Butadiene Polymerization Catalyst

1. Field of the Invention

This invention relates to a catalyst system and a process for using same, useful for the stereospecific polymerization of butadiene. More specifically, the catalyst comprises an organoaluminum compound, a Lewis base, and a Ti(+4) halogen - M.sub.n I.sub.m composition. This catalyst system is useful in the production of polybutadiene homopolymers or copolymers in which the ratio of trans-1,4 to cis-1,4 units can be varied over wide limits and especially within the particularly desirable trans-1,4 range of 50 to 90% of total unsaturation.

2. Description of the Prior Art

Ziegler-type multi-component catalyst systems such as transition metal halides combined with organo metallic compounds have been known for well over a decade. The ability of such catalysts to polymerize butadiene to elastomeric products has also been recognized, but the polymers thus produced have generally been characterized by high cis-1,4 content rather than trans-1,4 addition units. Thus, for example, in U.S. Pat. No. 3,471,461, there is disclosed the use of TiX.sub.3.nAlI.sub.3 -xAl(alkyl).sub.3 catalyst systems for the polymerization of butadiene, in which the steric arrangement of the polybutadiene is predominantly that resulting from cis-1,4 addition.

Other catalysts, particularly based on vanadium compounds, capable of producing polybutadienes containing more than 90% trans-1,4 addition units have also been developed, but the polymers thus produced have been characterized by resinous or plastic rather than elastomeric properties.

We had previously discovered that certain trivalent titanium halide catalyst systems, used in conjunction with particular bases, produced polybutadienes containing a predominant amount of 1,4-addition units. These catalyst systems are taught in U.S. pat. No. 3,663,450 and U.S. Pat. No. 3,779,944.

In U.S. Pat. No. 3,663,450, a partially reduced titanium halide, notably TiCl.sub.3, is precomplexed with a Lewis base and forms a final catalyst capable of producing high cis-1,4 polybutadiene, while in U.S. Pat. No. 3,779,944 the Lewis base is not precomplexed with the titanium halide component and may be added to the polymerization zone in any order with respect to the latter component. This difference is important in that it provides for the production of polybutadienes having variable amounts of trans-1,4 addition units, up to about 90% in the polymer.

It is also known that certain titanium tetrahalides are useful in polymerizing butadiene. Generally, they produce polybutadienes containing 85-95% cis-1,4 addition units. Typical of these are a series of iodine-containing catalyst systems, such as TiI.sub.4 -AlR.sub.3, TiX.sub.4 -I.sub.2 -AlR.sub.3, and TiX.sub.4 -AlI.sub.3 -AlR.sub.3, where X is chlorine or bromine. These systems have been published in several patents. See Belgian Patent 551,851; Belgian Patent 602,496; U.S. Pat. No. 3,245,976, and British Patent 1,138,840.

Previously, when Lewis bases were added to titanium tetrahalide-type catalysts in amounts necessary to effect a noticeable change in the stereochemistry of the polymerization reaction, there was always an accompanying drastic decrease in the polymerization rate and catalyst efficiency.

The inventors have now found surprisingly that the addition of certain Lewis bases to a general catalyst system comprised of an organoaluminum, having the formula AlR.sub.a X.sub.3-a, and a Ti(+4) halide - M.sub.n I.sub.m composition causes a decrease in the cis-1,4 addition of the butadiene monomer and a corresponding increase in the trans-1,4 addition without the catalyst efficiency being adversely affected to any significant extent.

The inventors have also found that the novel catalyst system is capable not only of efficiently producing polybutadienes containing larger proportions of trans-1,4 addition units than those titanium based catalysts disclosed in the prior art, but also of producing predominantly elastomeric polybutadienes containing 50-90% of such units. This is a particularly surprising and significant discovery. Such polymers of corresponding over-all compositions which have been prepared in the past with the help of other catalysts or polymerization methods have generally exhibited a considerable amount of crystallinity and plastic character. While the inventors do not wish to be bound by any particular explanation for the difference in physical properties between the 50-90% trans-1,4 polybutadienes prepared according to this invention and those of the prior art, it is believed that the much more elastomeric character of the former is the consequence of the more random distribution of the cis-1,4 and trans-1,4 units in the polymer molecules than in the polymers heretofore known.

