Unsaturated Polyesterstwisting a t various temperatures. The frequency of polymerizable groups in the polyester has a large influence on rigidity. Variation in this single chemical characteristic is shown to produce styrene copo1ymers that vary from rubbery polymers a t room temperature to polymers that remain rigid and difficult to deform up to temperatures as high as 225' C. Within relatively wide limits, the ratio of the polyester to styrene is a factor of lesser influence on the properties measured.
LITERATURE CITED
(1) Alfrey, T., and M e r e , E., Polgmer RuU., 1, 86 (1945). ( 2 ) Le Fhvre, R. J. W., "Dipole Moments; Their Measurement and
dpplication in Chemistry," 2nd Ed., London, Methuen, 1948.
( 3 ) Lewis, F. M., and Rfayo, F. R., J . Am, Chem, Sot,, 70, 1533 (1948). (4) Lewis, F. 31.,Wallins, C., Cummings, W., RriggJ. E. R.,and Mayo, F . R.,Ihid., 70, 1519 (1948). ACCEPTED J u n e 2, 1954.
RECEIVED for review March 31, 1954.
Maleic-Fumaric Isomerization in Unsaturated Polvesters J
S. S. FEUER, T. E. BOCKSTAHLER, C. A. BROWN, AND I. ROSENTHAL Rohm & Haas Co., Philadelphia 5, Pa.
Comparative reactivity and cured resin property data of selected polyester resins derived from maleic anhydride indicate that the reactive polyester unsaturation is actually the fumaric type. Evidence for the isomerization of maleic double bonds to the fumaric form during polyesterification is presented; using a polarographic method of analysis, only the fumaric double bond is found in two different polyesters prepared from maleic anhydride.
T
H E excellent copolymerization characteristics of styrene and unsaturated polyesters derived from maleic anhydride are well known and are responsible, in large measure, for the commercial success of resins of this type. The relatively poorer copolymerization characteristics of methyl methacrylate and the same unsaturated polyesters have probably been partly responsible for retarding the development of these systems. Yet, the literature ( 1 ) shows that while styrene copolymerizes well with maleic anhydride, maleic acid, and maleic half esters, styrene does not copolymerize well with maleic diesters. In fact, the tendency for homopolymerization is almost as great (Table I ) as for the methyl methacrylate-maleic diester system. [Where T I and ~p are both small ( < 1 ) the tendency for copolymerization is great. A large value (>1) for either rl or r2 indicates preference for homopolymerization over copolymerization. ] However, styrene does copolymerize well with fumaric diesters and fumaronitrile, and while no data are available for methyl methacrylate-fumaric diester, methyl methacrylate and fumaronitrile copolymerize poorly (11). This paper illustrates, from comparative reactivity and cured resin property data, that the fumaric double bond is responsible for the desirable performance characteristics of unsaturated polyester resins derived from maleic anhydride. In addition, an. alytical proof of the isomerization of the maleic double bond to the fumaric form during polyesterification a t temperatures of about 200" C. is presented. EXPERIMENTAL
Isomerization of Dimethvl Maleate. One hundred grams of dimethyl maleate (Commercial Solvents Corp.) were catalyzed mith 0.1% p-toluene sulfonic acid monohydrate (Eastman Kodak Co., practical grade, 90%) and heated a t reflux (1 atm.) for 32 hours. During this time the temperature rose from 180' to 200' C. On cooling 71 grams of a crystalline solid were reY
August 1954
covered. After washing with methanol, the crystals had a mixed melting point of 101O to 102" C. Jvith dimethyl fumarate (m.p. 102" C.). Polyester Reactivity. A 10 X 100 mm. test tube was filled to a height of 3 inches xvith resin catalyzed with 1% of benzoyl peroxide and an iron-constantan thermocouple was centered in the resin. The test tube was immersed in a constant temperature I?.)] and the temperature in bath maintained a t 180" F. ( & l o the center of the resin was recorded continuously on a Speedomax Recorder (Leeds & Sorthrup Co.). The maximum temperature reached was noted. Polyester Cur >d Resin Properties. Heat diPtortion temperature (H.D.T.) was measured on 0.250-inch thick sheets cast from resin catalyzed with 1% benzoyl peroxide and cured a t 60' C. for 17 hours and 120' C. for 1 hour. Standard test bars were prepared and tested in accordance with ASTlI Method D 648-45T using a heating rate of 2" C. per minute.
