Patent Publication Number: US-10322317-B2

Title: Rubber composition, crosslinked rubber molded product and golf ball

Description:
FIELD OF THE INVENTION 
     The present invention relates to a rubber composition, more specifically, a rubber composition containing a novel co-crosslinking agent. 
     DESCRIPTION OF THE RELATED ART 
     Examples of the method for increasing a flight distance of a golf ball on driver shots include a method of utilizing a core having high resilience, and a method of utilizing a core having a hardness distribution in which the hardness increases from the core center towards the core surface. The former method has an effect of increasing the initial speed of the golf ball, and the latter method has an effect of increasing the launch angle and lowering the spin rate of the golf ball. A golf ball showing a high launch angle and a low spin rate travels a great flight distance. 
     Examples of the method of obtaining a core having high resilience include a method of forming a core with a multi-layer structure and controlling the property of each layer, and a method of forming a core from a rubber composition having high resilience. The method of controlling the property of each layer of the core, for example, is described in Japanese Patent Publications No. 2001-87422 A and 2001-161858 A. Japanese Patent Publication No. 2001-87422 A discloses a multi-piece solid golf ball wherein an inner core layer of the golf ball has an elastic modulus in a range from 50 to 200 MPa, and an outer core layer of the golf ball has a lower elastic modulus than the inner core layer only by 15 to 100 MPa (refer to paragraphs 0020, 0022). In addition, Japanese Patent Publication No. 2001-161858 A discloses a multi-piece solid golf ball wherein if an amount of an organic sulfur compound blended in an innermost core layer and an outermost core layer of the golf ball with respect to 100 parts by weight of a base rubber, is Hi and Ho, respectively, the formula 0≤Ho/Hi&lt;1 is satisfied (refer to paragraph 0019). 
     As the method of forming a core from a rubber composition having high resilience, a technology of improving a base rubber blended in the rubber composition has been proposed. For example, Japanese Patent Publications No. 4062363 B and No. 2004-292667 A propose a rubber composition containing a high-cis polybutadiene as a base rubber, wherein the high-cis polybutadiene is synthesized by using a cobalt catalyst. Japanese Patent Publication No. 2006-241265 A proposes a rubber composition containing a high-cis polybutadiene (A) and a high-cis polybutadiene (B) in combination as a base rubber, wherein the high-cis polybutadiene (A) is synthesized by using a cobalt catalyst, and the high-cis polybutadiene (B) is synthesized by using a catalyst other than a cobalt catalyst (refer to paragraphs 0017, 0035). In addition, Japanese Patent Publications No. 4811274 B proposes a rubber composition containing a polybutadiene rubber (A) containing a syndiotactic polybutadiene, and a rubber other than (A) in combination as a base rubber (refer to paragraphs 0010, 0025). 
     SUMMARY OF THE INVENTION 
     Various rubber compositions for improving the resilience have been proposed. However, there is still room for improvement of the resilience performance. The present invention has been made in view of the above mentioned circumstances, and an object of the present invention is to provide a rubber composition providing a crosslinked rubber molded product excellent in the resilience performance. 
     The present invention that has solved the above problems provides a rubber composition containing (a) a base rubber, (b) a co-crosslinking agent and (c) a crosslinking initiator, wherein (b) the co-crosslinking agent contains a complex represented by a general formula (1).
 
((RCOO) 8 M 5 (OH) 2 ) n   (1)
 
     [In the formula (1), M is a metal atom, R is a hydrogen atom, an alkyl group having 1 to 18 carbon atoms, an alkenyl group having 2 to 18 carbon atoms or an alkynyl group having 2 to 18 carbon atoms, and n is an integer of 1 or more. A plurality of R and M may be identical to or different from each other. At least one of R is the alkenyl group having 2 to 18 carbon atoms or the alkynyl group having 2 to 18 carbon atoms.] 
     The complex represented by the general formula (1) has a high affinity to (a) the base rubber, and thus has a high dispersibility in (a) the base rubber. Accordingly, the crosslinking point after the crosslinking has a small size (about 0.4 nm). In addition, the complex represented by the general formula (1) has formed a structure of a crosslinking point in advance. Accordingly, the crosslinking point can be formed merely by the reaction with the base rubber, and the crosslinking efficiency is high. Thus, if the rubber composition according to the present invention is used, a crosslinked rubber molded product excellent in the resilience performance can be obtained. 
     The present invention also includes a crosslinked rubber molded product formed from the above rubber composition. Further, the present invention also includes a golf ball comprising a constituent member formed from the above rubber composition. 
     If the rubber composition according to the present invention is used, a crosslinked rubber molded product excellent in the resilience performance can be obtained. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows IR spectrum of zinc acrylate hydroxo cluster; 
         FIG. 2  shows X-ray diffraction spectrum of zinc acrylate hydroxo cluster; 
         FIG. 3  shows a structure of zinc acrylate hydroxo cluster determined by single crystal X-ray structural analysis; 
         FIG. 4  shows a relationship between a slab hardness and a rebound resilience of a crosslinked rubber molded product; 
         FIG. 5  shows a relationship between a slab hardness and a rebound resilience of a crosslinked rubber molded product; 
         FIG. 6  shows a relationship between a slab hardness and |log(E′/E″ 2 )|of a crosslinked rubber molded product; 
         FIG. 7  shows a three dimensional structure of the assembled complex of the structural formula (2); and 
         FIG. 8  shows an Oak ridge thermal-ellipsoid plot program of the crystal structure of the assembled complex. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     [Rubber Composition] 
     The rubber composition according to the present invention contains (a) a base rubber, (b) a co-crosslinking agent and (c) a crosslinking initiator, wherein (b) the co-crosslinking agent contains a complex represented by a general formula (1) which will be described later. 
     A metal salt of an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms, which has been widely used as a co-crosslinking agent so far, is an ionic crystal, thus it has a low affinity to the base rubber and has a low dispersibility in the base rubber. In addition, such the metal salt of the α,β-unsaturated carboxylic acid forms crosslinking points by forming a graft chain by the oligomerization of the co-crosslinking agents and the reaction with the base rubber. Accordingly, each crosslinking point has a great size (about 4 nm), and the crosslinking efficiency is low. On the contrary, the complex represented by the general formula (1) has a high affinity to (a) the base rubber, and thus has a high dispersibility in (a) the base rubber. Accordingly, the crosslinking point after the crosslinking has a small size (about 0.4 nm). In addition, the complex represented by the general formula (1) has formed a structure of a crosslinking point in advance. Accordingly, the crosslinking point can be formed merely by the reaction with the base rubber, and the crosslinking efficiency is high. Thus, if the rubber composition according to the present invention is used, a crosslinked rubber molded product excellent in the resilience performance can be obtained. 
     ((a) Base Rubber) 
     As (a) the base rubber, a natural rubber and/or a synthetic rubber may be used. Examples of the synthetic rubber include a diene rubber such as polybutadiene rubber (BR), polyisoprene rubber (IR), styrene polybutadiene rubber (SBR), chloroprene rubber (CR), butyl rubber (IIR) and acrylonitrile butadiene rubber (NBR); and a non-diene rubber such as ethylene propylene rubber (EPM), ethylene-propylene-diene rubber (EPDM), urethane rubber, silicone rubber, acrylic rubber, epichlorohydrin rubber, polysulfide rubber, fluorine rubber and chlorosulfonated polyethylene rubber. These synthetic rubbers may be used solely, or at least two of them may be used in combination. 
     (a) The base rubber preferably contains the natural rubber and/or the diene rubber. The total amount of the natural rubber and/or the diene rubber in (a) the base rubber is preferably 50 mass % or more, more preferably 70 mass % or more, and even more preferably 90 mass % or more. It is also preferred that (a) the base rubber consists of the natural rubber and/or the diene rubber 
     (a) The base rubber preferably contains the polybutadiene rubber. Particularly preferred is a high-cis polybutadiene having a cis-1,4-bond which is beneficial to the resilience in an amount of 40 mass % or more, preferably 80 mass % or more, and more preferably 90 mass % or more. The amount of the high-cis polybutadiene in (a) the base rubber is preferably 50 mass % or more, more preferably 70 mass % or more. 
     The high-cis polybutadiene preferably contains a 1,2-vinyl bond in an amount of 2.0 mass % or less, more preferably 1.7 mass % or less, and even more preferably 1.5 mass % or less. If the amount of the 1,2-vinyl bond is too large, the resilience may be lowered. 
     The high-cis polybutadiene preferably includes one synthesized using a rare-earth element catalyst. When a neodymium catalyst, which employs a neodymium compound of a lanthanum series rare-earth element compound, is used, a polybutadiene rubber having the cis-1,4 bond in a high amount and the 1,2-vinyl bond in a low amount is obtained with excellent polymerization activity. Such polybutadiene rubber is particularly preferred. 
     The high-cis polybutadiene preferably has a molecular weight distribution Mw/Mn (Mw: weight average molecular weight, Mn: number average molecular weight) of 2.0 or more, more preferably 2.2 or more, even more preferably 2.4 or more, and most preferably 2.6 or more, and preferably has a molecular weight distribution Mw/Mn of 6.0 or less, more preferably 5.0 or less, even more preferably 4.0 or less, and most preferably 3.4 or less. If the molecular weight distribution (Mw/Mn) of the high-cis polybutadiene is excessively low, the workability may be lowered, and if the molecular weight distribution (Mw/Mn) of the high-cis polybutadiene is excessively high, the resilience may be lowered. It is noted that the measurement of the molecular weight distribution is conducted by gel permeation chromatography (“HLC-8120GPC” available from Tosoh Corporation) using a differential refractometer as a detector under the conditions of column: GMHHXL (available from Tosoh Corporation), column temperature: 40° C. and mobile phase: tetrahydrofuran, and calculated by converting based on polystyrene standard. 
     The high-cis polybutadiene preferably has a Mooney viscosity (ML 1+4 (100° C.)) of 30 or more, more preferably 32 or more, and even more preferably 35 or more, and preferably has a Mooney viscosity (ML 1+4 (100° C.)) of 140 or less, more preferably 120 or less, even more preferably 100 or less, and most preferably 80 or less. It is noted that the Mooney viscosity (ML 1+4 (100° C.)) in the present invention is a value measured according to JIS K6300 using an L rotor under the conditions of: a preheating time of 1 minute; a rotor rotation time of 4 minutes; and a temperature of 100° C. 
     ((b) Co-Crosslinking Agent) 
     (b) The co-crosslinking agent contains a complex having a carboxylate as a ligand and represented by the general formula (1).
 
