methoxyphenyl group, a 1,1-diphenylethyl group, a phenoxyphenyl group, a naphthyl group, a phenylnaphthyl group, a phenanthrene group, and an anthracene group. R 1 and R 2 Specifically, it is preferable that each of the groups be a phenyl group.

[0027] In addition, in the general formula (1), R 3 represents a substituted or unsubstituted branched alkyl group having 4 to 20 carbon atoms, or a substituted cycloalkyl group having 6 to 30 carbon atoms. 3 is preferably a substituted or unsubstituted branched alkyl group having 4 to 15 carbon atoms, or a substituted cycloalkyl group having 6 to 15 carbon atoms. Specific examples of the substituted or unsubstituted branched alkyl group having 4 to 20 carbon atoms include a tert-butyl group, a tert-pentyl group, an isopentyl group, a 3-ethylpent-3-yl group, and a 3-methylpent-3-yl group. Substituents for the cycloalkyl group include an alkyl group having 1 to 12 carbon atoms, an alkoxyl group having 1 to 12 carbon atoms, and an alkylsilyl group having 1 to 12 carbon atoms. When the number of substituents is two or more, the substituents may be the same or different. Specific examples of substituted cycloalkyl groups having 6 to 30 carbon atoms include a hexamethylcyclohexyl group, a 2-isopropyl-5-methylcyclohexyl group (neomenthyl group), a pentamethylcyclopentyl group, a 2-cyclooctylethyl group, a methylcyclohexyl group, a methoxycyclohexyl group, and a phenylcyclohexyl group. 3 Specifically, is preferably a tert-butyl group or a 2-isopropyl-5-methylcyclohexyl group (neomenthyl group).

[0028] (c) In the specific catalyst composition, the phosphorus atom (P) of the specific phosphine compound is not complexed.

[0029] The amount of the specific catalyst composition used is preferably 400 to 4,000, more preferably 800 to 2,000, in terms of the molar ratio (BD / Al) of the aluminum atom (Al) of the (b) aluminoxane compound to 1,3-butadiene. A ratio (BD / Al) of less than 400 is economically disadvantageous, while a ratio exceeding 4,000 may result in reduced polymerization activity. Furthermore, the amount of the (b) aluminoxane compound used is preferably 4 to 200, more preferably 10 to 150, in terms of the molar ratio (Al / M) of the aluminum atom (Al) to the transition metal atom (M) of the (a) transition metal carboxylate. Furthermore, the amount of the (c) specific phosphine compound used is preferably 0.1 to 50, more preferably 0.5 to 20, and particularly preferably 1 to 20, in terms of the molar ratio (P / M) of the phosphorus atom (P) to the transition metal atom (M) of the (a) transition metal carboxylate.

[0030] The method for producing 1,2-polybutadiene of the present invention is carried out in the presence of a polymerization solvent, and an inert organic solvent is used as the polymerization solvent. Examples of the inert organic solvent include aromatic hydrocarbons such as benzene, toluene, xylene, and cumene, aliphatic hydrocarbons such as n-pentane, n-hexane, and n-butane, alicyclic hydrocarbons such as cyclopentane, methylcyclopentane, and cyclohexane, and mixtures thereof.

[0031] The polymerization temperature is usually -50°C to 120°C, preferably -20°C to 100°C. The polymerization reaction is preferably a continuous polymerization in which multiple units are connected in series. The monomer concentration in the solvent is usually 5 to 50% by mass, preferably 10 to 35% by mass. In addition, to produce 1,2-polybutadiene, care must be taken to minimize the inclusion of compounds that have a deactivating effect, such as oxygen, water, or carbon dioxide gas, in the polymerization system so as not to deactivate the catalyst and polymer.

[0032] In the method for producing 1,2-polybutadiene of the present invention, conjugated dienes other than 1,3-butadiene can also be used in combination in an amount of about 10% or less. Examples of conjugated dienes include 4-alkyl-substituted-1,3-butadiene and 2-alkyl-substituted-1,3-butadiene. Among these, examples of 4-alkyl-substituted-1,3-butadienes include 1,3-pentadiene, 1,3-hexadiene, 1,3-heptadiene, 1,3-octadiene, 1,3-nonadiene, and 1,3-decadiene. Representative examples of the 2-alkyl substituted-1,3-butadiene include 2-methyl-1,3-butadiene (isoprene), 2-ethyl-1,3-butadiene, 2-propyl-1,3-butadiene, 2-isopropyl-1,3-butadiene, 2-butyl-1,3-butadiene, 2-isobutyl-1,3-butadiene, 2-amyl-1,3-butadiene, 2-isoamyl-1,3-butadiene, 2-hexyl-1,3-butadiene, 2-cyclohexyl-1,3-butadiene, 2-isohexyl-1,3-butadiene, 2-heptyl-1,3-butadiene, 2-isoheptyl-1,3-butadiene, 2-octyl-1,3-butadiene, and 2-isooctyl-1,3-butadiene. Of these conjugated dienes, preferred conjugated dienes to be used in combination with 1,3-butadiene include isoprene and 1,3-pentadiene.

