USE OF A Mo-BASED CATALYST COMPOSITION TO CONTROL THE CHARACTERISTICS OF CONJUGATED DIENE POLYMERS

Abstract: A catalyst composition that is the combination of or the reaction product of ingredients including (a) a molybdenum-containing compound, (b) a hydrogen phosphite, and (c) a blend of two or more sterically distinct organoaluminum compounds is particularly useful for polymerizing conjugated dienes. When polymerizing 1,3-butadiene into syndiotactic 1,2-polybutadiene, the ratio of the organoaluminum compounds can be adjusted to vary the melting temperature and molecular weight of the polymer product.

Description: USE OF A Mo-BASED CATALYST COMPOSITION TO CONTROL THE CHARACTERISTICS OF CONJUGATED DffiNE POLYMERS BACKGROUND OF THE INVENTION The present invention relates generally to a process for polymerizing conjugated dienes, more particularly to a process that employs a Mo-based catalyst composition formed by combining (a) a molybdenum-containing compound, (b) a hydrogen phosphite, and (c) a blend of two or more sterically distinct organoaluminum compounds. Characteristics, such as melting tempera- ture and molecular weight, of conjugated diene polymers made by processes using this catalyst composition can be manipulated. Syndiotactic 1 ,2-polybutadiene (hereinafter "s-PBD") is a crystalline thermoplastic resin that has a stereoregular structure in which the side chain vinyl groups are located alternately on the opposite sides in relation to the polymeric main chain. s-PBD is a unique material that exhibits the properties of both plastics and rubber, and therefore it has many uses. For example, films, fibers, and various molded articles can be made from s-PBD. It can also be blended into and co-cured with natural or synthetic rubber. S-PBD can be made by solution, emulsion or suspension polymerization. The physical properties of s-PBD are largely determined by its melting temperature and molecular weight. Generally, s-PBD has a melting temperature within the range of about 195° to about 215°C, but due to processability considerations, it is generally desirable for s-PBD to have a melting temperature of less than about 195°C. Accordingly, there is a need for means to regulate the melting temperature and molecular weight of s-PBD. Various transition metal catalyst systems based on Co, Ti, V, Mo, and Cr for the preparation of s-PBD have been reported. The majority of these catalyst systems, however, have no practical utility because they have low catalytic activity or poor stereoselectivity, and in some cases they produce low molecular weight polymers or partially crosslinked polymers unsuitable for commercial use. The following two Co-based catalyst systems are known for the preparation of s-PBD on a commercial scale: (1) a system containing Co bis(acetylacetonate), triethylaluminum, water, and triphenylphosphine (U.S. Pat. Nos. 3,498,963 and 4,182,813), and (2) a system containing Co tris(acetylacetonate), triethylaluminum, and CS 2 (U.S. Pat. No. 3,778,424). These Co-based catalyst systems also have disadvantages. The first Co catalyst system referenced above yields s-PBD having very low crystallinity. Also, this catalyst system develops sufficient catalytic activity only when halogenated hydrocarbon solvents are used as the polymerization medium, and halogenated solvents present toxicity problems. The second Co catalyst system referenced above uses CS 2 as one of the catalyst components. Because of its low flash point, obnoxious smell, high volatility, and toxicity, CS 2 is difficult and dangerous to use, and requires expensive safety measures to prevent even minimal amounts escaping into the atmosphere. Furthermore, the s-PBD produced with this Co catalyst system has a very high melting temperature of about 200° to 210°C, which makes processing the polymer difficult. Although the melting temperature of the s-PBD produced with this Co catalyst system can be reduced by employing a catalyst modifier as a fourth catalyst component, the presence of this catalyst modifier has adverse effects on the catalyst activity and polymer yields. Accordingly, many restrictions are required for the industrial utilization of these Co-based catalyst systems. Coordination catalyst systems based on Mo-containing compounds, such as the combination of molybdenum acetylacetonate and triethylaluminum, have been known for some time, but they have shown very low catalytic activity and poor stereoselectivity for the polymerization of 1 ,3-butadiene. The product mixture often contains oligomers, low molecular weight liquid polymers, and partially cross- linked polymers. Therefore, these catalyst systems have little industrial utility. Because s-PBD is useful and the catalysts known heretofore in the art have many shortcomings, developing a new and significantly improved catalyst composition that has high activity and stereoselectivity for polymerizing 1 ,3- butadiene into s-PBD is desirable. Ability to control the melting temperature and molecular weight of the polymerization product also is desirable. SUMMARY OF THE INVENTION In general, the present invention provides a process for preparing conjugated diene polymers with desired characteristics, the process comprising the step of polymerizing conjugated diene monomers in the presence of a catalytically effective amount of a catalyst composition formed by combining (a) a Mo-containing compound, (b) a hydrogen phosphite, and (c) a blend of two or more sterically distinct organoaluminum (hereinafter "org-AI") compounds. The present invention also provides a method for controlling the melting temperature of a crystalline conjugated diene polymer prepared by polymerizing conjugated diene monomers with a catalyst composition that is formed by combining (a) a Mo-containing compound, (b) a hydrogen phosphite, and (c) a blend of two or more sterically distinct org-AI compounds. The method includes the steps of selecting at least one sterically hindered org-AI compound and at least one sterically non-hindered org-AI compound, combining the selected org-AI compounds to form ingredient (c) of the catalyst composition, and thereafter polymerizing the conjugated diene monomers with the catalyst composition. The present invention further provides a catalyst composition formed by a process comprising the step of combining (a) a Mo-containing compound, (b) a hydrogen phosphite, and (c) a blend of two or more sterically distinct org-AI compounds. The present invention also provides a catalyst composition that is the combination of or the reaction product of ingredients comprising (a) a Mo- containing compound, (b) a hydrogen phosphite, and (c) a blend of two or more sterically distinct org-AI compounds. Advantageously, the Mo-based catalyst composition utilized in the present invention has very high catalytic activity and stereoselectivity for polymerizing conjugated diene monomers such as 1 ,3-butadiene. This