Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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PROCESS FOR PREPARING HIGHER MOLECULAR WEIGHT POLYISOBUTYLENE
Description
The present invention relates to an improved process for preparing isobutene
homopolymers
having a weight-average molecular weight of 75 000 to 10 000 000 by
polymerization of
isobutene in the liquid phase in an inert solvent in the presence of a
polymerization catalyst
based on Lewis acids.
Efficient preparation processes which satisfy the specification for higher
molecular weight
polyisobutenes generally entail very low polymerization temperatures. A
typical process for
preparing higher molecular weight polyisobutenes is called the "BASF belt
process", in which
liquid isobutene together with boron trifluoride as a polymerization catalyst
and a high excess
of liquid ethene are passed onto a continuous steel belt of width from 50 to
60 cm, which is
configured in a trough shape by suitable guiding and is present in a gas-tight
cylindrical
casing. Constant evaporation of the ethene at standard pressure sets a
temperature of -
104 C. This fully removes the heat of polymerization. The evaporated ethene is
collected,
purified and recycled. The resulting polyisobutenes are freed of ethene which
still adheres
and residual monomers by degassing. The polymerization of this type leads to
virtually full
isobutene conversion.
In the BASF belt process, the polymerization temperature can be controlled
easily and
reliably owing to evaporative cooling, i.e. as a result of formation of large
vapor passages.
However, a disadvantage of the BASF belt process is that, owing to lack of
movement of the
reaction material on the belt, inadequate mixing of the reaction material and
hence no
product surface renewal takes place, which can have a disadvantageous effect
on the
product properties. This leads, for example, to inhomogeneous distribution of
the ethene
used for evaporative cooling and associated local overheating of the reaction
mixture as
soon as the ethene has vaporized. In addition, there may be explosive boiling
of the reaction
mixture when overheated regions and ethene-rich cold regions come into contact
with one
another, which then leads to soiling of the reactor wall as a result of
entrainment of
polymerizing reaction mixture. Another disadvantage is that the inhomogeneous
temperature
distribution causes unwanted broadening of the molecular weight distribution
of the polymer,
which is associated with unfavorable product properties. A further
disadvantage of the BASF
belt process is that the steel belt is subject to wear and thus causes high
maintenance costs.
A further disadvantage of the BASF belt process is that the reactor walls and
the product
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intake in the downstream workup section (usually an extruder) are not cooled;
since
polyisobutylene is highly tacky above its glass transition temperature, this
leads to significant
coverage of the reactor walls with sticky polymer, which necessitates an
increased level of
cleaning. A further disadvantage of the BASF belt process is that boron
trifluoride present in
.. the recycled ethene stream is highly corrosive at relatively high
temperatures, which causes
a high level of maintenance in the ethene workup circuit.
A further customary process for preparing higher molecular weight
polyisobutenes is the
"Exxon slurry process", in which the polymerization is performed at -80 to -85
C in a stirred
tank equipped with a cooling jacket which is charged with liquid ethene. The
catalyst system
used is anhydrous aluminum chloride in methyl chloride. Owing to the very
vigorous stirring,
the polymer is obtained as a slurry consisting of small droplets which flows
via an
intermediate vessel into a degassing vessel. Here, the slurry is treated with
steam and hot
water so that the volatile constituents (essentially unconverted isobutene and
methyl
chloride) can be removed and sent to reprocessing. The remaining liquid slurry
of the
polymer particles is worked up by removing catalyst residues, solvent residues
and
isobutene residues.
In the Exxon slurry process, although intensive mixing and product surface
renewal takes
.. place, the polymerization temperature is difficult to control solely by the
jacket cooling. Since
the polymer cannot completely be prevented from adhering to the reactor and
apparatus
walls, reactor and apparatus have to be cleaned from time to time.
The BASF belt process and the Exxon slurry process are described in detail in
Ullmann's
Encyclopedia of Industrial Chemistry, 5th edition, Vol. A21, pp. 555-561,
under
"polyisobutylenes".