[0033] When the polymerization reaction has progressed to the desired stage, a polymerization terminator such as an alcohol, an antioxidant, an antioxidant, an ultraviolet absorber, etc. are added to the reaction mixture, and the resulting polymer is then separated, washed, and dried according to a conventional method to obtain the target 1,2-polybutadiene.

[0034] [Product] The 1,2-polybutadiene obtained by the above production method has a 1,2-vinyl bond content of 30% by mass or more, preferably 35% by mass or more, and more preferably 40% by mass or more. The 1,2-vinyl bond content is measured according to the method described in the examples below.

[0035] The precipitation index temperature difference of 1,2-polybutadiene, calculated from (polymerization temperature - precipitation index temperature), is preferably 5 to 50°C, more preferably 10 to 50°C, and particularly preferably 30 to 40°C. The precipitation index temperature was determined by measuring the solution viscosity at 5°C intervals using a polymer solution prepared by diluting the polymer (1,2-polybutadiene) obtained by the above-described production method with cyclohexane to a solids concentration equivalent to that during polymerization, and plotting the solution viscosity logarithmically from high temperature to low temperature. When a point at which linearity is lost appears, the temperature just before that point on the high temperature side was determined as the precipitation index temperature. When the precipitation index temperature difference is within the above range, 1,2-polybutadiene has high solubility in the polymerization solvent at the intended polymerization temperature, and is therefore less likely to precipitate in the polymerization vessel during polymerization. As a result, there is no need to stop the polymerization during the polymerization step to clean up the polymerization vessel, and high productivity can be achieved with the above-described production method. Note that when the precipitation index temperature difference is 0°C or less, i.e., when the precipitation index temperature is equal to or higher than the polymerization temperature, 1,2-polybutadiene clearly precipitates in the polymerization vessel during polymerization, necessitating cleanup of the polymerization vessel.

[0036] The melting point of the 1,2-polybutadiene is preferably higher than 126° C., more preferably higher than 130° C., and particularly preferably higher than 135° C. The melting point is measured according to the method described in the examples below.

[0037] The molecular weight of 1,2-polybutadiene is preferably 100,000 to 600,000 in terms of polystyrene equivalent weight average molecular weight. If it is less than 100,000, strength properties will be poor, while if it exceeds 600,000, processability will be poor. The molecular weight can be adjusted by the ratio of aluminum atoms (Al) in the (b) aluminoxane compound to transition metal atoms (M) in the (a) transition metal carboxylate.

[0038] Furthermore, the 1,2-polybutadiene obtained by the above-described production method exhibits sufficient strength even when used alone without crosslinking, making it suitable for non-crosslinking molding applications such as industrial parts and film applications, such as injection molding and extrusion molding. Furthermore, because it has excellent crosslinking reactivity, it is also suitable for use in crosslinked rubber applications and as a reaction aid for vulcanized rubber. In this case, there are no particular limitations on the processing method, and mixing by melt kneading or the like using a roll, kneader, Banbury mixer, screw extruder, feeder ruder extruder, or the like, which are commonly used in resin and rubber processing, is possible. Furthermore, the 1,2-polybutadiene obtained in this manner is suitable for use as a modifier for other thermoplastic elastomers and resins.

[0039] Specific examples of the present invention will be described below, but the present invention is not limited to these examples. In the examples, "parts" and "%" are by weight unless otherwise specified.

[0040] Various measurements in the examples were performed using the following methods. The solid content ratio was measured by measuring the solid content when polymerization was performed under conditions that resulted in a polymerization conversion rate of 90% or higher, and was expressed as an index relative to Example 1, which was set to 100. The solid content ratio was measured by placing a sample on an aluminum dish whose mass had been measured in advance, heating the aluminum dish on a hot plate at 170°C for 5 minutes, and determining the solid content ratio in the sample from the mass before and after heating. The vinyl bond content (1,2-bond content) and cis bond content (cis-1,4-bond content) of 1,2-polybutadiene were determined by infrared absorption spectroscopy (Morello method). The weight-average molecular weight (Mw) was measured using gel permeation chromatography (GPC) at 135°C in o-dichlorobenzene as a solvent, as a polystyrene-equivalent value. The melting point was determined using a DSC measurement device according to the ASTM method. The temperature at which the endothermic peak reached its minimum value was defined as the melting point. The precipitation index temperature was measured as described above. In addition, when the precipitation index temperature was lower than the polymerization temperature, the precipitation index was evaluated as "A", and when it was higher, it was evaluated as "B".