activity and selectivity, among other advantages, allows conjugated diene polymers, such as s-PBD, to be produced in very high yields with low catalyst levels after relatively short polymerization times. Significantly, the catalyst composition of this invention is very versatile. By blending two or more sterically distinct org-AI compounds, it is possible to produce crystalline conjugated diene polymers, such as s-PBD, with a wide range of melting temperatures and molecular weights, thus eliminating the need to add a melting temperature regulator or a molecular weight regulator that adversely affects the catalyst activity and the polymer yield. In addition, the catalyst composition utilized in this invention does not contain CS 2 . Therefore, the toxicity, objectionable smell, dangers, and expense associated with the use of CS 2 are eliminated. Further, the Mo-containing compounds utilized are generally stable, inexpensive, relatively innocuous, and readily available. Furthermore, the catalyst composition utilized in this invention has high catalytic activity in a wide variety of solvents including the environmentally-preferred non-halogenated solvents such as aliphatic and cycloaliphatic hydrocarbons. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The present invention is generally directed toward a process for synthesizing conjugated diene polymers by using a Mo-based catalyst composition. The preferred embodiments of this invention are directed toward the synthesis of crystalline conjugated diene polymers, such as s-PBD. The catalyst composition employed in the present invention is formed by combining (a) a Mo- containing compound, (b) a hydrogen phosphite, and (c) a blend of two or more sterically distinct org-AI compounds. In addition to the three catalyst ingredients (a), (b), and (c), other organometallic compounds or Lewis bases can also be added, if desired. The characteristics of the resulting conjugated diene polymer can be adjusted by selecting certain sterically distinct org-AI compounds. For example, the melting temperature of crystalline conjugated diene polymers can be adjusted by selecting certain sterically distinct org-AI compounds or by varying the molar ratio of the sterically distinct org-AI compounds. Various Mo-containing compounds or mixtures thereof can be employed as ingredient (a) of the catalyst composition utilized in this invention. It is generally advantageous to employ Mo-containing compounds that are soluble in hydrocarbon solvents such as aromatic hydrocarbons, aliphatic hydrocarbons, or cycloaliphatic hydrocarbons. Hydrocarbon-insoluble Mo-containing compounds, however, can be suspended in the polymerization medium to form the catalytically active species and are therefore also useful. The Mo atom in the Mo-containing compounds can be in various oxidation states ranging from 0 up to +6. Suitable types of Mo-containing compounds that can be utilized include, but are not limited to, Mo carboxylates, Mo organophos- phates, Mo organophosphonates, Mo organophosphinates, Mo carbamates, Mo dithiocarbamates, Mo xanthates, Mo β-diketonates, Mo alkoxides or aryloxides, Mo halides, Mo pseudo-halides, Mo oxyhalides, and organomolybdenum compounds. Some specific examples of suitable molybdenum compounds in each of the foregoing classes include carboxylates: formate, acetate, acrylate, methacrylate, valerate, gluconate, citrate, fumarate, lactate, maleate, oxalate, 2-ethylhexanoate, neodecanoate, naphthenate, stearate, oleate, benzoate, and picolinate; organophosphates: dibutyl phosphate, dipentyl phosphate, dihexyl phosphate, diheptyl phosphate, dioctyl phosphate, bis(l-methylheptyl) phosphate, bis(2- ethylhexyl) phosphate, didecyl phosphate, didodecyl phosphate, dioctadecyl phosphate, dioleyl phosphate, diphenyl phosphate, bis(p-nonylphenyl) phosphate, butyl (2-ethylhexyl) phosphate, (1-methylheptyl) (2-ethylhexyl) phosphate, and (2-ethylhexyl) (p-nonylphenyl) phosphate; organophosphonates: butyl phosphonate, pentyl phosphonate, hexyl phosphonate, heptyl phosphonate, octyl phosphonate, (1-methylheptyl) phosphonate, (2-ethylhexyl) phosphonate, decyl phosphonate, dodecyl phosphonate, octa- decyl phosphonate, oleyl phosphonate, phenyl phosphonate, (p-nonylphenyl) phosphonate, butyl butylphosphonate, pentyl pentylphosphonate, hexyl hexylphosphonate, heptyl heptylphosphonate, octyl octylphosphonate, (1- methylheptyl) (l-methylheptyl)phosphonate, (2-ethylhexyl) (2-ethylhexyl)- phosphonate, decyl decylphosphonate, dodecyl dodecylphosphonate, octa- decyl octadecylphosphonate, oleyl oleylphosphonate, phenyl phenylphos- phonate, (p-nonylphenyl) (p-nonylphenyl)phosphonate, butyl (2-ethylhexyl)- phosphonate, (2-ethylhexyl) butylphosphonate, (1-methylheptyl) (2-ethyl- hexyl)phosphonate, (2-ethylhexyl) (l-methylheptyl)phosphonate, (2-ethylhexyl) (p-nonylphenyl)phosphonate, and (p-nonylphenyl) (2-ethylhexyl)phos- phonate; organophosphinates: butylphosphinate, pentylphosphinate, hexylphosphinate, heptylphosphinate, octylphosphinate, (l-methylheptyl)phosphinate, (2-ethyl- hexyl)phosphinate, decylphosphinate, dodecylphosphinate, octadecylphos- phinate, oleylphosphinate, phenylphosphinate, (p-nonylphenyl)phosphinate, dibutylphosphinate, dipentylphosphinate, dihexylphosphinate, diheptylphos- phinate, dioctylphosphinate, bis(1-methylheptyl)phosphinate, bis(2-ethyl- hexyl)phosphinate, didecylphosphinate, didodecylphosphinate, dioctadecyl- phosphinate, dioleylphosphinate, diphenylphosphinate, bis(p-nonylphenyl)- phosphinate, butyl(2-ethylhexyl)phosphinate, (1 -methylheptyl)(2-ethyl- hexyl)phosphinate, and (2-ethylhexyl)(p-nonylphenyl)phosphinate; carbamates: dimethylcarbamate, diethylcarbamate, diisopropylcarbamate, dibutylcarbamate, and dibenzylcarbamate; dithiocarbamates: dimethyldithiocarbamate, diethyldithiocarbamate, diisopropyl- dithiocarbamate, dibutyldithiocarbamate, and dibenzyldithiocarbamate. xanthates: methylxanthate, ethylxanthate, isopropylxanthate, butylxanthate, and benzylxanthate; β-diketonates: acetylacetonate, trifluoroacetylacetonate, hexafluoroacetyl- acetonate, benzoylacetonate, 2,2,6,6-tetramethyl-3,5-heptanedionate, dioxide bis(acetylacetonate), dioxide bis(trifluoroacetylacetonate), dioxide bis(hexafluoroacetyiacetonate), dioxide bis(benzoylacetonate), and dioxide bis(2,2,6,6-tetramethyl-3,5-heptanedionate); alkoxides or aryloxides: methoxide, ethoxide, isopropoxide, 2-ethylhexoxide, phenoxide, nonylphenoxide, and naphthoxide; halides: hexafluoride, pentafluoride, tetrafluoride, trifluόride, pentachloride, tetrachloride, trichloride, tetrabromide, tribromide, triiodide, and diiodide; pseudo-halides: cyanide, cyanate, thiocyanate, and azide; and oxyhalides: oxytetrafluoride, dioxydifluoride, oxytetrachloride, oxytrichloride, dioxydichloride, oxytribromide, and dioxydibromide. The term "organomolybdenum compound" refers to any molybdenum compound containing at least one Mo-C bond. Some specific examples