Syndiotactic 1,2-polybutadiene ("s-PBD") is a crystalline thermoplastic resin that has a stereoregular structure in which the side-chain vinyl groups are located alternately on opposite sides in relation to the polymeric main chain. It 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 rubbers to improve the properties thereof. Generally, s-PBD has a melting temperature in 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.

Various transition metal catalyst systems based on Co, Ti, V, Cr, and Mo 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, produce low molecular weight polymers or partially crosslinked polymers unsuitable for commercial use.

Two Co-based catalyst systems are known for preparing s-PBD. The first includes a Co compound, a phosphine compound, an organoaluminum compound, and water. This catalyst system yields s-PBD having very low crystallinity. Also, this catalyst system develops sufficient catalytic activity only when the polymerization medium is a halogenated hydrocarbon solvent (s), and such solvents present toxicity problems.

The second catalyst system includes a Co compound, an organo- aluminum compound, and CS2. Because CS2 has a low flash point, obnoxious smell, high volatility, and toxicity, it 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 catalyst system has a melting temperature of about 200° to 210°C, which makes it difficult to process. Although the melting temperature of the s-PBD produced with this catalyst system can be reduced by employing a catalyst modifier, the use of this catalyst modifier has adverse effects on the catalyst activity and polymer yields.

Coordination catalyst systems based on Cr-containing compounds, such as the combination of chromium (III) acetylacetonate and triethylalumi- num are known. However, they have very low catalytic activity and poor stereoselectivity for polymerizing conjugated dienes. The product mixture often contains oligomers, low molecular weight liquid polymers or partially crosslinked polymers, all of which reduce the industrial utility of the system.

For example, known is a process for polymerizing 1,3-butadiene into amorphous 1,2-polybutadiene by using a catalyst system including a soluble Cr (ill) compound, a trialkylaluminum compound, and a dialkyl hydrogen phosphite. The resulting polymer product has an extremely high molecular weight and is partially a gel. Also known is a method for preparing elasto- mers by polymerizing trans-1,3-pentadiene and isoprene in the presence of a catalyst system including a soluble Cr compound, a trialkylaluminum compound, and a dihydrocarbyl hydrogen phosphite.

Developing a catalyst composition with high activity and stereo- selectivity for polymerizing 1,3-butadiene into s-PBD remains desirable.

SUMMARY OF THE INVENTION The present invention provides a catalyst composition that is the combination of, is the reaction product of, or is formed by combining ingredients that include a Cr-containing compound, an organomagnesium compound, and a silyl phosphonate.

The present invention further includes a process for forming conjugated diene polymers that includes polymerizing conjugated diene monomers in the presence of a catalytically effective amount of this catalyst composition.

Advantageously, the present catalyst composition does not contain CS2. Therefore, the toxicity, objectionable smell, dangers, and expense associated with the use of carbon disulfide are eliminated. In addition, the Cr-containing compounds are generally stable, inexpensive, and readily available. Further, the catalyst composition has high activity in a wide variety of solvents including environmentally preferred non-halogenated solvents such as aliphatic and cycloaliphatic hydrocarbons. Furthermore, s- PBD produced with this catalyst composition has a higher melting tempera- ture and higher syndiotacticity than the s-PBD produced with Cr-based catalyst systems of the prior art.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The catalyst composition is formed by combining a Cr-containing compound, an organomagnesium compound, and a silyl phosphonate. In addition to these three ingredients, other organometallic compounds or Lewis bases also can be added, if desired.

Various Cr-containing compounds or mixtures thereof can be employed in the catalyst composition. Preferably, these Cr-containing compounds are soluble in a hydrocarbon solvent such as aromatic hydrocarbons, aliphatic hydrocarbons, or cycloaliphatic hydrocarbons.

Hydrocarbon-insoluble Cr-containing compounds, however, can be suspended in the polymerization medium to form a catalytically active species and are therefore also useful.

The Cr atom in the Cr-containing compounds can be in various oxidation states including, but not limited to, the 0, +2, +3, +4, and +6 oxidation states. Chromous compounds, where the Cr atom is in the +2 oxidation state, and chromic compounds, where the Cr atom is in the +3 oxidation state, are preferred. Suitable Cr-containing compounds include, but are not limited to, the following classes of compounds: carboxylates, organophosphates, organophosphonates, organophosphinates, carbamates, dithiocarbamates, xanthates, (3-diketonates, alkoxide, aryloxides, halides, pseudo-halides, oxyhalides, and organochromium compounds.

Suitable chromium carboxylates, where the oxidation state of the associated Cr can be either +2 or +3, include formate, acetate, (meth) acrylate, valerate, gluconate, citrate, fumarate, lactate, maleate, oxalate, 2-ethylhexanoate, neodecanoate, naphthenate, stearate, oleate, benzoate, and picolinate.

