objectives; first, it frees the experimenter from the problem of obtaining a suitable reference standard in those cases where alternatively labeled forms may not be available, and second, it provides the mathematical certainty that the reference population contains a single population. To our knowledge, this reflexive procedure of generating a hybrid ratio between the fractions of an experimental distribution and a computed distribution with the same determinant characteristics has not been used as a diagnostic tool before. Its ability to dis-
cern separations between two components as low as 0.8 u indicates the possibility of analyzing doublet peaks in which the components differ in migration rate by as little as 13 of the peak width. This should provide the opportunity to measure extremely small differences in energy levels of molecular species undergoing chromatography. RECEIVED for review March 19, 1968. Accepted June 24, 1968. This work was supported by the U. S. Atomic Energy Commission.
Analysis of Ethylene Oxide and Propylene Oxide Adducts of Alkylphenols or Alcohols by Nuclear Magnetic Resonance, Gas-Liquid Chromatography, and Thin-Layer Chromatography Procedures F. John Ludwig, Sr. Research Laboratory, Petrolite Corp., 369 Marshall
Ave.,St. Louis, Mo. 63119
NMR methods have been developed to distinguish pCH3
I
RC,H,O(CH,CH,O),(CH,CHO),H CH3
(I)
from
p-RCGH,O-
(CH2CHO),(CH2CH20),H (11) and, in conjunction with infrared and H I decomposition-GLC procedures, to determine X , y, and the empirical formula of R. Individual components of the ethylene oxide, propylene oxide, or ethylene oxide-propylene oxide adducts of p-alkylphenols which contain fewer than 10 polyether units were separated as the trimethylsilyl ethers by means of gas-liquid chromatography on SE-52 Chromosorb G columns, and GLC-molecular weight distributions were calculated. Thin-layer chromatographic systems were developed to separate components of I and II with the sum ( x y ) less than 10,to estimate the molecular weight range of polypropylene glycols, and to separate different classes of oxyalkylates from each other. Gel permeation chromatography with Sephadex LH-20 was shown to be a promising method of measuring the molecular weight distributions in oxyalkylates and of separating oxyalkylate mixtures on the basis of molecular weight differences.
+
THEMOST IMPORTANT CLASS of nonionic surfactants consists of the reaction products of ethylene oxide (EtO) and/or propylene oxide ( P r o ) with compounds such as p-alkylphenols, glycols, or fatty alcohols which contain a reactive hydrogen atom. The commercial adducts, which are frequently termed oxyalkylates, are mixtures with rather broad molecular weight distributions ( I ) . Different products are obtained if E t 0 and P r o are added in two separate steps or as a mixture (2). The structural parameters which must be measured to characterize an oxyakylate containing both E t 0 and P r o groups include the following: (a) the composition of the starting material; (b) the total number of moles of oxide added per reactive hydrogen atom; (c) the weight or mole per cent of E t 0 and P r o ; (d) the order of addition of the oxides-Le., (1)
N. Shachat and H. L. Greenwald in “Nonionic Surfactants,”
,M.. J. Shick, Ed., Marcel Decker, New York, N. Y . ,1967, Chapter L. (2) I. R. Schmolka, ibid., Chapter 10. 1620
ANALYTICAL CHEMISTRY
as a mixture or consecutively as P r o , E t 0 or EtO, P r o ; and (e) the molecular weight distribution. Nadeau and Siggia (3) have recently reviewed instrumental methods of analysis of nonionic surfactants. Nadeau and Waszeciak ( 4 ) have summarized methods of separation of these products. Several investigators have used proton N M R spectrometry to measure primary hydroxyl/secondary hydroxyl ratios in polyalkylene oxides (5-8). Manatt et al. ( 9 ) employed l9F NMR spectrometry to determine these ratios in polyalkylene oxides which had been converted to trifluoroacetate esters. Gas-liquid chromatography (GLC), column chromatography, gel permeation chromatography, and thinlayer chromatography (TLC) have been used by numerous investigators to measure molecular weight distributions of oxyethylated p-alkylphenols or alcohols, and of polypropylene glycols (10-23). (3) (4) (5) (6) (7)