((RCOO) 8 M 5 (OH) 2 ) n   (1)
 
     [In the formula (1), M is a metal atom, O is an oxygen atom, H is a hydrogen atom, R is a hydrogen atom, an alkyl group having 1 to 18 carbon atoms, an alkenyl group having 2 to 18 carbon atoms or an alkynyl group having 2 to 18 carbon atoms, and n is an integer of 1 or more. A plurality of R and M may be identical to or different from each other. At least one of R is the alkenyl group having 2 to 18 carbon atoms or the alkynyl group having 2 to 18 carbon atoms.] 
     Examples of the metal atom (M) include an alkali metal such as lithium, sodium, potassium, rubidium and cesium; an alkaline earth metal such as calcium, strontium and barium; a transition metal such as scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, yttrium, zirconium, niobium, molybdenum, technetium, ruthenium, rhodium, palladium, silver, hafnium, tantalum, tungsten, rhenium, osmium, iridium, platinum and gold; and a base metal such as beryllium, magnesium, aluminum, zinc, gallium, cadmium, indium, tin, thallium, lead, bismuth and polonium. These metal atoms may be used solely, or at least two of them may be used in combination. Among them, as the metal atom, the metal atom having oxidation number of +2 is preferable, and beryllium, magnesium, calcium, zinc, barium, cadmium or lead is more preferable. In the formula (1), a plurality of metal atoms (M) may be identical to or different from each other, and are preferably all identical to each other. 
     Examples of the alkyl group having 1 to 18 carbon atoms include methyl group, ethyl group, propyl group, butyl group, pentyl group, hexyl group, heptyl group, octyl group, nonyl group, decyl group, undecyl group, dodecyl group, tridecyl group, tetradecyl group, pentadecyl group, hexadecyl group, heptadecyl group, and octadecyl group. The alkyl group having 1 to 18 carbon atoms may have a linear structure, a branched structure or a cyclic structure, and the linear structure is preferable. 
     Examples of the alkenyl group having 2 to 18 carbon atoms include ethenyl group (vinyl group), 1-propenyl group, 2-propenyl group, isopropenyl group, 3-butenyl group, 4-pentenyl group, 5-hexenyl group, 6-heptenyl group, 7-octenyl group, 8-nonenyl group, 9-decenyl group, 10-undecenyl group, 11-dodecenyl group, 8-tridecenyl group, 12-tridecenyl group, 13-tetradecenyl group, 8-pentadecenyl group, 14-pentadecenyl group, 15-hexadecenyl group, 8-heptadecenyl group, 10-heptadecenyl group, 16-heptadecenyl group, and 17-octadecenyl group. The alkenyl group having 2 to 18 carbon atoms may have a linear structure or a branched structure, and the linear structure is preferable. As the alkenyl group having 2 to 18 carbon atoms, an alkenyl group having one carbon-carbon double bond is preferable, and an alkenyl group having a carbon-carbon double bond at a terminal thereof is more preferable. The alkenyl group preferably has 8 or less carbon atoms, more preferably 6 or less carbon atoms, and even more preferably 4 or less carbon atoms. Preferable examples of the alkenyl group having 2 to 18 carbon atoms include vinyl group, isopropenyl group, 1-propenyl group and 2-propenyl group. 
     Examples of the alkynyl group having 2 to 18 carbon atoms include ethynyl group, 1-propynyl group, 2-propynyl group, 3-butynyl group, 4-pentynyl group, 5-hexynyl group, 6-heptynyl group, 7-octynyl group, 8-nonynyl group, 9-decynyl group, 10-undecynyl group, 11-dodecynyl group, 8-tridecynyl group, 12-tridecynyl group, 13-tetradecynyl group, 8-pentadecynyl group, 14-pentadecynyl group, 15-hexadecynyl group, 8-heptadecynyl group, 10-heptadecynyl group, 16-heptadecynyl group, and 17-octadecynyl group. The alkynyl group having 2 to 18 carbon atoms may have a linear structure or a branched structure, and the linear structure is preferable. As the alkynyl group having 2 to 18 carbon atoms, an alkynyl group having one carbon-carbon triple bond is preferable, and an alkynyl group having a carbon-carbon triple bond at a terminal thereof is more preferable. The alkynyl group preferably has 8 or less carbon atoms, more preferably has 6 or less carbon atoms, and even more preferably has 4 or less carbon atoms. Preferable examples of the alkynyl group having 2 to 18 carbon atoms include ethynyl group, 1-propynyl group and 2-propynyl group. 
     In the formula (1), at least one of R is the alkenyl group having 2 to 18 carbon atoms or the alkynyl group having 2 to 18 carbon atoms. In other words, the complex represented by the formula (1) has one or more carbon-carbon unsaturated bonds. If the complex represented by the formula (1) has one or more carbon-carbon unsaturated bonds, the complex represented by the formula (1) is capable of reacting with (a) the base rubber. The number of the alkenyl group having 2 to 18 carbon atoms or alkynyl group having 2 to 18 carbon atoms in the above R is preferably 2 or more, more preferably 4 or more, even more preferably 5 or more, particularly preferably 6 or more, and most preferably 8 or more. If the complex represented by the formula (1) has two or more carbon-carbon unsaturated bonds, the complex represented by the formula (1) is capable of crosslinking a plurality of molecular chains of (a) the base rubber. 
     In the formula (1), at least one of R is preferably an alkenyl group having 2 to 18 carbon atoms and a carbon-carbon double bond at a terminal thereof, or an alkynyl group having 2 to 18 carbon atoms and a carbon-carbon triple bond at a terminal thereof. The number of the alkenyl group having 2 to 18 carbon atoms and a carbon-carbon double bond at a terminal thereof or alkynyl group having 2 to 18 carbon atoms and having a carbon-carbon triple bond at a terminal thereof in the above R is preferably 2 or more, more preferably 4 or more, even more preferably 5 or more, particularly preferably 6 or more, and most preferably 8 or more. 
     In the formula (1), a plurality of R may be identical to or different from each other, and are preferably all identical to each other. 
     In the formula (1), a complex in which n is 1 is a basic structural unit of the complex, and a complex in which n is an integer of 2 or more is an assembled complex formed by assembling the basic structural units. The above n is an integer of 1 or more. If the above n is 2 or more, a strong crosslinking point can be formed, and thus the obtained crosslinked rubber has enhanced durability. In the assembled complex (n is an integer of 2 or more), the upper limit of n is preferably one hundred million or less, more preferably one million or less, and even more preferably ten thousand or less, without any limitation. 
     Examples of the complex represented by the formula (1) include a complex (zinc acrylate hydroxo cluster) in which all the R are vinyl group and the metal atom (M) is zinc; and a complex (zinc methacrylate hydroxo cluster) in which all the R are isopropenyl group and the metal atom (M) is zinc. 
     The complex represented by the formula (1) is preferably a complex having a structure represented by a structural formula (2). 
     
       
         
         
             
             
         
       
     
     [In the formula (2), M 1  to M 5  are identical to or different from each other and represent a metal atom, O represents an oxygen atom, H represents a hydrogen atom, R 1  to R 8  are identical to or different from each other and represent a hydrogen atom, an alkyl group having 1 to 18 carbon atoms, an alkenyl group having 2 to 18 carbon atoms or an alkynyl group having 2 to 18 carbon atoms, n is an integer of 1 or more, and at least one of R 1  to R 8  is the alkenyl group having 2 to 18 carbon atoms or the alkynyl group having 2 to 18 carbon atoms.]. 
     In the formula (2), the atomic group of the adjacent structural units is depicted outside the brackets. In the structural formula (2), the bond shown in the dashed line which is attached to the solid line is a hybrid bond of the carboxylate group. In addition, in the structural formula (2), the covalent bond and the coordination bond are both shown in a solid line. 
     In the assembled complex (n is an integer of 2 or more), the upper limit of n is preferably one hundred million or less, more preferably one million or less, and even more preferably ten thousand or less, without any limitation. 
     The assembled complex represented by the structural formula (2) has a three-dimensional structure represented by formulae (2-1) ( FIGS. 7 ) and (2-2) ( FIG. 8 ). The formula (2-2) shows ORTEP (Oak Ridge Thermal-Ellipsoid Plot Program) of the crystal structure of the assembled complex. Based on these structural formulae, it is found that the assembled complex represented by the structural formula (2) has a three-dimensional structure in which four structural units bond to one structural unit. 
     In addition, when n is 1, the complex represented by the formula (1) is preferably a complex represented by a formula (3). 
     
       
         
         
             
             
         
       
     