Suitable chromium organophosphates, where the oxidation state of the associated Cr can be either +2 or +3, include dibutyl phosphate, dipentyl phosphate, dihexyl phosphate, diheptyl phosphate, dioctyl phosphate, bis (1- 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-nonyl- phenyl) phosphate.

Suitable chromium organophosphonates, where the oxidation state of the associated Cr can be either +2 or +3, include butyl phosphonate, pentyl phosphonate, hexyl phosphonate, heptyl phosphonate, octyl phosphonate, (1-methylheptyl) phosphonate, (2-ethylhexyl) phosphonate, decyl phospho- nate, dodecyl phosphonate, octadecyl phosphonate, oleyl phosphonate, phenyl phosphonate, (p-nonylphenyl) phosphonate, butyl butylphosphonate, pentyl pentylphosphonate, hexyl hexylphosphonate, heptyl heptylphospho- nate, octyl octylphosphonate, (1-methylheptyl) (1-methylheptyl) phosphonate, (2-ethylhexyl) (2-ethylhexyl) phosphonate, decyl decylphosphonate, dodecyl dodecylphosphonate, octadecyl octadecylphosphonate, oleyl oleylphospho- nate, phenyl phenylphosphonate, (p-nonylphenyl) (p-nonylphenyl) phospho- nate, butyl (2-ethylhexyl) phosphonate, (2-ethylhexyl) butylphosphonate, (1- methylheptyl) (2-ethylhexyl) phosphonate, (2-ethylhexyl) (1-methylheptyl)- phosphonate, (2-ethylhexyl) (p-nonylphenyl) phosphonate, and (p-nonyl- phenyl) (2-ethylhexyl) phosphonate.

Suitable chromium organophosphinates, where the oxidation state of the associated Cr can be either +2 or +3, include butylphosphinate, pentyl- phosphinate, hexylphosphinate, heptylphosphinate, octylphosphinate, (1- methylheptyl) phosphinate, (2-ethylhexyl) phosphinate, decylphosphinate,

dodecylphosphinate, octadecylphosphinate, oleylphosphinate, phenyl- phosphinate, (p-nonylphenyl) phosphinate, dibutylphosphinate, dipentyl- phosphinate, dihexylphosphinate, diheptylphosphinate, dioctylphosphinate, bis (1-methylheptyl) phosphinate, bis (2-ethylhexyl) phosphinate, didecyl- phosphinate, didodecylphosphinate, dioctadecylphosphinate, dioleylphos- phinate, diphenylphosphinate, bis (p-nonylphenyl) phosphinate, butyl (2- ethylhexyl) phosphinate, (1-methylheptyl) (2-ethylhexyl) phosphinate, and (2- ethylhexyl) (p-nonylphenyl) phosphinate.

Suitable chromium (thio) carbamates, where the oxidation state of the associated Cr can be either +2 or +3, include dimethyl (dithio) carbamate, diethyl (dithio) carbamate, diisopropyl (dithio) carbamate, dibutyl (dithio)- carbamate, and dibenzyl (dithio) carbamate.

Suitable chromium xanthates, where the oxidation state of the associated Cr can be either +2 or +3, include methylxanthate, ethylxanthate, isopropylxanthate, butylxanthate, and benzylxanthate.

Suitable chromium ß-diketonates, where the oxidation state of the associated Cr can be either +2 or +3, include acetylacetonate, trifluor- acetylacetonate, hexafluoroacetylacetonate, benzoylacetonate, and 2,2,6,6- tetramethyl-3, 5-heptanedionate; also suitable are chromium (VI) dioxide bis (acetylacetonate), chromium (VI) dioxide bis (trifluoroacetylacetonate), chromium (VI) dioxide bis (hexafluoroacetylacetonate), chromium dioxide (VI) bis (benzoylacetonate), and chromium (VI) dioxide bis (2,2,6,6-tetramethyl- 3,5-heptanedionate).

Suitable chromium alkoxide or aryloxides, where the oxidation state of the associated Cr can be either +2 or +3, include methoxide, ethoxide, iso- propoxide, 2-ethylhexoxide, phenoxide, nonylphenoxide, and naphthoxide.

Suitable chromium halides include chromium (VI) hexafluoride, chromium (V) pentafluoride, chromium (IV) tetrafluoride, chromium (lif) trifluoride, chromium (V) pentachloride, chromium (IV) tetrachloride, chromium (III) trichloride, chromium (IV) tetrabromide, chromium (lil) tribromide, chromium (III) triiodide, and chromium (li) diiodide.

Suitable chromium pseudo-halides, where the oxidation state of the associated Cr can be either +2 or +3, include cyanide, (thio) cyanate, and azide.