It was an object of the present invention to provide an easily performable,
efficient and
economically viable process for preparing higher molecular weight isobutene
homopolymers,
which, with regard to the product parameters to be established, such as
molecular weight,
polydispersity and residual monomer content, allows reliable control of the
polymerization
and affords an easily purifiable and efficiently manageable product, in
particular one which
does not stick prior to workup. Due to the comparatively intense mixing of the
reaction
mixture, the polymerization should be performed in a customary closed reactor
and in the
.. disperse phase in an immiscible fluid or mixed homogeneously in a miscible
fluid, i.e. in a
suitable solvent or diluent.
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The polymerization of isobutene to give higher molecular weight isobutene
homopolymers in
customary closed reactors in solvents or diluents is also known from other
documents as well
as the Exxon slurry process described. For instance, DE-A 2 061 289 discloses
an isobutene
polymerization process in which isobutene is polymerized between 0 C and -160
C in an
inert diluent such as ethylene, methane, ethane or propane by means of boron
trifluoride as
a catalyst in the presence of a solution of formaldehyde in an alcohol such as
isobutanol as a
molecular weight regulator in a reaction flask to give higher molecular weight
polyisobutene.
In the monograph "Polymerization and Polycondensation Processes, Advances in
Chemistry
Series 34" (1961), J. P. Kennedy and R. M. Thomas describe, in their article
"Cationic
Polymerization at Ultralow Temperatures", on pages 111-119, the polymerization
of
isobutene in a propane-isopentane mixture in a cooled reactor at -30 C to -190
C by means
of an aluminum trichloride catalyst to give higher molecular weight
polyisobutene. Aluminum
trichloride has the disadvantage that, as a nonvolatile catalyst, it
complicates the subsequent
purification of the polyisobutene. Reaction accelerators or chain length
regulators are not
used.
The literature article "Fundamental Studies on Cationic Polymerization IV ¨
Homo- and Co-
polymerizations with Various Catalysts" by J. P. Kennedy and R. G. Squires in
Polymer 6,
pages 579-587, 1965 discloses that isobutene can be polymerized under boron
trifluoride
catalysis in alkyl chloride solvents at -30 C to -146 C in the presence of
isoprene to give
higher molecular weight polyisobutene. Reaction accelerators are not used.
The object of the present invention is achieved by a process for preparing
isobutene
homopolymers having a weight-average molecular weight of 75 000 to 10 000 000
by
polymerization of isobutene in the liquid phase in an inert solvent in the
presence of a
polymerization catalyst based on Lewis acids, which comprises, in a
polymerization reactor,
at the same time
(a) performing the polymerization at temperatures of -80 C to -190 C,
(b) using, as an inert solvent, one or more Ci to C8 hydrocarbons or one
or more
halogenated Ci to C8 hydrocarbons or a mixture thereof and
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(c) using, as a polymerization catalyst, a Lewis acid complex based on
boron trifluorid, on
iron halides, on aluminum trihalides or on aluminum alkyl halides or a Lewis
acid in
combination with organic sulfonic acids as initiators,
and additionally
(d) performing the polymerization in the presence of at least one reaction
accelerator in the
form of an ethylenically saturated hydrocarbon compound comprising at least
one
oxygen atom and no abstractable proton, and/or
(e) performing the polymerization in the presence of at least one chain
length regulator
comprising at least one tertiary olefinic carbon atom.
In a preferred embodiment, measures (d) and (e) are both performed.
In the context of the present invention, isobutene homopolymers are understood
to mean
those polymers which, based on the polymer, are composed of isobutene to an
extent of at
least 98 mol%, preferably to an extent of at least 99 mole&
For the use of isobutene, or of the isobutenic monomer mixture as the monomer
to be
polymerized, suitable isobutene sources are, more particularly, pure isobutene
which
generally comprises at most 0.5% by volume of residual impurities such as 1-
butene, 2-
butenes, butane, water and/or C1- to C4- alkanols. However, it is also
possible in principle to
use isobutenic technical C4 hydrocarbon streams, for example, C4 raffinates,
C4 Cuts from
isobutane dehydrogenation, 04 cuts from steamcrackers and from FCC crackers
(fluid
catalyzed cracking), provided that they have been substantially freed of 1,3-
butadiene
present therein. Suitable technical C4 hydrocarbon streams comprise generally
less than
500 ppm, preferably less than 200 ppm, of butadiene. The isobutene from such
technical
C4 hydrocarbon streams is polymerized here substantially selectively to the
desired isobutene
homopolymer without incorporation of significant amounts of other C4 monomers
into the
polymer chain. Typically, the isobutene concentration in the technical 04
hydrocarbon
streams mentioned is in the range from 40 to 60% by weight. However, the
process
according to the invention can in principle also be operated with isobutenic
C4 hydrocarbon
streams which comprise less isobutene, for example, only 10 to 20% by weight.