of suitable organomolybdenum compounds include tris(allyl)molybdenum, tris(meth- allyl)molybdenum, tris(crotyl)molybdenum, bis(cyclopentadienyl)molybdenum, bis(ethylbenzene)molybdenum, bis(pentamethylcyclopentadienyl)molybdenum, bis(pentadienyl)molybdenum, bis(2,4-dimethylpentadienyl)molybdenum, bis(allyl)tricarbonylmolybdenum, (cyclopentadienyl)(pentadienyl)molybdenum, tetra(1-norbomyl)molybdenum (trimethylenemethane)tetracarbonylmolybdenum, bis(butadiene)dicarbonylmolybdenum, (butadiene)tetracarbonylmolybdenum, and bis(cyclooctatetraene)molybdenum. Useful hydrogen phosphite compounds that can be employed as ingredient (b) of the catalyst composition utilized in this invention are acyclic hydrogen phosphites, cyclic hydrogen phosphites, and mixtures thereof. In general, acyclic hydrogen phosphites may be represented by the following keto-enol tautomeric structures: where R 1 and R 2 , which may be the same or different, are monovalent organic groups. Preferably, R 1 and R 2 are hydrocarbyl groups such as, but not limited to, alkyl, cycloalkyl, substituted cycloalkyl, alkenyl, cycloalkenyl, substituted cyclo- alkenyl, aryl, allyl, substituted aryl, aralkyl, alkaryl, and alkynyl groups, with each group preferably containing from 1 C atom, or the appropriate minimum number of C atoms to form the group, up to 20 C atoms. These hydrocarbyl groups may contain heteroatoms such as, but not limited to, N, O, Si, S, and P. The acyclic hydrogen phosphites exist mainly as the keto tautomer (shown on the left), with the enol tautomer (shown on the right) being the minor species. The equilibrium constant for the above-mentioned tautomeric equilibrium is dependent upon factors such as the temperature, the types of R 1 and R 2 groups, the type of solvent, and the like. Both tautomers may be associated in dimeric, trimeric or oligomeric forms by hydrogen bonding. Either of the two tautomers or mixtures thereof can be employed. Representative and non-limiting examples of suitable acyclic hydrogen phosphites include dimethyl hydrogen phosphite, diethyl hydrogen phosphite, dibutyl hydrogen phosphite, dihexyl hydrogen phosphite, dioctyl hydrogen phosphite, didecyl hydrogen phosphite, didodecyl hydrogen phosphite, dioctadecyl hydrogen phosphite, bis(2,2,2-trifluoroethyl) hydrogen phosphite, diisopropyl hydrogen phosphite, bis(3,3-dimethyl-2-butyl) hydrogen phosphite, bis(2,4- dimethyl-3-pentyl) hydrogen phosphite, di-t-butyl hydrogen phosphite, bis(2- ethylhexyl) hydrogen phosphite, dineopentyl hydrogen phosphite, bis(cyclo- propylmethyl) hydrogen phosphite, bis(cyclobutylmethyl) hydrogen phosphite, bis(cyclopentylmethyl) hydrogen phosphite, bis(cyclohexylmethyl) hydrogen phosphite, dicyclobutyl hydrogen phosphite, dicyclopentyl hydrogen phosphite, dicyclohexyl hydrogen phosphite, dimethyl hydrogen phosphite, diphenyl hydrogen phosphite, dinaphthyl hydrogen phosphite, dibenzyl hydrogen phosphite, bis(1- naphthylmethyl) hydrogen phosphite, diallyl hydrogen phosphite, dimethallyl hydrogen phosphite, dicrotyl hydrogen phosphite, ethyl butyl hydrogen phosphite, methyl hexyl hydrogen phosphite, methyl neopentyl hydrogen phosphite, methyl phenyl hydrogen phosphite, methyl cyclohexyl hydrogen phosphite, methyl benzyl hydrogen phosphite, and the like. Mixtures of the above dihydrocarbyl hydrogen phosphites may also be utilized. In general, cyclic hydrogen phosphites contain a divalent organic group that bridges between the two O atoms single-bonded to the P atom. These cyclic hydrogen phosphites may be represented by the following keto-enol tautomeric structures: where R 3 is a divalent organic group. Preferably, R 3 is a hydrocarbylene group such as, but not limited to, alkylene, cycloalkylene, substituted alkylene, substituted cycloalkylene, alkenylene, cycloalkenylene, substituted alkenylene, substituted cycloalkenylene, arylene, and substituted arylene groups, with each group preferably containing from 1 C atom, or the appropriate minimum number of C atoms to form the group, up to 20 C atoms. These hydrocarbylene groups may contain heteroatoms such as, but not limited to, N, O, Si, S, and P. The cyclic hydrogen phosphites exist mainly as the keto tautomer (shown on the left), with the enol tautomer (shown on the right) being the minor species. The equilibrium constant for the above-mentioned tautomeric equilibrium is dependent upon factors such as the temperature, the types of R 3 group, the type of solvent, and the like. Both tautomers may be associated in dimeric, trimeric or oligomeric forms by hydrogen bonding. Either of the two tautomers or mixtures thereof can be used. Cyclic hydrogen phosphites may be synthesized by the transesterification reaction of an acyclic dihydrocarbyl hydrogen phosphite (usually dimethyl hydrogen phosphite or diethyl hydrogen phosphite) with an alkylene diol or an arylene diol. Procedures for this transesterification reaction are well known to those skilled in the art. Typically, the transesterification reaction is carried out by heating a mixture of an acyclic dihydrocarbyl hydrogen phosphite and an alkylene diol or an arylene diol. Subsequent distillation of the side-product alcohol (usually methanol or ethanol) that results from the transesterification reaction leaves the new-made cyclic hydrogen phosphite. Examples of suitable cyclic alkylene hydrogen phosphites are 2-oxo-(2H)-5-butyl-5-ethyl-1 ,3,2-dioxaphosphorinane, 2-oxo-(2H)-5,5-dimethyl- 1 ,3,2-dioxaphosphorinane, 2-oxo-(2H)-1 ,3,2-dioxaphosphorinane, 2-oxo-(2H)-4- methyl-1 ,3,2-dioxaphosphorinane, 2-oxo-(2H)-5-ethyl-5-methyl-1 ,3,2-dioxaphosphorinane, 2-oxo-(2H)-5,5-diethyl-1 ,3,2-dioxaphosphorinane, 2-oxo-(2H)-5-methyl- 5-propyl-1 ,3,2-dioxaphosphorinane, 2-oxo-(2H)-4-isopropyl-5,5-dimethyl-1 ,3,2- dioxaphosphorinane, 2-oxo-(2H)-4,6-dimethyl-1 ,3,2-dioxaphosphorinane, 2-oxo- (2H)-4-propyl-5-ethyl-1 ,3,2-dioxaphosphorinane, 2-oxo-(2H)-4-methyl-1 ,3,2-dioxa- phospholane, 2-oxo-(2H)-4,5-dimethyl-1 ,3,2-dioxaphospholane, 2-oxo-(2H)- 4,4,5,5-tetramethyl-1 ,3,2-dioxaphospholane, and the like. Mixtures of the above cyclic alkylene hydrogen phosphites may also be utilized. Examples of suitable cyclic arylene hydrogen phosphites are 2-oxo-(2H)- 4,5-benzo-1 ,3,2-dioxaphospholane, 2-oxo-(2H)-4,5-(3'-methylbenzo)-1 ,3,2- dioxaphospholane, 2-oxo-(2H)-4,5-(4'-methylbenzo)-1 ,3,2-dioxaphospholane, 2- oxo-(2H)-4,5-(4'-terf-butylbenzo)-1 ,3,2-dioxaphospholane, 2-oxo-(2H)-4,5- naphthalo-1 ,3,2-dioxaphospholane, and the like. Mixtures of the above cyclic arylene hydrogen phosphites may also be utilized. As noted above, ingredient (c) of the catalyst composition includes a blend of two or more org-AI compounds that have distinct steric hindrance. Use of org-AI compounds that are soluble in hydrocarbon solvent generally is preferred. As used herein, the term "org-AI compound" refers to any Al compound containing at least one Al-C bond. In a preferred embodiment, ingredient (c) of the catalyst composition is formed by combining at least one org-AI compound that is sterically hindered with at least one org-AI compound that is sterically less hindered or, more simply stated, non-hindered. The org-AI compounds employed to form ingredient (c) are generally characterized by containing at least one organic group that is attached to an aluminum atom via a carbon atom. The structure of these organic groups determines whether the org-AI compound is sterically hindered or non-hindered for purposes of this invention. The structures of these organic groups are best explained with reference to the following figure, which shows an organic group attached to an aluminum atom: \ Al r a pβ where C α is the α carbon and C β is the β carbon. In general, the steric hindrance of an org-AI compound is determined by the substitution patterns of the α and β carbons. An org-AI compound is sterically hindered where the α carbon is secondary or tertiary, i.e., has only one or no hydrogen atom bonded thereto. Also, an org-AI compound is sterically hindered where the β carbon has only one or o hydrogen atom bonded thereto. On the other hand, an org-AI compound is non-hindered where the α carbon is primary, i.e., has two hydrogen atoms bonded thereto, and the β carbon has at least two hydrogen atoms bonded thereto. Other non-hindered organic groups include CH 3 or CH 2 F. Non-limiting examples of sterically hindered organic groups include isopropyl, isobutyl, t-butyl, 2-ethylhexyl, neopentyl, cyclohexyl, 1-methylcyclo- pentyl, and 2,6-dimethylphenyl groups. Non-limiting examples of non-hindered organic groups include methyl, ethyl, n-propyl, n-butyl, n-hexyl, and n-octyl groups. Those skilled in the art understand that an org-AI compound may include both hindered and non-hindered organic groups because the Al atom generally has a valence of three as shown in the foregoing figure. If the org-AI compound includes both hindered and non-hindered organic groups, then, for purposes of this specification, the compound is deemed to be both sterically hindered and non- hindered because both the hindered and non-hindered organic groups are believed to have an impact on the characteristics of the resulting polymer. A preferred class of org-AI compounds that can be utilized to form ingredient (c) of the catalyst composition utilized in this invention is represented by the general formula AIRnX 3 -n, where each R independently is a monovalent organic group attached to the Al atom via a C atom, n is an integer of from 1 to 3, and each X independently is a hydrogen atom, a halogen atom, a carboxylate group, an alkoxide group, or an aryloxide group. Preferably, each R is a hydrocarbyl group such as, but not limited to, alkyl, cycloalkyl, substituted cycloalkyl, alkenyl, cycloalkenyl, substituted cycloalkenyl, aryl, allyl, substituted aryl, aralkyl, alkaryl, and alkynyl groups, with each group preferably containing from 1 C atom, or the appropriate minimum number of C atoms to form the group, up to about 20 C atoms. Also, these hydrocarbyl groups may contain heteroatoms such as O, S, N, Si, and P. Preferably, each X is a carboxylate, alkoxide, or aryloxide group, with each group preferably containing from 1 C atom, or the appropriate minimum number of C atoms to form the group, up to about 20 C atoms. Thus, some suitable types of org-AI compounds that can be utilized include, but are not limited to, trihydrocarbylaluminum, dihydrocarbylaluminum hydride, hydrocarbylaluminum dihydride, hydrocarbylaluminum dihalide, dihydrocarbylaluminum halide, dihydrocarbylaluminum carboxylate, hydrocarbylaluminum bis(carboxylate), dihydrocarbylaluminum alkoxide, hydrocarbylaluminum dialkoxide, dihydrocarbylaluminum aryloxide, hydrocarbylaluminum diaryloxide, and the like, and mixtures thereof. Trihydrocarbylaluminum compounds are generally preferred. Some specific examples of org-AI compounds that can be utilized include trimethylaluminum, triethylaluminum, triisobutylaluminum, tri-n-propylaluminum, triisopropylaluminum, tri-n-butylaluminum, tri-t-butylaluminum, tri-n-hexylaluminum, tri-n-octylaluminum, tricyclohexylaluminum, triphenylaluminum, tri-p-tolylaluminum, tribenzylaluminum, trineopentylaluminum, tris(1 -methylcyclopentyl)aluminum, tris(2,6-dimethylphenyl)aluminum, diethylphenylaluminum, diethyl-p-tolylaluminum, diethylbenzylaluminum, ethyldiphenylaluminum, ethyldi-p-tolylaluminum, ethyldibenzylaluminum, diethylaluminum hydride, di-n-propylaluminum hydride, diisopropylaluminum hydride, di-n-butylaluminum hydride, diisobutylaluminum hydride, di-n-octylaluminum hydride, diphenylaluminum hydride, di-p-tolylaluminum hydride, dibenzylaluminum hydride, phenylethylaluminum hydride, phenyl-n-propyl- aluminum hydride, phenylisopropylaluminum hydride, phenyl-n-butylaluminum hydride, phenylisobutylaluminum hydride, phenyl-n-octylaluminum hydride, p- tolylethylaluminum hydride, p-tolyl-n-propylaluminum hydride, p-tolylisopropyl- aluminum hydride, p-tolyl-n-butylaluminum hydride, p-tolylisobutylaluminum hydride, p-tolyl-n-octylaluminum hydride, benzylethylaluminum hydride, benzyl-n- propylaluminum hydride, benzylisopropylaluminum hydride, benzyl-n-butyl- aluminum hydride, benzylisobutylaluminum hydride, and benzyl-n-octylaluminum hydride, ethylaluminum dihydride, n-propylaluminum dihydride, isopropylaluminum dihydride, n-butylaluminum dihydride, isobutylaluminum dihydride, n-octylaluminum dihydride, dimethylaluminum chloride, diethylaluminum chloride, diisobutylaluminum chloride, dimethylaluminum bromide, diethylaluminum bromide, dimethylaluminum fluoride, diethylaluminum fluoride, methylaluminum dichloride, ethylaluminum dichloride, isobutylaluminum dichloride, methylaluminum dibromide, ethylaluminum dibromide, methylaluminum difluoride, ethylaluminum difluoride, methylaluminum sesquichloride, ethylaluminum sesquichloride, isobutylaluminum sesquichloride, dimethylaluminum hexanoate, diethylaluminum octoate, diisobutylaluminum 2-ethylhexanoate, dimethylaluminum neodecanoate, diethylaluminum stearate, diisobutylaluminum oleate, methylaluminum bis(hexanoate), ethylaluminum bis(octoate), isobutylaluminum bis(2-ethylhexanoate), methylaluminum bis(neodecanoate), ethylaluminum bis(stearate), isobutylaluminum bis(oleate), dimethylaluminum methoxide, diethylaluminum methoxide, diisobutyl- aluminum methoxide, dimethylaluminum ethoxide, diethylaluminum ethoxide, diisobutylaluminum ethoxide, dimethylaluminum phenoxide, diethylaluminum phenoxide, diisobutylaluminum phenoxide, methylaluminum dimethoxide, ethylaluminum dimethoxide, isobutylaluminum dimethoxide, methylaluminum diethoxide, ethylaluminum diethoxide, isobutylaluminum diethoxide, methyl- aluminum diphenoxide, ethylaluminum diphenoxide, isobutylaluminum diphenoxide, tris(fluoromethyl)aluminum, and the like, and mixtures thereof. Another class of org-AI compounds that can be utilized to form ingredient (c) of the catalyst