Suitable chromium oxyhalides include chromium (VI) oxytetrafluoride, chromium (VI) dioxydifluoride, chromium (VI) oxytetrachloride, chromium (V) oxytrichloride, chromium (VI) dioxydichloride, chromium (V) oxytribromide, and chromium (VI) dioxydibromide.

The term"organochromium compound"refers to any chromium compound containing at least one covalent Cr-C bond. Suitable organo- chromium compounds include tris (allyl) chromium (ill), tris (methallyl)- chromium (lit), tris (crotyl) chromium (lil), bis (cyclopentadienyl) chromium (li), bis (pentamethylcyclopentadienyl) chromium (Il), bis (benzene) chromium (0), bis (ethylbenzene) chromium (0), bis (mesitylene) chromium (0), bis (penta- dienyl) chromium (lI), bis (2,4-dimethylpentadienyl) chromium (li), bis (allyl) tri- carbonylchromium (ll), (cyclopentadienyl) (pentadienyl) chromium (li), tetra (1- norbornyl) chromium (IV), (trimethylenemethane) tetracarbonylchromium (II), bis (butadiene) dicarbonylchromium (0), (butadiene) tetracarbonylchromium (0), and bis (cyclooctatetraene) chromium (0).

Various organomagnesium compounds or mixtures thereof can be used in the catalyst composition. The term"organomagnesium compound" refers to any magnesium compound containing at least one covalent Mg-C bond. Organomagnesium compounds that are soluble in a hydrocarbon solvent are preferred.

A preferred class of organomagnesium compounds is represented by the general formula MgR12 where each R'independently is a mono-valent organic group attached to the Mg atom via a C atom. Preferably, each R1 is a hydrocarbyl group such as, e. g., alkyl, cycloalkyl, substituted cycloalkyl, alkenyl, cycloalkenyl, substituted cycloalkenyl, aryl, substituted aryl, aralkyl, alkaryl, allyl, and alkynyl groups, with each group preferably containing from 1 C atom (or the appropriate minimum number of C atoms to form the group in question) up to about 20 C atoms. These hydrocarbyl groups may contain heteroatoms such as, but not limited to, N, O, Si, S, and P.

Specific examples of organomagnesium compounds represented by the general formula MgR12 include dimethylmagnesium, diethylmagnesium, di-n-propylmagnesium, diisopropylmagnesium, di-n-butylmagnesium, di-sec- butylmagnesium, diisobutylmagnesium, di-t-butylmagnesium, di-n-hexylmag- nesium, di-n-octylmagnesium, diphenylmagnesium, di-p-tolylmagnesium, and dibenzylmagnesium. Commercial dibutylmagnesium is particularly useful on the grounds of its availability and its solubility in (cyclo) aliphatic hydrocarbon solvents. Commercial dibutylmagnesium is actually an organometallic oligomer and is a mixture of n-butyl, sec-butyl, and n-octyl groups bonded to the Mg atom.

Another class of organomagnesium compounds is represented by the general formula R2MgX where R2 is a mono-valent organic group attached to the Mg atom via a C atom and X is a H atom, a halogen atom, a carboxylate group, an alkoxide group, or an aryloxide group. Preferably, R2 is a hydro- carbyl group such as, e. g., alkyl, cycloalkyl, substituted cycloalkyl, alkenyl, cycloalkenyl, substituted cycloalkenyl, aryl, substituted aryl, aralkyl, alkaryl, allyl, and alkynyl groups, with each group preferably containing from 1 C atom (or the appropriate minimum number of C atoms to form the group in question) up to about 20 C atoms. These hydrocarbyl groups may contain heteroatoms such as, but not limited to, N, O, Si, S, and P. Preferably, X is a carboxylate group, an alkoxide group, or an aryloxide group, with each group preferably containing 1 to 20 C atoms.

Suitable organomagnesium compounds represented by the general formula R2MgX include, but are not limited to, hydrocarbylmagnesium hydride, hydrocarbylmagnesium halide, hydrocarbylmagnesium carboxylate, hydrocarbylmagnesium alkoxide, and hydrocarbylmagnesium aryloxide.

Specific examples of organomagnesium compounds represented by the general formula R2MgX include methylmagnesium hydride, ethylmag- nesium hydride, butylmagnesium hydride, hexylmagnesium hydride, phenyl- magnesium hydride, benzylmagnesium hydride, methylmagnesium chloride, ethylmagnesium chloride, butylmagnesium chloride, hexylmagnesium chloride, phenylmagnesium chloride, benzylmagnesium chloride, methyl-

magnesium bromide, ethylmagnesium bromide, butylmagnesium bromide, hexylmagnesium bromide, phenylmagnesium bromide, benzylmagnesium bromide, methylmagnesium hexanoate, ethylmagnesium hexanoate, butyl- magnesium hexanoate, hexylmagnesium hexanoate, phenylmagnesium hexanoate, benzylmagnesium hexanoate, methylmagnesium ethoxide, ethyl- magnesium ethoxide, butylmagnesium ethoxide, hexylmagnesium ethoxide, phenylmagnesium ethoxide, benzylmagnesium ethoxide, methylmagnesium phenoxide, ethylmagnesium phenoxide, butylmagnesium phenoxide, hexyl- magnesium phenoxide, phenylmagnesium phenoxide, benzylmagnesium phenoxide, and the like, and mixtures thereof.