The purpose of this invention therefore is to provide a new catalyst for controlling the steric configuration of the 1,4 addition units of polymers of conjugated dienes, and copolymers thereof, and a process for employing same to produce such polymers and in particular to a polybutadiene containing a high percentage of trans-1,4 addition units and having predominantly elastomeric properties.

In general, this invention relates to a catalyst system broadly belonging to the Ziegler group, comprising a non-reduced salt of titanium, an organometallic compound and an iodine-containing compound or iodine corresponding to the generalized formula

where the halide may be I, Cl, or Br. When the halide is I, M.sub.n I.sub.m may be omitted. M is either aluminum or tin (+4), n is 0 or 1, and m is the valence of M and when n=0, m=2. In the organoaluminum component, R is an alkyl having 1 to 12 carbon atoms, X is Cl, Br, or I and a is any number from 2 to 3.

To the basic catalyst system is added certain Lewis bases such as tetrahydrothiophene, tetrahydrothiopyran, 2-methyl tetrahydrothiophene, tetrahydrofuran, tetrahydropyran, 2,5-dimethyl tetrahydrofuran, and the like. The ratio of Lewis base to titanium in the catalyst mixture controls the steric arrangement of the monomer units, and may range from 1:1 to 500:1, depending upon the amount of trans-1,4 structural units desired in the polybutadiene.

Particular examples of the titanium halide -M.sub.n I.sub.m component are:

where X is Cl or Br

where X is Cl or Br

where X is Cl or Br

The catalyst system of this invention is principally useful for the polymerization of conjugated diolefin monomers, such as 1,3-butadiene. However, the catalysts are also useful for forming copolymers of conjugated dienes with other olefin monomers.

The Lewis bases that may be used are heterocyclic thia, aza and oxa compounds, specifically cyclic thioethers, cyclic ethers, and their derivatives. Such compounds include tetrahydrothiophene, tetrahydrothiopyran, tetrahydrofuran, tetrahydropyran, 2,5-dimethyl tetrahydrofuran, 3-phenyl tetrahydrofuran, 3-ethyl-4-propyl tetrahydrofuran, 2,5-dimethyl-3-chloro tetrahydrofuran, 2-methyl tetrahydrothiophene, 3-phenyl tetrahydrothiophene, 3-ethyl-4-propyl tetrahydrothiophene and 2,5-dimethyl-3-chloro tetrahydrothiopyran. Particularly preferred among these compounds is tetrahydrothiophene; however, other cyclic thioethers may also be advantageously employed.

As a general rule, the higher the concentration of the Lewis base in the polymerization zone, the higher will be the trans-1,4 content of the polymer until a certain upper limit has been reached. Thus, from 50-90% of the resulting polybutadiene may be of the trans-1,4 configuration.

The organoaluminum compounds that can be advantageously used for making the catalysts of this invention are trialkylaluminums, such as trimethylaluminum, triethylaluminum, triisobutylaluminum, trihexylaluminum, triisoprenylaluminum, etc. Mixtures of trialkylaluminums and dialkylaluminum halides and alkoxides may also be successfully employed. Among suitable dialkylaluminum compounds to be used in conjunction with trialkylaluminums may be mentioned dialkylaluminum halides, particularly dialkylaluminum iodides, dialkylaluminum alkoxides, etc.

The hydrocarbon diluents used in making the polybutadiene of the present invention should be liquids at the conditions of temperature and pressure used in the polymerization reaction. Suitable diluents include C.sub.4 to C.sub.10 saturated aliphatic or cycloaliphatic hydrocarbons, such as n-butene, n-pentane, n-heptane, isooctane, n-decane, cyclohexane, methylcyclohexane, etc. and aromatic compounds such as benzene, toluene, xylene, tetralin isopropyl benzene, etc. In a batch polymerization, the diluent can be added to the reaction zone either prior to, with, or subsequent to the monomer. The same holds true for the aluminum alkyl component.

However, when AlI.sub.3 or SnI.sub.4 is used as a component of the catalyst (species IV and V, respectively), they are normally added prior to addition of monomer and aluminum alkyls.

The Lewis base may be added to the polymerization zone in virtually any sequence with the other ingredients, but it is generally preferably to add at last the Ti(halide).sub.4 and monomer before adding the Lewis base. However, in a continuous process, the base would normally be added with either the monomer or the aluminum alkyl. In such a continuous system, the titanium tetrahalide along with solvent and either I.sub.2, AlI.sub.3 or the SnI.sub.4, depending on the particular catalyst combination used, would be added as a mixture to the reactor.