H. G. Nadeau and S. Siggia, ibid., Chapter 26. H. G. Nadeau and P. H. Waszeciak, ibid., Chapter 27. T. F. Page and W. E. Bresler, ANAL.CHEM.,36, 1981 (1964). A. Mathias, Anal. Chim, Acta, 31, 598 (1964). V. W. Goodlet, ANAL.CHEM., 37, 431 (1965). (8) A. Mathias and N. Mellor, ibid., 38, 472 (1966). (9) S. L. Manatt, D. D. Lawson, J. D. Ingham, J. D. Rapp, and J. P. Hardy, ibid., p 1063. (10) L. C. Case and N. H. Rent, Polymer Letters, 2,417 (1964). (1 1) L. Gildenberg and J. R. Trowbridge, J. Amer. Oil Chem. SOC., 42,69 (1965). (12) J. K. Weil, A. J. Stirton, and E. A. Barr, ibid., 43, 157 (1966). (13) J. Tornquist. Acta Cheni. Scand., 20, 572 (1966). (14) M. K. Withers, J. Gus C/iromatogr., 6, 242 (1968). (15) J. Schaefer, R. J. Katnik, and R. J. Kern, J . Anier. Chem. SOC., 90, 2476 (1 968). (16) K. Burger, Z. Anal. Chew., 196 (4), 259 (1963). (17) J. Borecky, CoNeci. Czech. Cliem. Commim., 30, 2549 (1965). (18) N. E. Skelly and W. B. Crummett, J. Chromutogr., 21, 257 (1966). (19) K. Burger, Z. Anal. Cliem., 224,421 (1966). (20) K. Burger, ibid., p 425. (21) R. J. Morris and H. E. Persinger, J . Polymer Sei., Part A , 1, 1041 (1963). (22) M. Hori, K. Kondo, and T. Yoshida, Tukedu Kenykusho Nempo, 24, 194 (1965). (23) E. J. Quinn, H. W. Osterhoudt, J. S. Heckles, and D. C . Ziegler, ANAL.CHEM., 40, 547 (1968).
The following results and conclusions from these studies were of assistance t o us in developing methods of determining oxyalkylate composition. The hydroxyl end groups of polypropylene oxides which are prepared by propylene glycol alkoxide catalyst are predominantly secondary (9). The primary hydroxyl contents of two block copolymers of CH3 formula HO(CH2CH20),(CHCH20),(CH2CH20),H and of molecular weight about 3500 were 25 =t5 and 50 =t5 z (6). The terminal secondary hydroxyl group in P r o adducts is less reactive than the terminal primary hydroxyl group in ~O)~H E t 0 adducts (12). The product ~ - C I ~ H ~ ~ O ( Econsists of individual adducts which contain from 1 t o 15 E t 0 units as determined by G L C of the trimethylsilyl ether (13). The molecular weight distributions in p-RC6HdO(EtO),H become more narrow as x increases from 4 t o 50 (20). The molecular weight distribution in polypropylene glycol of molecular weight 2000 extends from about 1200 t o 2200 (21, 22). The NMR, GLC, and TLC procedures which are described below were developed t o determine the composition of E t 0 and/or P r o adducts of p-alkylphenols or alcohols used as surfactants. The primary and secondary hydroxyl contents of these oxyalkylates were measured from the proton N M R spectra of their trifluoroacetate esters. From these values and from N M R analysis of the mole per cent EtO, the sequence of addition of the oxides to the p-alkylphenol or alcohol reactant was determined. By G L C of the trimethylsilyl ethers, from 7 to 14 components were separated in the E t 0 and/or Pro adducts of p-alkylphenols which contained an average of about 3 t o 8 moles total oxide. The TLC system which was used by Burger (16) and Skelly and Crummett (18) to separate oxyethylates was found t o resolve components of the EtO-Pro adducts of p-alkylphenols containing 1 t o 10 moles total oxide. Precoated aluminum oxide TLC plates were used t o separate individual compounds in oxyalkylated p alkylphenols and t o estimate the molecular weight range of polyalkylene glycols of number average molecular weights 1200 t o 3600. EXPERIMENTAL
Apparatus. The proton N M R spectra were obtained with the Varian A60A N M R spectrometer at ambient probe temperature. Infrared spectra were run o n the Beckman IR-4 spectrophotometer. The Hewlett-Packard Model 720 gas chromatograph with a Disc Integrator was used for the G L C studies. Molecular weights were obtained with the Hewlett-Packard vapor pressure osmometer Model 301A. Brinkmann Instrument Co. TLC apparatus and precoated TLC plates were used. Mallinckrodt Chemical Works "Chrom AR" plates for TLC were also employed. Gel filtration separations were made in a Sephadex solvent resistant column Type SR 25/45, Pharmacia Fine Chemicals, Inc. Materials. P h o n i c s were obtained from the Wyandotte Chemical Co., Niax PPG-425 from Union Carbide Co., and polyalkylene glycols E-200, E-600, and P-3000 from Dow Chemical Co. All of the other oxyalkylates were experimental surfactants of the Petrolite Corp. All products were solvent-free and used as received. Esterification Procedure. The trifluoroacetate esters were prepared by a 2-hour reflux of a solution containing 2.0 ml of oxyalkylate, 2.0 ml of trifluoroacetic anhydride (Eastman White Label), and about 0.3 gram of dried sodium trifluoroacetate (Aldrich Chemical Co.). After evaporation of unreacted anhydride at about 70 "C, a n aliquot of this reaction mixture was diluted with CDC13 or distilled CC14 for NMR measurement. The 2-hour reflux was found to be