Useful silyl phosphonate compounds that can be used in the catalyst composition include acyclic silyl phosphonates, cyclic silyl phosphonates, and mixtures thereof. Acyclic silyl phosphonates may be represented by the following structure: where each R3 independently is a H atom or a mono-valent organic group.

Preferably, each R3 is a hydrocarbyl group such as, e. g., alkyl, cycloalkyl, substituted cycloalkyl, alkenyl, cycloalkenyl, substituted cycloalkenyl, aryl, substituted aryl, aralkyl, alkaryl, allyl, and alkynyl groups, with each group preferably containing from 1 C atom (or the appropriate minimum number of C atoms to form the group in question) up to about 20 C atoms. These hydrocarbyl groups may contain heteroatoms such as, e. g., N, O, Si, S, and P. The acyclic silyl phosphonates may be associated in dimeric, trimeric or oligomeric forms by hydrogen bonding.

Suitable acyclic silyl phosphonates include one or more of bis (tri- methylsilyl) phosphonate, bis (dimethylsilyl) phosphonate, bis (triethylsilyl) phosphonate, bis (diethylsilyl) phosphonate, bis (tri-n-propylsilyl) phospho- nate, bis (di-n-propylsilyl) phosphonate, bis (triisopropylsilyl) phosphonate, bis (diisopropylsilyl) phosphonate, bis (tri-n-butylsilyl) phosphonate, bis (di-n- butylsilyl) phosphonate, bis (triisobutylsilyl) phosphonate, bis (diisobutylsilyl) phosphonate, bis (tri-t-butylsilyl) phosphonate, bis (di-t-butylsilyl) phospho- nate, bis (trihexylsilyl) phosphonate, bis (dihexylsilyl) phosphonate, bis (tri- octylsilyl) phosphonate, bis (dioctylsilyl) phosphonate, bis (tricyclohexylsilyl) phosphonate, bis (dicyclohexylsilyl) phosphonate, bis (triphenylsilyl) phospho- nate, bis (diphenylsilyl) phosphonate, bis (tri-p-tolylsilyl) phosphonate, bis (di- p-tolylsilyl) phosphonate, bis (tribenzylsilyl) phosphonate, bis (dibenzylsilyl) phosphonate, bis (methyldiethylsilyl) phosphonate, bis (methyidi-n-propylsilyl) phosphonate, bis (methyldiisopropylsilyl) phosphonate, bis (methyidi-n-butyl- silyl) phosphonate, bis (methyidiisobutylsilyl) phosphonate, bis (methyldi-t- butylsilyl) phosphonate, bis (methyidiphenytsityt) phosphonate, bis (dimethyl- ethylsilyl) phosphonate, bis (dimethyl-n-propylsilyl) phosphonate, bis (di- methylisopropylsilyl) phosphonate, bis (dimethyl-n-butylsilyl) phosphonate, bis (dimethylisobutylsilyl) phosphonate, bis (dimethyl-t-butylsilyl) phospho- nate, bis (dimethylphenylsilyl) phosphonate, bis (t-butyldiphenylsilyl) phospho- nate, bis [tris (2-ethylhexyl) silyl] phosphonate, bis [bis (2-ethylhexyl) silyl] phos- phonate, bis [tris (nonylphenyl) silyl] phosphonate, bis [tris (2,4,6-trimethyl- phenyl) silyl] phosphonate, bis [bis (2,4,6-trimethylphenyl) silyl] phosphonate, bis [tris (4-fluorophenyl) silyl] phosphonate, bis [bis (4-fluorophenyl) silyl] phos- phonate, bis [tris (pentafluorophenyl) silyl] phosphonate, bis [tris (trifluorometh- yl) silyl] phosphonate, bis [tris (2,2,2-trifluoroethyl) silyl] phosphonate, bis [tris- (trimethylsilyl) silyl] phosphonate, bis [tris (trimethylsilylmethyl) silyl] phospho- nate, bis [tris (dimethylsilyl) silyl] phosphonate, bis [tris (2-butoxyethyl) silyl] phosphonate, bis (trimethoxysilyl) phosphonate, bis (triethoxysilyl) phospho- nate, bis (triphenoxysilyl) phosphonate, bis [tris (trimethylsilyloxy) silyl] phos- phonate, bis [tris (dimethylsilyloxy) silyl] phosphonate, or mixtures thereof.

