Ind. Eng. Chem. Prod. Res. Dev. 1981, 20, 336-338
330
Preparation of Aromatic Polyesters of Hindered Phenols by Acid Interchange Polycondensation. 1 Solvent Effects
.
Robert W. Stackman Celanese Research Company, Summit, New Jersey 0790 1
The preparation of high molecular weight aromatic polyesters from tetrachlorobisphenol A diacetate, the ester of a sterically hindered bisphenol, with mixtures of iso- and terephthalic acids, by an acid interchange polycondensation has previously been unsuccessful. These polymers as prepared by conventional methods are high melting materials which begin to decompose at temperatures at or below their melting points. This paper describes the evaluation of a number of solvents as potential reaction media for the acid Interchange condensation. Diphenyl ether and benzophenone were found to act as solvents both for the reactants and for the polymer. These solvents at low concentrations (50% or less by weight based on polymer) serve as volatile plasticizers, lowering the melting point of the polyester and providing chain end mobility. in summary, we have been able to demonstrate the Preparation of high molecular weight polymers from the transacylatlon reaction on the acetate ester of a hindered bisphenol. While the molecular weights of the polymers prepared in this program are not sufficiently high to be of commercial interest, we have developed a viable approach to thier preparation. futther refinement of the process will therefore be directed toward the evaluation of catalysts in order to obtain still higher molecular weight products.
slows the condensation, at temperatures sufficiently low to prevent decomposition of the polymer or decarboxylation of the reactive acid end groups, and (2) steric hindrance from the two ortho chlorines lowers the rate of acid exchange, allowing terminating reactions to become competitive. Despite these potential problems, it was felt that solutions could be found to these problems. Two of the approaches considered were (1)use of a high-boiling solvent or plasticizer to lower the melt viscosity and (2) discovery of an efficient catalyst system to increase the rate of the reaction. The use of high-boiling solvents in the preparation of aromatic polyesters from diacid chloride condensation with diphenol has been widely reported. Kantor and Holub (1964)have reported a variety of high-boiling solvents such as the chlorinated diphenyl ethers (Arochlor), benzophenone, or biphenyls to be solvents for the effective preparation of aromatic polyesters. Wilson (1972) has reported on the preparation of a polyester from tetrachlorobisphenol A with aromatic acid chlorides in tetrachloroethane solution in the presence of Lewis acid catalyst. Any reference to the use of such solvents for a transacylation polycondensation, however, has not been found. In the present work we have examined the effects of the use of high-boiling liquids as solvents or plasticizers for the preparation of polyesters of the hindered bisphenol, tetrachlorobisphenol A, by a transacylation process.
Introduction The preparation of aromatic polyesters by the acid interchange reaction between the diacetate ester of a diphenol with a free dicarboxylic acid has been known for over 30 years (Drewitt and Lincoln, 1949). The reaction, as described, can utilize either a diacetate that is preformed or generated in situ from a diphenol along with acetic anhydride. CATALYST
HOArOH
t 2ICH3C)zO
n
n
n
-
CH3C-OArOCCH3
0
0 0
II I1 HOCRCOH
0
This method was limited for some time to use with aliphatic dicarboxylic acids whose polymers have relatively low melting points and melt viscosities. In the instances where high melting all aromatic polyesters from aromatic dicarboxylic acids were desired, this method led to only low molecular weight, discolored polymers. The low molecular weight can be attributed to a number of factors. These range from crystallization of the oligomer to decarboxylation of the aromatic acids and/or decomposition of the diphenolic compound. Conix (1959) has reported the use of this reaction to prepare all aromatic polyesters from 4,4’-dicarboxyl diphenyl ether or 4,4’-isopropylidene di(benzoic acid) with bisphenol A. It is noted that “the extent of melt polycondensation is limited by the extremely high melt viscosity of the polymers, which prevents effective stirring of the reaction mass.” The highest molecular weights obtained in this study correspond t~ an I.V. of 0.8 dL/g. Levine and Temin (1959) also prepared polyesters from the diacetates of bisphenols, and later Temin (1961) reported on the polymerziations of sterically hindered bisphenols. He concluded that hindered phenols would not undergo the acid exchange reaction to yield high molecular weight polyesters. One would conclude, therefore, that the synthesis of tetrachlorobisphenol A polyesters would not proceed satisfactorily for two reasons: (1)the high melting point of the polymer (>300“C) prevents adequate mixing and 0 1 9 6 - 4 3 2 l I 8 1 f 1220-0336$O1.25/0
Discussion In the initial experiments mixtures or iso- and terephthalic acids were condensed with tetrachlorobisphenol A diacetate (TCBPA-DA) in both the presence and absence of solvents or catalysts and under a variety of conditions. While acetic acid evolution occurred, none of the isolated products was of high molecular weight as measured by inherent viscosity. These experiments demonstrated that TCBPA-DA was capable of undergoing a transacylation reaction. Thus with the feasibility demonstrated, the problem then became one of preventing any process which would hinder transacylation from occurring and of finding effi0
1981 American Chemical Society
Ind. Eng. Chem. Prod.
