Method for the production of an initiator composition for retarded anionic polymerization

Disclosed are a process for the preparation of an initiator composition comprising an alkali metal organyl and an aluminum organyl and a process for the polymerization of anionically polymerizable monomers.

The present invention relates to a process for the preparation of an
 initiator composition comprising an alkali metal organyl and an aluminum
 organyl and to a process for the polymerization of anionically
 polymerizable monomers.
 Anionic polymerizations typically proceed very rapidly, so that they are
 difficult to control on an industrial scale owing to the considerable
 amount of heat generated. Lowering the polymerization temperature results
 in an excessive increase in viscosity, in particular with a concentrated
 solution. Reducing the initiator concentration increases the molecular
 weight of the polymer formed. Controlling the reaction by appropriate
 dilution of the monomers results in higher solvent requirement and lower
 space-time yields.
 It has therefore been proposed to include in the anionic polymerization
 initiators various additives to influence the polymerization rate.
 The effect of Lewis acids and Lewis bases on the rate of the anionic
 polymerization of styrene was described in Welch, Journal of the American
 Chemical Society, Vol 82 (1960), pages 6000-6005. For instance, it has
 been found that small amounts of Lewis bases such as ethers and amines
 accelerate the n-butyllithium-initiated polymerization of styrene at
 30.degree. C. in benzene, whereas Lewis acids such as zinc and aluminum
 alkyls reduce the polymerization rate or, when used in superstoichiometric
 amounts, stop the polymerization completely.
 U.S. Pat. No. 3,716,495 discloses initiator compositions for the
 polymerization of conjugated dienes and vinylaromatics where a more
 efficient use of the lithium alkyl as initiator is achieved by the
 addition of a metal alkyl of a metal of group 2a, 2b or 3a of the Periodic
 Table of the Elements, such as diethyl zinc, and polar compounds such as
 ethers or amines. The manner in which the individual initiator components
 are added to the polymerization system is said to be uncritical.
 Earlier patent application PCT/EP97/04497, unpublished at the priority date
 of the present invention, describes continuous processes for the anionic
 polymerization or copolymerization of styrene or diene monomers using
 alkali metal alkyl as polymerization initiator in the presence of an at
 least bivalent element as a retarder.
 Various initiator mixtures which may comprise alkali metals, alkaline earth
 metals, aluminum, zinc or rare earth metals are known, for example, from
 EP-A 0 234 512 for the polymerization of conjugated dienes with a high
 degree of 1,4-trans-linking. German Offenlegungsschrift 26 28 380 teaches,
 for example, the use of alkaline earth aluminates as cocatalyst in
 conjunction with an organolithium initiator for the preparation of the
 polymers or copolymers of conjugated dienes having a high
 trans-1,4-linkage content and low 1,2-linkage or 3,4-linkage contents.
 This is said to lead to an increase in polymerization rate.
 The use of additives such as aluminum alkyls which have a strong retarding
 effect on the anionic polymerization requires exact dosage and temperature
 control. A slight underdosage may lead to an insufficient retardation of
 the reaction rate, whereas a slight overdosage may completely stop the
 polymerization.
 Separate addition, or insufficient mixing-in, of the individual initiator
 components to a monomer solution may lead to poor dispersion, particularly
 at high monomer concentrations, and house to different local
 concentrations of the individual initiator components. Before a
 homogeneous dispersion of the initiator components can be achieved, the
 polymerization may already be initiated in some regions, whereas the
 polymerization is strongly retarded or has not yet started in others. This
 may lead to large local temperature increases and irreproducible molecular
 weight distributions.
 It is an object of the present invention to provide a process for the
 preparation of an initiator composition comprising an alkali metal organyl
 and an aluminum organyl to make it possible to polymerize anionically
 polymerizable monomers, in particular styrene, in a reproducible manner
 with respect to polymerization rate and molecular weight distribution.