Cyclic silyl phosphonates contain a ring structure formed by joining two Si atoms together or by bridging the two Si atoms with one or more divalent organic groups. These cyclic silyl phosphonates may be represented by the following structure: where each R4 independently is a H atom or a mono-valent organic group and R5 is a bond between the Si atoms or a divalent organic group. Bicyclic silyl phosphonates may be formed by joining two R4 groups, and therefore the term cyclic silyl phosphonate includes multi-cyclic silyl phosphonates.

Preferably, each R4 is a hydrocarbyl group such as, e. g., alkyl, cycloalkyl, substituted cycloalkyl, alkenyl, cycloalkenyl, substituted cycloalkenyl, aryl, substituted aryl, aralkyl, alkaryl, allyl, and alkynyl groups, with each group preferably containing from 1 C atom (or the appropriate minimum number of C atoms to form the group in question) up to about 20 C atoms. These hydrocarbyl groups may contain heteroatoms such as, e. g., N, O, Si, S, and P. Preferably, R5 is a hydrocarbylene group such as, e. g., alkylen, substi- tuted alkylen, cycloalkylene, substituted cycloalkylene, alkenylene, substi- tuted alkenylene, cycloalkenylene, substituted cycloalkenylene, arylen, 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 in question) up to about 20 C atoms. These hydrocarbylene groups may contain heteroatoms such as, e. g., N, 0, Si, S, and P. The cyclic silyl phosphonates may be associated in dimeric, trimeric or oligomeric forms by hydrogen bonding.

Suitable cyclic silyl phosphonates are 2-oxo- (2H)-4, 5-disila-1, 3,2- dioxaphospholane, 2-oxo- (2H)-4, 5-disila-4, 4,5,5-tetramethyl-1,3,2-dioxa- phospholane, 2-oxo- (2H)-4, 5-disila-4, 4,5,5-tetraethyl-1,3,2-dioxaphos- pholane, 2-oxo- (2H)-4, 5-disila-4, 4,5,5-tetraphenyl-1,3,2-dioxaphospholane, 2-oxo- (2H)-4, 5-disila-4, 4,5,5-tetrabenzyl-1,3,2-dioxaphospholane, 2-oxo- (2H)-4,5-disila-4,5-dimethyl-1,3,2-dioxaphospholane, 2-oxo- (2H)-4, 5-disila- 4,5-diethyl-1,3,2-dioxaphospholane, 2-oxo-(2H)-4, 5-disila-4, 5-diphenyl-1,3,2- dioxaphospholane, 2-oxo- (2H)-4, 5-disila-4, 5-dibenzyl-1, 3,2-dioxaphos- pholane, 2-oxo- (2H)-4. 5-disila-4-methyl-1, 3,2-dioxaphospholane, 2-oxo- (2H)-4,6-disila-1,3,2-dioxaphosphorinane, 2-oxo- (2H)-4, 6-disila-4, 4,6,6- tetramethyl-1, 3,2-dioxaphosphorinane, 2-oxo- (2H)-4, 6-disila-4, 4,6,6-tetra- ethyl-1, 3,2-dioxaphosphorinane, 2-oxo- (2H)-4, 6-disila-4, 4,6,6-tetraphenyl- 1,3,2-dioxaphosphorinane, 2-oxo- (2H)-4, 6-disila-4, 4,6,6-tetrabenzyl-1,3,2- dioxaphosphorinane, 2-oxo- (2H)-4, 6-disila-4, 6-dimethyl-1, 3,2-dioxaphos- phorinane, 2-oxo- (2H)-4, 6-disila-4, 6-diethyl-1, 3,2-dioxaphosphorinane, 2- oxo- (2H)-4, 6-disila-4, 6-diphenyl-1, 3,2-dioxaphosphorinane, 2-oxo- (2H)-4, 6- disila-4, 6-dibenzyl-1, 3,2-dioxaphosphorinane, 2-oxo- (2H)-4, 6-disila-5, 5- dimethyl-1, 3,2-dioxaphosphorinane, 2-oxo-(2H)-4, 6-disila-5, 5-diethyl-1, 3,2- dioxaphosphorinane, 2-oxo- (2H)-4, 6-disila-5, 5-diphenyl-1, 3,2-dioxaphos- phorinane, 2-oxo- (2H)-4, 6-disila-5, 5-dibenzyl-1, 3,2-dioxaphosphorinane, 2- oxo- (2H)-4, 6-disila-5-ethyl-5-methyl-1, 3,2-dioxaphosphorinane, 2-oxo- (2H)- 4,6-disila-5-methyl-5-propyl-1,3,2-dioxaphosphorinane, 2-oxo- (2H)-4, 6- disila-5-butyl-5-ethyl-1, 3,2-dioxaphosphorinane, 2-oxo- (2H)-4, 6-disila-4- isopropyl-5, 5-dimethyl-1, 3,2-dioxaphosphorinane, 2-oxo- (2H)-4, 6-disila-4- propyl-5-ethyl-1, 3,2-dioxaphosphorinane, 2-oxo- (2H)-4, 6-disila-4-methyl- 1,3,2-dioxaphosphorinane, or mixtures thereof.