The
isobutenic monomer mixture may comprise small amounts of contaminants such as
water,
carboxylic acids or mineral acids without any critical yield or selectivity
losses. It is
appropriate to the purpose to avoid accumulation of these impurities by
removing such
5
harmful substances from the isobutenic monomer mixture, for example, by
adsorption on solid
adsorbents such as activated carbon, molecular sieves or ion exchangers.
The Lewis acid complexes which are to be used as a polymerization catalyst
according to
measure (c) and are based on iron halides, on aluminum trihalides or on
aluminum alkyl
halides, and Lewis acids which are to be used as a polymerization catalyst in
combination with
organic sulfonic acids as initiators, are described in detail in WO
2012/072643 A2. The iron
halide, aluminum trihalide and aluminum alkyl halide complexes mentioned
comprise, as well
as the Lewis acid, a donor in the form of an organic compound having at least
one ether
function or a carboxylic ester function. This combination of Lewis acids,
especially of boron
trifluoride, iron halides, aluminum trihalides or aluminum alkyl halides, with
organic sulfonic
acids as initiators comprise at least one organic sulfonic acid of the general
formula Z-S03H in
which Z is a 01-C20-alkyl radical, 01-020-haloalkyl radical, 06-C8-cycloalkyl
radical, 06-C20-aryl
radical or a C7-020-aralkyl radical; a typical organic sulfonic acid of this
kind is methanesulfonic
acid.
According to measure (c), the polymerization catalyst used, however, is
preferably a complex
of boron trifluoride and a proton source. Suitable proton sources of this
kind, which assume
the function of an activator or moderator in the catalyst complex, are in
particular ethers,
especially C1- to C4-dialkyl ethers such as diethyl ether, and alcohols,
especially low molecular
weight monohydric aliphatic alcohols. In a particularly preferred embodiment,
the
polymerization catalyst used is a complex of boron trifluoride and a Cl- to C3-
alkanol, e.g.
methanol, ethanol, n-propanol or isopropanol. The proton sources used may also
be mixtures
of the ethers and/or alcohols mentioned.
The boron trifluoride and the proton source may be premixed and added to the
polymerization
reactor already as an active complex. Alternatively, however, the boron
trifluoride [in gaseous
or liquid form or in an inert solvent or diluent, for example dissolved in an
inert solvent according
to measure (b)] and the proton source may also be supplied separately to the
polymerization
medium.
The amount of polymerization catalyst to be used is guided substantially by
the type of catalyst
and by the reaction conditions, especially the reaction temperature and the
desired molecular
weight of the polymer. It can be determined on the basis of a few sample tests
for the respective
reaction system. In general, the polymerization catalyst is used in amounts of
0.0001 to 1% by
weight, especially 0.0005 to 0.5% by weight, in particular 0.001 to 0.1% by
weight, based in
each case on the Lewis acid content or boron trifluoride content in the
catalyst complex and
on isobutene used.
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The proton source can be used in a substoichiometric, stoichiometric or
superstoichiometric
amount in relation to the boron trifluoride. Typical molar ratios of proton
source to boron
trifluoride are in the range from 0.3:1 to 3:1, especially 0.5:1 to 2:1, in
particular 0.7:1 to 1.3:1
(based in each case on one proton equivalent of the proton source). Just like
the amount of
reaction accelerator according to measure (d) and of chain length regulator
according to
measure (e), it is also possible for the amount of proton source according to
measure (c) to
influence the establishment of the molecular weight to be achieved in the
isobutene
homopolymer and also to serve for controlled establishment of the molecular
weight thereof.