Res. Dev., Vol. 20, No. 2, 1981 337
Table I. Effect of Solvents uDon TransacslationC
solvent
solubilityu reaction conditions parameter, ( ~ a l / c m ~ ) " ~temp, "C time, h
11.3
__
250 275 300 220
5 7 6 5
230
10.5
220
5
polymer properties black tar black tar no polymer no acetic acid evolution black tar
188 216
8.4
__
200 220
15 5
no polymer formed no polymer formed
259
10.1
250
7
I.V. = 0.35 dL/gb light tan polymer
bp, "C
sulfolane
285
13.4
diphenyl acetone N-methyl pyrrolidone hexamethyl phosphoramide benzonitrile triethylene glycol dimethyl ether diphenyl ether
330 202
a Buttell (1975). Determined as 0.1%concentration in 10/7 (wt/wt) phenol/tetrachlorothane. Reactions conducted using 50 mL of solvent, 0.1 mol of tetrachlorobisphenol A diacetate; 0.07 mol of isophthalic acid; 0.03 mol of terephthalic acid; 0.01 g of manganese acetate.
cient catalysts for the condensation to allow the process to proceed under practical conditions (time, temperature, etc.). The initial approach to this problem was to eliminate the occurrence of the processes which would prevent further polycondensation by retnoval of active chain ends. These processes might be chemical, such as decarboxylation of the aromatic acid or decomposition of the phenol acetate moiety, or they might be purely physical processes, such as crystallization of either the oligomeric candensates or one of the reactants (such as terephthalic acid, so that approach of the active chain ends is prohibited. Of these processes the chemical degradations can likely be prevented or a t least minimized by conducting the condensation under mild conditions. On the other hand, the prevention of termination by active end immobilization or unavailability due to crystallization would be favored by use of higher temperatures, above the melting point, by conducting the condensation in the presence of a solvent for the reactants or by use of a melting point depressant (plasticizer) for the oligomeric products. It was known from earlier polymer studies, both in this laboratory and from published reporta (Wilson, 19721, that chlorinated hydrocarbons are good solvents for the TCBPA polyesters. Attempts to use the higher boiling members as reaction media were unsuccessful. This was probably due to a number of factors ranging from (1) a lack of solubility of the phthalic acids to (2) the inability of the reaction mixture to reach sufficiently high temperatures in the presence of these solvents. As an approach to the problem of terephthalic acid unavailability to the reaction site (due to insolubility in the reaction mixture), a series of potential terephthalic acid solvents were examined is possible reaction media. While there are relatively few nonbasic solvents for terephthalic acid (Harper and Janik, 19701, its solubility in many neutral solvents does increase to an appreciably level at elevated temperatures (Towle et al., 1964);even a low level of terephthalic acid in solution should enhance the rate of the transesterification reactioh. The results of these experiments are suinmarized in Table I. With the exception of diphenyl ether, none of the solvents selected on the basis of boiling point and solubility parameter proved useful as polymerization media. Reactions carried out in diphenyl ether showed a rapid acetic acid evolution at temperatures of 210-230 "C which eventually (after about 1 h) resulted in a clear, slightly viscous reaction mixture. Continued heating at increasing temperatures up to 300 "C produced a viscous polymer
Table 11. Evaluation of High-hiling Solvents as Transacylation Media reaction conditions polymer solvent diphenyl sulfone diphenyl sulfone benzophenone benzophenone benzophenone Arochlor
bp, "C 375
color black
5
300
0.21
305 305 305 -320
1.4 8 24 7
300 300 300 300
Arochlor
-320
8
250
Arochlor
-320
_-
8
300
0.33 tan 0.26 offwhite 0.34 tan no acetic acid evolution no acetic evolution diphenyl ether added after 4 h ; acetic acid evolution began immediately 0.21 brown
259
7
250
0.35
Arochlor/diphenyl ether diphenyl ether
375
time, temp, I.V. h "C dL/ga 5 275 0.12
black
light tan
(I Determined as 0.1%concentration in 1017 (wt/wt) phenol/tetrochloromethane.