The present catalyst composition has very high catalytic activity for polymerizing conjugated dienes, such as 1,3-butadiene, into polymers, such as s-PBD, over a wide range of catalyst concentrations and catalyst ingredient ratios. The polymers having the most desirable properties, however, are obtained within a narrower range of catalyst concentrations and catalyst ingredient ratios. Further, it is believe that the three catalyst

ingredients may interact to form an active catalyst species. Accordingly, the optimum concentration for any one catalyst ingredient depends on the concentrations of the other catalyst ingredients. The molar ratio of the organomagnesium compound to the Cr-containing compound (Mg/Cr) can be varied from about 1: 1 to about 50: 1, more preferably from about 2: 1 to about 30: 1, and even more preferably from about 3: 1 to about 20: 1. The molar ratio of the silyl phosphonate to the Cr-containing compound (P/Cr) 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.

The catalyst composition is formed by combining or mixing the three ingredients. Although an active catalyst species is believed to result from this combination, the degree of interaction or reaction between the various ingredients is not known with any certainty. Therefore, the term"catalyst composition"encompasses 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 may be formed in situ by adding the three ingredients to a solution containing monomer and solvent, or simply bulk monomer, in either a stepwise or simultaneous manner. When adding the ingredients in a stepwise manner, the sequence in which the ingredients are added is not critical. Preferably, however, the organomagnesium compound is added first, followed by the Cr-containing compound and then the silyl phosphonate.

The ingredients may be pre-mixed outside the polymerization system at an appropriate temperature, which is generally from about-20° to about 80°C, with the resulting catalyst composition added to the monomer solution.

The catalyst composition may be pre-formed in the presence of monomer such as 1,3-butadiene. That is, the three ingredients are pre- mixed in the presence of a small amount of 1,3-butadiene monomer at an appropriate temperature, e. g., from about-20° to about 80°C. The amount of 1,3-butadiene monomer, per mole of the Cr-containing compound, used

for pre-forming the catalyst can range from about 1 to about 500 moles, more preferably from about 5 to about 250 moles, and even more preferably from about 10 to about 100 moles. The resulting catalyst composition is added to the remainder of the 1,3-butadiene monomer to be polymerized.

The catalyst composition may be formed by using a 2-stage procedure. The first stage involves reacting the Cr-containing compound with the organomagnesium compound in the presence of a small amount of 1,3-butadiene monomer at an appropriate temperature, e. g., from about-20° to about 80°C. In the second stage, the foregoing reaction mixture and the silyl phosphonate are charged in either a stepwise or simultaneous manner to the remainder of the 1,3-butadiene monomer to be polymerized.

An alternative 2-stage procedure may also be employed. A Cr-ligand complex is first formed by pre-combining the Cr-containing compound with the silyl phosphonate. Once formed, this Cr-ligand complex is then combined with the organomagnesium compound to form the active catalyst species. The Cr-ligand complex can be formed separately or in the presence of the 1,3-butadiene monomer to be polymerized. This complex- ation can be conducted at any convenient temperature at normal pressure but, for an increased rate of reaction, this reaction preferably occurs at room temperature or above. The time required for the formation of the Cr-ligand complex is usually within the range of about 10 to about 120 minutes after mixing the Cr-containing compound with the silyl phosphonate. The tem- perature and time used for the formation of the Cr-ligand complex depends on several variables including the particular starting materials and solvent (s) employed. Once formed, the Cr-ligand complex can be used without isolation from the complexation mixture. If desired, however, the Cr-ligand complex may be isolated from the complexation mixture before use.

When a solution of the 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 preferably is employed. The organic solvent 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. Desirably, an organic solvent that is inert with respect to the catalyst composition is used. 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 hydrocarbon solvents include n-pentane, n-hexane, n-heptane, n-octane, n-nonane, n-decane, isopentane, iso- hexanes, isopentanes, isooctanes, 2,2-dimethylbutane, petroleum ether, kerosene, petroleum spirits, and the like. Non-limiting examples of cyclo- aliphatic 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 preferred.

The catalyst composition exhibits very high catalytic activity for the polymerization of 1,3-butadiene into s-PBD although other conjugated dienes also can be polymerized with the Cr-based catalyst composition.

Examples of conjugated dienes that can be polymerized include 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 copolymerization.