The isobutene homopolymers prepared by the process according to the invention
preferably
have a weight-average molecular weight (Mw) of 150 000 to 8 000 000,
especially of 250 000
to 6 000 000, in particular of 400 000 to 5 000 000. Alternatively, they
preferably have a
number-average molecular weight (Me) (determined by gel permeation
chromatography) of
25 000 to 2 000 000, more preferably of 45 000 to 1 500 000, especially of 55
000 to
1 000 000, in particular of 65 000 to 750 000.
In general, the isobutene homopolymers prepared by the process according to
the invention
have a polydispersity (PDI = Mw/Mn) of 2 to 20, especially of 3 to 15, in
particular of 5 to 10.
According to measure (a), the polymerization process according to the
invention is performed
in the liquid polymerization medium at temperatures of -80 C to -190 C. In a
preferred
embodiment, it is performed at temperatures close to the lower limit of the
abovementioned
temperature range, specifically at -130 C to -190 C, in particular at less
than -160 C to -
185 C, especially at -165 C to -180 C, in a typical procedure at -168 C to -
173 C. In an
alternative preferred embodiment, the process is performed at temperatures of -
100 C to -
150 C, preferably at -105 C to -147 C, in particular at -110 C to -140 C,
especially at -115 C
to -135 C, in a typical procedure at -120 C to -130 C. The controlled low
polymerization
temperatures have an advantageous effect on the product properties. The
temperature
establishment in the precooling of the starting materials used, especially of
the isobutene,
can under some circumstances likewise influence the course of the
polymerization and the
results achieved; the isobutene to be used is cooled typically to temperatures
of -70 C to -
140 C, especially to -70 C to -100 C.
The reaction medium is advantageously cooled to the abovementioned
temperatures by
external cooling. In a preferred embodiment, therefore, measure (a) is
executed by bringing
the polymerization medium to the required low temperature and keeping it there
during the
polymerization by means of a separate cooling circuit. The separate cooling
circuit, which in
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terms of design is usually implemented as an outer cooling jacket around the
polymerization
reactor, is generally operated with liquid nitrogen or liquefied air as the
coolant.
The polymerization is performed generally at a pressure of 500 mbar to 5 bar,
especially at a
pressure of 800 mbar to 2 bar. Most advantageously and also most economically
viably, the
polymerization reactor is operated at or close to ambient pressure (standard
pressure). A
slightly elevated pressure can bring advantages in the case of some of the
possible inert
solvents. Even though a mode of operation of the polymerization at elevated
pressure is
possible in principle, higher pressures, especially those over 5 bar,
generally do not bring
any additional advantages.
According to measure (b), particular inert solvents or mixtures of such inert
solvents are used
in the liquid polymerization medium. The term "inert solvents" shall be
understood here to
mean not just fluids in which isobutene dissolves homogeneously in a liquid
phase but also
.. fluids with which isobutene is immiscible and is present in the dispersed
form. Suitable inert
solvents of this kind are firstly Ci to C8 hydrocarbons, preferably Ci to C5
hydrocarbons,
especially C2 to 04 hydrocarbons, which are typically saturated or
monoethylenically
unsaturated and generally have a linear or lightly branched structure. If they
are ethylenically
unsaturated, they must not of course themselves polymerize under the reaction
conditions of
the present invention; they normally have only primary and/or secondary
olefinic carbon
atoms. Typical examples of such Ci to C8 hydrocarbons are methane, ethane,
ethene,
propane, propene, n-butane, isobutane, n-pentane, 2-methylbutane, 2,3-
dimethylbutane, 2-
methylpentane, 3-methylpentane, 3-ethylpentane, 2,2-dimethylpentane, 2,3-
dimethylpentane, 2,4-dimethylpentane, 2-methylhexane, 3-methylhexane, 3-ethyl-
2-
methylpentane, 2,2-dimethylhexane, 2,3-dimethylhexane, 3,3-dimethylhexane, 4-
methylheptane, 2,2,3-trimethylpentane and 3-methylheptane. Other suitable
inert solvents of
this kind are halogenated Ci to C8 hydrocarbons, preferably halogenated Ci to
C5
hydrocarbons, especially fluorinated and/or chlorinated Ci to CB or Ci to Cs
hydrocarbons
such as methyl chloride, methyl fluoride, difluoromethane, dichloromethane,
fluoroethane, 1-
fluoropropane, 1,1,1,2,3,3,3-heptafluoropropane, octafluoropropane or 1-
fluorobutane;
particularly useful here are perfluorinated Ci to C8 or Ci to Cs hydrocarbons
or those C1 to C8
or Ci to 05 hydrocarbons in which at least half of the hydrogen atoms have
been replaced by
fluorine atoms. It is also possible to use mixtures of C1 to C8 or Ci to 05
hydrocarbons,
mixtures of halogenated Ci to C8 or Ci to C5 hydrocarbons or mixtures of one
or more Ci to
C8 or C1 to 05 hydrocarbons and one or more halogenated Ci to 08 or Ci to C5
hydrocarbons.