solution which was only slightly discolored, usually slightly tan. While the results obtained with diphenyl ether solvent were promising, the poymer obtained was of too low a molecular weight to be a useful commerical product. In an attempt to drive the transacylation reaction to still higher molecular weight, solvents with higher boiling points were sought which would function similarly to diphenyl ether. Three potential solvents which were evaluated are Arochlor, diphenyl sulfone, and benzophenone. The results of these experiments are summarized in Table 11. It was found that benzophenone was also a good solvent for the system. Molecular weights in the range of those achieved with diphenyl ether were obtained. The use of higher boiling solvent, however, did result in the polymer products being darker colored than were those from diphenyl ether. No advantage, therefore, was noted for benzophenone. The difficulties involved in handling of this solid solvent and its removal from the reaction mixture precluded its further use in this program. Polymerization reactions using diphenyl sulfone resulted in low molecular weight, badly discolored products. Therefore this solvent was not used further.
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Ind. Eng. Chem. Prod. Res. Dev., Vol.
20, No. 2, 1981
The transacylation reaction did not occur when chlorinated biphenyl (Arochlor) was used as the reaction solvent (i.e., no acetic acid evolution was noted). However, the condensations run in this solvent resulted in some further information on the diphenyl ether system. In one reaction a quantity of diphenyl ether was added to the Arochlor reaction mixtures after several hours. Immediately upon the addition of the diphenyl ether, a vigorous acetic acid evolution began, the acids dissolved, and a viscous solution formed. One explanation for this behavior may be a lack of solubility of the two phthalic acids in the Arochlor. Upon addition of the diphenyl ether a portion of the acid mixture dissolves and the reaction proceeds. Any alternative explanation must account for an apparent inhibition of the condensation in the presence of Arochlor, as opposed to the slow condensation even in the absence of any solvent or diluent. The resultant polymer proved to be of unexceptional molecular weight, and no advantage could be found for the use of this mixed solvent system. In order to demonstrate that the diphenyl ether was in fact an inert solvent or diluent for the condensation mixture, mixtures of diphenyl ether and tetrachlorobisphenol A diacetate were scanned by NMR while being heated to 255 "C for periods of up to 48 h. No in-growth of any extraneous nmr signals was found during this period. Since the normal polymerization periods in this study were usually less than 7 h, it is unlikely that any reactions between diphenyl ether and the tetrachlorobisphenol A occurred which could lead to end capping or generation of species which could terminate polycondensation. While all of the other solvents seem to have some deficiencies such as accelerating decomposition of the monomers, retarding acetic acid evolution, being difficult to handle or recover, diphenyl ether stands out in this area as being unique. It offers a combination of inertness toward the monomers, sufficient solubility for the reactants to allow rapid condensation (as evidenced by the rapid acetic acid evolution) solubility for the formed polymer, to permit adequate mixing under relatively mild conditions, along with the ability to be readily recovered by vacuum stripping from the polymer at the end of the reaction. These properties have therefore pemitted the demonstration of a plasticized melt polycondensation reaction for the preparation of a polyester having a melting point near its decomposition temperature and having an extremely high melt viscosity. In summary, we have been able to demonstrate the preparation of high molecular weight polymers from the transacylation reaction on the acetate ester of a hindered bisphenol. While the molecular weights of the polymers prepared in this program are not sufficiently high to be of commerical interest, we have developed a viable approach to their preparation. Further