Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymerizing in tubular or loop reactor
Reexamination Certificate
1999-02-17
2002-09-03
Wu, David W. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymerizing in tubular or loop reactor
C526S173000, C526S183000, C526S335000, C526S346000, C502S157000
Reexamination Certificate
active
06444762
ABSTRACT:
The present invention relates to a particularly economical and safe process for the preparation of a styrene or diene polymer or a styrene-diene block copolymer by anionic, preferably continuous polymerization of the corresponding monomers using an alkali metal alkyl compound as polymerization initiator.
It is known in general terms that although anionic polymerization proceeds to completion, ie. to 100% conversion, it also proceeds very quickly. Apart from selecting the lowest possible temperature, the reaction rate could only be reduced by selecting a lower concentration of the polymerization initiator; however, this would result in the formation of only a few, very long chain molecules, ie. an undesirably high molecular weight would be obtained. Owing to the considerable heat evolution and the difficulty of dissipating the heat from a viscous solution, restriction of the reaction temperature is not very effective.
An excessively high reaction temperature has particularly disadvantageous consequences, especially in block copolymerization, since thermolysis disrupts the formation of uniform block copolymers and, if a coupling reaction is intended after the polymerization, the coupling yield would be unfavorably low.
The temperature must thus be controlled by appropriate dilution of the monomers, but this means that the reaction space needed becomes unnecessarily large, ie. the anionic polymerization can only be carried out at relatively low space-time yields in spite of the high reaction rate that can be achieved.
In the case of batch polymerization (ie. in a stirred reactor), the temperature could also be regulated via the monomer feed rate. However, this is virtually impossible in a continuous process without simultaneously changing other parameters.
It has hitherto been found satisfactory, in the cases where anionic polymerization cannot be avoided, ie. in block copolymerization, to carry out the reaction, in spite of the unpopularity of the batch method, in a stirred reactor, as described, for example, in German Patents 13 01 496 and 25 50 227, and to adjust the reaction rate by regulating the feed of fresh monomer. A major potential problem here is forced chain termination caused by the slightest impurities, which results in the desired structure not being achieved. If it is desired to prepare, for example, three-block copolymers, mixtures containing a proportion of two-block copolymers or homopolymers are obtained instead of the pure product. These mixtures have, for example, lower tear strengths.
In the processes described, in addition, only dilute polymer solutions are prepared since solutions having a high solids content do not allow good heat dissipation in the stirred reactor and are also difficult to handle, for example during discharge of the finished product from the reaction space.
Continuous anionic polymerization has recently been investigated by, inter alia, Priddy and Pirc (J. Appl. Polym. Sci.; 37 (1989) 392-402) using the example of continuous polymerization of styrene with n-butyllithium in ethylbenzene in a continuously stirred tank reactor at from 90 to 110° C. The mean residence time is greater than 1.5 hours. The authors also mention the difficulties arising if the polymerization is carried out in tubular reactors, which, owing to heat exchange, must have very small diameters. In particular, deposits of polymers of very high molecular weight occur on the tube walls. In addition, the authors mention the abovementioned fact that temperatures above 110° C. result in thermolysis through elimination of Li—H.
European Patent 592 912 claims that a higher monomer concentration or a better space-time yield can be obtained by a continuous process in a so-called SMR reactor, a tubular reactor with internals which promote cross-mixing. However, the examples given likewise use only relatively dilute polymer solutions; it was apparently impossible, even using a tubular reactor, to dissipate the heat of reaction sufficiently quickly.
In order to reduce the residual monomer content, polystyrene prepared by continuous bulk or solution free-radical polymerization processes must—as is always the case in free-radical polymerization—subsequently be freed from residual monomer (“degassed”) by means of an extruder or thin-film evaporator. For thermodynamic reasons, however, depolymerization occurs at the high temperatures prevailing in the degassing apparatuses, which means that the residual styrene content is generally above 500 ppm (Kunststoff-Handbuch, Vol. 4, Polystyrene, Carl Hanser-Verlag 1996, page 124).
It is known that a significantly lower concentration of residual monomers can be achieved by anionic polymerization. However, anionic polymerization generally gives a molecular weight distribution which is too narrow for technical polymers, resulting in a low melt flow index and poorer flow behavior during processing.
The effect of Lewis acids and Lewis bases on the rate of anionic polymerization of styrene at 30° C. has been reported by Welch in J.A.C.S. 82 (1960), 6000-6005. For example, it has been found that small amounts of Lewis bases, such as ethers and amines, accelerate the n-butyllithium-initiated polymerization of styrene, whereas small or even stoichiometric amounts of Lewis acids, such as alkylzinc and alkylaluminum compounds, reduce or even completely suppress the polymerization rate. Hsieh and Wang, Macromolecules 19 (1966), 299-304, have investigated the complexing of approximately stoichiometric amounts of dibutylmagnesium with alkyllithium or the living polymer chain in the presence and absence of THF and have found that dibutylmagnesium reduces the polymerization rate of styrene and butadiene without affecting the stereochemistry.
U.S. Pat. No. 3,716,495 discloses initiator compositions for the polymerization of conjugated dienes and vinylaromatic compounds where more effective utilization of alkyllithium compounds as initiator is achieved by addition of, for example, diethylzinc and polar compounds, such as ethers or amines. However, this U.S. Patent requires a large amount of solvent and a reaction time of several hours, and accordingly the space-time yields are correspondingly low.
It is an object of the present invention to find an anionic polymerization process, in particular a continuous process and in particular one which can be carried out at a high rate even at high temperature, which allows control of the reaction rate even at high monomer concentrations—in some cases even in a solvent-free environment—and is therefore distinguished by being particularly economical, and which enables the preparation of polymers having a particularly low residual monomer content.
We have found that this object is achieved in accordance with the invention by polymerizing styrene or butadiene and/or technical equivalents thereof anionically and preferably continuously using alkali metal alkyl or aryl compounds in the presence of an alkyl- or arylmetal compound of an element which occurs in at least divalent form, in particular from the second or third main group or second subgroup of the Periodic Table.
These alkyl- or arylmetal compounds of elements which occur in at least divalent form are also referred to below as rate regulators or retarders.
The alkali metal alkyl or aryl compounds can also be replaced as initiator by an initiator-capable, low-molecular-weight product of the reaction of the alkyl- or arylmetal compound.
The direct subject of the invention is a process for the continuous anionic polymerization or copolymerization of styrene or diene monomers using an alkali metal alkyl compound as polymerization initiator, in the presence of an alkyl- or arylmetal compound of an element which occurs in at least divalent form as rate regulator.
Addition of the rate regulator (retarder) according to the invention allows the reaction rate to be reduced or the temperature increased, without disadvantages for the polymer properties, to the extent that the heat of polymerization liberated can be controlled, even at a high monomer concentration, and/or a high
Deffieux Alain
Desbois Philippe
Fischer Wolfgang
Fontanille Michel
Gausepohl Hermann
BASF - Aktiengesellschaft
Cheung William
Keil & Weinkauf
Wu David W.
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