Molecular sieve abstract
In the solution polymerization process wherein monomers comprising
at least one conjugated diene are polymerized followed by coupling
with a polyfunctional coupling agent which results in some alkyl
chloride formation and solvent recovered from said polymerization
is used in a subsequent solution polymerization process, the improvement
comprising contacting said solvent with a molecular sieve to remove
alkyl chloride therefrom before said solvent is used in said subsequent
polymerization.
Molecular sieve claims
What is claimed is:
1. In the solution polymerization process wherein (1) monomers
comprising at least one conjugated diene are polymerized in a first
polymerization in a solvent in the presence of an organoalkali metal
initiator followed by coupling with a polyfunctional coupling agent
which results in some alkyl chloride formation, and (2) solvent
is recovered from said first polymerization and used in a subsequent
solution polymerization process wherein monomers selected from the
group consisting of conjugated dienes and vinyl-substituted aromatic
compounds are polymerized in the presence of an organoalkali metal
initiator, the improvement comprising contacting said solvent from
said first polymerization with a molecular sieve to remove alkyl
chloride therefrom before said solvent is used in said subsequent
solution polymerization process.
2. A process according to claim 1 wherein said molecular sieve
has an effective pore size in the range of about 4 to about 11 angstrom
units.
3. A process according to claim 2 wherein said polyfunctional coupling
agent is tin tetrachloride or silicon tetrachloride.
4. A process according to claim 3 wherein said molecular sieve
is an X type molecular sieve.
5. A process according to claim 4 wherein said molecular sieve
has a formula of Na.sub.86 [(AlO.sub.2).sub.86 (SiO.sub.2)].276H.sub.2
O.
6. A process according to claim 2 wherein the coupling following
said first polymerization results in a branched block copolymer
of the formula
Wherein A represents a block of one monomer and B represents a
block of another monomer, Y is an atom or group of atoms derived
from the polyfunctional coupling agent, and x is an integer of greater
than 3 representing the number of functional groups of the coupling
agent that attach Y to the A-B segments of the polymer.
7. A process according to claim 6 wherein A represents a non-elastomeric
block formed from vinyl-substituted aromatic compounds, and B represents
an elastomeric block formed from conjugated dienes.
8. A process according to claim 7 wherein said molecular sieve
is an X type molecular sieve.
9. A process according to claim 8 wherein said molecular sieve
has a formula of Na.sub.86 [(AlO.sub.2).sub.86 (SiO.sub.2).sub.106
].276H.sub.2 O.
10. A process according to claim 9 wherein said coupling agent
is silicon tetrachloride and said initiator is n-butyllithium.
11. A process according to claim 10 wherein said subsequent polymerization
is directed toward making the same polymer as that made in said
first polymerization.
12. A process according to claim 11 wherein said solvent comprises
cyclohexane.
13. A process according to claim 12 wherein A represents a block
prepared from styrene and B represents a block from 13-butadiene.
14. A process according to claim 13 wherein the weight ratio of
13-butadiene monomer to styrene monomer in the range of about 70/30
to about 52/48.
15. A process according to claim 2 wherein said first polymerization
results in the formation of a random copolymer.
16. A process according to claim 15 wherein said monomers in said
first polymerization are 13-butadiene and styrene.
17. A process according to claim 16 wherein said coupling agent
is tin tetrachloride and said initiator is n-butyllithium.
18. A process according to claim 17 wherein said subsequent polymerization
is directed toward making the same polymer as that made in said
first polymerization.
19. A process according to claim 18 wherein said solvent comprises
cyclohexane.
20. A process according to claim 19 wherein the weight ratio of
13-butadiene monomer to styrene monomer is about 75/25.
Molecular sieve description
MOLECULAR SIEVE PURIFICATION OF RECYCLED SOLVENT
This invention relates to preparing conjugated diene polymers by
solution polymerization. In another aspect, this invention relates
to solution polymerization processes wherein the solvent from one
solution polymerization is used in preparing conjugated diene polymers
in a subsequent solution polymerization.
The production of conjugated diene polymers by solution polymerization
is well known in the art. Some examples of such processes are disclosed
in U.S. Pat. Nos. 3078254; 3281383; 3393182; 3639521; 3639517;
and 4086298 the disclosures of which are incorporated herein
by reference. Typically, the polymerization process involves polymerizing
monomers comprising at least one conjugated diene in a solvent in
the presence of a suitable organoalkali metal initiator.
In commercial operations, it has proven advantageous to recover
the solvent from previous polymerizations for reuse in subsequent
polymerizations. The present applicant has observed that the reuse
of the solvent which was obtained from certain solution polymerizations
which employ polyfunctional coupling agents can result in undesirable
variations in the properties of the polymers produced in subsequent
solution polymerizations. The present invention is based upon the
applicant's discovery that the variations in the subsequently produced
polymers are due to alkyl chlorides that are formed as a byproduct
of the coupling step in the preceding polymerization process.