     [In the formula (3), M 1  to M 5  are identical to or different from each other and represent a metal atom, O represents an oxygen atom, H represents a hydrogen atom, R 1  to R 8  are identical to or different from each other and represent a hydrogen atom, an alkyl group having 1 to 18 carbon atoms, an alkenyl group having 2 to 18 carbon atoms or an alkynyl group having 2 to 18 carbon atoms, and at least one of R 1  to R 8  is the alkenyl group having 2 to 18 carbon atoms or the alkynyl group having 2 to 18 carbon atoms.] 
     In the structural formula (3), the bond shown in the dashed line which is attached to the solid line is a hybrid bond of the carboxylate group. In addition, in the structural formula (3), the covalent bond and the coordination bond are both shown in a solid line. 
     The complex represented by the formula (3) is the basic structural unit. The assembled complex (n is an integer of 2 or more) in the formula (2) and the complex (n is 1) represented by the formula (3) exist in the solvent in an equilibrium state, and the existing proportion thereof varies depending on the solvent, temperature and concentration. The complex represented by the formula (3) changes toward the assembled complex (n is an integer of 2 or more) which is more stable. For example, in a solid state, the assembled complex (n is an integer of 2 or more) which is more stable is obtained, and in a low concentration solution, the proportion of the complex represented by the formula (3) becomes high because it is stabilized by the solvent. 
     Examples of the metal atoms represented by M 1  to M 5  in the formulae (2) or (3) include those listed as M in the formula (1). Among them, as the metal atom, the metal atom having oxidation number of +2 is preferable, and beryllium, magnesium, calcium, zinc, barium, cadmium or lead is more preferable. The metal atoms represented by M 1  to M 5  may be different from each other, but are preferably all the same metal atom. 
     Examples of the alkyl group having 1 to 18 carbon atoms represented by R 1  to R 8  in the formulae (2) or (3) include those listed as R in the formula (1). The alkyl group having 1 to 18 carbon atoms may have a linear structure, a branched structure or a cyclic structure, and the linear structure is preferable. 
     Examples of the alkenyl group having 2 to 18 carbon atoms represented by R 1  to R 8  in the formulae (2) or (3) include those listed as R in the formula (1). The alkenyl group having 2 to 18 carbon atoms may have a linear structure or a branched structure, and the linear structure is preferable. As the alkenyl group having 2 to 18 carbon atoms, an alkenyl group having a carbon-carbon double bond at a terminal thereof is preferable. The alkenyl group preferably has 8 or less carbon atoms, more preferably 6 or less carbon atoms, and even more preferably 4 or less carbon atoms. Preferable examples of the alkenyl group having 2 to 18 carbon atoms include vinyl group, isopropenyl group, 1-propenyl group and 2-propenyl group. 
     Examples of the alkynyl group having 2 to 18 carbon atoms represented by R 1  to R 8  in the formulae (2) or (3) include those listed as R in the formula (1). The alkynyl group having 2 to 18 carbon atoms may have a linear structure or a branched structure, and the linear structure is preferable. As the alkynyl group having 2 to 18 carbon atoms, an alkynyl group having a carbon-carbon triple bond at a terminal thereof is preferable. The alkynyl group preferably has 8 or less carbon atoms, more preferably 6 or less carbon atoms, and even more preferably 4 or less carbon atoms. Preferable examples of the alkynyl group having 2 to 18 carbon atoms include ethynyl group, 1-propynyl group and 2-propynyl group. 
     In the formulae (2) or (3), at least one of R 1  to R 8  is the alkenyl group having 2 to 18 carbon atoms or the alkynyl group having 2 to 18 carbon atoms. The number of the alkenyl group having 2 to 18 carbon atoms or alkynyl group having 2 to 18 carbon atoms in the above R 1  to R 8  is preferably 2 or more, more preferably 4 or more, even more preferably 5 or more, particularly preferably 6 or more, and most preferably 8. 
     In the formulae (2) or (3), at least one of R 1  to R 8  is preferably an alkenyl group having 2 to 18 carbon atoms and a carbon-carbon double bond at a terminal thereof, or an alkynyl group having 2 to 18 carbon atoms and a carbon-carbon triple bond at a terminal thereof. The number of the alkenyl group having 2 to 18 carbon atoms and a carbon-carbon double bond at a terminal thereof or alkynyl group having 2 to 18 carbon atoms and a carbon-carbon triple bond at a terminal thereof in the above R 1  to R 8  is preferably 2 or more, more preferably 4 or more, even more preferably 5 or more, particularly preferably 6 or more, and most preferably 8. 
     In the formulae (2) or (3), R 1  to R 8  may be identical to or different from each other, and are preferably all identical to each other. 
     In the formulae (2) or (3), R 1  to R 8  are preferably vinyl group or isopropenyl group, and the metal atom (M) is preferably a metal atom having oxidation number of +2, more preferably zinc. 
     (b) The co-crosslinking agent may further contain another co-crosslinking agent than the complex represented by the general formula (1), unless the other co-crosslinking agent impairs the effect of the present invention. Examples of the other co-crosslinking agent include an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or a metal salt thereof. The α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or the metal salt thereof has an action of crosslinking a rubber molecule by graft polymerization to a base rubber molecular chain. Examples of the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms include acrylic acid, methacrylic acid, fumaric acid, maleic acid, and crotonic acid. 
     Examples of the metal constituting the metal salt of the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms include a monovalent metal ion such as sodium, potassium and lithium; a divalent metal ion such as magnesium, calcium, zinc, barium and cadmium; a trivalent metal ion such as aluminum; and other metal ion such as tin and zirconium. The metal component may be used solely or as a mixture of at least two of them. Among them, as the metal component, the divalent metal such as magnesium, calcium, zinc, barium, cadmium or the like is preferred. This is because use of the divalent metal salt of the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms easily generates a metal crosslinking between the rubber molecules. 
     In the case that the other co-crosslinking agent is used, the amount of the complex represented by the general formula (1) in (b) the co-crosslinking agent is preferably 5 mass % or more, more preferably 20 mass % or more, even more preferably 50 mass % or more, and most preferably 80 mass % or more. It is noted that it is also preferred that (b) the co-crosslinking agent of the rubber composition consists of the complex represented by the general formula (1). 
     The amount of (b) the co-crosslinking agent in the rubber composition is preferably 1 part by mass or more, more preferably 3 parts by mass or more, and even more preferably 5 parts by mass or more, and is preferably 50 parts by mass or less, more preferably 45 parts by mass or less, and even more preferably 40 parts by mass or less, with respect to 100 parts by mass of (a) the base rubber. If the amount of (b) the co-crosslinking agent is less than 1 part by mass, the amount of (c) the crosslinking initiator which will be described later must be increased in order to obtain an appropriate hardness of the constituent member formed from the rubber composition, which tends to lower the resilience of the molded product of the crosslinked rubber. On the other hand, if the amount of (b) the co-crosslinking agent exceeds 50 parts by mass, the constituent member formed from the rubber composition tends to become too hard. 
     ((c) Crosslinking Initiator) 
     (c) The crosslinking initiator is blended in order to crosslink (a) the base rubber component. As (c) the crosslinking initiator, an organic peroxide is preferred. Examples of the organic peroxide include peroxy ketal, dialkyl peroxide, diacyl peroxide, and peroxy ester. Examples of the peroxy ketal include 1,1-di(t-hexylperoxy) cyclohexane, 1,1-di(t-butylperoxy) cyclohexane, n-butyl-4,4-di-(t-butylperoxy valerate), and 1,1-bis(t-butylperoxy)-3,3,5-trimethyl cyclohexane. Examples of the dialkyl peroxide include dicumyl peroxide, di(2-t-butylperoxyisopropyl) benzene, t-butylcumyl peroxide, 2,5-dimethyl-2,5-di(t-butylperoxy) hexane, 2,5-dimethyl-2,5-di(t-butylperoxy) hexyne-3, and di-t-butyl peroxide. Examples of the peroxy ester include 2,5-dimethyl-2,5-di(benzoylperoxy) hexane, t-hexylperoxy benzoate, and t-butylperoxy benzoate. These organic peroxides may be used solely, or at least two of them may be used in combination. Among them, dicumyl peroxide is preferably used. 
     The amount of (c) the crosslinking initiator is preferably 0.2 part by mass or more, more preferably 0.5 part by mass or more, and even more preferably 0.7 part by mass or more, and is preferably 50 parts by mass or less, more preferably 45 parts by mass or less, even more preferably 40 parts by mass or less, and most preferably 35 parts by mass or less, with respect to 100 parts by mass of (a) the base rubber. If the amount of (c) the crosslinking initiator is less than 0.2 part by mass, the constituent member formed from the rubber composition becomes so soft that the resilience of the crosslinked rubber molded product may be lowered, and if the amount of (c) the crosslinking initiator exceeds 50 parts by mass, the amount of (b) the co-crosslinking agent which has been described above must be decreased in order to obtain an appropriate hardness of the constituent member formed from the rubber composition, which may lower the resilience of the crosslinked rubber molded product or worsen the durability of the crosslinked rubber molded product. 
     ((d) Metal Compound) 
     The rubber composition may further contain (d) a metal compound. Examples of (d) the metal compound includes a metal hydroxide such as magnesium hydroxide, zinc hydroxide, calcium hydroxide, sodium hydroxide, lithium hydroxide, potassium hydroxide and copper hydroxide; a metal oxide such as magnesium oxide, calcium oxide, zinc oxide and copper oxide; and a metal carbonate such as magnesium carbonate, zinc carbonate, calcium carbonate, sodium carbonate, lithium carbonate and potassium carbonate. (d) The metal compound may be used solely or as a mixture of at least two of them. As (d) the metal compound, the metal oxide is preferable, at least one member selected from the group consisting of magnesium oxide, calcium oxide and zinc oxide is more preferable. 
     The amount of (d) the metal compound in the rubber composition is preferably 0.5 part by mass or more, more preferably 1 part by mass or more, and even more preferably 1.5 parts by mass or more, and is preferably 20 parts by mass or less, more preferably 15 parts by mass or less, and even more preferably 10 parts by mass or less, with respect to 100 parts by mass of (a) the base rubber. 
     ((e) Carboxylic Acid and/or Salt Thereof) 
     The rubber composition may further contain (e) a carboxylic acid and/or a salt thereof. If (e) the carboxylic acid and/or the salt thereof is contained, the hardness distribution of the obtained crosslinked rubber molded product can be controlled. Examples of (e) the carboxylic acid and/or the salt thereof include an aliphatic carboxylic acid, an aliphatic carboxylic acid salt, an aromatic carboxylic acid, and an aromatic carboxylic acid salt. (e) The carboxylic acid and/or the salt may be used solely or as a mixture of at least two of them. It is noted that (e) the carboxylic acid and/or the salt thereof excludes the complex represented by the general formula (1) and the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or the metal salt thereof which are used as (b) the co-crosslinking agent. 
     The aliphatic carboxylic acid may be a saturated aliphatic carboxylic acid (hereinafter, sometimes referred to as “saturated fatty acid”), or an unsaturated aliphatic carboxylic acid (hereinafter, sometimes referred to as “unsaturated fatty acid”). Further, the aliphatic carboxylic acid may have a branched structure or a cyclic structure. The saturated fatty acid preferably has 6 or more and 24 or less carbon atoms, more preferably 18 or less carbon atoms, and even more preferably 13 or less carbon atoms. The unsaturated fatty acid preferably has 6 or more carbon atoms, more preferably 7 or more carbon atoms, and even more preferably 8 or more carbon atoms, and preferably has 24 or less carbon atoms, more preferably 18 or less carbon atoms, and even more preferably 13 or less carbon atoms. 
     Examples of the aromatic carboxylic acid include a carboxylic acid having a benzene ring in the molecule, and a carboxylic acid having an aromatic heterocycle in the molecule. The aromatic carboxylic acid may be used solely or as a mixture of at least two of them. Examples of the carboxylic acid having a benzene ring include an aromatic carboxylic acid having a carboxyl group directly bonding to a benzene ring, an aromatic-aliphatic carboxylic acid having an aliphatic carboxylic acid bonding to a benzene ring, a polynuclear aromatic carboxylic acid having a carboxyl group directly bonding to a fused benzene ring, and a polynuclear aromatic-aliphatic carboxylic acid having an aliphatic carboxylic acid bonding to a fused benzene ring. Examples of the carboxylic acid having an aromatic heterocycle include a carboxylic acid having a carboxyl group directly bonding to an aromatic heterocycle. 
     As the aliphatic carboxylic acid salt or aromatic carboxylic acid salt, a salt of the above-mentioned aliphatic carboxylic acid or aromatic carboxylic acid can be used. Examples of the cation component of the salt include a metal ion, an ammonium ion, and an organic cation. The cation component may be used solely or as a mixture of at least two of them. Examples of the metal ion include a monovalent metal ion such as sodium, potassium, lithium and silver; a divalent metal ion such as magnesium, calcium, zinc, barium, cadmium, copper, cobalt, nickel and manganese; a trivalent metal ion such as aluminum and iron; and other metal ion such as tin, zirconium and titanium. Among them, the metal ion is preferably the divalent metal ion, and more preferably magnesium, zinc or calcium. 