The production of s-PBD is accomplished by polymerizing 1,3- butadiene in the presence of a catalytically effective amount of the catalyst composition. 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, a 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 Cr-containing compound used, per 100 g 1,3-butadiene monomer, can be varied from about 0.01 to about 2

mmol, more preferably from about 0.05 to about 1.0 mmol, and even more preferably from about 0.1 to about 0.5 mmol.

The polymerization of 1,3-butadiene is preferably carried out in an organic solvent as the diluent. Accordingly, a solution polymerization system may be employed in which both the 1,3-butadiene monomer 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 organic solvent in addition to the amount of organic solvent that may be used in preparing the catalyst composition is usually added to the polymeri- zation system. The additional organic solvent may be the same as or different from the organic solvent used in preparing the catalyst composition.

Desirably, an organic solvent that is inert with respect to the catalyst compo- sition employed is selected. Exemplary hydrocarbon solvents have been set forth above.

The concentration of 1,3-butadiene monomer to be polymerized is not limited to a special range. Preferably, however, the concentration of the 1,3- butadiene monomer present at the beginning of the polymerization should be 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.

The polymerization of 1,3-butadiene may also be carried out by means of bulk polymerization, which refers to a polymerization environment where no solvents are employed. The bulk polymerization can be conducted either in a condensed liquid phase or in a gas phase.

In performing the polymerization of 1,3-butadiene, a molecular weight regulator may be employed to control the molecular weight of the s-PBD. 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 s-PBD having a wide range of molecular weights. Suitable molecular weight regulators include, but are not limited to, a-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; non-conjugated diolefins such as 1,6-octadiene, 5-methyl- 1,4-hexadiene, 1,5-cyclooctadiene, 3,7-dimethyl-1,6-octadiene, 1,4-cyclo- hexadiene, 4-vinylcyclohexene, 1,4-pentadiene, 1,4-hexadiene, 1,5-hexa- diene, 1,6-heptadiene, 1,2-divinylcyclohexane, 5-ethylidene-2-norbornene, 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 1,3-butadiene monomer (phm), is from about 0.01 to about 10 phm, prefer- ably from about 0.02 to about 2 phm, and more preferably from about 0.05 to about 1 phm.

The molecular weight of the resulting s-PBD can also be effectively controlled by conducting the polymerization of 1,3-butadiene monomer in the presence of H2. In this case, the partial pressure of H2 is preferably from about 1 to about 5000 kPa.

The polymerization of 1,3-butadiene may be carried out as a batch, continuous, or semi-continuous process. In the semi-continuous process, 1,3-butadiene monomer is intermittently charged as needed to replace that monomer already polymerized. In any case, the polymerization is preferably conducted under anaerobic conditions by using an inert protective gas such as N2, Ar or He, with moderate to vigorous agitation. The polymerization temperature may vary widely from a low temperature (e. g.,-10°C or below) to a high temperature (e. g., 100°C or above) with a preferred temperature range being from about 20° to about 90°C. The heat of polymerization may be removed by external cooling, cooling by evaporation of the 1,3-butadiene 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 100 to about 1000 kPa.

Once a desired conversion is achieved, the polymerization can be stopped by adding 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-tert-butyl-4-methylphenol may be added along with, before or after the addition of the terminator. The amount of antioxidant employed is usually in the range of 0.2 to 1% by weight of the polymer product. When the polymerization has been stopped, the s-PBD can be recovered from the polymerization mixture by utilizing conventional procedures of desolventization and drying. For instance, the s- PBD may be isolated from the polymerization mixture by coagulation of the polymerization mixture with an alcohol such as methanol, ethanol, or iso- propanol, or by steam distillation of the solvent and the unreacted 1,3- butadiene, followed by filtration. The polymer product is dried to remove residual amounts of solvent and water.

The s-PBD produced has many uses. It can be blended into and co- cured with various natural or synthetic rubbers to improve the properties thereof. For example, it can be incorporated into elastomers to improve the green strength of 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 the s-PBD into rubber compositions utilized in the supporting carcass has particular utility in preventing or minimizing this distortion. In addition, incorporation of s-PBD into tire tread compositions can reduce the heat build-up and improve the tear and wear resistance thereof. It also is useful in the manufacture of films and packaging materials and in many molding applications.

To demonstrate the practice of the present invention, the following examples have been prepared and tested. The examples should not, however, be viewed as limiting the scope of the invention.

EXAMPLES Example 1 Bis (trimethylsilyl) phosphonate was synthesized by reacting anhydrous H3PO3 with hexamethyldisiloxane in the presence of anhydrous ZnCI2 as catalyst. Anhydrous H3PO3 (33.1 g, 0.404 mol), hexamethyl- disiloxane (98.4 g, 0.606 mol), anhydrous ZnCI2 (2.0 g, 0.015 mol), and

benzene (240 mL) were mixed in a round-bottom reaction flask connected to a Dean-Stark trap and a reflux condenser. The mixture was heated to reflux for 24 hours with continuous removal of water via the trap.