Accordingly, an object of the present invention is to provide an
improvement in the quality of polymers that are produced when using
previously used polymerization solvent. Still another object is
to remove a process variable which can cause variations in the quality
of polymer product. Other objects, advantages, and features of this
invention will be apparent to those skilled in the art from the
following discussion.
The present invention thus represents an improvement in the solution
polymerization process wherein (1) monomers comprising at least
one conjugated diene are polymerized in a first polymerization in
a solvent in the presence of an organoalkali metal initiator followed
by coupling with a polyfunctional coupling agent which results in
alkyl chloride formation; and (2) solvent is recovered and used
in a subsequent solution polymerization process wherein monomers
selected from the group consisting of conjugated dienes and vinyl-substituted
aromatic compounds are polymerized in the presence of an organoalkali
metal initiator. The improvement comprises contacting the solvent
from the first polymerization with a molecular sieve to remove alkyl
chloride from the solvent prior to the use of the solvent in the
subsequent polymerization.
The present invention is applicable to any of the prior art polymerization
processes wherein the coupling step results in the formation of
alkyl chlorides. Although, the mechanism is not totally understood,
it is believed that the alkyl chlorides form as a result of interaction
between chloride in the coupling agent and alpha-olefins or possibly
residual monomer in the polymerization mixture.
The subsequent polymerization can be the same as or different than
the first polymerization. It is not necessary that subsequent polymerization
be one employing a polyfunctional coupling agent. The extent to
which the alkyl chlorides affect the properties of the polymers
in the subsequent polymerization can vary depending upon the particular
type of polymer being produced in that polymerization.
The monomers employed in the polymerizations can be any of those
recognized as suitable in the prior art. The conjugated diene monomers
generally contain 4 to 12 carbon atoms per molecule and preferably
4 to 8 carbon atoms per molecule. Examples of such compounds include
13-butadiene, isoprene, 23-dimethyl-13-butadiene, piperylene,
3-butyl-13-octadiene, 2-phenyl-13-butadiene, and the like. Other
comonomers such as vinyl-substituted aromatic compounds can be employed
with the conjugated diene monomers. Vinyl-substituted aromatic compounds
generally contain 8 to 18 carbon atoms per molecule. Examples of
such compounds include styrene, 3-methylstyrene, 4-n-propylstyrene,
4-cyclohexylstyrene, 4-dodecylstyrene, 2-ethyl-4-benzylstyrene,
4-ptolylstyrene, 4-(4-phenyl-n-butyl)styrene, 1-vinylnaphthalene,
2-vinylnaphthalene, and the like. The vinyl-substituted aromatic
compounds can contain alkyl, cycloalkyl, and aryl substituents,
and combinations thereof such as alkaryl in which the total number
of carbon atoms in the combined substituents is generally not greater
than 12. The conjugated dienes can be polymerized alone or in admixture
with vinyl-substituted aromatic compounds to form any of the homopolymers,
random copolymers or block copolymers known in the art. Monomers
which are currently preferred are butadiene, isoprene, and styrene.
The term block copolymers as used herein is intended to include
both linear and branched copolymers. Included within the term block
copolymer are the polymers of the formulas A-B and (A-B).sub.x Y,
wherein A and B denote different polymer blocks, Y is an atom or
group of atoms derived from a polyfunctional coupling agent and
x represents the number of functional groups of the coupling agent
and is an integer of at least 2. Block polymers in which x is 3
or greater are branched block copolymers. The term teleblock polymers
is used herein to denote those block polymers which have identical
block portions on opposite ends of the polymers.
The polymers of the above-listed compounds are prepared by contacting
the monomer or monomers which it is desired to polymerize with an
organoalkali metal compound, including mono and polyalkali metal
compounds in the presence of a hydrocarbon diluent. The organoalkali
metal compounds preferably contain from 1 to 4 alkali metal atoms
per molecule. While organometallic compounds of any of the alkali
metals can be employed, organolithium compounds are preferred. The
alkali metals include lithium, sodium, potassium, rubidium, and
cesium.
The preferred class of organolithium initiators are those of the
formula RLi wherein R is a hydrocarbon radical selected from the
group consisting of aliphatic, cycloaliphatic, and aromatic radicals
containing from 1 to 20 carbon atoms, although higher molecular
weight initiators can be used. Examples of such initiators include
methyllithium, n-butyllithium, n-decyllithium, phenyllithium, naphthyllithium,
p-tolyllithium, cyclohexyl lithium, eicosyllithium, and the like.