In a preferred embodiment, as measure (b), the inert solvent used is ethane,
ethene,
propane, propene, n-butane, isobutane or a mixture thereof.
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In an alternative preferred embodiment, as measure (b), the inert solvent used
is
1,1,1,2,3,3,3,-heptafluoropropane, octafluoropropane or a mixture thereof.
The weight ratio of isobutene to the inert solvents according to measure (b)
in the
polymerization reactor is generally 1:0.1 to 1:50, preferably 1:0.1 to 1:40,
in particular 0.1:1 to
1:20, especially 1:0.5 to 1:10.
According to measure (d), the polymerization is performed in the presence of
one or more
reaction accelerators. Such a reaction accelerator is a compound which, under
the selected
polymerization conditions, influences and thus controls the catalytic activity
of the boron
trifluoride in the desired manner. Such reaction accelerators are saturated
hydrocarbon
compounds which comprise at least one oxygen atom, preferably as an ether
oxygen atom
or as part of a carbonyl function. In a preferred embodiment, as measure (d),
the
polymerization is performed in the presence of at least one reaction
accelerator selected
from ketones, aldehydes, ethers, acetals and hemiacetals. Typically, such
reaction
accelerators are low molecular weight compounds having 1 to 40, especially
having 1 to 16
and in particular having 1 to 8 carbon atoms; the structure thereof may be
open-chain or
cyclic; they may be aliphatic, aromatic or heteroaromatic in nature.
Typical representatives of such reaction accelerators are ketones such as
acetone,
butanone, cyclohexanone, acetophenone or benzophenone, aldehydes such as
formaldehyde, trioxane, paraformaldehyde, acetaldehyde, propionaldehyde,
butyraldehyde,
benzaldehyde, cyclohexylaldehyde or glyoxal, dialkyl ethers such as dimethyl
ether, diethyl
ether or di-n:butyl ether, cyclic ethers such as tetrahydrofuran or dioxane,
and acetals and
hemiacetals which are obtainable by reaction of the abovementioned ketones and
aldehydes
with alcohols such as methanol, ethanol, n-propanol, isopropanol, n-butanol,
isobutanol, sec-
butanol or tert-butanol. Very particular preference is given to using
formaldehyde as such a
reaction accelerator.
The reaction accelerators mentioned can usually be used in an advantageous
manner
together with one or more medium molecular weight alcohols, especially
monohydric
aliphatic, cycloaliphatic or araliphatic alcohols, in particular Ca to Cio
alcohols, e.g. n-butanol,
isobutanol, sec-butanol, tert-butanol, n-pentanol, n-hexanol, n-octanol, 2-
ethylhexanol, n-
decanol, 2-propylheptanol, cyclohexanol or benzyl alcohol. Firstly, such
medium molecular
weight alcohols ¨ similarly to the low molecular weight alcohols used as a
proton source
according to measure (c) ¨ act as activators or moderators in the catalyst
complex, but
usually with weaker activating action; secondly, they function as solvents for
the reaction
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accelerators. When aldehydes or ketones are used as reaction accelerators, the
abovementioned medium molecular weight alcohols, and also some of the low
molecular
weight alcohols mentioned, can form acetals or hemiacetals or ketals (ketone
acetals)
therewith, and these likewise act as reaction accelerators. If formaldehyde is
used as a
reaction accelerator, it is possible to use a corresponding alcoholic
solution, e.g.