necessary to obtain complete esterification of oxyalkylates with molecular weights greater than 1500-2000. Trimethylsilyl Etherification. An approximately 0.5-gram sample of oxyalkylate was dissolved in 0.5 ml of anhydrous pyridine in a 10-ml flask. To this solution were added 1.0 ml of hexamethyldisilazane and 2.0 ml of trimethylchlorosilane. After standing for 15 minutes a t room temperature with occasional shaking, 5 ml of water were added, and the solution was extracted with about 3 ml of hexane. The hexane phase was heated 10-15 minutes at 60-70 "C under reduced pressure in a Rinco evaporator to remove pyridine. GLC Analysis of Oxyalkylate-Trimethylsilyl Ethers. Dual stainless steel, 2-ft x 0.25-inch columns, which were packed with 5z SE-52 on 60-80 mesh, acid-washed, silanized Chromosorb G were used. The helium carrier gas flow rate was 55 ml/minute at 35-lb pressure. The temperatures of the thermal conductivity detectors and injection ports were 390 and 385 "C, respectively. Samples were injected at an initial oven temperature of 100 "C. The temperature of the columns was programmed from 100 to 370 "C at a 5 deg/minute rate. GLC Analysis of Oxyalkylated p-Alkylphenol-HI Reaction Products. The procedure of Borecky ( 2 4 ) was modified by substitution of a GLC method for his paper chromatographic analysis of the HI-oxyalkylate reaction products as follows: Two drops of oxyalkylated p-alkylphenol and 1.0 ml of concentrated hydriodic acid were sealed in a 4-inch-long test tube and then heated for 30 minutes at 145 "C in an oil bath. After cooling, the tube was opened, and 10% aqueous sodium thiosulfate was added dropwise until the solution became colorless. The solution was extracted with 1.0 ml of benzene. An aliquot of the benzene phase, which was concentrated 2-3fold by evaporation, was injected into the chromatograph. Dual stainless steel columns, 2- ft x 0.25-inch which were packed with 4% LAC-446 on 60-80 mesh, acid-washed, silanized Chromosorb G , were used. G C operating conditions were: (a) Initial column temperature 100 "C; (b) Program rate 5 deg,iminute; (c) He (35 Ib) 55 m l p i n u t e ; (d) TC detectors 280 "C; and (e) Injection ports 260 "C. Thin-Layer Chromatography Procedures. About 3 p1 of a 0.5-1 .O wt/wt oxyalkylate-chloroform solution were spotted on the TCL plate with a Lang-Levy micropipet. Mixed solvent systems were prepared volumetrically in about 400-ml batches with A R solvents as received. About 30 minutes prior to the first TLC run of the day, a 30-ml aliquot of the stock solvent mixture was added to the cylindrical developing chamber which was lined with filter paper. Each batch of the 20- X 5-cm precoated plates was heated at 105 "C for at least 16 hours, and was spotted immediately after cooling. To ensure reproducible results, it was necessary to heat plates which had been stored several weeks in a desiccator over Drierite. Some batch to batch variations in precoated TLC plates were observed. In general, the separations were quite sensitive to small changes in solvent composition and/or adsorbent activity. The Dragendorffs reagent which was used to visualize the separated TLC zones was prepared by mixing in order 50 ml of 0.606% bismuth hydroxy nitrate in 71% acetic acid, 50 ml of 50% aqueous KI, 200 ml of glacial acetic acid, and 500 ml of distilled water. Excess spray reagent was readily removed from the adherent Brinkmann precoated aluminum oxide F2j4plates by washing them with water, leaving yellowbrown zones on a white background. Gel Permeation Procedure. About 0.1-gram samples of oxyalkylate were passed through a 43- X 2.5-cm column of Sephadex LH-20. Methanol was used as eluant at a rate of 0.95 ml/minute. Five-mi fractions were collected volumetrically by means of an automatic siphon in a G M In-
(24) J. Borecky, Micr.oc/iini.Acta, 5, 824 (1962). VOL. 40, NO. 11, SEPTEMBER 1968
1621
8.0
6.0
7.0
5.0
4.0 PPM (6)
P.0
3.0
1.o
0
CH3 Figure 1. NMR spectrum of pC~HllC~H~0(CH2CH20)~.~1(CH2CHO)l,oH strument Co. Microflex fraction collector. The delivery of 5.0-ml volumes was verified by volumetric collection of methanol fractions from the solvent reservoir. Two methods were used to analyze these fractions. For separations of oxyalkylated p-alkylphenols, the fractions were diluted quantitatively with methanol to final volumes (5-100 ml) such that the UV absorbance a t 278 mp was about 0.5. For separations of oxyalkylates which have no aromatic structure, the solvent was evaporated and the weights of the individual fractions were measured. Infrared spectra of selected solvent-free fractions were run to identify the structure of the isolated oxyalkylate. RESULTS AND DISCUSSION
NMR Studies.