The polymerization reactions can be conducted under any suitable
polymerization conditions. Generally, the temperature is in the
range between -100.degree. and +175.degree. C., preferably -75.degree.
and +125.degree. C. The polymerization can be conducted under autogenous
pressure. It is usually desirable to operate at pressures sufficient
to maintain the polymerization recipe ingredients substantially
in the liquid phase.
Any suitable solvents can be employed in the polymerizations. Typically,
the solvents are hydrocarbons selected from aromatics, paraffins,
cycloparaffins, and mixtures thereof, containing 4 to 10 carbon
atoms per molecule. Examples include isobutane, n-pentane, cyclohexane,
benzene, toluene, xylene, naphthalene, and the like.
Non-limiting examples of polyfunctional coupling agents which can
result in the formation of alkyl chlorides during the coupling step
include silicon tetrachloride and tin tetrachloride, especially
where those coupling agents are employed in excess of the stoichiometric
polymer-lithium present, as for example in those polymerization
processes in which maximum coupling is desired.
The solvent that is to be contacted with the molecular sieve material
can be recovered in any suitable manner. Typically, it is obtained
as the overhead from the flashing of the polymer cement resulting
from the polymerization. Preferably, the solvent that is contacted
with the molecular sieve is substantially free of water, i.e., containing
no more than about 10 ppm of water, preferably no more than 5 ppm.
In the practice of this invention any molecular sieve material
can be employed that is effective in removing alkyl chlorides from
the solvent. Applicable materials are the crystalline alumino-silicates
which have been heated to remove water of hydration. The adsorbent
materials may be prepared by any of the well known methods in the
art. Of the three classes of crystalline zeolites, fibrous, laminar,
and rigid three-dimensional anionic networks, the last mentioned
class only is suitable in this invention. Examples of such materials
include chabazite, phacolite, gmelinite, harmotone, and the like,
or suitable modifications thereof, produced by base exchange. The
literature contains many references to the composition and adsorbing
action of molecular sieves. Generally speaking, molecular sieves
are alkali metal or alkaline earth metal alumino-silicates and can
be either natural or synthetic in origin. Said materials have large
numbers of submicroscopic cavities interconnected by many smaller
pores or channels which are extremely uniform in size. In operation,
the generally accepted explanation for the action of the molecular
sieves is that adsorption takes place within said pores, and only
those materials having molecular diameters small enough to enter
said pores are retained by the zeolite. Hence, the name molecular
sieves. Generally, the molecular sieves applicable in the present
invention are those having pore diameters in the range of about
4 to about 11 angstrom units. The 4A, 10X, and 13X type molecular
sieves available from Linde Company have pore diameters in that
range. The currently preferred molecular sieve is the Linde 13X
type molecular sieve.
The molecular sieve materials can be employed in granular form,
such as 1/16 to 1/4 inch pellets, or in finely divided form, such
as about 200 mesh. The contacting of the dry solvent with the molecular
sieve materials can be carried out in any suitable zone, such as
a fixed bed, moving bed, or the like.
The conditions employed for contacting the dry solvent with the
molecular sieve materials can vary depending upon the concentration
of alkyl chloride, the desired degree of removal of alkyl chloride,
and other factors that will be readily apparent to those skilled
in the use of molecular sieves. The temperature of contacting will
generally be in the range of about 70.degree. to about 200.degree.
F., preferably about 70.degree. to about 150.degree. F., and more
preferably about 80.degree. to about 100.degree. F. The contacting
pressure is not critical and generally will be within the range
of from atmospheric to about 600 psig. Generally, it is preferred
to employ a pressure sufficient to maintain the solvent in the liquid
phase and sufficient to provide for normal pressure drops through
the bed when a fixed bed is employed. Generally, the contacting
can be carried out at liquid space velocities within the range of
from 0.5 to 10.0 preferably 0.8 to 1.2 volumes of solvent per volume
of molecular sieve per hour.
The molecular sieve adsorbent materials employed in the practice
of the invention can be regenerated in any suitable manner such
as by heating, and/or contacting with a suitable gas, for example,
hydrogen. The hydrogen employed in many instances can conveniently
be obtained from a catalytic reforming unit. Regeneration temperatures
within the range of from about 200.degree. to about 600.degree.
F., preferably 300.degree. to 450.degree. F., can be utilized. Said
regeneration can be carried out at any suitable pressure. However,
when a gas such as hydrogen from a catalytic reforming unit is employed
as the regeneration medium it is generally preferred to carry out
the regeneration at approximately the same system pressure employed
in the catalytic reforming operation, for example, within the range
of from about 400 to about 1000 psig. It is also within the scope
of the invention to employ suitable liquid regeneration medium.