formaldehyde in isobutanol. If medium molecular weight alcohols of this kind
are used, the
weight ratio thereof relative to the reaction accelerator is generally 0.05:1
to 15:1, but
preferably 0.1:1 to 5:1, especially 0.5:1 to 2.5:1, in particular 0.75:1 to
1.5:1.
The reaction accelerator itself is normally used in amounts of 0.0001 to 1% by
weight,
preferably 0.0003 to 0.75% by weight, especially 0.0005 to 0.5% by weight, in
particular
0.001 to 0.1% by weight, based in each case on isobutene used.
According to measure (e), the polymerization is performed in the presence of
at least one
chain length regulator, which is normally an ethylenically unsaturated system
and comprises
one or more tertiary olefinic carbon atoms ¨ optionally in addition to one or
more primary
and/or secondary olefinic carbon atoms. Usually, such chain length regulators
are mono- or
polyethylenically unsaturated hydrocarbons having 5 to 30, especially having 5
to 20 and in
particular having 5 to 16 carbon atoms; the structure thereof may be open-
chain or cyclic.
Typical representatives of such chain length regulators are isoprene (2-methyl-
1,3-
butadiene), 2-methyl-2-butene, diisobutene, triisobutene, tetraisobutene and 1-
methylcyclohexene. In a preferred embodiment, as measure (e), the
polymerization is
performed in the presence of isoprene and/or diisobutene as chain length
regulators.
Diisobutene (isooctene) is typically understood to mean the isomer mixture of
2,4,4-trimethyl-
1-pentene and 2,4,4-trimethy1-2-pentene; the individually used 2,4,4-trimethy1-
1-pentene and
2,4,4-trimethy1-2-pentene isomers also of course likewise act as chain length
regulators
according to measure (e). Through the amount of the chain length regulators
used in
accordance with the invention, it is possible in a simple manner to adjust the
molecular
weight of isobutene homopolymers obtained: the higher the amount of chain
length
regulators, the lower the molecular weight will generally be. The chain length
regulator
typically controls the molecular weight by being incorporated into the polymer
chain at an
earlier or later stage and thus leading to chain termination at this site.
The chain length regulator is used normally in amounts of 0.0001 to 2% by
weight, especially
0.0005 to 1% by weight, in particular 0.001 to 0.5% by weight, based in each
case on
isobutene used.
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The process according to the invention for preparing isobutene homopolymers ¨
including
the subsequent workup steps ¨ can be performed batchwise or continuously.
The polymerization reactors used for the process according to the invention
may in principle
5 be all batchwise or continuous reactor types suitable for such liquid
phase polymerizations,
for example stirred tanks, stirred tank cascades, kneaders, extruders, tubular
reactors or loop
reactors.
It is advantageous to perform the process according to the invention at high
conversions, if at
10 all possible at full conversion or near full conversion, for example at
a conversion of the
isobutene used to the desired product of 85% to 100%, especially of 90% to
100%. However,
it is also possible ¨ especially in continuous mode ¨ to perform the process
according to the
invention with partial conversion, for example at a conversion of the
isobutene used to the
desired product of 10% to 85%, especially of 30% to 60%. In a preferred
embodiment, the
polymerization conditions for the process according to the invention are
selected such that
the isobutene used is converted in the polymerization reactor with a
conversion of at least
90%, especially of at least 95%, in particular of at least 99%, to isobutene
homopolymers
having a weight-average molecular weight of 75 000 to 10 000 000.
