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Description  |
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BACKGROUND OF THE INVENTION
The present invention relates to a process for the production of
selectively hydrogenated polymers of conjugated dienes and more
particularly to such a process utilizing a titanium hydrogenation
catalyst.
The hydrogenation or selective hydrogenation of conjugated diene polymers
has been accomplished using any of the several hydrogenation processes
known in the prior art. For example the hydrogenation has been
accomplished using methods such as those taught, for example, in U.S. Pat.
Nos. 3,494,942; 3,634,594; 3,670,054; 3,700,633 and Re. 27,145, the
disclosure of which patents are incorporated herein by reference. These
methods known in the prior art for hydrogenating polymers containing
ethylenic unsaturation and for hydrogenating or selectively hydrogenating
polymers containing aromatic and ethylenic unsaturation, involve the use
of a suitable catalyst, particularly a catalyst or catalyst precursor
comprising a Group VIII metal.
In the methods described in the foregoing patents, a catalyst is prepared
by combining a Group VIII metal, particularly nickel or cobalt, compound
with a suitable reducing agent such as an aluminum alkyl. Also, while
aluminum alkyls are the preferred reducing agents, it is known in the
prior art that alkyls and hydrides of metals of Groups I-A, II-A and III-B
of the Periodic Table of the Elements are effective reducing agents,
particularly lithium, magnesium and aluminum. In general, the Group VIII
metal compound is combined with Group I-A, II-A or III-B metal alkyl or
hydride at a concentration sufficient to provide Group I-A, II-A and/or
III-B metal to Group VIII metal ratios within the range from about 0.1:1
to about 20:1, preferably from about 1:1 to about 10:1. As indicated in
the foregoing patents, the hydrogenation catalyst is generally prepared by
combining the Group VIII metal compound and the reducing agent in a
suitable solvent or diluent at a temperature within the range from about
20.degree. C. to about 60.degree. C. before the catalyst is fed to the
hydrogenation reactor.
In 1985, Kishimoto et al. disclosed (in U.S. Pat. No. 4,501,857) that
selective hydrogenation of the unsaturated double bonds in conjugated
diolefin polymers could be achieved by hydrogenating such polymers in the
presence of at least one bis(cyclopentadienyl)titanium compound and at
least one hydrocarbon lithium compound wherein the hydrocarbon lithium
compound can be an added compound or a living polymer having a lithium
atom in the polymer chain. European patent application 0,339,986 discloses
that similar hydrogenation activity can be accomplished with the same
titanium compounds in combination with an alkoxy lithium compound which
can either be added directly or as a reaction mixture of an organo lithium
compound with an alcoholic or phenolic compound. The use of these catalyst
systems was said to be advantageous because the catalysts were said to be
highly active so that they were effective even in such a small amount as
not to affect adversely the stability of a hydrogenated polymer and
require no deashing step. Further, the hydrogenation was said to be able
to be carried out under mild conditions.
In U.S. Pat. No. 4,673,714, bis(cyclopentadienyl)titanium compounds were
disclosed which preferentially hydrogenate the unsaturated double bonds of
conjugated diolefins but do not require the use of an alkyl lithium
compound. These titanium compounds were bis(cyclopentadienyl)titanium
diaryl compounds. The elimination of the need for the hydrocarbon lithium
compound was said to be a significant advantage of the invention disclosed
in the '714 patent.
SUMMARY OF THE INVENTION
The present invention provides a process for the hydrogenation of
conjugated diolefin polymers which first involves the polymerization or
copolymerization of such monomers with an organo alkali metal
polymerization initiator in a suitable solvent thereby creating a living
polymer. The living polymer is terminated by the addition of hydrogen.
Finally, selective hydrogenation of the unsaturated double bonds in the
conjugated diolefin units of the terminated polymer is carried out in the
presence of at least one bis(cyclopentadienyl)titanium compound preferably
of the formula:
##STR1##
wherein R.sub.1 and R.sub.2 are the same or different and are selected
from the group consisting of halogen groups, C.sub.1 -C.sub.8 alkyl and
alkoxy groups, C.sub.6 -C.sub.8 aryloxy groups, aralkyl, cycloalkyl
groups, silyl groups and carbonyl groups. The hydrogenation step is
carried out in the absence of hydrocarbon lithium and alkoxy lithium
compounds.