The reaction flask was then connected to a distillation head and a receiving flask. The benzene and unreacted hexamethyldisiloxane were removed by distillation at atmospheric pressure.

The remaining crude product was distilled under vacuum, yielding bis (trimethylsilyl) phosphonate as a colorless liquid (51.7 g, 0.228 mol, 57% yield). The identity of the product was established by NMR spectroscopy.

'H NMR data (CDC13, 25°C, referenced to (CH3) 4Si) :. 6.85 (doublet, 1JHp = 699 Hz, 1 H, H-P), 0.31 (singlet, 18 H, CH3). 13p NMR data (CDC13, 25°C, referenced to external 85% H3PO4).-14. 0 (doublet, 1JHp = 698 Hz).

Example 2 Bis (triethylsilyl) phosphonate was synthesized by reacting anhydrous H3PO3 with hexaethyldisiloxane in the presence of anhydrous ZnCI2 as catalyst. Anhydrous H3PO3 (22.1 g, 0.269 mol), hexaethyldisiloxane (99.5 g, 0.404 mol), anhydrous ZnCI2 (1.33 g, 0.010 mol), and toluene (230 mL) were mixed in a round-bottom reaction flask connected to a Dean-Stark trap and a reflux condenser. The mixture was heated to reflux for 29 hours, with continuous removal of water via the trap.

The reaction flask was connected to a distillation head and a receiving flask. The toluene and unreacted hexaethyldisiloxane were removed by distillation at atmospheric pressure.

The remaining crude product was distilled under vacuum, yielding bis (triethylsilyl) phosphonate as a colorless liquid (67.9 g, 0.219 mol, 81 % yield). The identity of the product was established by NMR spectroscopy.

1H NMR data (CDC13, 25°C, referenced to (CH3) 4Si) :. 6.92 (doublet, 1JHp = 695 Hz, 1 H, H-P), 1.01 (triplet, 3JHH = 7.4,18 H, CH3), 0.76 (quartet, 3JHH =

7.4,12 H, CH2). 3P NMR data (CDC13, 25°C, referenced to external 85% H3 PO4).-14. 5 (doublet, I JHP = 695 Hz).

Example 3 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 N2, the bottle was charged with 236 g of a 1,3-butadiene/ hexanes blend containing 21.2% by weight of 1,3-butadiene. Catalyst ingredients were added to the bottle in the following order: (1) 0.90 mmol dibutylmagnesium, (2) 0.15 mmol chromium (lil) 2-ethylhexanoate, and (3) 0.60 mmol bis (trimethylsilyl) phosphonate. The bottle was tumbled for 15 hours in a water bath maintained at 50°C.

The polymerization was terminated by addition of 10 mL isopropanol containing 1.0 g 2,6-di-tert-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.

The yield of the polymer was 40.6 g (81 % yield).

As measured by DSC, the polymer had a melting temperature of 150°C. The 1H and 13C NMR analysis of the polymer indicated a 1,2-linkage content of 78% and a syndiotacticity of 79%.

Example 4 The procedure described in Example 3 was repeated except that bis (triethylsilyl) phosphonate was substituted for bis (trimethylsilyl) phosphonate.

Yield of the polymer was 43.3 g (87% yield).

As measured by DSC, the polymer had a melting temperature of 146°C. The 1H and 13C NMR analysis of the polymer indicated a 1,2-linkage content of 82% and a syndiotacticity of 71 %.

Comparative Example 5 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 N2 gas, the bottle was charged with 255 g hexanes and 236 g of a 1,3-butadiene/hexanes blend containing 21.2% by weight of 1,3- butadiene. Catalyst ingredients were added to the bottle in the following order: (1) 0.75 mmol triethylaluminum, (2) 0.050 mmol chromium (ill) 2-ethyl- hexanoate, and (3) 0.33 mmol dineopentyl hydrogen phosphite. The bottle was tumbled for 4 hours in a water bath maintained at 50°C.

The polymerization was terminated by addition of 10 mL isopropanol containing 1.0 g 2,6-di-tert-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.

The yield of the polymer was 48.0 g (96% yield).

As measured by DSC, the polymer had a melting temperature of 97°C. The 1H and 13C NMR analysis of the polymer indicated a 1,2-linkage content of 81 % and a syndiotacticity of 66%.

Comparison of the analytical data of the s-PBD products obtained in Examples 3 and 4 with the analytical data of the s-PBD product obtained in Comparative Example 5 indicates that the Cr-based catalyst composition of the present invention produces s-PBD of higher quality as shown by the significantly higher melting temperature and higher syndiotacticity than are obtained with the Cr-containing catalyst systems of the prior art.