 We have found that this object is achieved by a process for the preparation
 of an initiator composition comprising an alkali metal organyl and an
 aluminum organyl, which comprises homogeneously mixing the metal organyls,
 dissolved in inert hydrocarbons, and aging at a temperature in the range
 from 0 to 120.degree. C. for at least 2 minutes.
 The initiator composition prepared in this manner is particularly useful
 for the polymerization of anionically polymerizable monomers.
 Alkali metal organyls which may be used are mono-, bi- or multifunctional
 alkali metal alkyls, aryls or aralkyls customarily used as anionic
 polymerization initiators. It is advantageous to use organolithium
 compounds such as ethyllithium, propyllithium, isopropyllithium,
 n-butyllithium, sec-butyllithium, tert-butyllithium, phenyllithium,
 diphenylhexyllithium, hexamethylenedilithium, butadienyllithium,
 isoprenyllithium, polystyryllithium or the multifunctional compounds
 1,4-dilithiobutane, 1,4-dilithio-2-butene or 1,4-dilithiobenzene. The
 amount of alkali metal organyl required depends on the desired molecular
 weight, the type and amount of the other metal organyls used and the
 polymerization temperature and is typically in the range from 0.0001 to 5
 mol percent, based on the total amount of monomers.
 Aluminum organyls which may used are those of the formula R.sub.3 Al,
 wherein the radicals R are each, independently of one another, hydrogen,
 halogen, C.sub.1- C.sub.20 -alkyl or C.sub.6- C.sub.20 -aryl. Preferred
 aluminum organyls are aluminum trialkyls such as triethylaluminum,
 triisobutylaluminum, tri-n-butylaluminum, triisopropylaluminum or
 tri-n-hexylaluminum. Particular preference is given to using
 triisobutylaluminum. It is also possible to use aluminum organyls which
 are formed by partial or complete hydrolysis, alcoholysis, aminolysis or
 oxidation of alkyl- or arylaluminum compounds or those which carry
 alkoxide, thiolate, amide, imide or phosphide groups. Examples are
 diethylaluminum N,N-dibutylamide, diethylaluminum ethoxide,
 diisobutylaluminum ethoxide,
 diisobutyl-(2,6-di-tert-butyl-4-methyl-phenoxy)aluminum (CAS No.
 56252-56-3), methylaluminoxane, isobutylated methylaluminoxane,
 isobutylaluminoxane, tetraisobutyldialuminoxane, or
 bis(diisobutyl)aluminum oxide.
 The molar ratios of the metal organyls with respect to each other may vary
 within wide limits, but depend primarily on the desired retardation
 effect, the polymerization temperature, the monomer composition and
 concentration and the desired molecular weight.
 The molar ratio of aluminum to alkali metal is advantageously in the range
 from 0.2 to 4.
 In the process of the invention, use is made primarily of alkali metal
 organyls and aluminum organyls and, if desired, magnesium organyls.
 Barium, calcium or strontium organyls are preferably only present in
 ineffective amounts not having a significant effect on the polymerization
 rate or copolymerization parameters. Nor should transition metals or
 lanthanoids, especially titanium, be present in significant amounts.
 The inert hydrocarbon used may be aliphatic, cycloaliphatic or aromatic.
 Preference is given to using solvents in which the metal alkyls are
 commercially available in the form of a solution. Particular preference is
 given to using pentane, hexane, heptane, cyclohexane, ethylbenzene or
 toluene.
 The initiator components are advantageously used in the solution
 concentrations in which they are commercially available or, for a quicker
 establishment of equilibrium, in a more diluted form. Preference is given
 to concentration where the sum of all metal organyls is in the range from
 0.01 to 2 mol/1, based on the initiator composition.
 The temperature depends on the concentration, the type of the metal
 organyls and the solvent. It is usually possible to use any temperature
 between the freezing point and boiling point of the mixture. It is
 advantageous to use a temperature in the range from 0 to 120.degree. C.,
 preferably in the range from 20 to 80.degree. C.