DETAILED DESCRIPTION OF THE INVENTION
As is well known, polymers containing both aromatic and ethylenic
unsaturation can be prepared by copolymerizing one or more polyolefins,
particularly a diolefin, by themselves or with one or more alkenyl
aromatic hydrocarbon monomers. The copolymers may, of course, be random,
tapered, block or a combination of these, as well as linear, star or
radial.
As is well known, polymers containing ethylenic unsaturation or both
aromatic and ethylenic unsaturation may be prepared using anionic
initiators or polymerization catalysts. Such polymers may be prepared
using bulk, solution or emulsion techniques. In any case, the polymer
containing at least ethylenic unsaturation will, generally, be recovered
as a solid such as a crumb, a powder, a pellet or the like. Polymers
containing ethylenic unsaturation and polymers containing both aromatic
and ethylenic unsaturation are, of course, available commercially from
several suppliers.
In general, when solution anionic techniques are used, conjugated diolefin
polymers and copolymers of conjugated diolefins and alkenyl aromatic
hydrocarbons are prepared by contacting the monomer or monomers to be
polymerized simultaneously or sequentially with an anionic polymerization
initiator such as Group IA metals, their alkyls, amides, silanolates,
napthalides, biphenyls and anthracenyl derivatives. It is preferred to use
an organoalkali metal (such as sodium or potassium) compound in a suitable
solvent at a temperature within the range from about -150.degree. C. to
about 300.degree. C., preferably at a temperature within the range from
about 0.degree. C. to about 100.degree. C. Particularly effective anionic
polymerization initiators are organolithium compounds having the general
formula:
RLi.sub.n
wherein: R is an aliphatic, cycloaliphatic, aromatic or alkyl-substituted
aromatic hydrocarbon radical having from 1 to about 20 carbon atoms; and n
is an integer of 1 to 4.
Conjugated diolefins which may be polymerized anionically include those
conjugated diolefins containing from 4 to about 12 carbon atoms such as
1,3-butadiene, isoprene, piperylene, methylpentadiene, phenylbutadiene,
3,4-dimethyl-1,3-hexadiene, 4,5-diethyl-1,3-octadiene and the like.
Conjugated diolefins containing from 4 to about 8 carbon atoms are
preferred for use in such polymers. Alkenyl aromatic hydrocarbons which
may be copolymerized include vinyl aryl compounds such as styrene, various
alkyl-substituted styrenes, alkoxy-substituted styrenes, 2-vinyl pyridine,
4-vinyl pyridine, vinyl naphthalene, alkyl-substituted vinyl naphthalenes
and the like.
In general, any of the solvents known in the prior art to be useful in the
preparation of such polymers may be used. Suitable solvents, then, include
straight- and branched-chain hydrocarbons such as pentane, hexane,
heptane, octane and the like, as well as, alkyl-substituted derivatives
thereof; cycloaliphatic hydrocarbons such as cyclopentane, cyclohexane,
cycloheptane and the like, as well as, alkyl-substituted derivatives
thereof; aromatic and alkyl-substituted derivatives thereof; aromatic and
alkyl-substituted aromatic hydrocarbons such as benzene, naphthalene,
toluene, xylene and the like; hydrogenated aromatic hydrocarbons such as
tetralin, decalin and the like; halogenated hydrocarbons, particularly
halogenated aromatic hydrocarbons, such as chlorobenzene, chlorotoluene
and the like; linear and cyclic ethers such as methyl ether, methyl ethyl
ether, diethyl ether, tetrahydrofuran and the like.
