Polymerization of 1,3-butadiene to trans-1,4-polybutadiene with organolithium and alkali metal alkoxide

Trans-1,4-polybutadiene is a thermoplastic resin rather than a rubber at room temperature by virtue of its high degree of crystallinity. Because trans-1,4-polybutadiene contains many double bonds in its backbone, it can be blended and cocured with rubbers. It is accordingly an attractive alternative to conventional fillers such as carbon black, which are typically utilized in compounding rubber. By utilizing the catalyst system and techniques of this invention, trans-1,4-polybutadiene can be synthesized at a high level of conversion. This invention specifically relates to a process for the synthesis of trans-1,4-polybutadiene by an anoinic polymerization process which comprises: (a) adding an organolithium compound and an alkali metal alkoxide as a catalyst system to a polymerization medium containing 1,3-butandiene monomer and an organic solvent; and (b) allowing the 1,3-butandiene monomer to polymerize at a temperature which is within the range of about -10.degree. C. to about 45.degree. C. to produce the trans-1,4-polybutadiene.

BACKGROUND OF THE INVENTION 
Trans-1,4-polybutadiene (TPBD) is a thermoplastic resin by virtue of its 
high level of crystallinity. Because it contains many double bonds in its 
polymeric backbone, it can be blended and cocured with rubber. TPBD is 
similar to syndiotactic-1,2-polybutadiene in this respect. 
TPBD is normally prepared utilizing transition metal catalysts or rare 
earth catalysts. The synthesis of TPBD with transition metal catalysts is 
described by J. Boor Jr., "Ziegler-Natta Catalysts and Polymerizations", 
Academic Press, New York, 1979, Chapters 5-6. The synthesis of TPBD with 
rare earth catalysts is described by D. K. Jenkins, Polymer, 26, 147 
(1985). However, molecular weight control is difficult to achieve with 
such transition metal or rare earth catalysts and monomer conversions are 
often very modest. 
Better molecular weight control could be achieved by utilizing an anionic 
polymerization system to produce the TPBD. There is typically an inverse 
relationship between the catalyst level utilized and the molecular weight 
attained when anionic polymerization systems are used. Such an anionic 
polymerization system is disclosed in U.S. Pat. No. 4,225,690. The 
catalyst system disclosed therein is based on a dialkylmagnesium compound 
which is activated with a potassium alkoxide. However, only a minor amount 
of the polymer produced with such dialkyl magnesium based catalyst systems 
is TPBD. In other words, the small amount of TPBD produced utilizing such 
catalyst systems is always accompanied by major amounts of hexane-soluble 
polybutadiene of mixed microstructure. 
SUMMARY OF THE INVENTION 
The present invention relates to a technique for synthesizing TPBD at high 
levels of conversion by an anionic polymerization process. This anionic 
polymerization technique is attractive because molecular weight can be 
controlled by simply varying the catalyst level. It is also attractive 
because higher molecular weights can be obtained than can be reached 
utilizing typical coordination catalysts. 
The subject invention discloses a catalyst system which can be utilized in 
the polymerization of 1,3-butadiene monomer into trans-1,4-polybutadiene, 
said catalyst system being comprised of an organolithium compound and an 
alkali metal alkoxide, wherein the molar ratio of the organolithium 
compound to the alkali metal alkoxide is within the range of about 2:3 to 
about 1:10. 
The present invention further discloses a process for the synthesis of 
trans-1,4-polybutadiene by an anionic polymerization process which 
comprises polymerizing 1,3-butadiene monomer in an organic solvent at a 
temperature which is within the range of about -10.degree. C. to about 
45.degree. C. in the presence of an organolithium compound and an alkali 
metal alkoxide. 
The present invention more specifically relates to a process for the 
synthesis of trans-1,4-polybutadiene by an anionic polymerization process 
which comprises: (a) adding an organolithium compound and an alkali metal 
alkoxide as a catalyst system to a polymerization medium containing 
1,3-butadiene monomer and an organic solvent: and (b) allowing the 
1,3-butadiene monomer to polymerize at a temperature which is within the 
range of about -10.degree. C. to about 50.degree. C. to produce the 
trans-1,4-polybutadiene. 
DETAILED DESCRIPTION OF THE INVENTION 
The polymerizations of the present invention will normally be carried out 
in a hydrocarbon solvent which can be one or more aromatic, paraffinic, or 
cycloparaffinic compounds. These solvents will normally contain from 4 to 
10 carbon atoms per molecule and will be liquids under the conditions of 
the polymerization. Some representative examples of suitable organic 
solvents include pentane, isooctane, cyclohexane, normal hexane, benzene, 
toluene, xylene, ethylbenzene, and the like, alone or in admixture. 