The NMR spectrum of p-CSHl1C6H4OCH3
I
(CH2CH20)o.91(CH2CH0)1.0H is shown in Figure 1. The quartet of peaks between 6.7 and 7.2 ppm is due to four protons on the para-substituted aromatic ring, the peak at 5.14 ppm t o OH, the peaks between 3.2 and 4.2 ppm t o OCH and OCH2 protons, and the 12 or more peaks between 0.4 and 1.9 ppm to methyl group protons in P r o and t o -CH2-C and CH3-C protons in the alkyl group. The calculated and measured proton ratios for this compound are given in Table I. The good agreement between these values confirms that the average oxyalkylate molecule does contain 0.91 mole of E t 0 and 1.0 mole of P r o in accord with the weights of reactants used in the synthesis. In oxyalkylated p-alkylphenols which contain less than about 20 moles of oxide, the NMR area per proton may be calculated
Table I. NMR Proton Area Ratios in CH3
I
p-t-CsHiiCsHaO(CHzCHzO)o,a(CH?CHO)1.OH Ratio Measured Theory CHjCHO-, CsHii/OCH*,OCH 2.14 2.12 CH,CHO-, CBHI1/ArH 3.65 3.50 CHaCHO-, C,Hii/OH 14.6 14.0 1.71 1.65 OCH2,OCH/ArH 6.84 6.60 OCHZ, OCHjOH ArH/OH 4.00 4.00
1622
0
ANALYTICAL CHEMISTRY
from the area of four aromatic protons between 6.7 and 7.2 ppm, and then the number of protons in (OCH OCHZ ArCH) groups and, after correction for ArCH protons, (CH,CHO CH2 - C CHI - C) groups can be determined. Hence, if the type of alkyl group can be identified from a separate determination, a complete average structure for the P r o or Pro-Et0 adducts ofp-alkylphenols which contain up t o 20 moles of oxide can be deduced from the NMR spectrum. There are three methods of determining the structure of the p-alkyl group which we have found useful. First, if the oxyalkylated p-alkylphenol contains less than 12-15 moles of P r o , certain alkyl groups such as t-butyl, amyl, t-octyl, and branched-nonyl can be identified from their NMR pattern of peaks in the 0.5-1.5 ppm region. Note the (CH&C peak at 1.25 ppm in Figure 1, for example. Second, the number and intensities of infrared CH3-C symmetric deformation absorptions between 7.1 and 7.4 p and the relative intensities of the 6.6-, 6.8-, and 7.1-7.4-p bands in oxyalkylatedp-alkylphenols with less than about six polyether units may be used t o identify the alkyl group, particularly by comparison with spectra of known compounds. Finally, the polyether chain may be cleaved with HI, and the alkylphenolic decomposition products identified from a comparison of their GLC elution times with those of known p-alkylphenols. The following results which were obtained for commercial oxyalkylates show that the original p-alkylphenol reactant is recovered as one of the decomposition products:
+
+
+
+
Starting Compound 1. p-Octylphenol-5 mole E t 0 adduct 2. p-Nonylphenol-9 mole E t 0 adduct 3. p-Butylphenol-7 mole E t 0 adduct Phenolic Decomposition Products
1. Phenol, p-t-butylphenol, 2,4-di-t-butylphenolp-octylphenol 2. Phenol, p-nonylphenol 3. Phenol, p-t-butylphenol EtO-z In EtO-Pro block or random copolymers, the P r o may be calculated from infrared intensities as well as from NMR areas of the CH3CHO and (-CHZO >CHO-) peaks. For example, in Pluronics of molecular weights
+
I L
8.0
1.0
6.0
4.0
5.0
PPM
Figure 2.