[0041] Example 1 Polymerization of 1,3-Butadiene In a dry nitrogen atmosphere, 30 g of 1,3-butadiene (BD) and 114 g of cyclohexane (S / M = 3.8) were placed in a 300 ml pressure bottle. A 0.5% toluene solution of cobalt octoate (as Co atoms), a 1.0% toluene solution of (S)-(+)-neomenthyldiphenylphosphine, and a 1.0% toluene solution of methylaluminoxane (as Al atoms) were added to the bottle so that the molar ratios of BD / Al were 500, Al / Co (atomic ratios) were 25, and P / Co were 1.0, respectively. Polymerization was carried out for 120 minutes in a thermostatic water bath at 80°C. The reaction was terminated by adding a small amount of ethanol as a terminator. Next, 0.3 parts of 2,6-di-t-butyl-p-cresol per 100 parts of the polymer was added, and the mixture was heated on a hot plate to remove the solvent, yielding a polymer. The polymerization conversion was calculated from the yield. The results are shown in Table 1.

[0042] Comparative Example 1: In a dry nitrogen atmosphere, 30 g of 1,3-butadiene (BD) and 114 g of cyclohexane (S / M = 3.8) were placed in a 300 ml pressure bottle. A 0.5% toluene solution of cobalt octoate (as Co atoms), a 1.0% toluene solution of cyclohexyldiphenylphosphine, and a 1.0% toluene solution of methylaluminoxane (as Al atoms) were added so that the molar ratios of BD / Al were 500, Al / Co (atomic ratios) were 30, and P / Co were 1.0, respectively. Polymerization was carried out for 120 minutes in a thermostatic water bath at 40°C. A decrease in fluidity due to precipitation of the polymer solution occurred. The reaction was terminated by adding a small amount of ethanol as a terminator. Next, 0.3 parts of 2,6-di-t-butyl-p-cresol per 100 parts of polymer was added, and the mixture was heated on a hot plate. The solvent was removed to obtain a polymer, and the polymerization conversion was calculated from the yield. The results are shown in Table 1.

[0043] Comparative Example 2: In a dry nitrogen atmosphere, 24 g of 1,3-butadiene (BD) and 132 g of cyclohexane (S / M = 5.5) were placed in a 300 ml pressure bottle. A 0.5% toluene solution of cobalt octoate (as Co atoms), a 1.0% toluene solution of cyclohexyldiphenylphosphine, and a 1.0% toluene solution of methylaluminoxane (as Al atoms) were added so that the molar ratios of BD / Al were 500, Al / Co (atomic ratios) were 30, and P / Co were 1.0, respectively. Polymerization was carried out for 120 minutes in a thermostatic water bath at 40°C. The reaction was terminated by adding a small amount of ethanol as a terminator. Next, 0.3 parts of 2,6-di-t-butyl-p-cresol per 100 parts of polymer was added, and the mixture was heated on a hot plate. The solvent was removed to obtain a polymer, and the polymerization conversion was calculated from the yield. The results are shown in Table 1.

[0044] Comparative Example 3: In a dry nitrogen atmosphere, 30 g of 1,3-butadiene (BD) and 114 g of cyclohexane (S / M = 3.8) were placed in a 300 ml pressure bottle. A 0.5% toluene solution of cobalt octoate (as Co atoms), a 1.0% toluene solution of cyclohexyldiphenylphosphine, and a 1.0% toluene solution of methylaluminoxane (as Al atoms) were added so that the molar ratios of BD / Al were 500, Al / Co (atomic ratios) were 50, and P / Co were 1.0, respectively. Polymerization was carried out for 120 minutes in a thermostatic water bath at 70°C. The reaction was terminated by adding a small amount of ethanol as a terminator. Next, 0.3 parts of 2,6-di-t-butyl-p-cresol per 100 parts of polymer was added, and the mixture was heated on a hot plate. The solvent was removed to obtain a polymer, and the polymerization conversion was calculated from the yield. The results are shown in Table 1.