 The aging of the metal organyls is important for their reproducible use in
 anionic polymerization. Experiments showed that initiator solutions which
 are mixed separately or just prior to the initiation of the polymerization
 result in poorly reproducible polymerization conditions and polymer
 properties. It is believed that the aging process observed is caused by a
 complexation of the metal organyls which proceeds slower than the mixing
 process. In the concentration and temperature ranges described above, an
 aging time of about 2 minutes is usually sufficient. It is preferred to
 age the homogeneous mixture for at least 5 minutes, in particular at least
 20 minutes. However, aging the homogeneous mixture for several hours, e.g.
 from 1 to 480 hours, is not usually harmful either.
 It is also possible in the process of the invention to additionally add
 styrene. In this case an oligomeric polystyryl anion is obtained with the
 metal organyls complexed at its chain end. Preference is given to using
 styrene in an amount in the range from 10 to 1000 mol%, based on the
 alkali metal organyl.
 The initiator components may be mixed in any mixing apparatus, preferably
 in those which may be pressurized with inert gas. Examples of suitable
 mixers are stirred tanks equipped with anchor stirrers or shaker
 containers. Heatable tubes equipped with static mixing elements are
 particularly suitable for continuous preparation. The mixing process is
 necessary to mix the initiator components homogeneously. It is possible
 but not strictly necessary to continue mixing during aging. It is also
 possible to carry out the aging process in a continuous stirred tank
 reactor or in a tube section, the volume of which, together with the flow
 rate, determines the aging time.
 The initiator compositions are suitable for the polymerization of
 anionically polymerizable monomers. The initiator composition is
 preferably used for the homopolymerization or copolymerization of
 vinylaromatic monomers and dienes. Preferred monomers are styrene,
 .alpha.-methylstyrene, p-methylstyrene, ethylstyrene, tert-butylstyrene,
 vinyltoluene or 1,1-diphenylethylene, butadiene, isoprene,
 2,3-dimethylbutadiene, 1,3-pentadiene, 1,3-hexadiene or piperylene or
 mixtures thereof.
 The amount of initiator composition required depends on the desired
 molecular weight, the type and amount of the further metal organyls used
 in addition to the alkali metal organyl, and the polymerization
 temperature, and is usually in the range from 0.0001 to 5 mol%, based on
 the alkali metal organyl content and the total amount of monomers.
 The polymerization may be carried out in the presence of a solvent.
 Suitable solvents are the aliphatic, cycloaliphatic or aromatic
 hydrocarbons having from 4 to 12 carbon atoms which are generally used for
 anionic polymerization, such as pentane, hexane, heptane, cyclohexane,
 methylcyclohexane, isooctane, decalin, benzene, alkylbenzenes such as
 toluene, xylene, ethylbenzene or cumene or suitable mixtures. Obviously,
 the solvent must have the high purity which is typically required for the
 process. The solvents may be dried over aluminum oxide or molecular sieve
 and/or distilled prior to use to remove protic substances. The solvent
 from the process is preferably reused after condensation and the
 abovementioned purification.
 It is possible to adjust the retarding effects within wide temperature
 ranges via the composition and amount of the metal organyls. It is
 therefore also possible to carry out the polymerization at initial monomer
 concentrations in the range from 50 to 100% by volume, particularly from
 70 to 100% by volume, which lead to highly viscous polymer solutions and
 require higher temperatures at least at higher conversions.
 After the polymerization is completed, the living polymer ends may be
 capped with a chain terminator. Suitable chain terminators are protic
 substances or Lewis acids, such as water, alcohols, aliphatic or aromatic
 carboxylic acids and inorganic acids such as carbonic acid or boric acid.
 The target products may be homopolymers or copolymers and mixtures thereof.
 Polystyrene and styrene/butadiene block copolymers are preferably
 obtained. The process of the invention may also be used to prepare
 high-impact polystyrene (HIPS), in which case polybutadiene,
 styrene/butadiene block copolymers or mixtures thereof may be used as
 rubbers.