3.0
2.0
1 .o
0
(6)
NMR spectrum of E-600 and PPG-425 mixture I11 trifluoroacetate ester
1000-4000, the infrared absorbance ratios in CCl? solutions A7.25/A3.4a, A7.2j/A7.41, Ae.g~/A3.4a,and A6.85/&.25 were found to be linear functions of the E t 0 content in the range 10 to 80%. These results for Pluronics were used to prepare calibration curves of absorbance ratio U S . per cent EtO. The NMR spectrum of the trifluoroacetate ester of a mixture of polyethylene glycol E-600 and polypropylene glycol PPG-425 is shown in Figure 2. The four peaks of Area A i between about 5.1 and 5.5 ppm are attributed to the CH3
I
-0CH2-CHOCOCF3 proton (in italics) and those of Area A2 between about 4.4 and 4.7 ppm to the OCH2-CH2OCOCF3 protons adjacent to the ester group. Then the primary O H and secondary O H contents may be calculated from the equation: Primary OH = -A P (1) Secondary OH 2A1 In agreement with other workers (9), only secondary hydroxyl groups were detected in polypropylene glycols or oxypropylated p-alkylphenols. To evaluate the accuracy of this determination, we prepared a series of mixtures of E-600 and PPG-425 containing 22 to 77 % primary OH. The results of the NMR analyses of these mixtures are presented in Table 11. Differences of z2=!= between the measured and calculated primary O H contents were observed. T o check the reproducibility of the procedure, measurements of the primary hydroxyl group content of P - ~ - C ~ H ~ C ~ H ~ O ( P ~ O ) ~were . ~ ( made E ~ O )on ~ ,five ~ H separate preparations of the trifluoroacetate ester, operating at R F fields of 0.015-0.018 mG. The mean of these five determinations was 55.0%, and the standard deviation was 0.62%. For compounds of molecular weight greater than about 1000, measurement of the areas A2 and A I must be done at rather high spectrum amplitudes where noise may contribute to integrator base line instability. For the purpose of determination of the order of oxide addition, this accuracy and precision were adequate. This procedure was used to measure primary OH/secondary OH ratios in three different classes of E t 0 and P r o reaction products : (a) HO(EtO),(PrO),(EtO),H and p-C6H40(PrO)y(EtO),H; (b) RO(EtO),(PrO),H and p-R-C6H40(EtO)Z(PrO),H ; and (c) oxyalkylated alcohols which were synthesized by addition of a weighed mixture of E t 0 and P r o to the hydroxyl compound and alkaline catalyst. These results are
summarized in Tables 111, IV, and V, respectively. Several conclusions may be drawn from these data. First, the PrOterminated oxyalkylated p-alkylphenols or alcohols do not contain measurable amounts of primary OH when the number of moles of P r o per equivalent of OH is greater than about 1.5. Second, the terminal O H groups in EtO-terminated oxyalkylated p-alkylphenols containing approximately 1.41.5 moles E t 0 per equivalent of OH are approximately 50x primary-50 secondary. In EtO-terminated oxyalkylated glycols which contain from about 10 to 50 mol % EtO, measurable amounts of secondary hydroxyl groups are found, ranging from 45 in Pluronic L-61 to about 15 in Pluronic L-44, for example. This result may be interpreted in terms of the greater reactivity of primary OH groups than secondary OH groups in reaction with alkylene oxides. In other words, when E t 0 is added to a polyoxypropylene derivative, it tends to form long chains (-CH2CHPO-), rather than distribute itself uniformly among the terminal P r o groups, leaving CH3
x
I
-OCH2CHOH terminal groups on unreacted molecules. Finally, the primary OH content of an oxyalkylate which is synthesized by a reaction of a mixture of E t 0 and P r o with a fatty alcohol or glycol increases as the mole per cent E t 0 increases. For example, the EtO-Pro mixed adduct of dipropylene glycol with 25 mol E t 0 contained l l % primary Et0 OH while the adduct of Alfol 16-18 with 90 mol contained 6 4 z primary OH. A comparison of the data in Tables 111-V shows that, except for those Pro-terminated or
Table 11. Measured and Calculated Primary Hydroxyl Contents of Mixtures of Polyethylene Glycol 600 and Polypropylene Glycol 425 Primary hydroxyl content Calculated" Measured 76.8 75.4b 61.2 61. 5b 47.6 49. I C 34.9 35.8" 22.8 24. 7c a Calculated from weights of oxyalkylates and from measured hydroxyl values of 185 for E-600 and 236 for PPG 425. b Average of three determinations. c Average of two determinations.