     The organic cation is a cation having a carbon chain. The organic cation is not particularly limited, and examples thereof include an organic ammonium ion. Examples of the organic ammonium ion include a primary ammonium ion such as stearyl ammonium ion, hexyl ammonium ion, octyl ammonium ion and 2-ethylhexyl ammonium ion; a secondary ammonium ion such as dodecyl(laury) ammonium ion and octadecyl(stearyl) ammonium ion; a tertiary ammonium ion such as trioctyl ammonium ion; and a quaternary ammonium ion such as dioctyldimethyl ammonium ion and distearyldimethyl ammonium ion. These organic cations may be used solely or as a mixture of at least two of them. 
     Examples of the aliphatic carboxylic acid and/or the salt thereof include a saturated fatty acid and/or a salt thereof, and an unsaturated fatty acid and/or a salt thereof. The saturated fatty acid and/or the salt thereof is preferred, caprylic acid (octanoic acid), pelargonic acid (nonanoic acid), capric acid (decanoic acid), lauric acid, myristic acid, palmitic acid, stearic acid, behenic acid, and/or potassium salt, magnesium salt, calcium salt, aluminum salt, zinc salt, iron salt, copper salt, nickel salt, cobalt salt of these saturated fatty acids are more preferred. Preferable examples of the unsaturated fatty acid and/or the salt thereof include palmitoleic acid, oleic acid, linoleic acid, arachidonic acid, and/or potassium salt, magnesium salt, calcium salt, aluminum salt, zinc salt, iron salt, copper salt, nickel salt, cobalt salt of these unsaturated fatty acids. 
     Preferable examples of the aromatic carboxylic acid and/or the salt thereof include benzoic acid, butylbenzoic acid, anisic acid (methoxybenzoic acid), dimethoxybenzoic acid, trimethoxybenzoic acid, dimethylaminobenzoic acid, chlorobenzoic acid, dichlorobenzoic acid, trichlorobenzoic acid, acetoxybenzoic acid, biphenylcarboxylic acid, naphthalenecarboxylic acid, anthracenecarboxylic acid, furancarboxylic acid, thenoic acid, and/or potassium salt, magnesium salt, calcium salt, aluminum salt, zinc salt, iron salt, copper salt, nickel salt, cobalt salt of these aromatic carboxylic acids. 
     The amount of (e) the carboxylic acid and/or the salt thereof, for example, is preferably 1 part by mass or more, more preferably 2 parts by mass or more, and even more preferably 3 parts by mass or more, and is preferably 30 parts by mass or less, more preferably 20 parts by mass or less, and even more preferably 15 parts by mass or less, with respect to 100 parts by mass of (a) the base rubber. 
     ((f) Organic Sulfur Compound) 
     The rubber composition may further contain (f) an organic sulfur compound. Examples of (f) the organic sulfur compound include at least one compound selected from the group consisting of thiophenols, thionaphthols, polysulfides, thiurams, thiocarboxylic acids, dithiocarboxylic acids, sulfenamides, dithiocarbamates, thiazoles, and metal salts thereof. (f) The organic sulfur compound is preferably an organic sulfur compound having a thiol group (—SH) or a metal salt thereof, and more preferably thiophenols, thionaphthols or metal salts thereof. 
     Examples of the thiols include thiophenols and thionaphthols. Examples of the thiophenols include thiophenol; thiophenols substituted with a fluoro group, such as 4-fluorothiophenol, 2,5-difluorothiophenol, 2,6-difluorothiophenol, 2,4,5-trifluorothiophenol, 2,4,5,6-tetrafluorothiophenol and pentafluorothiophenol; thiophenols substituted with a chloro group, such as 2-chlorothiophenol, 4-chlorothiophenol, 2,4-dichlorothiophenol, 2,5-dichlorothiophenol, 2,6-dichlorothiophenol, 2,4,5-trichlorothiophenol, 2,4,5,6-tetrachlorothiophenol and pentachlorothiophenol; thiophenols substituted with a bromo group, such as 4-bromothiophenol, 2,5-dibromothiophenol, 2,6-dibromothiophenol, 2,4,5-tribromothiophenol, 2,4,5,6-tetrabromothiophenol and pentabromothiophenol; thiophenols substituted with an iodo group, such as 4-iodothiophenol, 2,5-diiodothiophenol, 2,6-diiodothiophenol, 2,4,5-triiodothiophenol, 2,4,5,6-tetraiodothiophenol and pentaiodothiophenol; and metal salts thereof. As the metal salt, a zinc salt is preferred. 
     Examples of the thionaphthols (naphthalenethiols) include 2-thionaphthol, 1-thionaphthol, 1-chloro-2-thionaphthol, 2-chloro-1-thionaphthol, 1-bromo-2-thionaphthol, 2-bromo-1-thionaphthol, 1-fluoro-2-thionaphthol, 2-fluoro-1-thionaphthol, 1-cyano-2-thionaphthol, 2-cyano-1-thionaphthol, 1-acetyl-2-thionaphthol, 2-acetyl-1-thionaphthol, and metal salts thereof. Among them, 2-thionaphthol, 1-thionaphthol, and metal salts thereof are preferable. The metal salt is preferably a divalent metal salt, more preferably a zinc salt. Specific examples of the metal salt include zinc salt of 1-thionaphthol and zinc salt of 2-thionaphthol. 
     The polysulfides are organic sulfur compounds having a polysulfide bond, and examples thereof include disulfides, trisulfides and tetrasulfides. The polysulfides are preferably diphenyl polysulfides. 
     Examples of the diphenyl polysulfides include diphenyl disulfide; diphenyl disulfides substituted with a halogen group, such as bis(4-fluorophenyl)disulfide, bis(2,5-difluorophenyl)disulfide, bis(2,6-difluorophenyl)disulfide, bis(2,4,5-trifluorophenyl)disulfide, bis(2,4,5,6-tetrafluorophenyl)disulfide, bis(pentafluorophenyl)disulfide, bis(4-chlorophenyl)disulfide, bis(2,5-dichlorophenyl)disulfide, bis(2,6-dichlorophenyl)disulfide, bis(2,4,5-trichlorophenyl)disulfide, bis(2,4,5,6-tetrachlorophenyl)disulfide, bis(pentachlorophenyl)disulfide, bis(4-bromophenyl)disulfide, bis(2,5-dibromophenyl)disulfide, bis(2,6-dibromophenyl)disulfide, bis(2,4,5-tribromophenyl)disulfide, bis(2,4,5,6-tetrabromophenyl)disulfide, bis(pentabromophenyl)disulfide, bis(4-iodophenyl)disulfide, bis(2,5-diiodophenyl)disulfide, bis(2,6-diiodophenyl)disulfide, bis(2,4,5-triiodophenyl)disulfide, bis(2,4,5,6-tetraiodophenyl)disulfide, and bis(pentaiodophenyl)disulfide; and diphenyl disulfides substituted with an alkyl group, such as bis(4-methylphenyl)disulfide, bis(2,4,5-trimethylphenyl)disulfide, bis(pentamethylphenyl)disulfide, bis(4-t-butylphenyl)disulfide, bis(2,4,5-tri-t-butylphenyl)disulfide, and bis(penta-t-butylphenyl)disulfide. 
     Examples of the thiurams include thiuram monosulfides such as tetramethylthiuram monosulfide; thiuram disulfides such as tetramethylthiuram disulfide, tetraethylthiuram disulfide and tetrabutylthiuram disulfide; and thiuram tetrasulfides such as dipentamethylenethiuram tetrasulfide. Examples of the thiocarboxylic acids include a naphthalene thiocarboxylic acid. Examples of the dithiocarboxylic acids include a naphthalene dithiocarboxylic acid. Examples of the sulfenamides include N-cyclohexyl-2-benzothiazole sulfenamide, N-oxydiethylene-2-benzothiazole sulfenamide, and N-t-butyl-2-benzothiazole sulfenamide. 
     (f) The organic sulfur compound may be used solely, or at least two of them may be used in combination. (f) The organic sulfur compound is preferably the thiophenols and/or the metal salts thereof, the thionaphthols and/or the metal salts thereof, the diphenyl disulfides and the thiuram disulfides, and more preferably 2,4-dichlorothiophenol, 2,6-difluorothiophenol, 2,6-dichlorothiophenol, 2,6-dibromothiophenol, 2,6-diiodothiophenol, 2,4,5-trichlorothiophenol, pentachlorothiophenol, 1-thionaphthol, 2-thionaphthol, diphenyl disulfide, bis(2,6-difluorophenyl)disulfide, bis(2,6-dichlorophenyl)disulfide, bis(2,6-dibromophenyl)disulfide, bis(2,6-diiodophenyl)disulfide and bis(pentabromophenyl)disulfide. 
     The amount of (f) the organic sulfur compound is preferably 0.05 part by mass or more, more preferably 0.1 part by mass or more, and is preferably 5.0 parts by mass or less, more preferably 2.0 parts by mass or less, with respect to 100 parts by mass of (a) the base rubber. If the amount of (f) the organic sulfur compound is less than 0.05 part by mass, the effect of adding (f) the organic sulfur compound is not obtained, and thus the resilience of the golf ball may not improve. In addition, if the amount of (f) the organic sulfur compound exceeds 5.0 parts by mass, the obtained golf ball has so great compression deformation amount that the resilience thereof may be lowered. 
     (Other Component) 
     The rubber composition may further contain additives such as a pigment, a filler for adjusting weight, an antioxidant, a peptizing agent and a softener, where necessary. In addition, the rubber composition may contain a rubber powder which is obtained by pulverizing a golf ball core or offcuts produced when preparing a core. 
     Examples of the pigment blended in the rubber composition include a white pigment, a blue pigment, and a purple pigment. As the white pigment, titanium oxide is preferably used. The type of titanium oxide is not particularly limited, but rutile type is preferably used because of the high opacity thereof. In addition, the amount of titanium oxide is preferably 0.5 part by mass or more, more preferably 2 parts by mass or more, and is preferably 8 parts by mass or less, more preferably 5 parts by mass or less, with respect to 100 parts by mass of (a) the base rubber. 
     It is also preferred that the rubber composition contains both a white pigment and a blue pigment. The blue pigment is blended in order to cause white color to be vivid, and examples thereof include ultramarine blue, cobalt blue, and phthalocyanine blue. In addition, examples of the purple pigment include anthraquinone violet, dioxazine violet, and methyl violet. 
     The filler blended in the rubber composition is mainly used as a weight adjusting agent for adjusting the weight of the crosslinked rubber molded product, and may be blended where necessary. Examples of the filler include an inorganic filler such as zinc oxide, barium sulfate, calcium carbonate, magnesium oxide, tungsten powder, and molybdenum powder. The amount of the filler is preferably 0.5 part by mass or more, more preferably 1 part by mass or more, and is preferably 30 parts by mass or less, more preferably 25 parts by mass or less, and even more preferably 20 parts by mass or less, with respect to 100 parts by mass of (a) the base rubber. This is because if the amount of the filler is less than 0.5 part by mass, the weight adjusting tends to become difficult, and if the amount of the filler exceeds 30 parts by mass, the weight proportion of the rubber component becomes low and thus the resilience tends to be lowered. 
     The amount of the antioxidant is preferably 0.1 part by mass or more and 1 part by mass or less with respect to 100 parts by mass of (a) the base rubber. In addition, the amount of the peptizing agent is preferably 0.1 part by mass or more and 5 parts by mass or less with respect to 100 parts by mass of (a) the base rubber. 
     [Process of Preparing the Rubber Composition] 
     (Preparation of the Complex Represented by the General Formula (1)) 
     Examples of the process of preparing the complex represented by the general formula (1) include a process of contacting at least one fatty acid component selected from the group consisting of a fatty acid, a fatty acid metal salt and a hydrate of a fatty acid metal salt, with a metal oxide (a first process); and a process of contacting a fatty acid metal salt with a metal oxide to obtain a precursor (fatty acid metal oxo cluster) followed by reacting this precursor with water (a second process). 
     (1) The First Process 
     Examples of the process of contacting at least one fatty acid component selected from the group consisting of a fatty acid, a fatty acid metal salt and a hydrate of a fatty acid metal salt, with a metal oxide include a preparing process comprising: a step of dissolving or dispersing at least one ligand component selected from the group consisting of a fatty acid, a fatty acid metal salt and a hydrate of a fatty acid metal salt, and the metal oxide in a first solvent and stirring the resultant reaction liquid (reaction step); a step of removing an insoluble substance from the reaction liquid (insoluble substance removal step); a step of charging a second solvent in the reaction liquid and filtering the precipitate (precipitation step); and a step of removing the solvent from the filtrate (drying step). 
     (1-1) Reaction Step 
     In the reaction step, at least one ligand component selected from the group consisting of a fatty acid, a fatty acid metal salt and a hydrate of a fatty acid metal salt, and the metal oxide are dissolved or dispersed in a first solvent, and the resultant reaction liquid is stirred. In this step, the ligand component and the metal oxide are contacted in the solvent to generate the complex represented by the general formula (1). 
     The ligand component is not particularly limited, as long as it is capable of forming the ligand in the general formula (1). The ligand component may be used solely, or at least two of them may be used in combination 
     Examples of the fatty acid include a saturated fatty acid having 1 to 19 carbon atoms, and an unsaturated fatty acid having 3 to 20 carbon atoms. Examples of the saturated fatty acid include methanoic acid, ethanoic acid, propanoic acid, butanoic acid, pentanoic acid, hexanoic acid, heptanoic acid, octanoic acid, nonanoic acid, decanoic acid, undecanoic acid, dodecanoic acid, tridecanoic acid, tetradecanoic acid, pentadecanoic acid, hexadecanoic acid, heptadecanoic acid, octadecanoic acid, and nonadecanoic acid. Examples of the unsaturated fatty acid include an unsaturated fatty acid having a carbon-carbon double bond such as propenoic acid (acrylic acid), 2-methylprop-2-enoic acid (methacrylic acid), 2-butenoic acid, 3-butenoic acid, 4-pentenoic acid, 5-hexenoic acid, 6-heptenoic acid, 7-octenoic acid, 8-nonenoic acid, 9-decenoic acid, 10-undecenoic acid, 11-dodecenoic acid, 12-tridecenoic acid, 9-tetradecenoic acid, 13-tetradecenoic acid, 14-pentadecenoic acid, 9-hexadecenoic acid, 15-hexadecenoic acid, 16-heptadecenoic acid, 9-octadecenoic acid, 11-octadecenoic acid, 17-octadecenoic acid and 18-nonadecenoic acid; and an unsaturated fatty acid having a carbon-carbon triple bond such as propiolic acid, 3-butynoic acid, 4-pentynoic acid, 5-hexynoic acid, 6-heptynoic acid, 7-octynoic acid, 8-nonynoic acid, 9-decynoic acid, 10-undecynoic acid, 11-dodecynoic acid, 12-tridecynoic acid, 9-tetradecynoic acid, 13-tetradecynoic acid, 14-pentadecynoic acid, 9-hexadecynoic acid, 15-hexadecynoic acid, 16-heptadecynoic acid, 9-octadecynoic acid, 11-octadecynoic acid, 17-octadecynoic acid, and 18-nonadecynoic acid. 