However, the catalyst systems of this invention can also be used in bulk 
polymerizations. 
In the solution polymerizations of this invention, there will normally be 
from 5 to 35 weight percent monomers in the polymerization medium. Such 
polymerization media are, of course, comprised of the organic solvent and 
1,3-butadiene monomer. In most cases, it will be preferred for the 
polymerization medium to contain from 10 to 30 weight percent monomers. It 
is generally more preferred for the polymerization medium to contain 20 to 
25 weight percent monomer. 
Polymerization is started by adding an organolithium compound and an alkali 
metal alkoxide to the polymerization medium. Such polymerizations can be 
carried out utilizing batch, semi-continuous, or continuous techniques. In 
a continuous process additional 1,3-butadiene monomer, catalyst, and 
solvent are continuously added to the reaction vessel being utilized. The 
polymerization temperature utilized will typically be within the range of 
about -10.degree. C. to about 45.degree. C. It is normally preferred for 
the polymerization medium to be maintained at a temperature which is 
within the range of about 0.degree. C. to about 40.degree. C. throughout 
the polymerization. It is typically most preferred for the polymerization 
temperature to be within the range of about 10.degree. C. to about 
30.degree. C. The pressure used will normally be sufficient to maintain a 
substantially liquid phase under the conditions of the polymerization 
reaction. 
The polymerization is conducted for a length of time sufficient to permit 
substantially complete polymerization of the 1,3-butadiene monomer. In 
other words, the polymerization is normally carried out until high 
conversions are realized. The polymerization can then be terminated using 
a standard procedure. 
The organolithium compounds which can be utilized are normally 
organomonolithium compounds. The organolithium compounds which are 
preferred can be represented by the formula: R-Li, wherein R represents a 
hydrocarbyl radical containing from 1 to about 20 carbon atoms. Generally, 
such monofunctional organolithium compounds will contain from 1 to about 
10 carbon atoms. Some representative examples of organolithium compounds 
which can be employed include methyllithium, ethyllithium, 
isopropyllithium, n-butyllithium, sec-butyllithium, n-octyllithium, 
tert-octyllithium, n-decyllithium, phenyllithium, 1-naphthyllithium, 
4-butylphenyllithium, p-tolyllithium, 4-phenylbutyllithium, 
cyclohexyllithium, 4-butylcyclohexyllithium, and 4-cycohexylbutyllithium. 
As a general rule, from about 0.1 to about 2 mmoles per 100 grams of 
butadiene monomer of the organolithium compound will be utilized. It is 
normally preferred for the organolithium compound to be present in an 
amount which is within the range of 0.2 to 1.5 mmoles per 100 grams of 
1,3-butadiene. Amounts in the range of about 0.5 to about 1.0 mmoles per 
100 grams of monomer are most preferred. 
The alkali metal alkoxide will typically contain from about 2 to about 12 
carbon atoms. It is generally preferred for the alkali metal alkoxide to 
contain from about 3 to about 8 carbon atoms. It is generally most 
preferred for the alkali metal alkoxide to contain from about 4 to about 6 
carbon atoms. Potassium t-amyloxide (potassium t-pentoxide) is a highly 
preferred alkali metal alkoxide which can be utilized in the catalyst 
systems of this invention. 
In the catalyst systems of this invention, the molar ratio of the 
organolithium compound to the alkali metal alkoxide will typically be 
within the range of about 2:3 to about 1:10. It is generally preferred for 
the molar ratio of the organolithium compound to the alkyl metal alkoxide 
to be within the range of about 1:2 to about 1:8. Molar ratios within the 
range of about 1:3 to about 1:6 are most preferred. The amount of catalyst 
employed will be dependent upon the molecular weight which is desired for 
the TPBD being synthesized. As a general rule with all anionic 
polymerizations, the molecular weight of the polymer produced is inversely 
proportional to the amount of catalyst utilized. 
In the TPBD produced by the process of this invention, at least 75% of the 
butadiene repeat units in the polymer are of the trans-1,4-isomeric 
structure. The TPBD made utilizing the catalyst system of this invention 
typically has a trans-isomer content of about 80% to about 95%. The TPBD 
produced has two distinct melting points. The first melting point is 
within the range of about 60.degree. C. to about 80.degree. C. and the 
second melting point is within the range of about 135.degree. C. to about 
155.degree. C.

This invention is illustrated by the following examples which are merely 
for the purpose of illustration and are not be regarded as limiting the 
scope of the invention or the manner in which it can be practiced. Unless 
specifically indicated otherwise, all parts and percentages are given by 
weight. 