 The block copolymers may be coupled using multifunctional compounds, such
 as polyfunctional aldehydes, ketones, esters, anhydrides or epoxides. The
 process of the invention may be carried out in any pressure- and
 temperature-resistant reactors, it being possible in principle to use
 backmixing or non-backmixing reactors (i.e. reactors having stirred tank
 or tubular reactor characteristics). Depending on the choice of initiator
 concentration and composition, the particular process route applied and
 other parameters, such as temperature and possible temperature profile,
 the process of the present invention leads to polymers having high or low
 molecular weight. It is possible to use, for example, stirred tanks, tower
 reactors, tube reactors and tubular reactors or tube bundle reactors with
 or without internals. Internals may be static or mobile. The process is
 preferably carried out continuously.
 The initiator compositions prepared according to the process of the
 invention make it possible to control the polymerization of anionically
 polymerizable monomers, in particular styrene, very effectively and to
 achieve reproducible polymer properties.

EXAMPLES:
 Preparation of Initiator Solution I1
 8 ml of a 1.6-molar s-butyllithium solution (sBuLi) in cyclohexane (from
 Aldrich) and 6.4 ml of a 1.6-molar solution of triisobutylaluminum (TIBA)
 in toluene (from Witco) were combined at 25.degree. C. and stirred prior
 to use for 10 hours.
 Example 1
 A 2.35 1 stirred tank equipped with an anchor stirrer was charged with 800
 g of styrene and 200 g of toluene under nitrogen and heated to 85.degree.
 C. with stirring. On reaching this temperature, initiator solution I1
 (molar Li/Al ratio=1/0.85) was added and the polymerization solution was
 kept at 85.degree. C. The conversion was 29% after 25 minutes. After 60
 minutes at 85.degree. C., the polymerization was terminated at a
 conversion of 52% by adding 4 ml of ethanol. The viscous polystyrene
 solution obtained had a number average molecular weight M.sub.n of 54,340
 g/mol and a polydispersity M.sub.w /M.sub.n of 1.29.
 Comparative Experiment 1
 Example 1 was repeated, except that the components of initiator solution I1
 were combined and added to the monomer solution within less than 1 minute.
 The polymerization mixture was heated up to 213.degree. C. within 5
 minutes.
 Example 2
 1.5 ml of sec-butyllithium (1.3 M in cyclohexane) and 2.3 ml of styrene
 were added to 30 ml of cyclohexane and the mixture was stirred for 4
 hours. Then 1.66 ml of a 1 M solution of triisobutylaluminum in
 cyclohexane (molar Al/Li ratio=0.85) were added and the solution was
 stirred at room temperature (25.degree. C.) for a further 4 hours. Glass
 ampoules were charged with 2.5 ml of this solution and 10 ml of styrene
 each, fused and stored in a heating bath at 150.degree. C. The ampoules
 were opened at different times and the polymerization was terminated by
 adding ethanol. The time/conversion curve obtained was used to calculate a
 half-life of 1 minute for the styrene conversion at 150.degree. C.
 Example 3
 Example 2 was repeated, except that 1.86 ml of a 1 M solution of
 triisobutylaluminum in cyclohexane was used (molar Al/Li ratio=0.95). The
 half-life at 150.degree. C. was more than 1 hour.
 Comparative Experiment 2
 0.75 ml of a sec-butyllithium solution (1.3 M in cyclohexane) and 0.6 ml of
 styrene were added to 200 ml of cyclohexane and stirred for 4 hours. 30 ml
 of this solution were transferred to a 100 ml flask equipped with a
 fused-on UV cell. The concentration of the polystyryllithium as determined
 by UV spectroscopy was [PS-LI]=4.8.times.10.sup.-3 M. This solution was
 mixed with 3.6 ml of a 0.06 M solution of Et.sub.2 AlOEt in cyclohexane
 (molar Al/Li ratio=1.5) and 2.5 ml of styrene. The decrease in the styrene
 concentration at 100.degree. C. was monitored by UV spectroscopy and
 analyzed according to a first order rate equation:
EQU ln ([styrene].sub.0 /[styrene])=k.sub.a * t
 A non-linear plot was obtained. The slope of the curve decreased with time.