While the TiX.sub.4 -nAlI.sub.3 -xAlR.sub.3 and Lewis base may be combined in many different ways to produce an active polymerization catalyst, a particularly preferred embodiment of this invention involves first dissolving the TiX.sub.4 -nAlI.sub.3 component in all or part of the polymerization diluent or solvent, then adding the monomer and finally adding the AlR.sub.3 and the Lewis base. Alternatively, the TiX.sub.4 -nAlI.sub.3 may be added to the polymerization diluent containing the monomer whereupon the AlR.sub.3 and the Lewis base may be added to form the complete catalyst and start the polymerization. Although the Lewis base may be successfully added before the trialkylaluminum, better results are usually obtained if the Lewis base is added either together with or after the trialkylaluminum, especially when the base is to be employed in the rather large concentrations required for production of polymers containing a predominant amount of trans-1,4 units. In such cases the base seems to interfere with and slow down the reaction between the alkyl metal and the TiX.sub.4 -nAlI.sub.3 component required for formation of the catalytically active species. By properly utilizing the latter mode of base addition, i.e., by adding the base after the polymerization has been initiated with the unmodified TiX.sub.3 -nAlI.sub.3 -xAlR.sub.3 catalyst system, it may actually be possible to obtain A-B type block copolymers in which the first (A) block has the high cis-1,4 structure characteristic of the polybutadienes made with the unmodified catalyst system and the second (B) block has the higher trans-1,4 structure characteristic of the polybutadienes made with the modified catalyst system. In this instance and in subsequent use, the term "modifier" refers to the Lewis base. On the other hand, if the Lewis base is added immediately after the AlR.sub.3, the polymerization may be initiated rapidly without any significant amount of rather pure cis-1,4 polybutadiene blocks being formed. Hence, this method is particularly suitable for the production of high trans-1,4 polybutadiene.

The same considerations are applicable to the catalyst species which use I.sub.2 or SnI.sub.4.

The total amount of catalyst employed in the polymerization of butadiene may vary within rather wide limits depending upon the particular conditions of polymerization, but is generally in the range of from about 0.001 to about 0.5 wt. %, preferably 0.01 to 0.2 wt. % based upon the total reaction mixture comprising the butadiene monomer to be polymerized and the reaction diluent.

In general, it will be found that, for the same catalyst composition, the molecular weight of the polymer decreases as the catalyst concentration is increased, provided, of course, that all the other conditions are kept constant, e.g., monomer concentration, temperature of polymerization, etc. Thus, variation in the amount of catalyst employed per unit of monomer may advantageously be used to control the molecular weight of the polymer obtained. Other factors, such as Al/Ti ratio, polymerization temperature, and the addition of chain transfer agents, can be used advantageously to control the molecular weight of the polymer product of the invention.

In component III (TiX.sub.4 -I.sub.2), the molar ratio of TiX.sub.4 to I.sub.2 can vary from about 1:1 to about 1:20. The preferred range is from about 1:1 to about 1:5. In Component II (TiI.sub.4) and Component III, the molar ratio of aluminum alkyl to TiX.sub.4 ranges from about 20:1 to about 1:1, more preferably from about 10:1 to about 2:1.

In Component IV (TiX.sub.4 -AlI.sub.3), the preferred molar ratio of TiX.sub.4 /AlI.sub.3 is in the range of about 1:1 to about 1:10 and the ratio of AlR.sub.3 to (TiX.sub.4 -AlI.sub.3) is preferably in the range of about 1.5:1 to about 4.5:1 and most preferably in the range of about 2:1 to about 4:1. The same holds true for Component V (TiX.sub.4 -SnI.sub.4).

For any given molar concentration of the titanium component, the ratio of Lewis base to titanium in the catalyst mixture controls the steric arrangement of the monomer units in the resulting polybutadiene and, in general, the greater the concentration of Lewis base employed, the higher the trans-1,4 content in the polymer up to a certain upper limit. Thus, to otain the same proportion of trans-1,4 unsaturation in different polymerizations involving varying concentrations of the titanium component, one will have to increase the ratio of Lewis base to titanium compound as the concentration of the latter is decreased. The molar ratio of Lewis base to titanium compound may vary from about 1:1 to about 500:1 depending upon the amount of trans-1,4 structural units desired in the polybutadiene and the concentration of the titanium component. Preferably, the molar ratio of Lewis base to titanium ranges from about 10:1 to about 200:1.

