INVESTIGAT ION 0 F M A LEATElFU MARATE ISOMERIZATION IN UNSATURATED POLYESTERS B Y NUCLEAR MAGNETIC RESONANCE L A R R Y G. C U R T I S , D A N I E L L. E D W A R D S , R O B E R T M . S I M O N S , ' P A R K E R J. T R E N T , A N D P A U L T. VON B R A M E R Chemicals Division, Eastman Chemical Products, h e . , Kingsport, Tenn.
The utility of nuclear magnetic resonance (NMR)spectroscopy to follow the extent of maleate (cis) to fumarate (trans) isomerization during preparation of standard unsaturated polyesters was demonstrated. NMR analysis indicated that most of the cis-trans isomerization occurred during the early stages of the cook, High cis-trans conversions (95%) were effected in cooks containing sterically hindered or low-reactivity glycols-e.g., propylene glycol. lower conversions (50%) were obtained with less sterically hindered glycols of greater reactivity such as diethylene glycol. The presence of aromatic dibasic acids increased the cistrans isomerization. In general, this study confirms earlier work cited in the literature in which the cis-trans conversions were determined b y other analytical techniques.
E A R L Y as 1929. in studying cis-trans interconversions, Carothers noted that on esterification of maleic acid at 200' C.: a transformation to the fumarate form occurred ( 7 ) . Since then, considerable study and research have been devoted by several investigators to the many aspects of maleate-fumarate isomerization as applied not only to simple diesters but also to the more complex polyester systems. In recent years, interest in cis-trans conversions in unsaturated polyesters has progressed from the academic stage to one of considerable practical significance due to the commercial development of these resins. T h e difference in reactivity between maleate and fumarate esters is ivell known, with the fumarates having the greater reactivity-for example, styrene copolymerizes readily with fumaric diesters but only with great difficulty with maleic diesters. In other properties of maleate and fumarate polyesters, the fumarates generally possess higher heat distortion temperatures, greater tensile strengths, higher hardness values, and better chemical resistance characteristics. Pol>-esters prepared from maleic anhydride, to a greater or lesser degree. exhibit reactivities and properties similar to those expected for polyesters prepared from fumaric acid. I t thus seems logical to assume that maleic unsaturation is converted to fumaric unsaturation during the course of the polyesterification. The fact that this cis-trans isomerization does occur has been fully demonstrated. It is reported that the excellent reactivity characteristics of maleic-based unsaturated poll-esters as well as the physical properties of the cured resins are? in fact, due to the presence of fumaric unsaturation ( 2 ; 4, 5). Recently, a group of Hungarian researchers reported that the type of glycol, the'type of aromatic acid, the time of polycondensation, and the final molecular weight of the polyester all exercise a considerable influence on the extent of cis-trans isomerization (7). These previous investigations have been someLvhat handi-
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Present address, Tennessee Eastman Co.. Kingsport. Tenn. I B E C PRODUCT RESEARCH A N D DEVELOPMENT
capped by the lack of suitable analytical methods. Past methods were based principally on polarographic and infrared techniques. T h e polarographic method. for example, depends on hydrolysis of the polyester prior to analysis, thus causing some doubt as to the isomeric composition, since some isomerization may have occurred during the hydrolysis step. T h e purpose of the study covered by this paper was to demonstrate the utility of high resolution nuclear magnetic resonance ( S M R ) in determining the maleate-fumarate ratio in unsaturated polyester systems. In a nuclear magnetic resonance spectrum of an unsaturated polyester. the maleic proton resonance peak is located at a different field strength than the fumaric proton resonance peak. In addition, the area under the absorption peak is proportional to the number of protons causing that absorption. Determination of the areas under the two peaks allows calculation of the relative amounts of cis and trans double bonds. M'ith this analytical method the effect of several variables on the isomerization reaction in unsaturated polyesters could be investigated T h e factors studied included the influence of different glycols, aromatic acids, degree of polymerization, and, in certain cases, reaction temperature. Six glycols with hydroxy groups of varying degrees of steric hindrance were used : 1,4-cyclohexanedimethanoI(CHDM). diethylene glycol (DG), 2,2-dimethyl-l:3-propanediol or neopentyl glycol (NPG), ethylene glycol (EG), 1,2-propylene glycol (PG), and 2.2,4-trimethyl-1,3-pentanediol (TMPD). (For brevity. subsequent references to these glycols are by the above abbreviations.) Each glycol was investigated in a maleic, a maleico-phthalic, and a maleic-isophthalic system. Conventional processing techniques and cure conditions were used throughout this study. During the course of the cooks samples were withdralvn and subjected to S M R analysis to determine the amount of isomerization at different degrees of polymerization. T h e cure characteristics and several physical properties of selected polyester samples were compared with those of polyesters of comparable molecular weight prepared from fumaric acid.