Conjugated diolefin polymers and conjugated diolefin-alkenyl aromatic
copolymers which may be used in the present invention include those
copolymers described in U.S. Pat. Nos. 3,135,716; 3,150,209; 3,496,154;
3,498,960; 4,145,298 and 4,238,202, the disclosure of which patents are
herein incorporated by reference. Conjugated diolefin-alkenyl aromatic
hydrocarbon copolymers which may be used in this invention also include
block copolymers such as those described in U.S. Pat. Nos. 3,231,635;
3,265,765 and 3,322,856, the disclosure of which patents are also
incorporated herein by reference. In general, linear and branched block
copolymers which may be used in the present invention include those which
may be represented by the general formula:
A.sub.z --(B--A).sub.y --B.sub.x
wherein:
A is a linear or branched polymeric block comprising predominantly
monoalkenyl aromatic hydrocarbon monomer units;
B is a linear or branched polymeric block containing predominantly
conjugated diolefin monomer units;
x and z are, independently, a number equal to 0 or 1;
y is a whole number ranging from 0 to about 15, and the sum of
x+z+y.gtoreq.2.
Polymers which may be treated in accordance with this invention also
include coupled and radial block copolymers such as those described in
U.S. Pat. Nos. 4,033,888; 4,077,893; 4,141,847; 4,391,949 and 4,444,953,
the disclosure of which patents are also incorporated herein by reference.
Coupled and radial block copolymers which may be treated in accordance
with the present invention include those which may be represented by the
general formula:
[B.sub.x --(A--B).sub.y --A.sub.z ].sub.n --C--P.sub.n'
wherein:
A, B, x, y and z are as previously defined; n and n' are, independently,
numbers from 1 to about 100 such that n+n'.gtoreq.3;
C is the core of the coupled or radial polymer formed with a polyfunctional
coupling agent; and
Each P is the same or a different polymer block or polymer segment having
the general formula:
B'.sub.x' --(A'--B").sub.y' --A".sub.z'
wherein:
A" is a polymer block containing predominantly monoalkenyl aromatic
hydrocarbon monomer units;
B' is a polymer block containing predominantly conjugated diolefin monomer
units;
A'--B" is a polymer block containing monoalkenyl aromatic hydrocarbon
monomer units (A') and conjugated diolefin monomer units (B"), the A'--B"
monomer units may be random, tapered or block and when A'--B" is block,
the A' block may be the same or different from A" and B" may be the same
or different from B';
x' and z' are, independently, numbers equal to 0 or 1; and
y' is a number from 0 to about 15, with the proviso that the sum of
x'+y'+z.gtoreq.1.
The radial polymers may, then, be symmetric or asymmetric.
In the production of all of the polymers described above, the
polymerization is herein terminated by utilizing hydrogen gas in place of
the conventionally used alcohol terminating agent. The living polymer, or
more accurately, the living end of the polymer chain, is terminated by the
addition of hydrogen thereto. The theoretical termination reaction is
shown using an S--B--S block copolymer for exemplary purposes:
S--B--S.sup.- Li.sup.+ +H.sub.2 .fwdarw.S--B--SH+LiH
As shown above, it is theorized that lithium hydride is formed during the
termination process. Formed in this manner, it is not a reactive
polymerization initiator. It is inert to polymerization and does not
interfere with the molecular weight control of the next polymerization
batch as alcohol can.
It is usually advisable to contact and vigorously mix the gas with the
polymerization solution at the end of the polymerization reaction. This
contact and vigorous mixing can be effected by adding the hydrogen gas
through spargers in a mixing vessel containing polymer solution. The time
of contact should be at least about ten seconds and preferably about
twenty minutes to allow sufficient contact time for the reaction to occur.
This is dependent upon the efficiency of the gas contacting equipment, gas
solubility, solution viscosity and temperature. Alternatively, a
continuous system could be employed whereby hydrogen is pumped into a
solution prior to going to a statically mixed plug flow reactor. Hydrogen
could also be dissolved in appropriate solution and added to the polymer
solution to be terminated. Another method would be to cause the hydrogen
to be absorbed into an absorption bed and then cause the polymer solution
to flow through the absorption bed. The hydrogen contact could also be
carried out by adding a material which gives off hydrogen upon
decomposition, i.e. diimide.
When this improvement is used, the problems of using alcohol, i.e. the
formation of lithium alkoxides and excess alcohol impurities, are avoided.