     Examples of the fatty acid metal salt include metal salts of the above fatty acids. Examples of the metal atom constituting the fatty acid metal salt include an alkali metal such as lithium, sodium, potassium, rubidium and cesium; an alkaline earth metal such as calcium, strontium and barium; a transition metal such as scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, yttrium, zirconium, niobium, molybdenum, technetium, ruthenium, rhodium, palladium, silver, hafnium, tantalum, tungsten, rhenium, osmium, iridium, platinum and gold; a base metal such as beryllium, magnesium, aluminum, zinc, gallium, cadmium, indium, tin, thallium, lead, bismuth and polonium. Among them, as the metal atom, the metal atom capable of forming a divalent metal ion is preferable, and beryllium, magnesium, calcium, zinc, barium, cadmium or lead is more preferable. These metal atoms may be used solely, or a mixture of at least two of them may be used. The fatty acid metal salt is preferably a fatty acid metal salt in which the metal ion is a divalent metal ion, more preferably an unsaturated fatty acid metal salt in which the metal ion is a divalent metal ion, even more preferably an acrylic acid or methacrylic acid metal salt in which the metal ion is a divalent metal ion, and most preferably zinc acrylate or zinc methacrylate. 
     Examples of the hydrate of the fatty acid metal salt include monohydrates, dihydrates, trihydrates and tetrahydrates of the above fatty acid metal salts. 
     In the fatty acid, the fatty acid constituting the fatty acid metal salt and the fatty acid constituting the hydrate of the fatty acid metal salt (hereinafter sometimes referred to as “fatty acid component”), the unsaturated fatty acid having a carbon-carbon double bond is preferably a fatty acid having one carbon-carbon double bond, and more preferably a fatty acid having a carbon-carbon double bond at a terminal thereof. In addition, in the fatty acid component, the unsaturated fatty acid having a carbon-carbon triple bond is preferably a fatty acid having one carbon-carbon triple bond, and more preferably a fatty acid having a carbon-carbon triple bond at a terminal thereof. 
     When two or more of the fatty acid components are used in combination, the amount of each fatty acid component can be suitably adjusted in accordance with the desirable complex. The amount of the unsaturated fatty acid in the fatty acid component is preferably 33 mol % or more, more preferably 50 mol % or more, and even more preferably 66 mol % or more. It is also preferable that all the fatty acid components are the unsaturated fatty acid. In addition, the amount of the unsaturated fatty acid having a carbon-carbon double bond in the fatty acid component is preferably 33 mol % or more, more preferably 50 mol % or more, and even more preferably 66 mol % or more. It is also preferable that all the fatty acid components are the unsaturated fatty acid having a carbon-carbon double bond. A a plurality of fatty acid components may be used in combination, but one fatty acid component is preferable. 
     Examples of the metal oxide include an alkali metal oxide such as lithium oxide, sodium oxide, potassium oxide, rubidium oxide and cesium oxide; an alkaline earth metal oxide such as calcium oxide, strontium oxide and barium oxide; a transition metal oxide such as scandium oxide, titanium oxide, vanadium oxide, chromium oxide, manganese oxide, iron oxide, cobalt oxide, nickel oxide, copper oxide, yttrium oxide, zirconium oxide, niobium oxide, molybdenum oxide, technetium oxide, ruthenium oxide, rhodium oxide, palladium oxide, silver oxide, hafnium oxide, tantalum oxide, tungsten oxide, rhenium oxide, osmium oxide, iridium oxide, platinum oxide and gold oxide; and a base metal oxide such as beryllium oxide, magnesium oxide, aluminum oxide, zinc oxide, gallium oxide, cadmium oxide, indium oxide, tin oxide, thallium oxide, lead oxide, bismuth oxide and polonium oxide. These metal oxides may be used solely, or a mixture of at least two of them may be used. Among them, as the metal oxide, the divalent metal oxide is preferable, and beryllium oxide, magnesium oxide, calcium oxide, zinc oxide, barium oxide, cadmium oxide or lead oxide is more preferable. 
     The first solvent is a solvent capable of dissolving the complex represented by the general formula (1) generated by the reaction. Examples of the first solvent include dichloromethane, 1,2-dichloroethane and chloroform. In addition, when the fatty acid metal salt is only used as the ligand component, water is necessarily added in the solvent. It is noted that when the fatty acid is used as the ligand component, water is generated by the reaction of the fatty acid with the metal oxide, and the hydrate of the fatty acid metal salt has water in its molecule. 
     The mixing ratio (molar ratio) (fatty acid component/metal oxide) of the fatty acid component to the metal oxide in the reaction liquid is preferably 1.0 or more, more preferably 1.5 or more, and even more preferably 2.0 or more, and is preferably 6.0 or less, more preferably 5.0 or less, and even more preferably 4.0 or less. If the mixing ratio falls within the above range, the desired complex can be produced in a good yield. 
     The amount of the solvent is preferably 1000 parts by mass or more, more preferably 2000 parts by mass or more, and even more preferably 3000 parts by mass or more, and is preferably 10000 parts by mass or less, more preferably 8000 parts by mass or less, and even more preferably 6000 parts by mass or less, with respect to 100 parts by mass of a total amount of the ligand component and the metal oxide. If the amount of the solvent is 1000 parts by mass or more, the yield of the complex is higher, and if the amount of the solvent is 10000 parts by mass or less, the synthetic workload can be lowered. 
     When water is added in the solvent, the amount of water is preferably 0.5 mole or more, more preferably 1.0 mole or more, and even more preferably 1.5 mole or more, and is preferably 10 mole or less, more preferably 8.0 mole or less, and even more preferably 6.0 mole or less, with respect to 1 mole of the metal oxide. 
     The liquid temperature when stirring the reaction liquid is preferably 0° C. or more, more preferably 10° C. or more, and even more preferably 20° C. or more, and is preferably 100° C. or less, more preferably 90° C. or less, and even more preferably 80° C. or less. If the liquid temperature is 0° C. or more, the reaction speed is greater and the yield of the complex is higher, and if the liquid temperature is 100° C. or less, the reaction of unsaturated bonds of the carboxylate is suppressed and thus the yield of the complex is higher. 
     The time of stirring the reaction liquid is preferably 1 hour or more, more preferably 3 hours or more, and even more preferably 12 hours or more, and is preferably 300 hours or less, more preferably 200 hours or less, and even more preferably 100 hours or less. 
     (1-2) Insoluble Substance Removal Step 
     In the insoluble substance removal step, the insoluble substance is removed from the reaction liquid after the reaction. The insoluble substance is considered to be the unreacted metal oxide or fatty acid remained in the solvent. In addition, if the liquid temperature when performing the reaction is high, it is considered that a polymer of the unsaturated fatty acid is also generated as the insoluble substance. 
     Examples of the method of removing the insoluble substance from the reaction liquid after the reaction include a method of filtering the reaction liquid. 
     (1-3) (Precipitation Step) 
     In the precipitation step, a second solvent is charged into the reaction liquid from which the insoluble substance has been removed, to precipitate the complex represented by the general formula (1). The complex is obtained by filtration. 
     The second solvent is not particularly limited, as long as it can selectively precipitate the complex represented by the general formula (1) in the reaction liquid. In other words, the solubility of the complex represented by the general formula (1) in the second solvent is lower than the solubility of the fatty acid or the like in the second solvent. Examples of the second solvent include hydrocarbons such as hexane, pentane, cyclohexane and heptane. 
     The amount of the second solvent may be suitably adjusted such that the complex represented by the general formula (1) can be precipitated. The amount of the second solvent is preferably 10 parts by mass or more, more preferably 20 parts by mass or more, and even more preferably 30 parts by mass or more, and is preferably 200 parts by mass or less, more preferably 150 parts by mass or less, and even more preferably 100 parts by mass or less, with respect to 100 parts by mass of the amount of the first solvent. 
     In addition, after the second solvent is charged, a part of the first solvent and second solvent may be removed to precipitate the complex represented by the general formula (1). As the method of removing a part of the first solvent and second solvent, drying under reduced pressure is preferable. When performing the drying under reduced pressure, the reaction liquid may be heated. The temperature of the reaction liquid when performing the drying is preferably 100° C. or less, more preferably 80° C. or less, and even more preferably 60° C. or less. 
     (1-4) Drying Step 
     In the drying step, the solvent is removed from the reaction liquid from which the insoluble substance has been removed. The complex represented by the general formula (1) is obtained by removing the solvent. 
     Examples of the method of removing the solvent include a method of drying under reduced pressure and a method of drying under heating, and the drying under reduced pressure is preferable. When performing the drying under reduced pressure, the reaction liquid may be heated. The temperature of the reaction liquid when performing the drying is preferably 100° C. or less, more preferably 80° C. or less, and even more preferably 60° C. or less. 
     (2) The Second Process 
     Examples of the process of obtaining a precursor (fatty acid metal oxo cluster) followed by reacting this precursor with water include a preparing process comprising: a step of dissolving or dispersing the fatty acid metal salt and the metal oxide in a first solvent and stirring the resultant reaction liquid; a step of removing an insoluble substance from the reaction liquid; a step of removing the solvent from the reaction liquid; and a step of reacting the obtained precursor with water. 
     (2-1) Reaction Step 
     In the reaction step, the fatty acid metal salt and the metal oxide are dissolved or dispersed in a first solvent, and the resultant reaction liquid is stirred. In this step, the fatty acid metal salt and the metal oxide are contacted in the solvent to generate the precursor of the complex represented by the general formula (1). 
     The fatty acid metal salt is not particularly limited, as long as it is capable of forming the ligand in the general formula (1). Preferable examples of the fatty acid metal salt are same as those listed in the first method. The fatty acid metal salt may be used solely, or at least two of them may be used in combination. 
     Examples of the embodiment of the fatty acid metal salt include an embodiment including one fatty acid and one metal ion; an embodiment including a plurality of fatty acids and one metal ion; an embodiment including one fatty acid and a plurality of metal ions; and an embodiment including a plurality of fatty acids and a plurality of metal ions. Among them, the embodiment including one fatty acid and one metal ion is preferable. 
     Preferable examples of the metal oxide are same as those listed in the first method. The combination of the fatty acid metal salt with the metal oxide is preferably, but is not limited to, a combination in which the metal constituting the fatty acid metal salt is identical to the metal constituting the metal oxide. The combination of the fatty acid metal salt with the metal oxide is preferably a combination of a fatty acid zinc salt with zinc oxide, more preferably a combination of an unsaturated fatty acid zinc salt with zinc oxide, and even more preferably a combination of zinc acrylate and/or zinc methacrylate with zinc oxide. 
     The first solvent is a solvent capable of dissolving the precursor of the complex represented by the general formula (1) generated by the reaction. Examples of the first solvent include dichloromethane, 1,2-dichloroethane and chloroform. 
     The mixing ratio (molar ratio) (fatty acid metal salt/metal oxide) of the fatty acid metal salt to the metal oxide in the reaction liquid is preferably 1.0 or more, more preferably 1.5 or more, and even more preferably 2.0 or more, and is preferably 6.0 or less, more preferably 5.0 or less, and even more preferably 4.0 or less. If the mixing ratio falls within the above range, the desired complex can be produced in a good yield. 
     The amount of the solvent is preferably 1000 parts by mass or more, more preferably 2000 parts by mass or more, and even more preferably 3000 parts by mass or more, and is preferably 10000 parts by mass or less, more preferably 8000 parts by mass or less, and even more preferably 6000 parts by mass or less, with respect to 100 parts by mass of a total amount of the fatty acid metal salt and the metal oxide. If the amount of the solvent is 1000 parts by mass or more, the yield of the complex is higher, and if the amount of the solvent is 10000 parts by mass or less, the synthetic workload can be lowered. 
     The liquid temperature when stirring the reaction liquid is preferably 0° C. or more, more preferably 10° C. or more, and even more preferably 20° C. or more, and is preferably 100° C. or less, more preferably 90° C. or less, and even more preferably 80° C. or less. If the liquid temperature is 0° C. or more, the reaction speed is greater and the yield of the complex is higher, and if the liquid temperature is 100° C. or less, the reaction of unsaturated bonds of the carboxylate is suppressed and thus the yield of the complex is higher. 
     The time of stirring the reaction liquid is preferably 1 hour or more, more preferably 3 hours or more, and even more preferably 12 hours or more, and is preferably 300 hours or less, more preferably 200 hours or less, and even more preferably 100 hours or less. 
     (2-2) Insoluble Substance Removal Step 
     In the insoluble substance removal step, the insoluble substance is removed from the reaction liquid after the reaction. The insoluble substance is considered to be the unreacted metal oxide or fatty acid metal salt remained in the solvent. In addition, if the liquid temperature when performing the reaction is high, it is considered that a polymer of the unsaturated fatty acid metal salt is also generated as the insoluble substance. Examples of the method of removing the insoluble substance from the reaction liquid after the reaction include a method of filtering the reaction liquid. 
     (2-3) Drying Step 
     In the drying step, the solvent is removed from the reaction liquid from which the insoluble substance has been removed. A mixture containing the precursor of the complex represented by the general formula (1) and the fatty acid metal salt is obtained by removing the solvent. Herein, for example, the precursor of the complex represented by the formulae (2) or (3) is a complex represented by a formula (4). 
     