Experimental
Polyester Preparation. Resins for this isomerization study were prepared in the laboratory under an inert atmosphere and controlled temperatures. Apparatus for the synthesis of the unsaturated polyesters consisted of a ?-liter. round-bottomed. four-necked reaction flask equipped with a stirrer, thermometer. nitrogen tube. and a packed, steam-jacketed condenser. Two water-cooled condensers were attached to the steam condenser by means of a Barrett water trap. 'l'he three series of resins Lvere prepared by the fusion process, \vith the isophthalic series requiring a two-stage method because of the insolubilit), of the isophthalic acid. No catalyst was emplo)-ed in the cooks. Ten per cent excess glycol was used in preparation of each resin. I n the isophthalic and ophthalic series a 1 to 1 molar ratio of aromatic acid to unsaturated acid \vas employed. \Vhen the viscosities and reaction times would permit. the cooks were continued until the solids acid number was between 20 and SO. 'I'his rangy \vas selected as representative of the acid number range of most commercial unsaturated polyester resins. Certain cooks kvere stopped a t higher acid numbers because of impractical reaction times at the reaction temperatures. 'Ihe gl>-col-maleate cooks \vere conducted at a top reaction temperature of 175' C., whereas the aromatic acid modified cooks were reacted a t a top temperature of 200' C . T h e TMPD-o-phthalic--maleate resins were not investigated because of unduly long reaction times and reaction conditions Fvhich were not comparable lvith those used in the preparation of the other resins. Samples of each resin were prepared as 15 to 20% solutions in acetone for N M R spectroscopic analysis-with one exception. Because of the extreme crystallinity of the 1,4-cyclohexanediniethanol (CHDM)-isophthalic-maleate. solution of this resin in acetone \vas not possible; consequently, N h l R analysis \vias not attempted.
The synthesis of each of the three resin series required somewhat different techniques. These differences were primarily related to the solubility of the acids or acid anhydrides used. The follo\ving examples typify the preparation of each series. GLYCOL-MALEATE The S . glycol maleates were prepared a t a reaction temperature of 175' C.: attained after an upheat cycle of 45 minutes. Careful control of the temperature was necessary in this series during the upheat cycle because of the initial exothermic reaction of the maleic anhydride. The first sample for N h l R spectroscopy was withdrawn from the reaction flask 30 minutes after the upheat cycle was complete. A total of six samples was taken during the course of the reaction. GLYCOI.-~-PHTH.ALIC-M.ALEATES. I n this series. an upheat cb-cle of 1 hour was used to bring the reaction to a top temperature of 200" C. Becaiise the maleic anhydride was initially charged with the o-phthalic anhydride and the glycol, an exothermic reaction \vas observed. necessitating careful control of the temperature during the upheat cycle. The first sample for NMR was prepared immediately after this exotherm had ceased. T h e second sample was withdrawn at the completion of the 1-hour upheat cycle. 4 total of six samples was taken periodically during the reaction. The folloLving reaction log of a propylene glycol-o-phthalic-maleate polyester is typical of those resins prepared in this study.
?'imp,
Acid
Hr.
.Vo.
A
0 0 1 9
0 5 0 0
250 152 109
Drgree of Polymerzzation 1 2
2 0
88
2 8 3 5
35
8 9
Funiarate Remainingu Presenth
70Acid 46 28 20 16 6
1
0 0 2
4
55 90 92
94 94
;Mol. WeightC
225 370 514 640 1600
H m ~ do n original acid number of charge and acid number of sample. By .V,MR analysis. All molecular weights. r u r p p l fhr j n u l on?. f o r rach resin prepared were calculated f r o m acid niimhiu. Finn1 molrculnr weiqhfs w r e determined ebulliometrically by b o i l i r i ~p o i n f r i j c in loliirne. See calculations section. '1 After 0..5 hr. of t o l d I - h r . iiphrnt cycle. ',
S r r cnlciilutions srction.