composition utilized in this invention is aluminoxanes. Aluminoxanes are well known in the art and comprise oligomeric linear aluminoxanes that can be represented by the general formula: and oligomeric cyclic aluminoxanes that can be represented by the general formula: where x is an integer of from 1 to about 100, preferably about 10 to about 50; y is an integer of from 2 to about 100, preferably about 3 to about 20; and each R 4 , independently is a monovalent organic group that is attached to the Al atom via a C atom. Preferably, each R 4 is a hydrocarbyl group such as, but not limited to, alkyl, cycloalkyl, substituted cycloalkyl, alkenyl, cycloalkenyl, substituted cycloalkenyl, aryl, allyl, substituted aryl, aralkyl, alkaryl, and alkynyl groups, with each group preferably containing from 1 C atoms, or the appropriate minimum number of C atoms to form the group, up to about 20 C atoms. These hydrocarbyl groups may contain heteroatoms such as, but not limited to, N, O, Si, S, and P. The number of moles of the aluminoxane refers to the number of moles of the Al atoms rather than the number of moles of the oligomeric aluminoxane molecules. This convention is commonly employed in the art of catalysis utilizing aluminoxanes. Aluminoxanes can be prepared by reacting trihydrocarbylaluminum compounds with water. This reaction can be performed according to known methods, such as (1) a method in which the trihydrocarbylaluminum compound is dissolved in an organic solvent and then contacted with water, (2) a method in which the trihydrocarbylaluminum compound is reacted with water of crystallization contained in, for example, metal salts, or water adsorbed in inorganic or organic compounds, and (3) a method in which the trihydrocarbylaluminum compound is added to the monomer or monomer solution that is to be polymerized, and then water is added. Some specific examples of suitable aluminoxane compounds that can be utilized include methylaluminoxane (MAO), modified methylaluminoxane (MMAO), ethylaluminoxane, n-propylaluminoxane, isopropylaluminoxane, n-butyl- aluminoxane, n-hexylaluminoxane, n-octylaluminoxane, isobutylaluminoxane, t- butylaluminoxane, neopentylaluminoxane, cyclohexylaluminoxane, 1-methyl- cyclopentylaluminoxane, 2,6-dimethylphenylaluminoxane, and the like, and mixtures thereof. Isobutylaluminoxane is particularly useful on the grounds of its availability and its solubility in aliphatic and cycloaliphatic hydrocarbon solvents. (MMAO can be formed by substituting about 20-80% of the methyl groups of MAO with C2-C12 hydrocarbyl groups, preferably with isobutyl groups, by using known techniques.) The catalyst composition has very high catalytic activity over a wide range of total catalyst concentrations and catalyst ingredient ratios. The polymers having the most desirable properties, however, are obtained within a narrower range of total catalyst concentrations and catalyst ingredient ratios. Further, it is believed that the three catalyst ingredients (a), (b), and (c) can interact to form an active catalyst species. Accordingly, the optimum concentration for any one catalyst ingredient is dependent upon the concentrations of the other catalyst ingredients. The molar ratio of the hydrogen phosphite to the Mo-containing compound (P/Mo) can be varied from about 0.5:1 to about 50:1 , more preferably from about 1:1 to about 25:1 , and even more preferably from about 2:1 to about 10:1. Where ingredient (c) of the catalyst composition comprises a blend of two or more org-AI compounds defined by the formula AIR n X3. n , as specified above, the molar ratio of the Al in the blend of org-AI compounds to the Mo-containing compound (Al/Mo) can be varied from about 1:1 to about 100:1 , more preferably from about 3:1 to about 50:1 , and even more preferably from about 5:1 to about 25:1. When ingredient (c) of the catalyst composition utilized in the present invention comprises a blend of two or more aluminoxanes, the molar ratio of the aluminum in the blend of aluminoxanes to the Mo-containing compound (Al/Mo) can be varied from about 5:1 to about 500:1 , more preferably from about 10:1 to about 200:1 , and even more preferably from about 20:1 to about 100:1. Despite the foregoing preferred embodiments, a blend of two or more org-AI compounds can be formed by combining compounds defined by the formula AIR n X3- n with aluminoxanes. As discussed above, the catalyst composition utilized in the present invention is preferably formed by combining the three ingredients (a), (b), and (c). Although an active catalyst species is believed to result from this combination, the degree of interaction or reaction between the various ingredients or components is not known with any great degree of certainty. Therefore, the term "catalyst composition" has been employed to encompass a simple mixture of the ingredients, a complex of the various ingredients that is caused by physical or chemical forces of attraction, a chemical reaction product of the ingredients, or a combination of the foregoing. The catalyst composition utilized in the present invention can be formed by combining or mixing the catalyst ingredients or components by using, for example, one of the following methods: 1) The catalyst composition may be formed in situ by adding the three catalyst ingredients to a solution containing monomer and solvent (or simply bulk monomer) in a stepwise or simultaneous manner. When adding the catalyst ingredients stepwise, the addition sequence is not critical. Preferably, however, the Mo-containing compound is added first, followed by the hydrogen phosphite, and then followed by the blend of two or more org-AI compounds. 2) The three ingredients may be pre-mixed outside the polymerization system at an appropriate temperature, generally from about -20° to about 80°C, and the resulting catalyst composition added to the monomer solution. 3) the catalyst composition may be pre-formed in the presence of monomer. That is, the three catalyst ingredients are pre-mixed in the presence of a small amount of monomer at an appropriate temperature, generally from about -20° to about 80°C. The amount of monomer used for the catalyst pre- forming can range from about 1 to about 500 moles, more preferably from about 4 to about 100 moles, and even more preferably from about 10 to about 50 moles, per mole of the Mo-containing compound. The resulting catalyst composition is added to the remainder of the monomer to be polymerized. 4) The catalyst composition may be formed by using a two-stage procedure. The first stage involves combining the Mo-containing compound and the blend of two or more org-AI compounds in the presence of a small amount of monomer at an appropriate temperature, generally from about -20° to about 80°C. In the second stage, the foregoing reaction mixture and the hydrogen phosphite are charged in either a stepwise or simultaneous manner to the remainder of monomer to be polymerized. 