Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acid esters
Reexamination Certificate
1997-12-03
2001-08-07
Killos, Paul J. (Department: 1623)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acid esters
C560S264000
Reexamination Certificate
active
06271413
ABSTRACT:
BACKGROUND AND SUMMARY OF THE INVENTION
This invention relates to a process for the preparation of polytetramethylene ether glycol (PTMEG) diesters of the formula R—CO—O(—CH
2
-CH
2
-CH
2
-CH
2
-O)
n
—COR
1
, wherein R and R
1
are identical or different and are an alkyl radical or a derivative thereof, and n is preferably an integer from 2 to 200, by polymerization of tetrahydrofuran (THF) in the presence of carboxylic acid anhydride using a neutral or slightly basic magnesium-aluminium hydrosilicate catalyst that has been activated by heating to 200-600° C. The natural minerals sepiolite and, in particular, attapulgite, are suitable raw materials for the new catalysts of this invention.
The polymerization of THF by means of oxonium ion catalysis became known as a result of the basic research work carried out by H. Meerwein et. al. (Angew. Chemie
72
, (1960), 927), and is treated comprehensively in the monograph “Polytetrahydrofuran” by P. Dreyfus, Gorden and Breach Sc. Publishers, New York, London, Paris 1982.
The German patent no. 2916653 describes a polymerization process for THF—that has been purified in a separate step—in which the polymerization takes place by means of bleaching earth in a fixed bed, in the presence of carboxylic acid anhydride. Bleaching earths are naturally occurring aluminium silicates with a cryptocrystalline three-layer structure of the montmorillonite mineral. The mineral, obtained from deposits thereof, exhibits physical and chemical properties which vary depending on the origin. This applies in particular to the activity of the catalysts, which is not constant but varies from batch to batch. Despite the low price, this poses a serious disadvantage with regard to the commercial use of bleaching earths as catalysts. Kaolin and zeolites, proposed in the PCT application no. WO 94/05719 for THF poymerizatation, have similar disadvantages. These minerals do not become active catalysts until they have undergone acid treatment. An added disadvantage of these minerals is that a commercially usable polymer is only obtained if the THF is extremely pure.
The object of the present invention was thus to simplify the industrial-scale polymerization of THF and to render it reproducible, at the same time retaining the advantages—particularly that of fixed-bed catalysis—of the method described in German patent no. 2916653.
Surprisingly, it was found that granular or pelletized sepiolite and attapulgite which are largely free from water and are either neutral-to-weakly-basic (pH=7-9.5) or else protonated as a result of acid treatment, when suspended, or, better, when introduced into a stationary, fixed catalyst bed, convert a mixture of THF and carboxylic acid anhydride reproducibly into the carboxylic ester of polytetrabutylene ether glycol at a high polymerization rate, the ester so formed having a low color index and the catalyst being active over an unusually long period of time. The THF does not need to be specially purified as in the case of bleaching earths, zeolites or kaolin, and even THF containing water (e.g. 1% water) can be polymerized. Through use of attapulgite, in particular, as catalyst, polymers are obtained which have a very low color index, a narrow molecular weight distribution and an extremely low content of crown ether imputities. Attapulgite is used with preference in the process according to the invention.
The catalyst remains active for a practically unlimited period of time, this being an added reason for the better environmental compatibility of the new process.
DETAILED DESCRIPTION OF THE INVENTION
Sepiolite and attapulgite are naturally widespread, hydrated magnesium-aluminium silicates, which catalyze THF polymerization without requiring special acid activation. Unlike the above-mentioned catalysts, these new catalysts are thus neutral or weakly basic, and since they contain no residual acid they are not corrosive. Their industrial use therefore has economic advantages, too. In the U.S. Pat. No. 5,210,283, the importance of subjecting bleaching earch catalysts to acid treatment is stressed and discussed in detail. It was not possible to develop the preferred embodiment of the process according to this invention, using neutral or weakly basic catalysts, without first overcoming this prejudice. Catalysts for the process of the invention include those known as Attapulgus Clay or the floridin earths. The preferred catalysts consist predominantly of the mineral attapulgite, made up of three-dimensional chains with the ideal chemical composition (OH
2
)
4
(OH)
2
Mg
5
Si
8
O
20
×4 H
2
O.
In contrast to clays and kaolin, attapulgite and sepiolite contain magnesium, an element which is more strongly basic than aluminium; accordingly, one would expect these catalysts to be cationically less effective. Surprisingly, however, they are extremely active in the cationic polymerization of THF.
Water is removed from natural attapulgite by means of extrusion and controlled calcination. By grinding and screening, the catalyst is obtained in granular form or as a powder, which can be used in a suspension process or as a fixed-bed catalyst.
Prior to their use, the catalysts to be used according to the invention should be calcined, e.g. for a period of 0.5 to 10 hours, at a temperature in the range from 200 to 600° C. In this way the water content is reduced and the reactivity increased.
The granules used in the fixed-bed range in size from 2.4-4.75 mm={fraction (4/8)} mesh. Attapulgite powder can also be stirred to a paste with water, pressed to pellets and calcined.
Instead of the naturally-occurring minerals, use can also be made of synthetic aluminium-magnesium hydrosilicates. These are obtainable from hydrolyzable mixtures of aluminium-magnesium-silicon compounds.
Only a small quantity of catalyst is required to polymerize THF. The dry catalyst is poured into a reactor, for example a tube or a shaft furnace. The dimensions of the catalyst bed are preferably selected according to the need to dissipate the heat of polymerization. It may also be useful to circulate all or some of the reaction product over the bed by pumping; in this way, by cooling or warming in a heat exchanger, isothermal reaction conditions can be ensured along the length of the furnace. In general, it is sufficient if the quantity of reaction product circulated hourly corresponds to about 3 to 10 times the reactor volume. During continuous polymerization, fresh feed in the form of THF and carboxylic anhydride is added to the circulating reaction product in an amount equivalent to 0.01-0.1 times the hourly circulating quantity thereof.
Another suitable type of reactor for polymerization reactions according to the invention is a rotating basket filled with granular catalyst; said basket is located in a reactor which can be controlled thermostatically and which may additionally be equipped with a stirring paddle.
Surprisingly, and a technical improvement on the procedure used hitherto and described, e.g., in German patent no. 29 16653, the products obtained using the process of the invention have a very narrow molecular weight distribution and a negligible quantity (less than 0.1 wt. %) of crown-ether impurities. While commercially available polytetramethylene ether glycol (PTMEG), with a molecular weight of 1000, is characterized by a polydispersity M
w
/M
n
of 1.6 to 1.8, products obtained according to the invention have a polydispersity M
w
/M
n
of 1.2 to 1.4. This is particularly suitable for the production of elastic polyurethane fibers or thermoplastic polyurethanes with excellent low temperature characteristics.
The essentially anhydrous attapulgite and sepiolite do not develop their catalytic activity until in the presence of the promotor carboxylic anhydride. It is of advantage to use only such carboxylic acid anhydrides as are derived from aliphatic or aromatic poly- and/or, preferably, monocarboxylic acids with 2 to 12 or preferably 2 to 8 carbon atoms. Examples of such anhydrides are acetic anhydride, propionic anhydride, butyric anhydride and also a
Fulbright & Jaworski LLP
Killos Paul J.
Korea PTG Co., Ltd.
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