Basically, all calculations involved in this C.AL,C~~LATIOSS, study were directly related to and derived from the acid numbers of the rzsins measured at various stages of the synthesis. Exceptions to this derivation were the determinations of final molecular Ivcights by the ebulliometric procedure and the fumarate concentration by N M R anal+. Acid Number (AN). The acid number of a resin is defined as the milligrams of potassium hydroxide (KOH) required to neutralize the free or unreacted carboxyl groups in 1 gram of the resin. It is determined by the following formula:
AS
=
ml. of KOH X normality of KOH X 56.1 wt. of sample, grams
Molecular Weight (MWav), The molecular weights reported in this study were calculated by the following formula: MMWav (approx.) =
56,100 acid number
.4s explained by Patton ( 6 ) in the derivation of this formula, this expression is valid for determinations of acid numbers of about 30 or above For this reason, the final molecular weight of each resin \vas determined ebulliometrically ( 3 ) . Except for the PG IPX 1f.A resin, these values correlated with the calculated molecular \I eights. Degree of Polvmerization (DP). T h e degree of polymerization indicates the number of times the glycol-acid repeating unit occurs in the resin molecule. I t is denoted as n in the following expresqion :
H-
[O-R-OOC-R-CO]n-O-R-OH
Degree of polymerization is calculated by the formula : DP
MWav __
=
molecular weight of repeating unit Per Cent .4cid Remaining. T h e value. per cent acid remaining, represents the unreacted carboxyls present in the resin and is expressed as a percentage of the total equivalents of acid charged to the reaction vessel. It is determined by the following expressions: charged X 56,100 Total acid number = total acid equivalents total weight of reactants, grams Per cent acid remaining =
m /O Rraction
G L Y C O L - I S O P H T H A L I C - ~ ~ ~ L E A T EPreparation S. of the isophthalic series involved a two-stage process. The glycol and isophthalic acid initially were charged to the reaction flask, and through an upheat cycle of 1 hour. brought to a reaction temperature of 200' C. The reaction \vas stopped a t this point, the mixture cooled to 150' C.. and the maleic anhydride charged to the flask. The temperature then was increased rapidly to 200' C . and the first sample for S M R analysis taken. A total of five samples was withdrawn periodically and prepared for analysis.
X 10G sample acid number -. --___ ~~
total acid number
Nuclear Magnetic Resonance (NMR) Determinations. All spectra were determined and recorded by means of a Varian '4-60 N M R spectrometer (product of Varian Associates). Samples of the polyesters were investigated a t room temperature as 15 to 2OYGsolutions in acetone. One-milliliter samples \vere employed for each N M R determination. Figure 1 shows portions of the N M R spectra of typical samples taken during the early and late stages of the cook. Peak A at 6 = 6.98 p.p.m. is produced by the olefinic protons of fumarate. and peak B a t 6 = 6.55 p.p.m. is due to the olefinic protons of maleate. The two peaks are well separated VOL. 3
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from each other, and their areas can be determined. The area of an NMR peak is proportional to the number of protons causing the peak; thus the relative concentrations of maleate and fumarate present in the sample can be determined. A
n
Discussion of Results
The results of this investigation generally confirm findings of Vancso-Szmercsanyi and coworkers (7). Isomerization of the maleate to the fumarate form occurred at the early stages of the polyesterification and the structure of the glycol as well as the presence of an aromatic dibasic acid had a profound effect on the degree of isomerization. Composite data are presented in Table I for the resins prepared and include reaction time and final acid number, degree of polymerization, molecular weights, and per cent fumarate. The curves in Figure 2 indicate the per cent fumarate formed in the different cooks during the course of the polyesterification. Examination of these data reveals that the structure of the glycol exhibits the greatest effect on the degree of cis-trans isomerization. In the entire series, the cooks containing CHDM had the least amount of isomerization, whereas either the propylene glycol or TMPD cooks showed the greatest amount. Although slight anomalies exist from one series to the next, the glycols might be listed in order of increasing effect on the cis-trans isomerization as: CHDM < DEG < NPG < EG < T M P D = PG. The results of this work clearly indicated that introduction of an aromatic dibasic acid into the system further enhanced the cis-trans conversion, with the phthalic molecule having a greater effect than the isophthalic molecule. The phthalicmodified series produced the best and most consistent results, since all the cooks in this series had acid numbers and molecular weights in the same range. A comparison of the glycol effects under conditions of minimum variability. therefore. is well illustrated by this series. The reaction times and molecular weights of the straight maleate cooks varied widely. No explanation for this inconsistency can be given, except that this series was prepared at
i;
Sample taken after 30min. of upheat cycle maleatf
end of cook
I;
I
fumarate
A fumarate 1
6.98 p . p m
Sample taken at
d a f lh i
6 55 p p
m.