TABLE I. RELATIVE REACTIVITIES OF MONOMER PAIRS (60' C.)
Mi diyi m e
M*
Ti('
rzb
Literature Cited
0 01 0 0 30 0.07 6 52 0.005 0 21 0.02 0.03 8 5 0 13 0.03 0 0 19 Fumaronitrile 0 0 19 Methyl mdthMaleic anhydride C 6 7 002 acrylate Diethyl maleate 20 0 Fumaronitrile 3 5 001 rate of 11.1, homopolymerisation a l l = ".1= rate of addition of MI free radical t o X 2 monouier hu rate of Mi homopolymeridation b rz = k. = k~ rate of addition t o M 2 t o UI monomer' c 750
(1) (1) (11)
c
I N D U S T R I A L A N D E N G I N E E R I N G C-HEMISTRY
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TABLE 11. COUPBRATIYE PERFORN.4NCE"
O F POLYESTER
polyesters prepared under certain reaction conditions m s implied by Kropa ( 8 ) , the conditions for, and extent of, such an isomerization were elucidated only very recently by Batzer and Mohr ( 2 ) . They found that under the high reaction temperatures (200" C.) used by Carothers ( 4 ) and others (3, l a ) , maleates are isomerized to fumarates and by using a solvent-refluu method of polyesterification at 100" C , they were able to prepare polyesters (hexamethylene glycol maleate) without appreciable isomerization. To check this, dimethyl maleate catalyzed with 0.1yop-toluene sulfonic acid monohydrate was refluxed a t 180" to 200" C. at atmospheric pressure for 32 hours. A 71% yield of crystalline dimethyl fumarate was obtained. Further circumstantial evidence may be gained by comparing reactivity test data and heat distortion temperatures (Tablr 11)for the following systems:
RESINS
PolyExothermb ester, Peak, HbD.T.. Monomer Polyester To F. C. 70 473 126 Styrene Resin A c 70 321 76 Methyl methacrylate Resin .4 98.7 Resin B d 60 406 Styrene 60 397 93.9 Styrene Resin Ce a Definite viscosity increase noted prior t o gelation. For test method see Proceedings of 6th Annual Technical Seesion, Reinforced Plastics Division, SPI,Feb. 28, 1951, Sec. 3, p. 1. with following exceptions: resin tube size 12 X 100 mm. and iron-constantan thermocouples were immersed directly in resin. C Polyester derived from maleic anhydride (no other dibasic acid charged t o the reactor). d Polyester made from phthalic and fumaric acids. Similar to resin B except t h a t maleic anhydride was used in place of fumaric acid.
POLAROGRAPHIC ANALYSIS
Apparatus and Reagents. The polarographic curves were obtained on a Leede & Sorthrup Electrochemograph using an H-cell with a saturated calomel reference electrode. Temperature was controlled to 1 0 . 5 ' C. The 1 M ammonium hydroxideammonium chloride buffer, pH 8.2, was measured Kith the lineoperated Leeds and Northrup pH meter. The diethyl maleate used was obtained from the Rohm & Haas Co. Sample Preparation. ii quantity of ester calculated to give, after saponification, 50 milliliters of a solution that is 0.2 to 2.0 m M in maleic or fumaric acid was added to a 50-ml. volumetric flask. The sample was dissolved in minimal amounts of 2B alcohol Kith the addition of small amounts of benzene when needed. Depending on the sample, 2 t o 5 ml. of a potassium hydroxide solution (0 2 to 0.512') were then added and the flask heated for 20 minutes on a steam bath. The samples were cooled, diluted to 50 ml. with a 131 ammonium chloride-ammonium hydroxide buffer, pH 8.2, and the p H checked. The sample was then added t o the cell, degassed with nitrogen, and run polarographically over the potential range -0.1 t o -1.85 volts. DISCUSS103
I n order t o explain the excellent reactivity characteristics of the styrene-maleic unsaturated polyester system two assumptions are possible:
1. In the polyester form the maleic double bond is more reactive than in simple diesters 2. Maleic unsaturation is converted t o the fumaric form by isomerization during polyesterification.
