Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Treating polymer containing material or treating a solid...
Reexamination Certificate
2000-03-29
2001-10-09
Acquah, Samuel A. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Treating polymer containing material or treating a solid...
C528S272000, C528S488000, C528S496000, C528S503000
Reexamination Certificate
active
06300467
ABSTRACT:
The invention relates to a process for preparing hydroxyl-containing polymers, in particular for preparing polytetrahydrofuran with terminal hydroxyl groups, from the corresponding acyloxy-containing polymers by transesterification with alcohols in the presence of an alkali metal- or alkaline earth metal-containing catalyst.
It is known from the prior art to prepare polytetrahydrofuran with terminal hydroxyl groups, referred to as PTHF for short hereinafter, by using cationic catalyst systems from tetrahydrofuran. The first step results in a polytetramethylene ether whose end groups are determined by the initiator system and reaction medium. These end groups in the initially resulting polymer must then be converted into alcohol functionalities. A conventional method used for this is transesterification using lower alcohols and initiated by alkaline catalysts. Sodium methoxide is known to be an effective transesterification catalyst.
If the intention is to carry out this transesterification in a continuous procedure, however, the considerable problem arises that the reaction solution foams greatly, resulting in the downstream column for removing methanol and the methanol/methyl acetate azeotrope no longer being able to perform its separation task owing to the presence of PTHF. The transesterification then proceeds in an uncontrolled fashion because considerable amounts of PTHF which have reacted to various degrees are driven back, so that the transesterification can be carried out only with considerably reduced capacity.
It is an object of the present invention, based on this prior art, to provide a process for preparing hydroxyl-containing polymers and, in particular, for preparing polytetrahydrofuran with terminal hydroxyl groups from the corresponding acyloxy-containing polymers by transesterification with alcohols, which permits continuous transesterification without interfering foam formation.
We have found that this object is achieved by a process in which the appropriate acyloxy-containing polymer is introduced together with the alcohol and the catalyst into a first prereactor and is converted therein at least partly to the hydroxyl-containing polymer, and the reaction solution is fed into the upper zone of a distillation column into whose lower zone alcohol in vapor phase is additionally fed.
The first prereactor means in this connection a reaction vessel in which the acyloxy-containing polymer can be reacted under catalytic conditions with the alcohol to give the hydroxyl-containing polymer and which is upstream of the distillation column.
It is true that WO 97,23559 has disclosed the preparation of polytetrahydrofuran with terminal hydroxyl groups by catalyzed transesterification with alcohols, in which the appropriate acyloxy-containing polymer, the catalyst and the alcohol are fed into the upper zone of a distillation column into whose lower zone alcohol in vapor phase is additionally fed. However, this management of the reaction is unsuitable for suppressing the severe foaming in the reaction solution during the transesterification. The difference from said process is, according to the invention, that the actual transesterification process is carried out essentially not in the distillation column but in the first prereactor. This prereactor acts as holdup vessel for establishing equilibrium for the conversion into the hydroxyl-containing polymers.
Downstream of this first prereactor is the distillation column which serves to strip out the methyl acetate which is formed, for example, in the transesterification, which shifts the reaction equilibrium in the direction toward the transesterified product to be formed in the reaction. Alcohol vapor is fed as stripping gas into the lower zone of the distillation column.
This division of the reaction into two zones, namely the actual transesterification in the first prereactor and the stripping out of the methyl acetate which is formed, for example, in the reaction, surprisingly results in a preparation process free of interference by foam formation.
The difference in the management of the reaction compared with the process disclosed in WO 97/23559 mentioned hereinbefore is that there is provision of a phase exchange of the coherent and disperse phase between the first prereactor on the one hand and the distillation column on the other hand.
In the first prereactor, the liquid represents the coherent phase and the vapor represents the disperse phase. In the distillation column, however, the coherent phase is the vapor in the form of the alcohol in vapor phase which is fed in, and the disperse phase is the liquid fed in the form of a downflowing film into the upper zone of the distillation column from the first prereactor.
When carrying out the process according to the invention, the reacted polymer is advantageously removed from the lower zone of the column after passing through it, while the alcohol vapor passes through the column in essentially the opposite direction. During this it reacts with, for example, the polytetrahydrofuran monoacetate essentially formed in the case of PTHF preparation, again bringing about a shift in the reaction equilibrium toward the wanted reaction product.
It is particularly preferred to connect the distillation column to at least one other prereactor into which is fed the reaction solution which passes through the column and is at least partly converted into the hydroxyl-containing polymer, and which is returned from there, after a predetermined holdup time, to the column.
It is possible to provide in another embodiment of the process according to the invention for each prereactor to have at least one inflow into the column and an outflow from the column, with at least one inflow being disposed so that it opens into the column above the at least one outflow. This management of the reaction achieves optimization of the yield.
It has emerged as particularly advantageous with a view to optimizing the reaction conversion if the distillation column is connected to at least three other prereactors into which is fed successively the reaction solution passing through the column and at least partly converted into the hydroxyl-containing polymer.
The process according to the invention can be used for a reaction employing polytetrahydrofuran diacetate, referred to as PTHF DiAc for short hereinafter, while the abbreviated PTHF means the reacted final product, as acyloxy-containing polymer, methanol as alcohol and sodium methoxide as alkali metal- or alkaline earth metal-containing catalyst. In this reaction, PTHF DiAc reacts in the first prereactor upstream of the column relatively quickly—as shown by kinetic investigations—to give the monoacetate, from which the diol PTHF is then formed.
There is thus transfer of a reaction solution which consists to a large extent of final product which has already been formed and to a smaller extent of the monoacetate and, additionally, methyl acetate, methanol and the catalyst used from the first prereactor into the column. In this case the ratio of final product which has already been formed to the monoacetate resulting as intermediate may vary depending on the degree of polymerization of the PTHF DiAc which is employed for conversion, for example, into PTHF 1000 or PTHF 2000.
It is possible to use a stirred vessel as first or further prereactor. However, a tubular reactor or another comparable reaction container is also suitable. It is possible in this case in particular to operate the first prereactor with separate heating. However, this is not absolutely necessary for carrying out the reaction. In addition, the first prereactor may already have a feed line for methanol vapour in order to introduce the latter into the first prereactor and thus speed up complete conversion to PTHF.
The invention is to be described in detail below with reference to exemplary embodiments depicted in the drawing.
REFERENCES:
patent: 4230892 (1980-10-01), Pruckmayr
patent: 5282929 (1994-02-01), Darai et al.
patent: 5852218 (1998-12-01), Dorai
patent: 5981688 (1999-11-01), Auer et a
Auer Heinz
Ciprian Jurgen
Franz Lothar
Franzischka Wolfgang
Iffland Gabriele
Acquah Samuel A.
BASF - Aktiengesellschaft
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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