1
B A
1
698p p
rn
655 p p r n .
I
Figure 1. NMR spectra of maleate-fumarate isomerization occurring during preparation of EG/PAA/MA polyester
100-
Table 1.
Properties of Resins Prepared for Isomerization Study Final Reaction Solids Degree MolecResin Time, Acid of Polyular FumComposition" Hr. Number merization Weight arate: 7,
CHDMiM.4 DEG/MA NPG/MA EG/MA PG/MA T M P D /MA CHDM/PAA/MA DEG/Pr\.4/MA NPG/PAA / M A EG/PAA/MA PG/PAA /MA DEG/IPAIMrl NPG/IP.!/MA EG/IPAIMA TMPD/IPA/M.A PG/IP.A/Mh
5.5 16.0 8.5 9.5 10.0 6.5 2 5 8 5 6 0 5 0 9.0
12.5 9.0
8.5 16.0 15.5
26 34 27 48 63 48 26 28 30 27 35 18.5 18 19 34 34.5
9.6 8.9 11.3 8.2
5.7 6.4 8 7 9 0 9 0 10 5
8.9 14.4 14.8 15.1 8.1 17.5
2160 1650 2070 1170 900
1450 2100 1900
1900 1900 1600 3040 3100 2517 2030 3200
33 50 50
75 93 93 52 82 85 84 94 65 72 71 96 95
CHD.Zrl. 7,4-cyclohexanedimethanol. DEG, diethylene glycol. EG, ethylene glycol. IPA, isophthalic acid. M A , maleic anhydride. S P G . 2:Z-dimdhyl- 7,3-$ropanedioi (neopentyl glycol). P A A , phthalic anhydride. PG, 7,2-$ropylen? glycol. T.MPD, 2,2,4-triQ
methyl- 7,3-pentanediol.
A /
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PRODUCT RESEARCH A N D DEVELOPMENT
LEG
Figure 2. Maleate-fumarate isomerization during polyesterification of glycol (2.2 moles) A. E. C.
220
TMPD
W i t h maleic anhydride (2.0 moles) With a-phthalic anhydride (1.0 mole) and moleic anhydride (1.0 mole) W i t h isophthalic acid (1.0 mole) and maleic anhydride (1.0 mole)
Table II.
Effect of Fumarate-Maleate
Polyestera
PG/IPA/FA PG/IPA/MA DEG/IPA/FA DEG/IPA/MA
Gardner Viscosity J 0
G I
Isomerization on Routine Properties of Propylene Glycol- and Diethylene Glycollsophthalate Polyesters
Gel time. min.
180' SPI Gel Test Time to peak Pxotherm. min.
Peak exotherm. ' F.
Heat Distortion Tern)., F.
Flexural Streneth.
5:20 5:20 4:30 4:40
7:47 7:44 6: 51 8:17
410 410 396 362
213 214 126 109
18,355 17,951 15,366 12,246
P.s:I'
'
Flexural Modulus. 105 P.S.I.