The conditions of the polymerization reaction can vary over a wide range. Generally, temperatures ranging from less than 0.degree.C to about 100.degree.C can be used; however, temperatures ranging from 5.degree. to 100.degree.C are preferred. Pressures ranging from subatmospheric to about 10 atmospheres can be employed depending primarily upon the vapor pressure of the diene and diluent in the polymerization reaction. A preferred range would, however, be from atmospheric to about 5 atmospheres. Reaction times ranging from a minute to 250 hours can be utilized depending primarily on the time needed for the desired monomer conversion under the polymerization conditions used; however, it is usually possible to achieve close to the maximum conversion obtainable in 24 hours or less.

The reaction vessel used for the polymerization can be constructed from any material that is inert to the reactants and is capable of withstanding the operating pressures. Reactors made of glass, stainless steel and glass lined steel may thus be employed.

Upon completion of the polymerization, the catalyst is deactivated by the addition of a small quantity of a suitable deactivating agent, such as a lower alkanol or a solution of an alkoxide of an alkali or alkaline earth metal, e.g., sodium isopropoxide, sodium ethoxide, potassium t-butoxide, etc. The polymer formed may be recovered from the polymerization mixture by standard techniques such as removal of the diluent by steam distillation or by addition of an anti-solvent to precipitate the polymer. The solid polymer obtained is then isolated by filtration, centrifugation, or similar methods.

The molecular weights, expressed as viscosity average molecular weight, of the butadiene polymers of the present invention range upwards from 100,000 and preferably from 150,000 to 3,000,000. The butadiene polymers contain reactive unsaturation and may be cured to form highly useful vulcanized materials of varying properties. Any one of a wide variety of curing procedures may be employed, such as sulfur curing or free radical curing.

In the uncured state, the instant butadiene polymers exhibit tensile strengths of the order of 150 to 2000 psi, with % elongation up to about 1300. The percent permanent set after breaking ranges from about 25 to about 300.

In the cured state, the instant butadiene polymers exhibit tensile strengths of the order of 1200 to 2500 psi, with % elongation up to about 900.

The polymers of this invention have many varied uses. They may be employed in the preparation of tires, inner tubes, hose and tubing, wire and cable coatings, as well as for a wide variety of coated or molded articles. Those polymers having from 70-90% trans unsaturation are particularly suitable for the preparation of injection-molded articles and possess thermoelastic properties.

This invention and its advantages will be better understood by reference to the following examples.

A number of butadiene polymerizations were carried out with tetrahydrothiophene (THT) modified TiCl.sub.4 -I.sub.2 catalyst systems, using aluminum triethyl as the aluminum alkyl. The polymerizations were carried out inside a nitrogen containing dry box in capped 1/2 gallon jars equipped with magnetic stirrers. The TiCl.sub.4, and 100g. of monomer were added to the reaction vessel, to which 450 ml benzene had first been added.

The AlEt.sub.3 and I.sub.2, which has been dissolved in 50 ml of benzene, were then added to the reactor followed immediately by the THT. The polymerizations were conducted at room temperature. Actual quantities of catalyst components, etc. and reaction times are shown in Table I, along with the results of the experiments.

The polymerizations were terminated by decomposition of the catalyst through the addition of 30 ml of a 0.2M solution of sodium isopropoxide in isopropanol. One-half gram of N-phenyl-2-naphthylamine dissolved in 500 ml benzene was then added as an oxidation inhibitor to the reaction medium, whereupon the polymer solution was transferred to an open pan and the polymer recovered by evaporation of the diluent, first at atmospheric pressure and room temperature and then in vacuo at about 50.degree.C.

As shown by the results in the table, the unmodified, i.e., THT free, catalysts produced high mol. wt. polybutadiene containing about 90% cis-1,4 addition units. All of the modified catalysts on the other hand produced polymers with a preponderant amount of trans-1,4 addition units, up to about 80%.

A number of butadiene polymerizations were carried out with THT, aluminum triethyl, TiCl.sub.4, and I.sub.2, according to the general procedure of Examples 1-4 but with differences in the order of addition of the various components.