VOL. 40, NO. 1 1 , SEPTEMBER 1968
1623
I
Table 111. Primary OH Contents of EtO-Terminated Oxyalkylated Glycols, H0(CH2CH,0),(CH2CH0),(CH2CH20),H,and EtOCH3
I
Terminated Oxyalkylated p-Alkylphenols, ArO(CH2CHO),(CH2CH20),H Mol E t 0 in Structure Calcd mol wt polyether chain Pluronic L-31 10600 12.8 b r C L-61 1940a 12.gbzd L-81 2810a 12.gb L-101 36100 12.g b L-42 1500a 24.8* L-33 1360~ 36. lb L-43 1710a 36. lb L-44 2000a 46.P P - ~ - C ~ H ~ C ~ H ~ O ( P ~ O ) ~ . O ( E ~ O ) ~ . ~ H 330 41.4 P - ~ - C ~ H ~ ~ C ~ H ~ ~ ( P ~ O ) ~ . ~ ( E ~ O ) ~330 . 46.7 p-t-CoH1sCsHaO(PrO)3.dEtO)s. OH 615 62.5 840 60.0 P - C - C ~ H ~ ~ C G H ~ OdEt ( P0~1 O 7 . SH )~. Mol wt calcd from “Pluronic Grid,” Wyandotte Chemical Co. E t 0 content calcd from “Pluronic Grid,” Wyandotte Chemical Co. NMR measured value E t 0 in L-31 21.3 mol %. NMR measured value E t 0 in L-61 12.0 mol %. Average of three determinations. Approximate value; area CHOCOCFI peaks was so small that accurate measurement was not possible. Average of five determinations.
z
’ e
Primary OH, 43. Se 55 52 48 64 77 78 85’ 550 56 100 100
C S
Table IV.
z
CH3
I I Primary OH Contents of Pro-Terminated Oxyalkylated Alcohols, RO(CHzCH,0),(CH2CHO),H or HO(CHCH20),CH3
CH3
I I (CH2CH20),(CH2CHO),H, and Pro-Terminated Oxyalkylated p-Alkylphenols, ArO(CHKHzO),(CHzCHO),H Starting Mol oxide/mol reactant compound Et0 Pr 0 1.3 9.2 TEGb 4.0 8.0 MeOH 4.0 7.0 MeOH 1.2 1.4 p-t-CoHisCsH4OH 22.0 25.0 TEG 1.4 1.6 p-CizHzsCe“0H 4.0 4.2 MeOH 10.0 5.0 Alfol 16-18 22.0 10.0 TEG 12.0 3.0 TEG 30.0 3.0 TEG = Mol wt calcd from weights of P r o and E t 0 added to starting material. * TEG: Triethylene glycol. c Composition of compound was verified from its NMR spectrum.
Calcd mol wt5 740 670 610 355 2570 415 450 lo00 1700 850 1660
Calcd % E t 0 in polyether chain 12.4 33.3 36.4 46.2 46.8 46.7~ 48.8 66.7 68.8 80.0 91.0
Primary OH, 0 0 0 0 0 0 0 0 0
11 21
Table V. Primary OH Contents of Oxyalkylated Alcohols with Simultaneous Addition of E t 0 and P r o to Starting Compound
Starting compound Alfol 16-18 Alfol 16-18 Alfol 16-18 Dipropylene glycol Dipropylene glycol Dipropylene glycol Dipropylene glycol
1624
0
Moles oxide/mole reactant Et0 Pro 1.04 9.58 2.14 8.47 3.17 7.44 3.77 1.88 11.0 3.66 4.51 0.90 13.7 2.00
ANALYTICAL CHEMISTRY
Mol
Calcd mol wt 750 760 780 430 920 430 1020
75
E t 0 in polyether chain 90.2 79.8 70.1 33.3 25.0 16.7 10.0
Primary OH, 64 45 31 19 11 7.5
0
z
T
UO’
Figure 4. Molecular weight distribution of trimethylsilyl ethers of oxyalkylated p a l k y l phenols p-t-C4HgC6H40(PrO)sH 0 p-t-CaHgCaH40(Et0)4H
Figure 3. Gas-liquid CH3
chromatogram of pt-C4H9C&140-
A p.t-CaH9CeHa0(Pr0)2(EtO)l,aH
fer, Katnik, and Kern (15) found that in polypropylene glycols the relative molar response factors for the trimethylsilyl ethers are unity when the number of P r o units in the glycol is greater than or equal to 4. EtO-Pro mixed oxyalkylates with less than about 10 mol The GLC-molecular weight distributions for the 3 to 4 mole EtO, the order of oxide addition can be determined by two adducts of p-t-butylphenol with EtO, P r o , or E t 0 and P r o NMR procedures: (a) measurement of the moles of E t 0 are shown in Figure 4. The pronounced differences in these and P r o per equivalent of OH and (b) measurement of the distributions may be related to the differences in reactivity of primary OH content. E t 0 or P r o with primary and secondary OH groups. GLC Studies. The chromatogram which was obtained GLC-molecular weights of 11 oxyalkylate trimethylsilyl for the trimethylsilyl ether of p-t-C4HsCsH40(PrO)2(Eto)~.4H ethers were calculated from the summation Z(Mi 2 i) where is reproduced in Figure 3. For the GLC conditions which Mi is the average molecular weight