5) In an alternative two-stage procedure, a Mo-ligand complex is first formed by pre-combining the Mo-containing compound with the hydrogen phosphite. Once formed, this Mo-ligand complex is combined with the blend of two or more org-AI compounds to form the active catalyst species. The Mo- ligand complex can be formed separately or in the presence of the monomer to be polymerized. This complexation reaction can be conducted at any convenient temperature at normal pressure, but for an increased rate of reaction, this reaction preferably is performed at room temperature or above. The temperature and time used for the formation of the Mo-ligand complex depend upon several variables including the particular starting materials and the solvent employed. Once formed, the Mo-ligand complex can be used without isolation from the complexation reaction mixture. If desired, however, the Mo-ligand complex may be isolated from the complexation reaction mixture before use. With respect to the catalyst ingredient (c), i.e., the blend of two or more org-AI compounds, it is advantageous to preform this blend by combining two or more org-AI compounds prior to mixing the blend with the other catalyst ingredients and the monomers that are to be polymerized. Nevertheless, the blend of two or more org-AI compounds can also be formed in situ. That is, the two or more org-AI compounds are combined at the time of polymerization in the presence of the other catalyst ingredients and the monomers to be polymerized. When a solution of the Mo-based catalyst composition or one or more of the catalyst ingredients is prepared outside the polymerization system as set forth in the foregoing methods, an organic solvent or carrier is preferably employed. Useful solvents include hydrocarbon solvents such as aromatic hydrocarbons, aliphatic hydrocarbons, and cycloaliphatic hydrocarbons. Non-limiting examples of aromatic hydrocarbon solvents include benzene, toluene, xylenes, ethylbenzene, diethylbenzene, mesitylene, and the like. Non-limiting examples of aliphatic hydro- carbon solvents include n-pentane, n-hexane, n-heptane, n-octane, n-nonane, n- decane, isopentane, isohexanes, isopentanes, isooctanes, 2,2-dimethylbutane, petroleum ether, kerosene, petroleum spirits, and the like. Non-limiting examples of cycloaliphatic hydrocarbon solvents include cyclopentane, cyclohexane, methyl- cyclopentane, methylcyclohexane, and the like. Commercial mixtures of the above hydrocarbons may also be used. For environmental reasons, aliphatic and cycloaliphatic solvents are highly preferred. The foregoing organic solvents may serve to dissolve the catalyst composition or ingredients, or the solvent may simply serve as a carrier in which the catalyst composition or ingredients may be suspended. As described above, the catalyst composition utilized in the present invention exhibits a very high catalytic activity for the polymerization of conjugated dienes. Some specific examples of conjugated diene that can be polymerized include 1,3-butadiene, isoprene, 1 ,3-pentadiene, 1 ,3-hexadiene, 2,3-dimethyl-1 ,3- butadiene, 2-ethyl-1 ,3-butadiene, 2-methyl-1,3-pentadiene, 3-methyl-1 ,3-pentadiene, 4-methyl-1 ,3-pentadiene, and 2,4-hexadiene. Mixtures of two or more conjugated dienes may also be utilized in co-polymerization. The preferred conjugated dienes are 1,3-butadiene, isoprene, 1,3-pentadiene, and 1 ,3-hexadiene. The most preferred monomer is 1 ,3-butadiene inasmuch as the catalyst composition utilized in this invention advantageously has very high catalytic activity and stereoselectivity for polymerizing 1,3-butadiene into s-PBD, and, as noted above, the melting temperature of the s-PBD can be adjusted. Production of conjugated diene polymers, such as s-PBD, is accomplished by polymerizing conjugated diene monomers in the presence of a catalytically effective amount of the foregoing catalyst composition. There are available a variety of methods for bringing the ingredients of the catalyst composition into contact with conjugated dienes as described above. To understand what is meant by a catalytically effective amount, the total catalyst concentration to be employed in the polymerization mass depends on the interplay of various factors such as the purity of the ingredients, the polymerization temperature, the polymerization rate and conversion desired, and many other factors. Accordingly, specific total catalyst concentration cannot be definitively set forth except to say that catalytically effective amounts of the respective catalyst ingredients should be used. Generally, the amount of the Mo-containing compound used can be varied from about 0.01 to about 2 mmol per 100 g conjugated diene monomers, more preferably from about 0.02 to about 1.0 mmol per 100 g conjugated diene monomers, and even more preferably from about 0.05 to about 0.5 mmol per 100 g conjugated diene monomers. Polymerization of conjugated diene monomers preferably is carried out in an organic solvent as the diluent. Accordingly, a solution polymerization system may be employed in which both the monomer to be polymerized and the polymer formed are soluble in the polymerization medium. Alternatively, a precipitation polymerization system may be employed by choosing a solvent in which the polymer formed is insoluble. In both cases, an amount of the organic solvent in addition to the organic solvent that may be used in preparing the catalyst composition is usually added to the polymerization system. The additional organic solvent may be either the same as or different from the organic solvent contained in the catalyst solutions. An organic solvent that is inert with respect to the catalyst composition employed to catalyze the polymerization generally is preferred. Suitable types of organic solvents that can be utilized as the diluent include, but are not limited to, aliphatic, cycloaliphatic, and aromatic hydrocarbons. Some representative examples of suitable aliphatic solvents include n-pentane, n- hexane, n-heptane, n-octane, n-nonane, n-decane, isopentane, isohexanes, isopentanes, isooctanes, 2,2-dimethylbutane, petroleum ether, kerosene, petroleum spirits, and the like. Some representative examples of suitable cycloaliphatic solvents include cyclopentane, cyclohexane, methyicyclopentane, methyl- cyclohexane, and the like. Some representative examples of suitable aromatic solvents include benzene, toluene, xylenes, ethylbenzene, diethylbenzene, mesitylene, and the like. Commercial mixtures of the above hydrocarbons may also be used. For environmental reasons, aliphatic and cycloaliphatic solvents are highly preferred. The concentration of conjugated diene monomers to be polymerized is not limited to a special range. Generally, however, the concentration of the monomers present in the polymerization medium at the beginning of the polymerization preferably is in a range of from about 