In an advantageous execution of the process according to the invention, the
isobutene
feedstock or an isobutenic hydrocarbon mixture is premixed in a separate
vessel together
with the proton source, especially a to C3-alkanol, and together with one
or more reaction
accelerators, especially selected from ketones, aldehydes, ethers, acetals and
hemiacetals,
and/or one or more chain length regulators, especially selected from isoprene
and
diisobutene, and added in the polymerization reactor to the inert solvent
which comprises the
boron trifluoride and has been cooled to polymerization temperature. It is
also particularly
advantageous to cool this mixture prior to addition to the polymerization
reactor. This
isobutenic mixture is added to the polymerization reactor such that the
external cooling
allows the desired polymerization temperature to be kept constant. Rapid and
complete
mixing of the isobutene into the continuous phase is crucial for effective
temperature control
and hence for the success of the process.
Evaporating nitrogen from the external cooling can then either be liquefied
again in a closed
circuit or ¨ without needing to undertake purification ¨ passed into the
environment. In the
case of renewed liquefaction of the recycled nitrogen stream, the low
temperature of the
evaporated nitrogen can be utilized advantageously and thus recovered. If the
recycling is
omitted, the refrigeration energy of the gaseous nitrogen can be utilized for
other cooling
purposes, for example for the cooling of the degassed end product.
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As an alternative to liquid nitrogen or liquefied air, it is also possible
when working within the
range of a polymerization temperature of -100 C to -150 C to work with other
external
coolants, for example based on halogenated hydrocarbons.
The isobutene homopolymers which have a weight-average molecular weight of 75
000 to
000 000 and are prepared in the process according to the invention are worked
up
typically by discharging the product from the polymerization reactor and ¨
optionally after a
suitable pretreatment ¨ by thermal purification of the product. The discharge
is
10 advantageously effected at very low temperatures. The discharge from the
reactor can be
undertaken, for example, with the aid of a mechanical discharge device such as
a discharge
screw. In a preferred embodiment, which is of significance especially in the
case of industrial
scale execution of the process according to the invention, the isobutene
homopolymers
obtained in the polymerization reactor are discharged from the polymerization
reactor at
temperatures of less than -80 C and subjected to a thermal purification
process at
temperatures of more than +80 C.
The thermal purification after the discharge of the product from the
polymerization reactor is
advantageously effected, in the case of industrial scale execution of the
process according to
the invention, by use of one or more extruders. In this case, the isobutene
homopolymers are
heated to temperatures of more than 80 C, especially more than 100 C. The
mechanical
action of the extruder shafts and of any internals in the extruder constantly
renews the inner
surface for better degassing of the volatile constituents in the product, such
as residual
monomers and solvents. The degassing and the purification of the product can
be facilitated
by applying a vacuum; more particularly, a pressure of less than 700 mbar is
employed for
this purpose, especially of less than 200 mbar and in particular of less than
100 mbar.
It is possible in principle to use all customary single-shaft and twin-shaft
and multishaft
extruders for the thermal purification of the isobutene homopolymers obtained.
In the case of
twin-shaft and multishaft extruders, the shafts may work in a corotatory or
contrarotatory
manner. The shafts in single-shaft and multishaft extruders are normally
equipped with
kneading and/or conveying elements. These apparatuses are generally self-
cleaning. The
shaft speeds are generally in the range from 10 to 500, and especially from 15
to 350
revolutions per minute. In a specific design, the shafts may be configured as
screw shafts
whose channels intermesh and whose internal shaft diameter is preferably
constant over the
entire length. Preferred construction materials for the extruders described
are steels or
stainless steels. It is also advantageous to introduce an inert gas, for
example nitrogen, into
one or more segments of the extruder in order to promote the degassing
operation.
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The process according to the invention has the advantage that the isobutene
homopolymers
obtained have only a low solubility in the solvent used (hydrocarbons and/or
halogenated
hydrocarbons) ¨ and this is especially true at low temperatures ¨ and hence
precipitate out
substantially in solid form. This precipitated solid has no tendency
whatsoever to stick at the
low temperatures used, and so the crude product can be discharged and
processed further
without difficulty since nowhere in the intake region of the product from the
reactor into the
workup section do temperatures exceed the glass transition temperature of the
polymer.