       
         
         
             
             
         
       
     
     [In the formula (4), M 11  to M 14  are identical to or different from each other and represent a metal atom, R 11  to R 16  are identical to or different from each other and represent a hydrogen atom, an alkyl group having 1 to 18 carbon atoms, an alkenyl group having 2 to 18 carbon atoms or an alkynyl group having 2 to 18 carbon atoms, and at least one of R 11  to R 16  is the alkenyl group having 2 to 18 carbon atoms or the alkynyl group having 2 to 18 carbon atoms.]. 
     In the structural formula (4), the bond shown in the dashed line which is attached to the solid line is a hybrid bond of the carboxylate group. In addition, in the structural formula (4), the covalent bond and the coordination bond are both shown in a solid line. 
     Specific examples of the metal atoms M 11  to M 14  and R 11  to R 16  in the formula (4) include those listed as M 1  to M 5  and R 1  to R 8  in the formulae (2) or (3). 
     Examples of the method of removing the solvent include a method of drying under reduced pressure and a method of drying under heating, and the drying under reduced pressure is preferable. When performing the drying under reduced pressure, the reaction liquid may be heated. The temperature of the reaction liquid when performing the drying is preferably 100° C. or less, more preferably 80° C. or less, and even more preferably 60° C. or less. 
     The second process may further comprise a step of purifying the precursor of the complex represented by the general formula (1). It is noted that when the step of purifying the precursor of the complex is comprised, the above-mentioned insoluble substance removal step may be omitted. Examples of the purification method include a method of removing the fatty acid metal salt from the reaction liquid in the preparing process (a method including a purification step); and a method of performing reprecipitation of the mixture of the precursor of the complex represented by the general formula (1) and the fatty acid metal salt obtained in the preparing process (a method including a reprecipitation step). Among them, the method of removing the fatty acid metal salt from the reaction liquid in the preparing process is preferable. 
     (2-4) Purification Step 
     In the purification step, a second solvent is charged into the reaction liquid from which the insoluble substance has been removed in the preparing process, and the resultant precipitate is removed. Impurities such as the fatty acid metal salt are precipitated by charging the second solvent into the reaction liquid. The purity of the finally obtained precursor of the complex represented by the general formula (1) can be enhanced by removing the precipitate. 
     The second solvent is not particularly limited, as long as it can selectively precipitate the fatty acid metal salt in the reaction liquid. In other words, the solubility of the precursor of the complex represented by the general formula (1) in the second solvent is higher than the solubility of the fatty acid metal salt in the second solvent. Examples of the second solvent include hydrocarbons such as hexane, pentane, cyclohexane and heptane. 
     The amount of the second solvent may be suitably adjusted such that the fatty acid metal salt can be precipitated. The amount of the second solvent is preferably 10 parts by mass or more, more preferably 20 parts by mass or more, and even more preferably 30 parts by mass or more, and is preferably 200 parts by mass or less, more preferably 150 parts by mass or less, and even more preferably 100 parts by mass or less, with respect to 100 parts by mass of the amount of the first solvent. 
     In addition, after the second solvent is charged, a part of the first solvent and second solvent may be removed to precipitate the fatty acid metal salt. As the method of removing a part of the first solvent and second solvent, drying under reduced pressure is preferable. When performing the drying under reduced pressure, the reaction liquid may be heated. The temperature of drying the reaction liquid is preferably 100° C. or less, more preferably 80° C. or less, and even more preferably 60° C. or less. 
     Examples of the method of removing the precipitated fatty acid metal salt include a method of filtering the reaction liquid. The precursor of the complex represented by the general formula (1) is obtained by removing the first solvent and the second solvent from the reaction liquid from which the precipitate has been removed, in the drying step. It is noted that the purification step may be performed several times depending on the desired purity of the precursor of the complex represented by the general formula (1). 
     (2-5) Reprecipitation Step 
     In the reprecipitation step, the reprecipitation of the mixture of the precursor of the complex represented by the general formula (1) and the fatty acid metal salt obtained in the preparing process is performed. Specifically, the mixture of the precursor of the complex represented by the general formula (1) and the fatty acid metal salt obtained in the preparing process is dissolved in the first solvent, the second solvent is charged into the resultant solution to precipitate the fatty acid metal salt, and the precipitate is removed. 
     As the first solvent and the second solvent used in the reprecipitation step, those listed in the reaction step and the purification step may be used. In addition, the preferable amount of the second solvent and the preferable method of removing the precipitate are identical to those in the purification step. The precursor of the complex represented by the general formula (1) is obtained by removing the solvent after the precipitate is removed. The preferable method of removing the solvent is identical to that in the drying step. It is noted that the reprecipitation step may be performed several times depending on the desired purity of the precursor of the complex represented by the general formula (1). 
     (2-6) Water-Adding Step 
     In the water-adding step, the complex represented by the general formula (1) is obtained by reacting the above obtained precursor of the complex represented by the general formula (1) with water. 
     Examples of the method of reacting the precursor with water include a method of exposing the precursor under a high humidity atmosphere. The humidity of the high humidity atmosphere is preferably 40% RH or more, more preferably 50% RH or more, and even more preferably 60% RH or more. In addition, the temperature of the high humidity atmosphere (atmospheric temperature) is preferably 0° C. or more, more preferably 10° C. or more, and even more preferably 20° C. or more, and is preferably 100° C. or less, more preferably 80° C. or less, and even more preferably 60° C. or less. 
     [Preparation of Rubber Composition] 
     The above rubber composition can be obtained by mixing and kneading (a) the base rubber, (b) the co-crosslinking agent, (c) the crosslinking initiator, and where necessary, (d) the metal compound, (e) the carboxylic acid and/or the salt thereof, (f) the organic sulfur compound and the other additives. The kneading can be conducted, without any limitation, for example, with a conventional kneading machine such as a kneading roll, a banbury mixer and a kneader. 
     [Crosslinked Rubber Molded Product] 
     The crosslinked rubber molded product according to the present invention is formed from the above rubber composition. The crosslinked rubber molded product can be formed by molding the kneaded rubber composition in a mold. The molding temperature is preferably 120° C. or more, more preferably 150° C. or more, and is preferably 250° C. or less. In addition, the pressure when performing the molding preferably ranges from 2.9 MPa to 11.8 MPa. The molding time preferably ranges from 10 minutes to 60 minutes. 
     The slab hardness of the crosslinked rubber molded product is preferably 20 or more, more preferably 25 or more, and even more preferably 30 or more, and is preferably 98 or less, more preferably 96 or less, and even more preferably 95 or less in Shore C hardness. If the slab hardness is 20 or more in Shore C hardness, the crosslinked structure is sufficiently formed and thus the crosslinked rubber molded product has enhanced durability, and if the slab hardness is 98 or less in Shore C hardness, the crosslinked rubber molded product has further enhanced resilience. 
     The Lupke type rebound resilience of the crosslinked rubber molded product is preferably 55% or more, more preferably 60% or more, and even more preferably 65% or more, and is preferably 98% or less, more preferably 95% or less, and even more preferably 90% or less. If the Lupke type rebound resilience is 55% or more, the crosslinked rubber molded product has further enhanced resilience, and if the Lupke type rebound resilience is 98% or less, the crosslinked rubber molded product is sufficiently hard and thus the durability thereof is enhanced. 
     The storage modulus E′ (Pa) of the crosslinked rubber molded product is preferably 3.0×10 7  Pa or more, more preferably 5.0×10 7  Pa or more, and even more preferably 10×10 7  Pa or more, and is preferably 40×10 7  Pa or less, more preferably 35×10 7  Pa or less, and even more preferably 30×10 7  Pa or less. If the storage modulus E′ (Pa) is 3.0×10 7  Pa or more, the crosslinked rubber molded product has further enhanced resilience, and if the storage modulus E′ (Pa) is 40×10 7  Pa or less, the crosslinked rubber molded product has suitable hardness. 
     The loss modulus E″ (Pa) of the crosslinked rubber molded product is preferably 0.001×10 7  Pa or more, more preferably 0.005×10 7  Pa or more, and even more preferably 0.01×10 7  Pa or more, and is preferably 3.0×10 7  Pa or less, more preferably 1.0×10 7  Pa or less, and even more preferably 0.5×10 7  Pa or less. If the loss modulus E″ (Pa) is 0.001×10 7  Pa or more, the crosslinked structure is sufficiently formed and thus the crosslinked rubber molded product has enhanced durability, and if the loss modulus E″ (Pa) is 3.0×10 7  Pa or less, the crosslinked rubber molded product has suitable viscosity and thus the resilience thereof is enhanced. 
     The slab hardness (Shore C) X and the Lupke type rebound resilience (%) Y of the above crosslinked rubber molded product preferably satisfy a relationship of Y≥87−0.283X. The crosslinked rubber molded product satisfying the above relationship is excellent in the resilience performance. 
     The slab hardness (Shore C) X and the Lupke type rebound resilience (%) Y of the crosslinked rubber molded product preferably satisfy a relationship of Y≥88−0.283X, and more preferably satisfies a relationship of Y≥89−0.283X. 
     The storage modulus E′ (Pa) and the loss modulus E″ (Pa), which are measured with a dynamic viscoelasticity measuring apparatus (tensile mode), and the slab hardness (Shore C) X of the crosslinked rubber molded product preferably satisfy a relationship of |log(E′/E″ 2 )|/X×100≤5.7. The crosslinked rubber molded product satisfying the above relationship has a great hardness and is excellent in the resilience performance. 
     The storage modulus E′ (Pa), the loss modulus E″ (Pa) and the slab hardness (Shore C) X of the crosslinked rubber molded product preferably satisfy a relationship of |log(E′/E″ 2 )|/X×100≤5.6, and more preferably satisfy a relationship of log(E′/E″ 2 )|/X×100≤5.5. 
     Examples of the application of the crosslinked rubber molded product include sports goods such as golf ball, tennis ball and grip, industrial products such as hose, belt and mat, shoe sole, tire, resin additive, antivibration rubber, and fender. Examples of the golf ball include a golf ball comprising a constituent member formed from the above rubber composition. 
     EXAMPLES 
     Next, the present invention will be described in detail by way of examples. However, the present invention is not limited to the examples described below. Various changes and modifications without departing from the spirit of the present invention are included in the scope of the present invention. 
     [Evaluation Method] 
     (1) Infrared Spectroscopic Analysis 
     The infrared spectroscopic analysis was carried out with a Fourier transform infrared spectrophotometer (“Spectrum One” available from PerkinElmer, Inc.) by a total reflection absorption measuring method (ATR method) using diamond as a prism of the total reflection absorption measurement. 
     (2) CHN Element Analysis 
     The element analysis was carried out with an organic trace element analyzer (Micro Corder JM10 type available from J-Science Lab Co., Ltd.). 
     (3) Zinc Amount Measurement 
     The zinc acrylate hydroxo cluster (0.1057 g) was weighed and put into a beaker with a volume of 100 ml, and 50 ml of distilled water was added to dissolve the complex. Into the resultant liquid, 10 ml of acetic acid-sodium acetate (pH 5) buffer was added, and some drops of a XO indicator (0.1 w/v % of xylenol orange solution for titration available from Wako Pure Chemical Industries, Ltd.: 0.1 g/100 ml=0.001396 M) were added. Finally, distilled water was added to adjust the liquid volume to 100 ml. The obtained liquid was titrated with 0.05 mol/l of an EDTA standard titrant (available from Dojin Chemical, Inc.). 
     (4) Powder X-Ray Diffraction 
     The X-ray diffraction measurement was carried out with a wide angle X-ray diffraction instrument (“RINT-TTR III type” available from Rigaku Corporation). The measuring sample was pulverized with an agate mortar. The measuring conditions were as follows. 
     X-ray source: CuKα X-ray 
     Tube voltage-tube current: 50 kV-300 mA 
     Step width: 0.02 deg. 
     Measuring speed: 5 deg./min 
     Slit system: light diffusion-light reception-light scattering: 0.5 deg.-opening-0.5 deg. 
     Monochromator: diffraction curve bent-crystal monochromator 
     (5) Slab Hardness (Shore C Hardness) 
     Sheets with a thickness of about 2 mm were prepared by heat molding the rubber composition, and stored at 23° C. for two weeks. It is noted that the heat molding was conducted according to the temperature and time shown in Tables 1, 2. At least three of these sheets were stacked on one another so as not to be affected by the measuring substrate on which the sheets were placed, and the hardness of the stack was measured with an automatic hardness tester (Digitest II, available from Bareiss company) using a detector of “Shore C”. 
     (6) Lupke Type Rebound Resilience 
     The rebound resilience test was carried out according to JIS K6255 (2013). Sheets with a thickness of about 2 mm were prepared by heat molding the rubber composition. It is noted that the heat molding was conducted according to the temperature and time shown in Tables 1, 2. A cylindrical test piece with a thickness of about 12 mm and a diameter of 28 mm was prepared by punching the sheet obtained above into a circular shape with a diameter of 28 mm, and stacking six of the obtained circular sheets. The test piece was stored at a temperature of 23° C. plus or minus 2° C. and a relative humidity of 50% plus or minus 5%. It is noted that the storing time was 12 hours for the rubber compositions No. 31 to 36 and 46 to 51, and 24 hours for the rubber compositions No. 1 to 16, 21 to 30 and 41 to 45. The rebound resilience of the obtained test piece was measured with a Lupke type rebound resilience tester (available from Ueshima Seisakusho Co., Ltd.). The planar part of the stacked test pieces obtained above was held by a mechanical fixing method during the measurement, and the measurement was carried out at a temperature of 23° C., relative humidity of 50%, impact end diameter of 12.50 mm plus or minus 0.05 mm, impact mass of 0.35 kg plus or minus 0.01 kg and impact speed of 1.4 m/s plus or minus 0.01 m/s. 
     (7) Measurement of Storage Modulus E′ (Pa) and Loss Modulus E″ (Pa) 
     The storage modulus E′ (Pa) and the loss modulus E″ (Pa) of the crosslinked rubber were measured under the following conditions. 
     Apparatus: Dynamic viscoelasticity measuring apparatus “Rheogel-E4000” available from UBM Co., Ltd. 
     Test piece: A sheet with a thickness of 0.5 mm was prepared by press molding the rubber composition under the conditions shown in Tables 1, 2, and a test piece was cut from the sheet to have a width of 4 mm and a length between clamps of 20 mm. 
     Measuring mode: tensile mode 
     Measuring temp.: 24° C. 
     Oscillation frequency: 10 Hz 
     Measuring strain: 0.05% 
     [Production of Zinc Acrylate Hydroxo Cluster] 
     Zinc oxide (0.125 kg, 1.54 mol) and zinc acrylate (0.955 kg, 4.60 mol) were added in 24.9 kg of dichloromethane, and stirred at 40° C. for 48 hours. It is noted that the solvent was refluxed. 48 hours later, the reaction liquid was filtered to remove the precipitate. 9.83 kg of hexane was added in the filtrate, and concentration under reduced pressure was performed until the liquid amount was reduced to about one-fourth. The concentrate was filtered, and the filtrate was concentrated and dried to obtain the precursor of acrylic acid hydroxo cluster. The precursor slowly reacted with water in an atmosphere of 20° C. to 30° C. and a humidity of 50% or more, to give zinc acrylate hydroxo cluster (631 g, yield of 57 mass %). 
     The infrared spectroscopic analysis, element analysis, zinc amount measurement and X-ray diffraction measurement were conducted for the above obtained zinc acrylate hydroxo cluster. The experimental results are each shown below. 
     IR spectrum peak: 413cm −1 , 469cm −1 , 688cm −1 , 829cm −1 , 972cm −1 , 1068cm −1 , 1274cm −1 , 1372cm −1 , 1435cm −1 , 1522cm −1 , 1568cm −1 , 1644cm −1   
     Anal. Calcd for C 24 H 26 O 18 Zn 5 : C, 31.02; H, 2.82. Found: C, 30.36; H, 2.77. 
       FIG. 1  shows IR spectrum. Based on the IR spectrum, the absorption attributed to the vinyl group of acrylate and the absorption attributed to the vibration of (HO)Zn 3  are confirmed. Further, two types of absorption attributed to the vibration of the carboxylate groups which have different coordination states from each other are confirmed. In addition, the measured value of the zinc amount is 35.7 mass %, which is very close to the theoretical value 35.2 mass %. Based on these results, it can be confirmed that the above prepared zinc acrylate hydroxo cluster is the compound represented by ((CH 2 CHCOO) 8 Zn 5 (OH) 2 ) n  (n is an integer of 1 or more). 
     The element analysis results show that the zinc acrylate hydroxo cluster contains carbon in an amount of 30.36 mass % and hydrogen in an amount of 2.77 mass %. The differences between the analysis results and the estimated values were 0.66 mass % for the carbon amount and 0.05 mass % for the hydrogen amount. Since the atomic compositions are very close to the estimated values, it can be confirmed that the zinc acrylate hydroxo cluster ((CH 2 CHCOO) 8 Zn 5 (OH) 2 ) n  (n is an integer of 1 or more) has a very high purity. 
       FIG. 2  shows X-ray diffraction spectrum. Based on the X-ray diffraction spectrum, it is confirmed that the zinc acrylate hydroxo cluster has a different crystal structure from zinc diacrylate. 
     The structure of the acrylic acid hydroxo cluster was determined by single crystal X-ray structural analysis. It is found that the acrylic acid hydroxo cluster has a structure shown in  FIG. 3 . It is noted that in  FIG. 3 , the arrow shows the symmetric center of this structure, and the part surrounded by the dotted line is one part of the adjacent molecules which are interacting with each other. 
     [Preparation of Rubber Composition] 
     Materials having the formulations shown in Tables 1-6 were kneaded to prepare rubber compositions. It is noted that the material temperature at the time of kneading the rubber compositions was set as 60° C. to 125° C. 
     