EXAMPLES 1-3 
A series of dry, nitrogen-filled 32 ounce (946 ml) septum sealed screw cap 
bottles were charged with 800 ml of an 18.9% solution of butadiene in 
mixed hexanes. These solutions had been passed several times over mixed 
silica/alumina under a nitrogen atmosphere. A 0.89M solution of potassium 
t-amyloxide in cyclohexane (obtained from Callery Chemical Company and 
treated with potassium metal) was injected into each of the bottles with a 
syringe. Then, 0.5 mmoles of a 2.5M solution of n-butyllithium in hexane 
was injected into each of the bottles. The molar ratio of the 
n-butyllithium to the potassium t-amyloxide used in each of the 
polymerizations is shown in Table I. 
The bottles were maintained at a temperature of 10.degree. C. and 
mechanically shaken for a polymerization time of at least 18 hours. The 
polymerizations were then shortstopped by injecting 10 ml of methanol and 
15 ml of a 5% W/V solution of butylated hydroxytoluene (BHT) in hexane 
into the bottles. The TPBD produced was then strained off and washed 
several times with hexane, the final wash (with soaking) being with a 1% 
solution of BHT in hexane. 
TABLE I 
______________________________________ 
Soluble 
n-butyllithium/potassium 
TPBD Polymer 
Example t-amyloxide (molar ratio) 
Yield Yield 
______________________________________ 
1 1:2 93% 3% 
2 1:3 98% 2% 
3 1:6 80% 20% 
______________________________________ 
The TPBD synthesized in these experiments was determined to have melting 
points at about 67.degree. C. and 151.degree. C., which means that it had 
a very high trans-1,4 content. A small amount of soluble polymer (medium 
vinyl polybutadiene) was also produced by these polymerizations. As can be 
seen, a maximum yield of TPBD was realized at a n-butyllithium to 
potassium t-amyloxide molar ratio of 1:3. As the molar ratio of the 
organolithium compound to the alkali metal alkoxide was reduced to 1:6, 
the amount of medium vinyl polybutadiene produced increased to 20%. In 
Example 2 where the ratio of n-butyllithium to potassium t-amyloxide was 
1:3, the TPBD yield attained was 98% with only 2% of the soluble polymer 
being produced. This series of experiments clearly shows that by utilizing 
the catalyst system of this invention that TPBD can be synthesized in high 
purity and at high yields. 
EXAMPLE 4 
The procedure of Example 3 was repeated in this experiment except for the 
polymerization being allowed to exotherm (by not being placed in a cooling 
bath). The TPBD yield attained was 89%. The TPBD produced had melting 
points at 68.degree. C. and 150.degree. C. 
EXAMPLES 5-7 
The general procedure described in Examples 1-3 was repeated in this series 
of experiments with the level of the potassium t-amyloxide being reduced 
as shown in Table II. 
TABLE II 
______________________________________ 
Soluble 
n-butyllithium/potassium 
TPBD Polymer 
Example t-amyloxide (molar ratio) 
Yield Yield 
______________________________________ 
5 1:2 38% 64% 
6 1:1.8 45% 58% 
7 1:1.6 69% 30% 
______________________________________ 
COMATIVE EXAMPLE 8 
The procedure described in Example 3 was repeated in this experiment except 
for the polymerization temperature being increased to 50.degree. C. The 
yield of TPBD was reduced to about 10% with most of the polymer produced 
being medium-vinyl polybutadiene. This experiment shows that it is 
important to keep the polymerization temperature below about 50.degree. C. 
COMATIVE EXAMPLES 9-10 
In these experiments, the level of potassium t-amyloxide was reduced to a 
very low level with the general procedure described in Examples 1-3 being 
utilized. The level of potassium t-amyloxide employed is shown in Table 
III. 
TABLE III 
______________________________________ 
Soluble 
n-butyllithium/potassium 
TPBD Polymer 
Example t-amyloxide (molar ratio) 
Yield Yield 
______________________________________ 
9 1:1.4 25% 68% 
10 1:1.3 0% 100% 
______________________________________ 
As can be seen, when the ratio of n-butyllithium to potassium t-amyloxide 
was increased to greater than about 2:3, yields were very poor. In Example 
10, the polymerization resulted in the formation of only medium-vinyl 
polybutadiene. 
While certain representative embodiments and details have been shown for 
the purpose of illustrating the present invention, it will be apparent to 
those skilled in this art that various changes and modifications can be 
made therein without departing from the scope of the present invention.