The examples which follow are intended to illustrate the present invention
without restricting
it.
Examples 1 to 12
A 1 liter three-neck flask with mechanical stirrer, inlet tube for dry,
gaseous nitrogen for
inertization, a thermocouple for temperature monitoring and a coolable
dropping funnel was
inertized with the aid of liquid nitrogen (in a Dewar flask positioned around
the flask) and then
cooled to -100 C. Subsequently, 300 ml of liquid propane was initially charged
in the flask
under a nitrogen atmosphere and 0.1 g of gaseous boron trifluoride was
introduced.
The Dewar flask under the flask stood on a height-adjustable lab jack.
Variation of the fill
level in the Dewar flask allowed the desired polymerization temperature T to
be established
without difficulty.
94.0 g (1.68 mol) of liquid isobutene were introduced into the dropping funnel
cooled with dry
ice (approx. -78 C) or liquid nitrogen (approx. -130 C). Subsequently, the
amounts of
methanol, isobutanol, formaldehyde (which had been freshly produced from
paraformaldehyde and was present dissolved in the methanol/isobutanol mixture)
and
diisobutene specified below in each case were metered in and mixed with the
isobutene in
the dropping funnel.
After the attainment of the desired polymerization temperature T in the flask,
the dropwise
addition of the dropping funnel contents was commenced while stirring. Each
droplet reacted
immediately and a fine solid was obtained to an increasing degree. The desired
reaction
temperature T was kept constant over the entire dropwise addition time by
raising or lowering
the Dewar flask by means of the jack and adding more liquid nitrogen.
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Once the entire dropping funnel contents have been added dropwise, the
contents of the
flask were allowed to thaw, in the course of which the propane solvent
(boiling point: -42 C)
evaporated. Subsequently, the crude product which had become tacky at room
temperature
was removed and it was freed of the residual solvent by heating in a drying
cabinet
(temperature: 160 C at 30 mbar, duration: 2 h). Thereafter, it was possible to
determine the
analytical data of the isobutene homopolymer obtained.
The table below indicates the temperatures, the amounts used and the
analytical data of the
products obtained in each case.
Example No. 1 2 3 4 5 6
Polymerization temp. T -170 C -170 C -170 C -170 C -170 C
-170 C
Isobutene precooling -78 C -78 C -78 C -130 C -130 C -130 C
Amount of methanol 0.15 ml 0.15 ml 0.15 ml 0.15 ml 0.15
ml 0.15 ml
Amount of isobutanol 0.05 ml 0.05 ml 0.05 ml 0.05 ml 0.05
ml 0.05 ml
Amount of formaldehyde 0.05 g 0.05 g 0.05 g 0.05 g 0.05 g 0.05
g
Amount of diisobutene 0.01 ml 0.02 ml 0.05 ml 0 ml 0.01 ml
0.05 ml
Molecular weight Mw 824000 796000 483000 5157000 3831000 2055000
Molecular weight Mn 123000 117000 68000 586000 421000 221000
Polydispersity D 6.7 6.8 7.1 8.8 9.1 9.3
Example No. 7 8 9 10 11 12
Polymerization temp. T -150 C -140 C -130 C -120 C -110 C
-100 C
lsobutene precooling -78 C -78 C -78 C -78 C -78 C -78 C
Amount of methanol 0.15 ml 0.15 ml 0.15 ml 0.15 ml 0.15
ml 0.15 ml
Amount of isobutanol 0.05 ml 0.05 ml 0.50 ml 0.50 ml 0.05
ml 0.05 ml
Amount of formaldehyde 0.05 g 0.05 g 0.05 g 0.50 g 0.50 g 0.50
g
Amount of diisobutene 0.01 ml 0.01 ml 0.01 ml 0.01 ml 0.01
ml 0.01 ml
Molecular weight Mw 823000 811000 803000 803000 766000 715000
Molecular weight Mr, 121000 121000 110000 103000 88000 73000
Polydispersity D 6.8 6.7 7.3 7.8 8.7 9.8