5.39 5.44 3.96 3.09
40y0styrene content.
lower reaction temperatures, 175' C. as compared with 200" C. for the cooks containing an aromatic acid. This temperature may not have been the optimum esterification temperature for some of the glycols. The diethylene glycol cooks, in particular, required unexpectedly long reaction times in all three series. Thic phenomenon may be explained, in part, by some decomposition observed during the reaction. Examination of molecular models shows that the chains in a maleate polyester are bent, while those in a fumarate polyester are straight and, therefore, probably in a more stable energy state. I t is logical, theri, to assume that sterically hindered glycols or those with less accessible secondary hydroxyl groups prefer to react with the trans isomer during polyesterification process. This preference accounts for the observed higher degree of cis-trans isomerization occurring during the cooks containing PG and T M P D . C H D M and diethylene glycol, on the other hand. offer considerably less steric hindrance and, therefore. would s h o ~ vless preference in reacting with either the maleate or fumarate forms. This assumption is in agreement with the linclings of this study of lower cis-trans conversion in cooks containing these glycols. Molecular models show that the hydroxy groups of C H D M are more widely separated and probably more accessible than those of DEG, thus accounting for greater reactivity and lower cistrans isomerization in the C H D M cooks. Inductive effects of the ether oxygen and possible intramolecular hydrogen bonding may also be contributing factors in accounting for the relative reactivities of these glycols. Similar consideration of special configurations indicates that the hydroxy groups of NPG and EG are intermediate in accessibility and thus would be expected to cause more isomerization of the maleate to the fumarate than C H D M or DEG. but less than P G or T M P D . This conclusion is in agreement with the results of this investigation. Inclusion of either phthalic or isophthalic groups as part of the polyesters contributes more steric effects, thereby causing the observed increase in cis-trans conversion. This effect is greater for the more hindered phthalic molecule. Additional Experiments
Brief investigations were made of the effects of reaction temperature on the cis-trans isomerization. -4 C H D M maleate polyester was prepared a t a top temperature of 200' C. rather than the usual 175" C . N M R analysis during the course of this reaction revealed that the rate of cis-trans conversion was accelerated by the higher temperature but, a t the same acid number level, the amount of conversion was about the same as at the lower reaction temperature. For example,
after 3.5 hours at the 175' C. reaction temperature, the acid number of the CHDM-maleate was 32.6 and 32y0 fumarate was formed. When the reaction temperature was increased to 200" C., the same acid number was reached after only 1 hour but the amount of fumarate formed was essentiallv the qame-34%. Of further interest and as added confirmation to this study, two additional polyesters were prepared in exactly the manner previously described. except that fumaric acid was used instead of maleic anhydride in the initial charge One polyester was based on PG/IPA/FA; the other on D E G 'Pa4A /FA Styrene solutions of these polyesters were prepared and cast The cure characteristics and certain physical properties of the castings were measured by standard techniques and compared with those of earlier described polyesters derived from maleic anhydride. The comparative data are shown in Table 11. As shown by these results, no significant difference existed in the properties of the propylene glycol polyesters whether prepared from maleic anhydride or fumaric acid. These data support the K M R findings that the cis-trans conversion is essentially complete in a PG-maleate resin. Conversely, comparison of the data on the two DEG resins shows significant property differences. indicating a lower degree of cis-trans isomerization in the D E G maleate polyester. As might be predicted, the resin prepared from fumaric acid showed increased cure reactivity, higher heat distortion temperature. and greater strength properties as compared with the maleate-based resin. Acknowledgment
The authors express their appreciation to V. W. Goodlett and Evelyn Simons of the Research Laboratories, Tennessee Eastman Co., for their assistance in conducting and reporting the N M R analyses. literature Cited
(1) Carothers. 11'. H., J . Am. Chem. SOL.51, 2560 (1929). (2) Feuer, S.S., Bockstahler, T. E., Brown, C. A,, Rosenthal, I.. Ind. Eng. Chem. 46, 1643-5 (1954). (3) Glover. C. A , . Stanlev. R. R.. Anal. Chem. 33. 447 11961). (4) Gordon, hl.,'Grieveio'n, B. M., McMillan, 'I. D.,' J . Polymer Sci. 18, 497-514 (1955). (5) Huggins, 13. G., Oronite Division, California Chemical Co.: San Francisco, Calif., private communication, 1962. (6) Patton, T. C., "Alkyd Resin Technology," Vol. 8, p. 107, Interscience. New York, 1962. (7) Vancso-Szmercsanyi, I., Maros-Greger, K., Makay-Bodi, E., J . Poiymer Sci. 53, 241-8 (1961). RECEIVED for review January 3, 1964 ACCEPTEDMay 7, 1964
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