 Towards the end of the conversion, the slope k.sub.a was 0.0035
 min.sup.-1. This ka value and the polystyryllithium concentration [PS-Li]
 gave a reaction rate constant k.sub.p =k.sub.a /[PS-Li].sup.0.5 &gt;0.05
 M.sup.-0.5 min.sup.-1.
 Example 4
 6.2 ml of a 0.06 M solution of Et.sub.2 AlOEt in cyclohexane were added to
 30 ml of a solution of polystyryllithium in cyclohexane having a [PS-Li]
 concentration of 6.2 .times.10.sup.-3 M as determined by UV spectroscopy
 and stirred at 100.degree. C. for 1h (molar Al/Li ratio=1.5). Further 2.5
 ml of styrene were then added. The decrease in styrene concentration at
 100.degree. C. was monitored by UV spectroscopy and analyzed as described
 in Comparative Example 2. The plot of ln ([styrene].sub.0 /[styrene]) as a
 function of time was linear over the whole conversion range. The slope
 k.sub.a of the straight line was 2.6=10.sup.-4 min.sup.-1. This k.sub.a
 value and the (PS-Li] concentration gave a reaction rate constant k.sub.p
 of 0.0033 M.sup.-0.5 min.sup.-1.
 Example 5
 1.2 ml of a sec-butyllithium solution (1.3 M in cyclohexane) and 0.9 ml of
 dry styrene were added to 200 ml of cyclohexane and stirred for 4 hours.
 30 ml of this solution were transferred to a 100 ml flask equipped with a
 fused-on UV cell. The concentration of the polystyryllithium as determined
 by UV spectroscopy was [PS-Li]=7.2.times.10.sup.-3 M. This solution was
 mixed with 2.3 ml of a 0.08 M solution of triisobutylaluminum in
 cyclohexane (molar Al/Li ratio=0.85). The UV-VIS spectrum of the solution
 was monitored in the UV cell at room temperature.
 An absorbence maximum of 287 nm was observed, which grew by 20% over a
 period of 2 h. After this period, a shoulder at about 330 nm had almost
 completely disappeared; the corresponding absorbence was reduced to about
 64% of the initial value over this period.
 Example 6
 A 1 l stirred tank equipped with an anchor stirrer was charged with 120 g
 of styrene and 480 g of toluene under a nitrogen atmosphere and heated to
 80.degree. C. with stirring. At the same time, an ampoule containing 10 ml
 of toluene and 0.5 ml of styrene was charged with 1.51 ml of a 1.6-molar
 s-butyllithium solution in cyclohexane and, after 10 minutes, with 1.42 ml
 of a 1.6-molar solution of triisobutylaluminum in toluene. The mixture was
 kept at 80.degree. C. for 5 minutes and then added to the stirred tank. At
 a constant temperature of 80.degree. C., the styrene conversion was 14%
 after 60 minutes, 36% after 115 minutes and 63% after 181 minutes. After
 360 minutes, the polymerization was terminated at a conversion of 92% by
 adding 4 ml of ethanol.
 Comparative Experiment 3
 A 1 l stirred tank equipped with an anchor stirrer was charged with 120 g
 of styrene and 480 g of toluene under a nitrogen atmosphere and heated to
 60.degree. C. with stirring. On reaching this temperature, 1.51 ml of a
 1.6-molar s-butyllithium solution in cyclohexane and 1.42 ml of a
 1.6-molar solution of triisobutylaluminum in toluene were added at the
 same time, but separately. After 3 minutes, the conversion was 51% and the
 temperature had risen to 77.degree. C. After 5 minutes, the conversion was
 61% and the temperature 72.degree. C., and after 40 minutes, the
 conversion was 83% and the temperature was 60.degree. C.
 Example 7
 The reactor used for the continuous polymerization was a double-jacketed 2
 1 stirred tank equipped with a standard anchor stirrer. The reactor was
 designed for a pressure of 60 bar and was kept at a specified temperature
 by heat-transfer medium to allow an isothermal polymerization. The
 initiator components were metered in via a common feed line using a static
 mixer. The feed line had a capacity of 160 ml, with a section containing
 100 ml being kept at 80.degree. C.