[0045] Example 2 Polymerization of 1,3-Butadiene In a dry nitrogen atmosphere, 30 g of 1,3-butadiene (BD) and 114 g of cyclohexane (S / M = 3.8) were placed in a 300 ml pressure bottle. A 0.5% toluene solution of cobalt octoate (as Co atoms), a 0.5% toluene solution of (S)-(+)-neomenthyldiphenylphosphine, and a 1.0% toluene solution of methylaluminoxane (as Al atoms) were added thereto so that the molar ratios of BD / Al were 500, Al / Co (atomic ratios) were 20, and P / Co were 1.0, respectively. Polymerization was carried out for 120 minutes in a constant-temperature water bath at 70°C. The reaction was terminated by adding a small amount of ethanol as a terminator. Next, 0.3 parts of 2,6-di-t-butyl-p-cresol per 100 parts of the polymer was added, and the mixture was heated on a hot plate to remove the solvent, yielding a polymer. The polymerization conversion was calculated from the yield. The results are shown in Table 1.

[0046] Example 3 Polymerization of 1,3-Butadiene In a dry nitrogen atmosphere, 30 g of 1,3-butadiene (BD) and 114 g of cyclohexane (S / M = 3.8) were placed in a 300 ml pressure bottle. A 0.5% toluene solution of cobalt octoate (as Co atoms), a 0.5% toluene solution of (S)-(+)-neomenthyldiphenylphosphine, and a 1.0% toluene solution of methylaluminoxane (as Al atoms) were added thereto so that the molar ratios of BD / Al were 500, Al / Co (atomic ratios) were 140, and P / Co were 0.3, respectively. Polymerization was carried out for 120 minutes in a thermostatic water bath at 50°C. The reaction was terminated by adding a small amount of ethanol as a terminator. Next, 0.3 parts of 2,6-di-t-butyl-p-cresol per 100 parts of the polymer was added, and the mixture was heated on a hot plate to remove the solvent, yielding a polymer. The polymerization conversion was calculated from the yield. The results are shown in Table 1.

[0047]

[0048] As is clear from Table 1, it was confirmed that in Examples 1 to 3, 1,2-polybutadiene having the desired melting point could be produced in the desired yield (solid content ratio) without precipitation of the polymer solution. On the other hand, in Comparative Example 1, in which cyclohexyldiphenylphosphine was used as the phosphine compound, the polymerization temperature had to be lowered to 40°C to produce 1,2-polybutadiene equivalent to that of Example 1 in terms of melting point and production amount. However, lowering the polymerization temperature resulted in a decrease in fluidity due to precipitation of the polymer solution. Furthermore, in Comparative Example 2, in which cyclohexyldiphenylphosphine was used as the phosphine compound, the S / M ratio had to be 5.5 to produce the same polymer by conventional technology without precipitation of the polymer solution, and the solid content ratio, which is an indicator of polymer productivity, decreased. Furthermore, in Comparative Example 3, in which cyclohexyldiphenylphosphine was used as the phosphine compound and the polymerization temperature was set to 70°C to prevent precipitation of the polymer solution, the melting point of the resulting polymer decreased with the increase in polymerization temperature, and the desired polymer could not be obtained.

Claims

1. A method for producing 1,2-polybutadiene, comprising the step of polymerizing 1,3-butadiene in the presence of a catalyst system containing (a) a carboxylate of a transition metal, (b) an aluminoxane compound, and (c) a phosphine compound represented by the following general formula (1): [In the above general formula (1), R 1 and R 2 represents a substituted or unsubstituted aryl group having 6 to 30 carbon atoms, and may be the same or different. 3 represents a substituted or unsubstituted branched alkyl group having 4 to 20 carbon atoms, or a substituted cycloalkyl group having 6 to 30 carbon atoms.

2. The method for producing 1,2-polybutadiene according to claim 1, characterized in that the transition metal constituting the carboxylate of the transition metal (a) is cobalt.

3. The method for producing 1,2-polybutadiene according to claim 1, characterized in that the 1,2-polybutadiene has a 1,2-vinyl bond content of 30% by mass or more.

4. A catalyst composition comprising (a) a carboxylate of a transition metal, (b) an aluminoxane compound, and (c) a phosphine compound represented by the following general formula (1): [In the above general formula (1), R 1 and R 2 represents a substituted or unsubstituted aryl group having 6 to 30 carbon atoms, and may be the same or different. 3 represents a substituted or unsubstituted branched alkyl group having 4 to 20 carbon atoms, or a substituted cycloalkyl group having 6 to 30 carbon atoms.

5. The catalyst composition according to claim 4, characterized in that the transition metal constituting the carboxylate of the transition metal (a) is cobalt.

Images