refinement of the process will therefore be directed toward the evaluation of catalysts in order to obtain still higher molecular weight products. Experimental Section Materials. Tetrachlorobisphenol A (TCBPA) was purchased from Dover Chemical Co., Dover, Ohio. Terephthalic acid was purchased from Amoco Chemical and isophthalic acid from Arco Chemical Co. Solvents and other chemicals were obtained from Fisher Chemical Co. Preparation of Tetrachlorobisphenol A Diacetate (TCBPA-DA). Three hundred sixty grams (1.0 mol) of tetrachlorobisphenol A was added slowly to 1020 g of (10 mol) of acetic anhydride, containing 0.5 mL of concentrated sulfuric acid in a 3-L beaker. The TCBPA dissovled exothermically and the resultant solution was heated to
100 "C for 2 h. At the end of that period the beaker was removed from the heat and the solution was allowed to cool slowly to crystallize the TCBPA-DA. The product was filtered off, washed several times with 300 mL of acetic acid in a Waring blendor, then washed four times with 1-L portions of distilled water. The white crystalline product had a melting point of 135 "C. Polycondensation General Procedure. To a 250-mL three-necked flask equipped with a stirrer, nitrogen inlet, and distillation head was added 46.0148 g (0.1 mol) of tetrachlorobisphenol A diacetate, 11.6294 g (0.07 mol) of isophthalic acid, 4.9840 g (0.03 mol) of terephthalic acid, 100 mL of diphenyl ether, 0.05 g of manganese acetate (anhydrous), and 2 mL of acetic anhydride. The stirrer shaft was connected to a constant speed motor equipped with a torque meter (Master Servodyne, Cole Parmer Co.) in order to monitor the viscosity increase in the reaction. The mixture was heated, by means of an oil bath, to about 220-240 "C, while a nitrogen sweep was maintained. As the temperature of the reaction mixture reached -210 "C, acetic acid began to distill out and the insoluble acids began to dissolve. Complete solubilization of the acids occurs after about 1h at 250 "C (bath temperature) during which time about 95% of the theoretical acetic acid is recovered (11.0-11.5 g) .
The clear reaction solution increases in viscosity as the bath temperature is increased to 300 "C. Nitrogen sweep was adjusted so that the diphenyl ether refluxed just below the take-off of the distillation head. Reaction was allowed to continue for 20 h during which time the viscosity of the mixture increased. At the end of this period a vacuum was applied and the diphenyl ether solvent was removed. The polymer solidified during this distillation. Vacuum was maintained for 1h while the temperature was kept at 300 "C. At the end of this period the polymer was cooled, the flask broken, and the polymer recovered. Additional purifcation of the polymer may be accomplished by dissolving in methylene chloride and precipitation by addition to an excess of either methanol or acetone in a Waring blendor. Inherent viscosities were determined in a 0.1% solution in a 10/7 (wt ratio) phenol/tetrachloroethanesolvent. Acknowledgment The author wishes to express his appreciation to the Management of the Celanese Research Company for permission to publish this work, to Edward Kuczynski for technical assistance, to Maria Lykes for preparation of the manuscript, and to Arnold Rosenthal and Martin Epstein for support and encouragement. Literature Cited Bunell, H. "Polymer Handbook", Brandrud and Immergut, Ed., 2nd ed.; Why: New York, 1975; I V 337-1V 360. Conix, A. Ind. Eng. Chem. 1959, 57, 147. Drewitt, J. G. N.; Lincoln, T. British Patent 621 102, Apr 4, 1949, to British Celanese. Harper, J. J.; Janik, P. J . Chem. Eng. Data 1970, 15, 439. Kantor, S. W.; Holub, F. F. U.S. Patents 3 160604, 3 160605, 3 160602, all Dec 8, 1964. Levine, M.; Temin, S. C. J . Polym. Sei. 1959, 28, 179. Temin, S. C. J . Org. Chem. 1961, 26, 2518. Towle, P. H.; Baldwin, R. H.; Meyer, D. H. "Encyclopedia of Chemical Technology", 2nd ed.;Vol. 15, Wiley: New York; 1964; pp 444-87. Wilson, D. R. US. Patent 3702838. Nov 14, 1972.
Received for review September 25, 1980 Accepted November 20, 1980 Presented at the Second Chemical Congress of the North American Continent, Las Vegas, Nevada, Aug 24-29,1980, Division of Polymer Chemistry.