       
         
           
               
               
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Rubber composition No. 
                 1 
                 2 
                 3 
                 4 
                 5 
                 6 
                 7 
                 8 
                 9 
                 10 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Formulation 
                 (a) 
                 BR730 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
               
               
                 (parts by mass) 
                 (b) 
                 Zinc acrylate hydroxo cluster 
                 20.5 
                 20.5 
                 31 
                 31 
                 41 
                 41 
                 35 
                 30 
                 25 
                 20 
               
               
                   
                   
                 ZN-DA90S 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 5 
                 10 
                 15 
                 20 
               
               
                   
                 (d) 
                 Zinc oxide 
                 2.69 
                 2.69 
                 2.69 
                 2.69 
                 2.69 
                 2.69 
                 3.2 
                 3.7 
                 4.3 
                 4.8 
               
               
                   
                 (c) 
                 Dicumyl peroxide 
                 0.8 
                 0.8 
                 0.8 
                 0.8 
                 0.8 
                 0.8 
                 0.8 
                 0.8 
                 0.8 
                 0.8 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Crosslinking 
                 Acrylate (parts by mass) 
                 12.1 
                 12.1 
                 18.3 
                 18.3 
                 24.2 
                 24.2 
                 23.8 
                 23.9 
                 24.0 
                 24.1 
               
               
                 component 
                 Zinc (parts by mass) 
                 9.6 
                 9.6 
                 13.4 
                 13.4 
                 17.0 
                 17.0 
                 16.7 
                 16.7 
                 16.8 
                 16.8 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Evaluation 
                 Molding 
                 Temperature (° C.) 
                 150 
                 170 
                 150 
                 170 
                 150 
                 170 
                 150 
                 150 
                 150 
                 150 
               
               
                   
                 conditions 
                 Time (min) 
                 20 
                 20 
                 20 
                 20 
                 20 
                 20 
                 20 
                 20 
                 20 
                 20 
               
               
                   
                 Slab 
                 Shore C hardness 
                 60.4 
                 63.5 
                 69.0 
                 71.8 
                 78.6 
                 81.9 
                 82.1 
                 83.3 
                 84.4 
                 85.6 
               
               
                   
                 properties 
                 Lupke type rebound resilience (%) 
                 74.1 
                 72.9 
                 73.6 
                 72.2 
                 69.4 
                 67.8 
                 67.1 
                 66.5 
                 65.8 
                 65.2 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 Rubber composition No. 
                 11 
                 12 
                 13 
                 14 
                 15 
                 16 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Formulation 
                 (a) 
                 BR730 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
               
               
                 (parts by mass) 
                 (b) 
                 ZN-DA90S 
                 19.7 
                 19.7 
                 29.6 
                 29.6 
                 39.4 
                 39.4 
               
               
                   
                 (d) 
                 Zinc oxide 
                 5 
                 5 
                 5 
                 5 
                 5 
                 5 
               
               
                   
                 (c) 
                 Dicumyl peroxide 
                 0.8 
                 0.8 
                 0.8 
                 0.8 
                 0.8 
                 0.8 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Crosslinking 
                 Acrylate (parts by mass) 
                 12.1 
                 12.1 
                 18.2 
                 18.2 
                 24.3 
                 24.3 
               
               
                 component 
                 Zinc (parts by mass) 
                 9.6 
                 9.6 
                 12.4 
                 12.4 
                 15.2 
                 15.2 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Evaluation 
                 Molding 
                 Temperature (° C.) 
                 150 
                 170 
                 150 
                 170 
                 150 
                 170 
               
               
                   
                 conditions 
                 Time (min) 
                 20 
                 20 
                 20 
                 20 
                 20 
                 20 
               
               
                   
                 Slab 
                 Shore C hardness 
                 58.9 
                 62.6 
                 75.5 
                 79.2 
                 85.0 
                 89.1 
               
               
                   
                 properties 
                 Lupke type rebound resilience (%) 
                 69.5 
                 67.2 
                 65.4 
                 63.8 
                 61.7 
                 60.0 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 3 
               
               
                   
               
               
                 Rubber composition No. 
                 21 
                 22 
                 23 
                 24 
                 25 
                 26 
                 27 
                 28 
                 29 
                 30 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Formulation 
                 (a) 
                 BR730 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
               
               
                 (parts by mass) 
                 (b) 
                 Zinc acrylate hydroxo 
                 20 
                 20 
                 30 
                 30 
                 40 
                 40 
                 35 
                 30 
                 25 
                 20 
               
               
                   
                   
                 cluster 
               
               
                   
                   
                 ZN-DA90S 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 5 
                 10 
                 15 
                 20 
               
               
                   
                 (d) 
                 Zinc oxide 
                 2.69 
                 2.69 
                 2.69 
                 2.69 
                 2.69 
                 2.69 
                 3.2 
                 3.7 
                 4.3 
                 4.8 
               
               
                   