and 2 i is the GLC area were used, p-t-C4H&H40SiMe3 elutes at 143 “C,and p-tper cent of the ith component. The resulting values are comC4H9CeH40(Pr0)1gSiMe3 elutes after 8 minutes at the 370 “C pared with the calculated formula weights of these products hold temperature. When approximately equal weights of in Table VI. p-t-C4HgC6H40(Et0)4SiMe3 and P - ~ - C ~ H ~ C ~ H ~ O ( P ~ O ) ~ S ~ M ~ ~ To measure the reproducibility of these GLC-molecular were mixed and an aliquot was injected into the chromatoweights, four syntheses of the trirnethylsilyl ether of p-t-C4Hggraph, the peaks of the separated E t 0 adducts coincided with those of the individual P r o adducts. The EtO-Pro adducts of p-t-butylphenol also gave single peaks. Table VI. Comparison of Average Molecular Weights of T o calculate the molecular weight of a separated compoOxyalkylate-Trimethylsilyl Ethers Calculated from GLC nent of a p-alkylphenol EtO-Pro adduct trimethylsilyl ether, Molecular Weight Distribution with Formula Weights an average molecular weight per oxide unit, Msv, was calcuFormula lated from the equation: weight Av. of No. of GLC-mol 44.04 x 58.08 y Ma, = Compound4 ether detns wt of ether (2) X + Y p-t-C4H,C6HaO(EtO)aH 398 4 396 p-t-C4HBC6H40(Pr0)3H 396 1 396 where x and y are the total number of moles of E t 0 and P r o , p-t-CaHDCBH40(Pr0),.7SH 672 2 674.5 respectively, in the oxyalkylate molecular formula. If N$ ~ - ~ - C ~ H I C ~ H ~ O ( P ~ O ) ~ . ~402 ~ ( E ~ O 8) ~ . ~ ~396.5 H is the integral number of oxide units of the ith peak in the p-CsHllCeH40(Pr0)1,0~(EtO)~.olH 335 1 340 chromatogram, the average formula weight of the polyether P-C~H~~C~H~O(P~O)~.~(E~O)~.~H 400 1 408 fraction of component i was taken to be the product NiM,,, P-C&L~C~H~O(E~O)BH 556 1 564 1 424 P - C I H ~ ~ C ~ H ~ O (3dEt0)l.2H P~O)~. 424 assuming that the ratio (average number of moles E t 0 in P.CBH~~CBH~O(PTO)Z.~~(E~O)Z.~~H 556 2 538.5 “i”)/(the average number of moles P r o in “i”) is equal to the E-200 346 1 345 ratio x/y. The total average molecular weight M t of i is then 1 599 PPG-425 621b the sum of NzMayand the formula weights of R-cd&o49.86%p-t-CaH&~H40(EtO)~H 428.5 50.14% p-f-C4HaCeH40(PrO)aH 423c and -SI(CH&. The plot of GLC area per cent as a function of calculated average molecular weight constitutes what a The structures of the p-alkylphenols were verified from their NMR spectra. might be termed the GLC-molecular weight distribution of an * The molecular weight of this sample was calculated from its oxyalkylate. Because pure compounds were not available hydroxyl value of 236. for measurement of GLC area correction factors, the exact Number average molecular weight of mixture (Vapor Pressure relationship of G L C area per cents to the true weight per cents Oxmometer 301A): 420. of the separated components could not be determined. Shae-
( C H ~ C H ~ ) ~ . O ( C H Z C.4Si(CH& HZO)~
+
1
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Table VII. Comparison of GLC and TLC Separations of Oxyalkylated p-Alkylphenols into Individual Components Number of components separated TLC System System GLC" lb 2b Structure p-I-CaHsCsH,O(EtO)aH 9 6 2 9 2c p-CeHiyCsHaO(EtO)aH 11 p-t-C4HsCtiH40( Pr0)3H 7 Noned 5 p-t-C4HDCsH40(Pr0)4H 9 Noned 6 p-r-C4HoCsH40(Pr0),.;jH 14 Noned 3 p-t-C4HDCbH40(Pr0)~(Et0)1.4H 11 8 5 P - C ~ H ~ ~ C ~ H ~.J-l O(P~O)~ 9 7 4 ~-CDH~~C~H~~(E .OH ~ O ) ~ ( P 9~ O ) ~ 7 2e ~ - C D H ~ ~ C ~ H ~ O ( E ~ O ) ~ ( P ~ O )8Z H 7 2e Includes any unreacted p-alkylphenol reactant. Includes only spots separated from each other by more than 2-3 mm. The separated compounds had Rf values about 0.2 and 0.3 and were preceded by an unresolved zone from the origin to about 20 mm. Only a single zone