3 to about 80% by weight, more preferably from about 5 to about 50% by weight, and even more preferably from about 10 to about 30% by weight. Polymerization of conjugated diene monomers also can be carried out by means of bulk polymerization, which refers to a polymerization environment where no solvents are employed. Bulk polymerization can be conducted either in a condensed liquid phase or in a gas phase. When polymerizing conjugated diene monomers, a molecular weight regulator may be employed to control the molecular weight of the conjugated diene polymers to be produced. As a result, the scope of the polymerization system can be expanded in such a manner that it can be used for the production of conjugated diene polymers having a wide range of molecular weights. Suitable types of molecular weight regulators that can be utilized include, but are not limited to, α-olefins such as ethylene, propylene, 1-butene, 1-pentene, 1-hexene, 1- heptene, and 1-octene; accumulated diolefins such as allene and 1 ,2-butadiene; nonconjugated diolefins such as 1 ,6-octadiene, 5-methyl-1 ,4-hexadiene, 1 ,5- cyclooctadiene, 3,7-dimethyl-1 ,6-octadiene, 1 ,4-cyclohexadiene, 4-vinylcyclo- hexene, 1 ,4-pentadiene, 1 ,4-hexadiene, 1,5-hexadiene, 1 ,6-heptadiene, 1,2- divinylcyclohexane, 5-ethylidene-2-norbomene, 5-methylene-2-norbornene, 5- vinyl-2-norbornene, dicyclopentadiene, and 1 ,2,4-trivinylcyclohexane; acetylenes such as acetylene, methylacetylene and vinylacetylene; and mixtures thereof. The amount of the molecular weight regulator used, expressed in parts per hundred parts by weight of the conjugated diene monomers (phm), is from about 0.01 to about 10 phm, preferably from about 0.02 to about 2 phm, and more preferably from about 0.05 to about 1 phm. The molecular weight of the conjugated diene polymers to be produced can also be effectively controlled by polymerizing conjugated diene monomers in the presence of H 2 . In this case, the preferable partial pressure of H 2 is within the range of about 0.01 to about 50 atm. The polymerization of conjugated diene monomers according to this invention may be carried out as a batch process, a continuous process, or even a semi-continuous process. In the semi-continuous process, monomer is intermit- tently charged as needed to replace that monomer already polymerized. In any case, the polymerization is desirably conducted under anaerobic conditions by using an inert protective gas such as nitrogen, argon or helium, with moderate to vigorous agitation. The polymerization temperature employed in the practice of this invention may vary widely from a low temperature, such as -10°C or below, to a high temperature such as 100°C or above, with a preferred temperature range being from about 20° to about 90°C. Heat generated by polymerization may be removed by external cooling, cooling by evaporation of the monomer or the solvent, or a combination of the two methods. Although the polymerization pressure employed may vary widely, a preferred pressure range is from about 1 to about 10 atm. Once a desired conversion is achieved, the polymerization can be stopped by the addition of a polymerization terminator that inactivates the catalyst. Typically, the terminator employed is a protic compound, which includes, but is not limited to, an alcohol, a carboxylic acid, an inorganic acid, water, or a mixture thereof. An antioxidant such as 2,6-di-ferf-butyl-4-methylphenol may be added along with, before or after the addition of the terminator. The amount of the antioxidant employed is preferably in the range of 0.2 to 1 % by weight of the polymer product. When the polymerization reaction has been stopped, the polymer can be recovered from the polymerization mixture by conventional procedures of desolventization and drying. For instance, the polymer may be isolated from the polymerization mixture by coagulation of the polymerization mixture with an alcohol such as methanol, ethanol, or isopropanol, or by steam distillation of the solvent and the unreacted monomer, followed by filtration. The polymer product is then dried to remove residual amounts of solvent and water. As noted above, a preferred embodiment of this invention is directed toward a process for the synthesis of crystalline conjugated diene polymers such as s-PBD. Advantageously, the melting temperature of the resulting crystalline conjugated diene polymers produced according to this invention can be manipu- lated by employing a blend of two or more sterically distinct org-AI compounds as ingredient (c) of the catalyst composition of this invention. In general, use of a sterically hindered org-AI compound within the catalyst composition has been found to yield a polymer having a relatively high melting temperature, and that the use of a sterically non-hindered org-AI compound within the catalyst composition gives rise to a polymer having a relatively low melting temperature. Surprisingly, by using a blend of two or more sterically dissimilar org-AI compounds, one can tailor the melting temperature of the resulting polymer. In other words, by using a blend of a sterically hindered org-AI compound that yields a polymer having a relatively high melting temperature and a sterically non-hindered org-AI compound that yields a polymer having a relatively low melting temperature, one can obtain a polymer whose melting temperature is somewhere between the relatively high and relatively low temperatures. For example, when an acyclic hydrogen phosphite is employed within the catalyst composition, the use of a sterically non-hindered org-AI compound generally yields a s-PBD polymer having a melting temperature of from about 90° to about 140°C. On the other hand, the use of a sterically hindered org-AI compound generally yields a s-PBD polymer having a melting temperature of from about 180° to about 210°C. By using a blend of a sterically hindered org-AI compound and a sterically non-hindered org-AI compound, one can obtain a s- PBD polymer whose melting temperature is somewhere between about 90° and about 210°C when an acyclic hydrogen phosphite is used within the catalyst composition. Advantageously, the process of this invention allows for the synthesis of s-PBD having a melting temperature from about 140° to about 180°C, more advantageously from about 145° to about 170°C, and even more advantageously from about 150° to about 160°C. Moreover, the desired melting temperature of the resulting crystalline conjugated diene polymer can be achieved by adjusting the molar ratio of the hindered to non-hindered org-AI compounds. In general, the melting temperature of the resulting polymer can be increased by increasing the molar ratio of the hindered to non-hindered org-AI compounds. Likewise, the melting temperature of the resulting polymer can be decreased by decreasing the molar ratio of the hindered to non-hindered org-AI compounds. In addition to adjusting the molar