                 (c) 
                 Dicumyl peroxide 
                 0.8 
                 0.8 
                 0.8 
                 0.8 
                 0.8 
                 0.8 
                 0.8 
                 0.8 
                 0.8 
                 0.8 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Crosslinking 
                 Acrylate (parts by mass) 
                 12.1 
                 12.1 
                 18.2 
                 18.2 
                 24.2 
                 24.2 
                 24.3 
                 24.3 
                 24.4 
                 24.4 
               
               
                 component 
                 Zinc (parts by mass) 
                 9.6 
                 9.6 
                 13.3 
                 13.3 
                 17.0 
                 17.0 
                 17.0 
                 17.0 
                 17.0 
                 17.0 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Evaluation 
                 Molding 
                 Temperature (° C.) 
                 150 
                 170 
                 150 
                 170 
                 150 
                 170 
                 150 
                 150 
                 150 
                 150 
               
               
                   
                 conditions 
                 Time (min) 
                 20 
                 20 
                 20 
                 20 
                 20 
                 20 
                 20 
                 20 
                 20 
                 20 
               
               
                   
                 Properties 
                 Slab hardness (Shore C) 
                 60.4 
                 63.5 
                 69.0 
                 71.8 
                 78.6 
                 81.9 
                 82.1 
                 83.3 
                 84.4 
                 85.6 
               
               
                   
                   
                 Lupke type rebound 
                 74.1 
                 72.9 
                 73.6 
                 72.2 
                 69.4 
                 67.8 
                 67.1 
                 66.5 
                 65.8 
                 65.2 
               
               
                   
                   
                 resilience (%) 
               
               
                   
                   
                 Rebound resilience/ 
                 1.23 
                 1.15 
                 1.07 
                 1.01 
                 0.88 
                 0.83 
                 0.818 
                 0.798 
                 0.780 
                 0.761 
               
               
                   
                   
                 slab hardness 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE 4 
               
               
                   
               
               
                 Rubber composition No. 
                 31 
                 32 
                 33 
                 34 
                 35 
                 36 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Formulation 
                 (a) 
                 BR730 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
               
               
                 (parts by mass) 
                 (b) 
                 ZN-DA90S 
                 19.65 
                 19.65 
                 29.55 
                 29.55 
                 39.4 
                 39.4 
               
               
                   
                 (d) 
                 Zinc oxide 
                 5 
                 5 
                 5 
                 5 
                 5 
                 5 
               
               
                   
                 (c) 
                 Dicumyl peroxide 
                 0.8 
                 0.8 
                 0.8 
                 0.8 
                 0.8 
                 0.8 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Crosslinking 
                 Acrylate (parts by mass) 
                 12.1 
                 12.1 
                 18.2 
                 18.2 
                 24.3 
                 24.3 
               
               
                 component 
                 Zinc (parts by mass) 
                 9.6 
                 9.6 
                 12.4 
                 12.4 
                 15.2 
                 15.2 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Evaluation 
                 Molding 
                 Temperature (° C.) 
                 150 
                 170 
                 150 
                 170 
                 150 
                 170 
               
               
                   
                 conditions 
                 Time (min) 
                 20 
                 20 
                 20 
                 20 
                 20 
                 20 
               
               
                   
                 Properties 
                 Slab hardness (Shore C) 
                 58.9 
                 62.6 
                 75.5 
                 79.2 
                 85.0 
                 89.1 
               
               
                   
                   
                 Lupke type rebound resilience (%) 
                 69.5 
                 67.2 
                 65.4 
                 63.8 
                 61.7 
                 60.0 
               
               
                   
                   
                 Rebound resilience/slab hardness 
                 1.18 
                 1.07 
                 0.87 
                 0.81 
                 0.73 
                 0.67 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 5 
               
               
                   
               
               
                 Rubber composition No. 
                 41 
                 42 
                 43 
                 44 
                 45 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Formulation 
                 (a) 
                 BR730 
                 100 
                 100 
                 100 
                 100 
                 100 
               
               
                 (parts by mass) 
                 (b) 
                 Zinc acrylate hydroxo cluster 
                 40 
                 35 
                 30 
                 25 
                 20 
               
               
                   
                   
                 ZN-DA90S 
                 — 
                 5 
                 10 
                 15 
                 20 
               
               
                   
                 (d) 
                 Zinc oxide 
                 2.69 
                 3.2 
                 3.7 
                 4.3 
                 4.8 
               
               
                   
                 (c) 
                 Dicumyl peroxide 
                 0.8 
                 0.8 
                 0.8 
                 0.8 
                 0.8 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Crosslinking 
                 Acrylate (parts by mass) 
                 24.2 
                 24.3 
                 24.3 
                 24.4 
                 24.4 
               
               
                 component 
                 Zinc (parts by mass) 
                 17.0 
                 17.0 
                 17.0 
                 17.0 
                 17.0 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Evaluation 
                 Molding 
                 Temperature (° C.) 
                 170 
                 150 
                 150 
                 150 
                 150 
               
               
                   
                 conditions 
                 Time (min) 
                 20 
                 20 
                 20 
                 20 
                 20 
               
               
                   
                 Slab 
                 Shore C hardness 
                 81.9 
                 82.1 
                 83.3 
                 84.4 
                 85.6 
               
               
                   
                 properties 
                 Lupke type rebound resilience (%) 
                 67.8 
                 67.1 
                 66.5 
                 65.8 
                 65.2 
               
               
                   
                   
                 Storage modulus E′ (×10 7  Pa) 
                 12.7 
                 15.6 
                 15.2 
                 15.4 
                 16.2 
               
               
                   
                   
                 Loss modulus E″ (×10 7  Pa) 
                 0.23 
                 0.20 
                 0.24 
                 0.26 
                 0.28 
               
               
                   
                   
                 | log(E′/E″ 2 ) | 
                 4.6 
                 4.4 
                 4.6 
                 4.7 
                 4.7 
               
               
                   
                   
                 |log(E′/E″ 2 )/hardness| × 100 
                 5.6 
                 5.4 
                 5.5 
                 5.5 
                 5.5 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE 6 
               
               
                   
               
               
                 Rubber composition No. 
                 46 
                 47 
                 48 
                 49 
                 50 
                 51 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Formulation 
                 (a) 
                 BR730 
                 100 
                 100 
                 100 
                 100 
                 100 
                 100 
               
               
                 (parts by mass) 
                 (b) 
                 ZN-DA90S 
                 19.65 
                 19.65 
                 29.55 
                 29.55 
                 39.4 
                 39.4 
               
               
                   
                 (d) 
                 Zinc oxide 
                 5 
                 5 
                 5 
                 5 
                 5 
                 5 
               
               
                   
                 (c) 
                 Dicumyl peroxide 
                 0.8 
                 0.8 
                 0.8 
                 0.8 
                 0.8 
                 0.8 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Crosslinking 
                 Acrylate (parts by mass) 
                 12.1 
                 12.1 
                 18.2 
                 18.2 
                 24.3 
                 24.3 
               
               
                 component 
                 Zinc (parts by mass) 
                 9.6 
                 9.6 
                 12.4 
                 12.4 
                 15.2 
                 15.2 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Evaluation 
                 Molding 
                 Temperature (° C.) 
                 150 
                 170 
                 150 
                 170 
                 150 
                 170 
               
               
                   
                 conditions 
                 Time (min) 
                 20 
                 20 
                 20 
                 20 
                 20 
                 20 
               
               
                   
                 Slab 
                 Shore C hardness 
                 58.9 
                 62.6 
                 75.5 
                 79.2 
                 85.0 
                 89.1 
               
               
                   
                 properties 
                 Lupke type rebound resilience (%) 
                 69.5 
                 67.2 
                 65.4 
                 63.8 
                 61.7 
                 60.0 
               
               
                   
                   
                 Storage modulus E′ (×10 7  Pa) 
                 3.3 
                 4.4 
                 11.5 
                 14.3 
                 16.9 
                 16.5 
               
               
                   
                   
                 Loss modulus E″ (×10 7  Pa) 
                 0.058 
                 0.093 
                 0.20 
                 0.25 
                 0.36 
                 0.52 
               
               
                   
                   
                 | log(E′/E″ 2 ) | 
                 4.0 
                 4.3 
                 4.6 
                 4.6 
                 4.9 
                 5.2 
               
               
                   
                   
                 |log(E′/E″ 2 )/hardness| × 100 
                 6.8 
                 6.9 
                 6.0 
                 5.8 
                 5.8 
                 5.9 
               
               
                   
               
            
           
         
       
     
     The materials used in Tables 1-6 are shown as follows. 
     BR730: high-cis polybutadiene (amount of cis-1,4 bond=96 mass %, amount of 1,2-vinyl bond=1.3 mass %, Moony viscosity (ML 1+4 (100° C.)=55, molecular weight distribution (Mw/Mn)=3) available from JSR Corporation 
     ZN-DA90S: zinc acrylate (a product coated with zinc stearate in an amount of 10 mass %) available from Nisshoku Techno Fine Chemical Co., Ltd. 
     Zinc oxide: “Ginrei R” available from Toho Zinc Co., Ltd. 
     Dicumyl peroxide: “Percumyl (register trademark) D” available from NOF Corporation 
     Tables 1, 2 show the hardness and rebound resilience of the slab formed from each rubber composition. In addition, Tables 1, 2 show the amount of the crosslinking component (the acrylate group in (b) the co-crosslinking agent, and the metal in (b) the co-crosslinking agent and (d) the metal compound) contained in each rubber composition.  FIG. 4  shows a relationship between the slab hardness and the rebound resilience of the crosslinked rubber molded product. The rubber compositions No. 1 to 10 are the cases in which (b) the co-crosslinking agent contains the complex represented by the general formula (1). The crosslinked rubber molded products (slabs) formed from these rubber compositions each have high resilience performance. 
     Tables 3, 4 show the slab hardness and rebound resilience of the crosslinked rubber molded product formed from each rubber composition. In addition, Tables 3, 4 show the amount of the crosslinking component (the acrylate group in (b) the co-crosslinking agent, and the metal in (b) the co-crosslinking agent and (d) the metal compound) contained in each rubber composition. Further,  FIG. 5  shows a relationship between the slab hardness and the rebound resilience of the crosslinked rubber molded product. The rubber compositions No. 21 to 30 are the cases in which (b) the co-crosslinking agent contains the complex represented by the general formula (1), and the slab hardness (Shore C) X and the Lupke type rebound resilience (%) Y satisfy the relationship of Y≥87−0.283X. The crosslinked rubber molded products formed from these rubber compositions also have excellent resilience performance. 
     Tables 5, 6 show the slab hardness, storage modulus and loss modulus of the crosslinked rubber molded product formed from each rubber composition. In addition, Tables 5, 6 show the amount of the crosslinking component (the acrylate group in (b) the co-crosslinking agent, and the metal in (b) the co-crosslinking agent and (d) the metal compound) contained in each rubber composition. Further,  FIG. 6  shows a relationship between the slab hardness and |log(E′/E″ 2 )| of the crosslinked rubber molded product. The rubber compositions No. 41 to 45 are the cases in which (b) the co-crosslinking agent contains the complex represented by the formula (1), and the storage modulus E′ (Pa) and loss modulus E″ (Pa) and the slab hardness (Shore C) X satisfy the relationship of |log(E′/E″ 2 )|/X×100≤5.7. The crosslinked rubber molded products formed from these rubber compositions also have excellent resilience performance. 
     If the rubber composition according to the present invention is used, a crosslinked rubber molded product excellent in the resilience performance can be obtained. Thus, the rubber composition according to the present invention can be utilized in sports goods such as golf ball, tennis ball and grip, industrial products such as hose, belt and mat, shoe sole, tire, resin additive, antivibration rubber, fender, and so on. 
     This application is based on Japanese Patent Applications No. 2016-250054, No. 2016-250055, No. 2016-250056 filed on Dec. 22, 2016, and No. 2017-129265, 2017-129266, No. 2017-129267 filed on Jun. 30, 2017, the contents of which are hereby incorporated by reference.