Furthermore, this process has been found to have significant advantage if
the polymer made is to be hydrogenated. It has been found that if the
present method is used, a bis(cyclopentadienyl)titanium hydrogenation
catalyst may be used without the necessity of a hydrocarbon lithium or
alkoxy lithium promoter, whether added with the catalyst or present in the
living polymer.
As stated above, the hydrogenation step of the present process is carried
out in the presence of a bis(cyclopentadienyl)titanium compound of the
formula:
##STR2##
wherein R.sub.1 and R.sub.2 are the same or different and are selected
from the group consisting of halogen groups, C.sub.1 -C.sub.8 alkyl and
alkoxy groups, C.sub.6 -C.sub.8 aryloxy groups, aralkyl, cycloalkyl
groups, silyl groups and carbonyl groups. The hydrogenation step is
carried out in the absence of hydrocarbon lithium and alkoxy lithium
compounds.
Specific bis(cyclopentadienyl) compounds which may be used in the present
invention include bis(cyclopentadienyl)titanium dichloride,
bis(cyclopentadienyl)titanium dibromide, bis(cyclopentadienyl)titanium
diiodide, bis(cyclopentadienyl)titanium difluoride,
bis(cyclopentadienyl)titanium dicarbonyl, bis(cyclopentadienyl)titanium
dimethyl, bis(cyclopentadienyl)titanium diethyl,
bis(cyclopentadienyl)titanium dibutyl (including n-butyl, sec-butyl,
tert-butyl), bis(cyclopentadienyl)titanium bis(trimethylsilylmethyl),
bis(cyclopentadienyl)titanium dibenzyl, bis(cyclopentadienyl)titanium
dihexyl, bis(cyclopentadienyl)titanium dimethoxide,
bis(cyclopentadienyl)titanium diethoxide, bis(cyclopentadienyl)titanium
dibutoxide, bis(cyclopentadienyl)titanium dipentoxide,
bis(cyclopentadienyl)titanium dineopentoxide, bis(cyclopentadienyl)
titanium diphenoxide, and all mixtures thereof. The preferred titanium
compound is bis(cyclopentadienyl) titanium dichloride because of ease of
handling, air stability and commercial availability.
This process will selectively hydrogenate conjugated diolefins without
hydrogenating alkenyl aromatic hydrocarbons to any degree. Hydrogenation
percentages of greater than 50% are easily obtained but it has been found
that in order to achieve hydrogenation percentages of greater than 95% as
is often desired, the alkali metal (for example, lithium) to titanium
ratio must be at least about 2:1 and preferably is from about 3 to 30.
There has to be sufficient alkali metal to ensure quick and sufficient
interaction between the two metals. A high viscosity (high molecular
weight) polymer may require a higher ratio because of the lesser mobility
of the metals in the polymer cement. If alkali metal hydride must be added
to increase the ratio, it can be made in situ by adding an organo alkali
metal compound and hydrogen to the polymer (i.e., sparge), either before
or after termination of the polymerization.
In general, the hydrogenation is carried out in a suitable solvent at a
temperature within the range of from about 0.degree. to about 120.degree.
C., preferably about 60.degree. to about 90.degree. C., and at a hydrogen
partial pressure within the range from about 1 psig to about 1200 psig,
preferably from about 100 to about 200 psig. Catalyst concentrations
within the range from about 0.01 mM(millimoles) per 100 grams of polymer
to about 20 mM per 100 grams of polymer, preferably 0.04 to 1 mM catalyst
per 100 grams of polymer, are generally used and contacting at
hydrogenation conditions is generally continued for a period of time
within the range from about 30 to about 360 minutes. Suitable solvents for
hydrogenation include, among others, n-heptane, n-pentane,
tetrahydrofuran, cyclohexane, toluene, hexane and benzene. Because of the
small amount of catalyst present in the polymer after hydrogenation, it is
not necessary to separate the hydrogenation catalyst and catalyst residue
from the polymer. However, if separation is desired, it may be carried out
using methods well known in the prior art. Hydrogenation may be carried
out in other manners such as batch processes, continuous processes, and
semi-continuous processes.