1. Methyl methacrylate-maleic unsaturated polyester A. 2. Styrene-same polyester A. 3. Styrene-maleic unsaturated polyester C. 4. Styrene-fumaric analog of polyester C (designated Polyester B). The exothermic peak and the heat distortion temperature (both of which are indicative of copolymerization potential) indicate that
I
1. Methyl methacrylate copolvmerizes much more poorly m-ith polyester A than does styrene. This can only be the case if the copolymerization tendencies of each monomer with the polvester unsaturation are vastly different. Both the methyl methacrylate-maleic diester and styrene-maleic diester copolymeriaations have been shown t o be similarly poor, whereas the copolymerization of methyl methacrylate with fumaric unsaturation is greatly inferior t o that of the styrene-fumarate ester system. Thus, polyester il probably contains fumarate unsaturation. 2. The styrene-maleic polyester C resin is nearly identical in performance t o its fumaric analog, styrene-polyester B. This could only be the case if both polyesters had the same type of unsaturation, and since coreactivity is good, the unsaturation type appears to be the fumaric rather than the maleic tvpe. Ebers and associates (6)have also reported comparative lieat degradation performance of analogous polyesters derived from maleic anhydride and fumaric acid as starting materials. While they recognized the difference in reactivity between the etyrenefumarate ester and styrene-maleate ester systems. they observed no difference in the heat aging characteristics of the two resinc: and concluded either that the copolymer effect was abqent or that some other degradation reaction occurred. Their results can easily be explained, however, in the light of the proposed maleic t o fumaric isomerization-i.e , both of their polyesters contained mostly fumarate unsaturation and hence were eseentially identical. For conclusive proof direct identification was obtained of the type and amounts of uneaturation present in polyesters based on maleic anhydride as a starting material. Since maleic and fumaric acids give distinct and separable polarographic waves in the alkaline region ( E l / 2of maleic arid = -1.33V. fumaric = -1.55V in 1M iYH&l-SH,OH buffer
On examining the first assumption, both Price ( I O ) and Len-is and Mayo (9) are of the opinion that the excellent copolymerization of maleic anhydride and fumaric esters with styrene is due to the ability of these materials t o assume a planar configuration that renders the double bond more susceptible to attack by a free radiral, and that stabilizes the activated radical (by resonance) after attack. I n the case of maleic diesters assumption of a planar form is highly improbable because of steric effects, and thus, copolynieriaability is greatly reduced. I t is difficult therefore t o visualize anv improvement in the reactivity of maleic double bonds by virtue of their TABLE 111. POLAROGRAPHIC DATAON POLYESTER RESINS incorporation into a polyester structure Estimated Final Concn. ~ _Concn.z _ (if anything, steric hindrance should be Ester, Maleic, KOH, of KOH, Eitz, id, m .I2 increased), except for terminal half Sample Grams mM M1. N Lv Y. pa Maleate Fumnrate esters. The last mentioned constitute Ethylmaleateb 0.0132 0.1 -1.332 6.73 .. 15.36 2 (0.203) Ethylmaleateb 0.0331 0.1 -1.332 6.80 38.40C 2 (0.535) only a relatively small fraction of the Resin D d 0.1539 0.1 -1.562 3.fi8 O'e i',i 10.0 5 (0.535) 0.1 -1.562 4.15 06 8 6 Resin Ef 0.181 10.0 2.5(0.535) total unsaturation in a high molecular weight polyester. a Based on tenfold dilution. b Rohm & Haas diethyl maleate, acid No. -0.21. Thus assumption 2 is the most logical C Solution diluted 25-fold before being run polarographically. d Flexible-type maleic anhydride derived polyester resin. explanation. 8 Estimated maximum of 3% maleic acid might have gone undetected. While the isomerization of maleic I Rigid-type maleio anhydride derived polyester resin. double bond t o the fumaric form in I .