 The stirred tank was continuously fed with 800 g/h of styrene and, via the
 common feed line, with a premixed initiator solution comprising 26 ml/h of
 a 0.16-molar s-butyllithium solution in cyclohexane/toluene (1/9), 24.7
 ml/h of a 0.16-molar solution of triisobutylaluminum in toluene, 180 g/h
 of toluene and 24 g/h of a 10% strength by weight solution of styrene in
 toluene (molar Li/Al ratio=1/0.92) and stirred (100 revolutions per
 minute) at a bulk temperature of 104.degree. C. The effluent from the
 stirred tank was conveyed into a stirred 4 liter tower reactor which was
 operated at an internal temperature of 109.degree. C. The effluent from
 the reactor was fed into a second 4 liter tower reactor. To set the
 temperature, two heating zones of equal length which were arranged in
 series were used, the internal temperature at the end of the first zone
 being 140.degree. C., and at the end of the second zone being 158.degree.
 C. The polymerization mixture was mixed with 20 g/h of a 10% strength by
 weight solution of methanol in toluene using a mixer at the outlet of the
 tower reactor, subsequently passed through a tube section heated to
 260.degree. C. and released into a vacuum pot kept at 25mbar via a
 pressure control valve. The melt was discharged via a screw conveyor and
 pelletized.
 A stable equilibrium state was reached in all parts of the unit after a few
 hours. The pressure drop across the whole unit was 2.2 bar. The solids
 content was 13.5% by weight at the outlet of the stirred tank and 40.4% by
 weight at the outlet of the tower reactor. The monomer conversion of the
 effluent was found to be complete. The polystyrene obtained had a
 molecular weight Mw of 167,000 g/mol and a polydispersity M.sub.w /M.sub.n
 of 2.62. The distribution was monomodal. Analysis showed a styrene content
 of less than 10 ppm, an ethylbenzene content of less than 10 ppm and a
 toluene content of 92 ppm.
 Comparative Experiment 4
 Example 7 was repeated, except that 26 ml/h of a 0.16-molar s-butyllithium
 solution in cyclohexane/toluene (1/9), 24.7 ml/h of a 0.16-molar solution
 of triisobutylaluminum in toluene and 180g/h of toluene (molar Li/Al
 ratio=0.95) were metered in via separate feed lines. When attempting to
 adjust the solids content to 13.5% by weight at the outlet of the stirred
 tank, the temperature had to be decreased to 83.degree. C. The solids
 content in the stirred tank varied in the range from 3-25% by weight in
 the course of several days. The bulk temperature could not be kept
 constant at 83.degree. C. Samples taken from the stirred tank showed
 significant variations in molecular weight distribution and sometimes
 bimodal or multimodal distributions.
 Example 8
 The initiator components were fed to a thermostatable coil having a length
 of 4m via a mixing element. The coil had a volume of 12.6 ml and led to
 the reactor.
 The stirred tank from Example 7 was continuously fed with 800 g/h of
 styrene, 180 g/h of toluene and, via the common feed line, with a premixed
 initiator solution comprising 23 ml/h of a 0.18-molar s-butyllithium
 solution in cyclohexane/toluene (1/9) and 44.3 ml/h of a 0.086-molar
 solution of triisobutylaluminum in toluene (molar Li/Al ratio=1/0.92) and
 stirred (100 revolutions per minute) at a bulk temperature of 109.degree.
 C. A constant operational state was reached after only a few hours. The
 solids content was 14%.
 Comparative Experiment 5
 Example 8 was repeated, except that the metal alkyls were introduced into
 the reactor via a feed line with a capacity of only 0.9 ml. The solids
 content in the reactor rose to 41% within only a few hours. It was found
 to be difficult to keep the solids content in the stirred tank and the
 bulk temperature at a constant level.