EXAMPLES
Homopolybutadiene, polystyrene-polybutadiene-polystyrene, and
polyisoprene-polystyrene block copolymers were terminated with hydrogen.
Typically, at the end of the polymerization reaction, the living polymer
cement was sparged with hydrogen gas (1.0 SCFM) from 5 to 60 minutes and
vigorously mixed. Generally, the temperature of the polymer cement was
60.degree. C. and no increase in temperature was observed during the
termination step. During the sparging stage of the reaction, the total
pressure in the reactor ranged from 40 to 100 psig of hydrogen.
Termination was confirmed by four independent methods. The first of these
was a simple colorimetric examination of the polymer cement. Styryllithium
living ends have an absorption maximum at 328 m.mu. and thus have a
distinct orange color which turn colorless when the living ends are
terminated. This was observed in hydrogen termination as samples were
pulled from the reactor and visually, as well as colorimetrically,
examined for color change. The second method for determining termination
was gel permeation chromatography (GPC). Analysis of the hydrogen
terminated polymers by GPC showed that there was no high molecular weight
polymer (HMP) formed. The absence of HMP generally indicates that the
polymer has not crosslinked. Crosslinking is a typical detrimental side
reaction in non-terminated polymer cements. Another method used to verify
termination was .sup.2 H NMR. Deuterium gas was used to terminate the
living polymer. During the termination, samples were submitted for .sup.2
H NMR analysis. Polymer termination was essentially complete when there
was no increase in deuterium incorporation. The last method employed
involved adding styrene monomer back to the terminated polymer. If any
living ends are still existing after the sparge, they will polymerize the
added monomer.
EXAMPLE 1
A 600 lb. batch of polystyrene-polybutadiene-polystyrene (S--B--S.sup.-
Li.sup.+) block copolymer 50,000 molecular weight was made by anionic
polymerization using sec-butyllithium as the initiator in a 150 gallon
pressurized reactor. The polymerization took place in a mixture of
cyclohexane and diethyl ether. The resulting polymer solution contained
20% polymer by weight.
At the end of the polymerization reaction, the reactor temperature was
approximately 60.degree. C. The reactor was sparged with hydrogen for
approximately 20 minutes. A colorimeter was used to determine when the
termination was complete since S--B--S.sup.- Li.sup.+ has a distinct
orange color. The colorimeter reading still showed "color" after 15
minutes of sparge time. At that time, the vent was closed and the reactor
pressured up to 80 psig with hydrogen. The temperature was raised to
decrease viscosity and improve mass transfer. The solution was mixed for
20 more minutes. During that time, the colorimeter reading dropped to
baseline which reflects a terminated polystyrene-polybutadiene-polystyrene
(S--B--S) polymer.
All hydrogenation runs were carried out under similar conditions unless
otherwise noted. A typical experimental hydrogenation run consisted of
pressure transferring to a 4-liter reactor a 12-25% by weight solution of
polymer. The temperature of the reactor was maintained at 70.degree. C. At
this point, bis(cyclopentadienyl)titanium dichloride, (Cp.sub.2
TiCl.sub.2), was added to the reactor as a toluene or cyclohexane slurry.
After addition of the catalyst, the reactor was pressurized to 140 psig
with hydrogen gas. The reaction was allowed to run for 3 hours, during
which time samples were drawn from the reactor and analyzed by proton NMR
to determine final percent conversion of olefin. Gel Permeation
Chromatography (GPC) was done on final samples to determine if there had
been any changes in molecular architecture.
EXAMPLES 2-7
Hydrogenation of Hydrogen Terminated Polymer with Varying Amounts of
Cp.sub.2 TiCl.sub.2 Catalyst
A polystyrene-polybutadiene-polystyrene type polymer of 50,000 molecular
weight was prepared as in Example 1. The polymer solution was 20% by
weight polymer. The polymer was hydrogenated with varying amounts of
catalyst as indicated in Table 1. The results of the hydrogenation run are
shown in Table 1.