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INDUSTRIAL A N D E N G I N E E R I N G C H E M I S T R Y
Vol. 46, No. 8
-
-Unsaturated
Polyesters-
used, a maximum of about 3% maleic acid might go undetected in the presence of large amounts of fumaric acid. The ability to detect the presence of maleic acid was checked by adding maleic acid to the saponified mixture. I t s presence was easily identified. Possible interferences by phthalic acid-a possible saponification product-was similarly checked. No interference was found (curve 7, Figure 1j. These results ar6 summarized in Table 111. -4s a result of the polarographic analyses there is proof of maleic to fumaric isomerization during polyesterification. CONCLUSIOS S
The excellent reactivity characteristics of maleic based unsaturated polyester resin-styrene systems is apparently due to the presence of fumaric unsaturation. The presence of the fumaric unsaturation in polyesters based on maleic anhydride is the result of isomerization during polyesterification. The properties of unsaturated polyester resins are probably dependent on the maleic-fumaric equilibrium reached during polyesterification. I n two different maleic unsaturated polyesters prepared under one set of similar conditions the conversion from maleic to fumaric unsaturation was found by polarographic analysis to be essentially complete. -0.5
-0.2
-1.0
-1.5
VOLTS
ACKNOWLEDGMENT
Figure 1. Composite Polarograms of Maleic and Fumaric Content of Polyesters
The authors are indebted t o E. R.I. Beavers and A. J. Canale of the Rohm 8;. Haas Laboratories for their helpful suggestions and constructive criticism,
+
1. Base solution pH 8.7 1.041 NHaCl “,OH Saponified resin D (d.5>%’KOH) Saponified diethyl maleate (0.2N KOH) Saponified resin D maleic acid 5. Saponified diethyl maleate (0.5N KOH) 6. Saponified resin E (0.5N KOH) 7. Saponified resin E maleic acid 4- phthalic acid
2. 3. 4.
+ +
p H = 8.2), and since the polarography of these substances has been extensively studied in various media ( 7 ) , polarographic analysis of the saponification products of two widely different maleicbased unsaturated polyesters was employed. The saponification was carried out under conditiom that do not cause isomerization during similar saponification of diethyl maleate (6). The results obtained are shown in Figure 1. Saponified diethyl maleate produces Elit values corresponding to maleic acid ( 1.33V), and no fumaric wave is obtained, thus indicating no isomerization during saponification (curves 3 and 5, Figure 1). The saponified polyesters produce waves that indicate the presence of only fumaric acid (Eli2 = 1.56V). N o maleic acid was detected (curves 2 and 6, Figure 1). Under the conditions
-
LITERATURE C I T E D
(1) Blfrey, T., Bohrer, J. J., and Mark, H., “Copolymerization,” pp. 34-7, New York, Interscience Publishers, 1952. (2) Batzer, H., and Mohr, B., Makromol. Chem., 8 , 217 (1952). ESG.CHEM., (3) Bradley, T. F., Kropa, E. L., Johnston, W. G., IND. 29, 1270 (1937). (4) Carothers, W. H., J . Am. Chem. SOC.,51, 2560 (1929). (5) Ebers, E. S., Brucksch, W. F., Elliott, P. AI., Holdsworth, R. S., and Robinson, H. W., IND.ENG.CHEM.,42, 114-19 (1950). (6) Elving, P. J., Martin, A, and Rosenthal, I., Anal. Chem., 25, 1082 (1953). (7) Elving, P. J.,and Rosenthal, I., Ibid., 25, 1082 (1953). (8) Kropa, F. L., India Rubber World, 118, 532-3 (1948). (9) Lewis, F. M., and Mayo, F. R., J . Am. Chem. SOC.,70, 1533 (1948). (10) Price, C. C., J . Polymer Sci., 1, 83 (1946). (11) Price, C. C., Gilbert, R. D., Ibid., 6, 577-81 (1952). (12) Vincent, H. L., I N D . ENO.CHEM.,29, 1267 (1937). RPCEIPED for review RIaroh 31, 1954.
ACCEPTED
June 4, 1954
END OF SYMPOSIUM
August 1954
INDUSTRIAL AND ENGINEERING CHEMISTRY
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