The order of addition was:

As the total amount of benzene used as diluent was 500 ml, 450 ml benzene was added at the beginning in Runs 6-10, while 500 ml was added at the beginning in Run 5.

The reactions were terminated, and product recovered, in the same manner as in Examples 1-4.

The results of the experiments are shown in Table II. It is apparent that the addition of THT causes a marked change in the stereospecificity from predominantly cis-1,4 to predominantly trans-1,4 addition.

When butadiene is polymerized in the same manner as in Example 2, substituting tetrahydrothiopyran for THT, the recovered polybutadiene has a trans-1,4 configuration of up to about 80%. The polybutadiene is recovered in good quantities and has a molecular weight in excess of 150,000 (as determined by the correlation of Johnson and Wolfangel, Ind. Eng. Chem., 44, 752 (1952).)

A series of polymerization experiments were conducted to prepare high trans-1,4 content polybutadiene. The catalyst system was TiCl.sub.4, AlI.sub.3, aluminum triethyl (AlEt.sub.3), and tetrahydrothiophene (THT).

All polymerizations were carried out with capped 1/2 gallon jars equipped with magnetic stirrers, the jars being inside a nitrogen-containing dry box. The order of addition of the various components was benzene, TiCl.sub.4, AlI.sub.3 and butadiene followed by AlEt.sub.3, which was added either alone or mixed with THT as indicated in Table III.

The polymerizations were terminated by decomposition of the catalyst through the addition of 30 ml of a 0.2M solution of sodium isopropoxide in isopropanol. One-half gram of N-phenyl-2-naphthylamine dissolved in 500 ml benzene was then added as an oxidation inhibitor to the reaction medium, whereupon the polymer solution was transferred to an open pan and the polymer recovered by evaporation of the diluent, first at room temperature and atmospheric pressure and then in vacuo at about 50.degree.C.

The interesting physical properties of polymers prepared according to this invention are reported in U.S. patent application Ser. No. 175,758, filed Aug. 27, 1971, which describes the preparation of similar polymers but with a different catalyst system.

Four runs were made following the procedure of Examples 12-18, substituting TiBr.sub.4 for the TiCl.sub.4.

The details of the experiments and the results are shown in Table IV.

Following the procedures and using the equipment of Examples 1-4, a series of experiments were conducted using the catalyst system:

with tetrahydrothiophene. Both TiCl.sub.4 (Examples 23-26) and TiBr.sub.4 (Examples 27 and 28) were used.

The polybutadiene was recovered in the same manner as previously described in Examples 1-4. The results are shown in Table V.

In Belgian Patent 551,851, it is shown that a catalyst system comprising TiI.sub.4 and a trialkyl aluminum polymerize 1,3-butadiene to yield a polybutadiene having 85-95% cis-1,4 configuration.

To show the unique advantage of the present invention, i.e., use of a Lewis base to modify the steric configuration of butadiene based polymers, a series of experiments were conducted polymerizing 1,3-butadiene. The catalyst system used was TiI.sub.4 -AlR.sub.3 using tetrahydrothiophene (THT). The mole ratio of THT to TiI.sub.4 was varied from 10 up to 30. The Al triethyl (AlEt.sub.3) and THT were premixed before addition to the reactor.

The polymerization was terminated and polymer recovered in the same manner as used in Examples 1-4.

The results are shown in Table VI.

Several butadiene-1,3 polymerizations were conducted with the catalyst system and procedure of Examples 29-33, above. However, the THT was added to the reaction mixture after the AlEt.sub.3. Results are in Table VII.

A polymerization was carried out as described in Example 26 except for the substitution of an equimolar amount (715.4 mg) of tetrahydrothiopyran for the THT. The polymerization reaction was allowed to proceed for 17 hours after which time 76.8g. of a tough rubbery polymer was recovered. The mol. wt. of the polymer was found to be 190,000 and the unsaturation distribution: vinyl -- 3.3%, cis-14.9%, and trans-81.8%.

A polymerization was carried out as described in Example 33 except for the substitution of an equimolar amount (920 mg) of tetrahydrothiopyran for the THT. The polymerization reaction was allowed to proceed for 17 hours after which time 63.1g. of a tough rubbery polymer was recovered. The mol. wt. of the polymer was found to be 225,000 and the unsaturation distribution: vinyl -- 6.1%, cis-13.3%, and trans-80.6%.