Another method which can be employed involves evacuation of the
contacting vessel. Still another method is to pass air or other
oxygen-containing gas preheated to a temperature within the range
of about 500.degree. to 800.degree. F. so as to burn off the adsorbed
materials under controlled conditions.
The objects and advantages of the present invention can be further
illustrated by the following examples.
EXAMPLE 1
In a commercial operation for producing a 52/48 weight ratio 13-butadiene/styrene
branched teleblock copolymer by solution polymerization and a subsequent
coupling step, the solvent comprising cyclohexane from each polymerization
step was recovered under conditions which minimized solvent loss.
The recovered solvent was then reused in subsequent polymerizations
producing additional amounts of the same copolymer. The copolymers
produced in the later polymerizations had lower Shore A hardness
and higher micrometer deflection properties. Also, the molecular
weight distribution in the polymers produced using recovered solvent
was different from those produced using fresh solvent.
Polymerization tests of initiator, monomers, solvent, and coupling
agent demonstrated that the problem was due to an impurity in the
recycle solvent. Numerous gas chromatographic analyses were made
on the recycle solvent. None of the analyses revealed the presence
of anything which was thought likely to cause the variations noted.
Only those hydrocarbons normally expected in the recycle solvent
were noted.
The coupling agent used in making this branched teleblock copolymer
was silicon tetrachloride.
The recycle solvent was fractionated into 13 fractions and the
chloride content of each fraction was determined. The presence of
chloride in at least the lighter fractions was confirmed, but the
concentration of chloride in the unfractionated recycle solvent
was less than the detectable limit of 10 mg/kg.
Certain of the fractions of recycle solvent were employed in laboratory
polymerizations. The only fractions that produced polymers having
the unusual molecular weight distribution were the two having chloride
content of at least 23 mg/kg.
A more detailed study of certain of the fractions using gas chromatography
and mass spectrometry revealed the presence of tertiary-butyl chloride.
The presence was not detected using only gas chromatography because
the tertiary-butyl chloride peak was overshadowed by a pentene peak.
In order to determine whether the tertiary butyl chloride was the
component causing the undesired variations, a series of laboratory
polymerizations were carried out using laboratory cyclohexane containing
various levels of tertiary-butyl chloride. The addition of tertiary-butyl
chloride to the solvent in amounts of 25 and 100 mg/kg resulted
in polymers having variations in molecular weight distribution analogous
to those observed in the commercial polymers prepared using recycled
solvent.
EXAMPLE II
In order to obtain a more accurate determination of the amount
of alkyl chlorides in the recycle solvent analysis was conducted
using a Tracor 560 gas chromatograph equipped with a Hall 700A electrolytic
conductivity detector. The adsorbents and electrolytes were selected
so that the Hall detector would not respond to non-halogen containing
compounds. This technique allowed for the recognition of alkyl chlorides
in amounts as low as 0.1 mg/kg. Alkyl chlorides noted in this manner
included tertiary-butyl chloride, tertiary-amyl chloride, isopropyl
chloride, and 2-chlorobutane, with tertiary-butyl chloride being
the major alkyl chloride at a level of about 42.5 mg/kg.
EXAMPLE III
Significant levels of tertiary-butyl chloride were also noted in
the recycle solvent from polymerizations producing 70/30 and 60/40
weight ratio 13-butadiene/styrene branched teleblock copolymers.
Both of these copolymers are formed using silicon tetrachloride
as a coupling agent. As in the production of the 52/48 13-butadiene/styrene
copolymers as described in Example I, the coupling agent in these
processes is used in excess of the stoichiometric amount to assure
maximum branching. The use of the chloride-containing recycle solvent
in the respective polymerization processes produced variations in
the hardness and stiffness of the polymers analogous to that observed
in the production of the 52/48 13-butadiene/styrene copolymer in
Example I.
EXAMPLE IV
Significant levels of tertiary-butyl chloride were also noted in
the recycle solvent from a polymerization producing a 75/25 weight
ratio 13-butadiene/styrene random copolymer which is coupled with
tin tetrachloride. The solvent from that polymerization was used
in a process for producing a 52/48 weight ratio 13-butadiene/styrene
linear random tapered block copolymer. The latter polymer is produced
using a fatty acid shortstopping agent rather than a tin or silicon
chloride coupling agent. The use of the chloride-containing solvent
resulted in a major modification of the 52/48 13-butadiene/styrene
linear, tapered block copolymer structure. Instead of obtaining
a linear, random tapered block copolymer having a butadiene/styrene
random tapered block at one end and a styrene block at the other
end, there was obtained a polymer having significant amounts of
butadiene/styrene random tapered blocks at each end, i.e., a polymer
having a polystyrene center block and two rubbery butadiene/styrene
random tapered copolymer end blocks. |