ratio of the hindered to non-hindered org- Al compounds, the melting temperatures of the resulting crystalline conjugated diene polymer can be manipulated by selecting certain org-AI compounds within the class of hindered compounds, by selecting certain org-AI compounds within the class of non-hindered compounds, or by selecting one or more from each class. The selected org-AI compounds are then combined to form ingredient (c) of the catalyst composition. The molecular weight, 1 ,2-linkage content, and syndiotacticity of the s-PBD can be increased by increasing the molar ratio of the hindered to non-hindered org-AI compounds within the blend of two or more sterically distinct org-AI compounds. s-PBD can be blended with various rubbers to improve the properties thereof. For example, it can be incorporated into elastomers to improve the green strength of components made from those elastomers, particularly in tires. The supporting or reinforcing carcass of tires is particularly prone to distortion during tire building and curing procedures. For this reason, incorporation of s-PBD into rubber compositions that are utilized in the supporting carcass of tires has particular utility in preventing or minimizing this distortion. In addition, incorporation of s- PBD into tire tread compositions can reduce heat build-up and improve the tear and wear characteristics of tires. s-PBD also is useful in the manufacture of films and packaging materials and in many molding applications. EXAMPLES Example 1 An oven-dried 1 L glass bottle was capped with a self-sealing rubber liner and a perforated metal cap. After the bottle was thoroughly purged with a stream of dry N 2 , the bottle was charged with 23 g hexanes and 227 g 1 ,3-butadiene/ hexanes blend containing 22.0% by weight 1 ,3-butadiene. The following catalyst ingredients were added to the bottle sequentially: (1) 0.15 mmol Mo 2- ethylhexanoate, (2) 0.60 mmol bis(2-ethylhexyl) hydrogen phosphite, and (3) 2.55 mmol tri-n-butylaluminum. The bottle was tumbled for 6 hours in a water bath maintained at 65°C. The polymerization was terminated by addition of 10 mL isopropanol containing 1.0 g 2,6-di-fe f-butyl-4-methylphenol. The polymerization mixture was coagulated with 3 L isopropanol. The resulting s-PBD was isolated by filtration and dried to a constant weight under vacuum at 60°C. Melting temperature was determined by DSC. The 1 ,2-linkage and syndiotacticity content of the polymer were determined by 1 H and 3 C NMR analysis. Weight average molecular weight (M w ), number average molecular weight (M n ), and polydispersity index were determined by means of GPC measurements. Examples 2-5 In Examples 2-5, the procedure described in Example 1 was repeated except that triisobutylaluminum/tri-n-butylaluminum mixtures having various molar ratios, i.e., 30:70, 50:50, 70:30, and 100:0 were substituted for the tri-n-butyl- aluminum used in Example 1. (The Mo/P/AI molar ratio for each of the samples was 1:4:17.) The org-AI component ratios, polymer yields, and physical properties of the resulting polymers from Examples 1-5 are summarized in Table I. Table I As shown in Table I, the melting temperature, molecular weight, 1 ,2-linkage content, and syndiotacticity of the s-PBD can be increased by increasing the molar ratio of triisobutylaluminum to tri-n-butylaluminum.

Claims: CLAIMS I claim: 1. A process for preparing conjugated diene polymers with desired characteristics, the process comprising polymerizing 1,3-butadiene, 1,3- pentadiene, 1 ,3-hexadiene, or mixtures thereof in the presence of a catalytically effective amount of a catalyst composition formed by combining: a) a molybdenum-containing compound; b) a hydrogen phosphite; and c) a blend of two or more sterically distinct organoaluminum compounds. 2. A method for controlling the melting temperature of a crystalline conjugated diene polymer that is prepared by polymerizing 1 ,3-butadiene, 1 ,3- pentadiene, 1 ,3-hexadiene, or mixtures thereof with a catalyst composition that is formed by combining (1) a molybdenum-containing compound, (2) a hydrogen phosphite, and (3) a blend of two or more sterically distinct organoaluminum compounds, the method comprising: a) selecting at least one sterically hindered organoaluminum compound and at least one sterically non-hindered organoaluminum compound; b) combining the selected organoaluminum compounds to form ingredient (3) of the catalyst composition; and c) thereafter polymerizing the conjugated diene monomers with the catalyst composition. 3. A catalyst composition formed by a process comprising combining a Mo-containing compound, a hydrogen phosphite, and a blend of two or more sterically distinct organoaluminum compounds. 4. The process of claim 1 or catalyst of claim 3, where the blend of two or more sterically distinct organoaluminum compounds includes at least one sterically hindered organoaluminum compound and at least one sterically non-hindered organoaluminum compound. 5. The process of claims 2 or 4, where the at least one sterically hindered organoaluminum compound is triisopropylaluminum, triisobutylaluminum, tri-t-butylaluminum, trineopentylaluminum, tricyclohexylaluminum, tris(1-methyl- cyclopentyl)aluminum, tris(2,6-dimethylphenyl)aluminum, isopropylaluminoxane, isobutylaluminoxane, t-butylaluminoxane, neopentylaluminoxane, cyclohexyl- aluminoxane, 1 -methylcyclopentylaluminoxane, 2,6-dimethylphenylaluminoxane, or mixtures thereof. 6. The process of claims 2 or 4, where the at least one sterically non- hindered organoaluminum compound is trimethylaluminum, triethylaluminum, tri-n- propylaluminum, tri-n-butylaluminum, tri-n-hexylaluminum, tri-n-octylaluminum, methylaluminoxane, ethylaluminoxane, n-propylaluminoxane, n-butylaluminoxane, n-hexylaluminoxane, n-octylaluminoxane, or mixtures thereof. 7. The process of claims 2 or 4, or catalyst of claim 3, where the hydrogen phosphite is a acyclic hydrogen phosphite that is defined by the following keto-enol tautomeric structures: where R 1 and R 2 independently are monovalent organic groups. 8. The process of claim 2, further comprising the step of increasing the molar ratio of the sterically hindered organoaluminum compounds to the sterically non-hindered organoaluminum compounds to increase the melting temperature of the crystalline conjugated diene polymer. 9. The process of claim 2, further comprising decreasing the molar ratio of the sterically hindered organoaluminum compounds to the sterically non- hindered organoaluminum compounds to decrease the melting temperature of the crystalline conjugated diene polymer. 10. The process of claims 8 and 9, where the conjugated diene is 1 ,3- butadiene, thereby forming syndiotactic 1 ,2-polybutadiene.

Inventors: LUO STEVEN, LUO, STEVEN
Applicants: BRIDGESTONE CORP, LUO STEVEN, BRIDGESTONE CORPORATION, LUO, STEVEN
Application year: 2001
Main classification: C12N9
IPC codes: C08F4, C08F36, C08F136