TABLE 1
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Ti (mM) Olefin
Example
(100 gram polymer)
Li:Ti ratio
Conversion, %
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2 0.04 56:1 97.2
3 0.08 28:1 98.5
4 0.16 14:1 99.6
5 0.48 5:1 99.3
6 0.80 3:1 98.5
7 2.24 1:1 87.5
______________________________________
FIG. 1 graphically represents the Table 1 hydrogenation runs by showing
percent olefin conversion over time. It was observed that catalyst
loadings ranging from 0.04 to 0.80 mM Ti per 100 grams polymer resulted in
>97% conversion of the olefin within 3 hours. As expected, the rate of
hydrogenation decreased with decreasing catalyst loading, with the lowest
catalyst loading of 0.04 mM Ti per 100 grams polymer requiring
substantially longer reaction time. At a catalyst loading of 2.24 mM Ti
per 100 grams polymer, it was observed that catalyst activity was
initially incredibly rapid but leveled off quickly at only 87.5%
conversion of the olefin. Noting this hydrogenation run, it appeared that
there existed an optimum titanium concentration which was dependent on the
concentration of LiH present from the polymerization termination step.
Whereas the runs made with 0.04 to 0.80 mM Ti per 100 grams polymer always
had an excess of LiH (Li:Ti ratios ranging from 56:1 to 3:1), the 2.24 mM
Ti per 100 grams polymer run calculated to be a 1:1 Li:Ti ratio.
EXAMPLE 8
Hydrogenation of Hydrogen Terminated Polymer with
Bis(Cyclopentadienyl)Titanium Diethoxide
A polystyrene-polybutadiene-polystyrene type polymer of 50,000 molecular
weight was prepared as in Example 1. The polymer solution was 20% by
weight polymer. Bis(cyclopentadienyl titanium diethoxide (0.33 mM Ti per
100 grams polymer), as a cyclohexane slurry, was added to the reactor.
After addition of the catalyst, the reactor was pressurized to 140 psig
hydrogen and the temperature was maintained at 70.degree. C. The
hydrogenation reaction was allowed to proceed for 3 hours. Final
conversion of the olefin was 98%.
EXAMPLE 9
Hydrogenation of Methanol Terminated Polymer Feed
A 5% by weight solution of a polystyrene-polybutadiene-polystyrene type
polymer of 50,000 molecular weight that had been terminated with methanol
was transferred to a 4 liter reactor. The solution was sparged with
hydrogen for 20 minutes. The contents of the reactor were heated to
40.degree. C. and the hydrogen pressure within the reactor was 70 psig.
Bis(cyclopentadienyl)titanium dichloride (3.2 mM Ti per 100 grams of
polymer) was added to the reactor as a catalyst/toluene slurry. During the
first 150 minutes, no hydrogenation occurred. After 150 minutes, 7.5 mM
of sec-butyl lithium was added and hydrogenation proceeded to 82%
conversion.
EXAMPLE 10
Hydrogenation of Methanol Terminated Polymer Feed
A 20% by weight solution of a polystyrene-polybutadiene-polystyrene type
polymer of 50,000 molecular weight that had been terminated with methanol
was transferred to a 4 liter reactor. The contents of the reactor were
heated to 40.degree. C. The solution was sparged with hydrogen for 20
minutes. The reactor temperature was maintained at 70.degree. C. and the
hydrogen pressure within the reactor was 140 psig.
Bis(cyclopentadienyl)titanium dichloride (0.8 mM Ti per 100 grams of
polymer) was added to the reactor as a catalyst/toluene slurry. During the
first 120 minutes no hydrogenation occurred. After 120 minutes, 15 mM of
sec-butyl lithium were added to the reactor and hydrogenation proceeded to
62% conversion.
EXAMPLE 11
Hydrogenation of a High Molecular Weight Hydrogen Terminated Polymer
A polystyrene-polybutadiene-polystyrene type polymer of 165,700 molecular
weight was prepared as in Example 1. The polymer solution was 12.1% by
weight polymer. The polymer was hydrogenated with
bis(cyclopentadienyl)titanium dichloride (0.16 mM Ti per 100 grams
polymer). The LiH:Ti ratio for the hydrogenation run was 4:1. The final
conversion of the olefin was 10%. This LiH:Ti is not high enough for such
a high molecular | | |