was observed from about 95 to 100 mm from the starting spot for a 120 mm solvent front travel. e The chromatogram contained two spots of R f values about 0.3 and 0.4 plus a zone from the starting spot to 30 mm consisting of three or more overlapping spots. CsH40(Pr0)2.0(EtO)l.4H were made. Duplicate GLC runs were made on each product. The mean of these eight determinations was 396.5, and the standard deviation was 2.62. The fairly good agreement between the calculated formula weights and the measured GLC-molecular weights indicates that the GLC-molecular weight distributions are at least approximately correct. TLC Studies. The following thin-layer chromatographic systems were found to be useful in characterizing the P r o and/or E t 0 adducts of p-alkylphenols and alcohols : System No.
TLC Adsorbent
1. 2.
Brinkmann precoated silica gel Mallinckrodt precoated silicic acid
3.
Brinkmann precoated aluminum oxide Solvent Composition
1. Water-saturated 2-butanone 2. 90 % Chloroform-5 % p-dioxane5 % water-saturated benzene 3. 42 % Chloroform-40.5 % benzene17.5 % acetone With TLC system 1, individual E t 0 or EtO-Pro adducts of p-alkylphenols containing less than 9-10 moles of oxide were separated completely from adjacent homologs. The R f value of the 1-mole adduct was about 0.8. Polyethylene glycols were found to have very low (less than 0.1) R f values in agreement with Burger's results (25). The p-alkylphenol-Pro adducts did not resolve, but eluted as a single zone about 10 m m in length which traveled 0.7-0.9 as far as the solvent front. Polypropylene glycols also eluted as single zones of 30-40-mm lengths. The highest molecular weight polypropylene glycol had the largest R,-e.g., P-3000 (Dow) traveled almost t o the solvent front. By means of this TLC system, certain mixtures of oxyalkylates could be (25) K. Burger, 2.Anal. Chern., 196, 251 (1963).
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ANALYTICAL CHEMISTRY
Figure 5. Thin-layer chromatogram of oxyalkylated p-alkylphenols on aluminum oxide
separated and the structural types of the components of the mixture identified. For example, a mixture of polypropylene glycol 2000 (large R/) and polyethylene glycol 2000 (small R f ) would be readily separated. A mixture of p-C9H19C6H40(Et0)6H and ~ - C & € ~ ~ C ~ H ~ O ( could P T O be )~H separated with resolution of 8-10 individual E t 0 p-nonyl phenol adducts at the same time. With TLC system 2, good resolution of p-C4H9C6H40(PrO),H was obtained for values of x ranging from 1 t o about 8. The R, value of the 1-mole adduct was about 0.55. Adducts which contained more than about 8-9 moles of P r o remained unresolved in a continuous zone which extended a distance of about 20 mm from the starting spot when the solvent front was allowed to travel 120 mm. Adducts of p-alkylphenols with either E t 0 or EtO-Pro were incompletely resolved. Polyalkylene glycols eluted as continuous zones; those of molecular weights greater than 1500 had R, values less than 0.1. These TLC separations with systems 1 and 2 preceded the GLC-molecular weight distribution studies which are discussed above. The G L C procedure offers two advantages. First, more individual compounds were separated by GLC than in either TLC systems 1 and 2 (see Table VII). Second, a quantitative analysis of each component in the range one t o 15 polyether units may be obtained from the G L C peak areas. The TLC procedures, however, can be used t o characterize certain mixtures of oxyalkylates which are of too high molecular weight to be eluted from G L C columns. Using the aluminum oxide precoated plates, System 3, we observed that the approximate molecular weight range of polypropylene glycols may be obtained, and a rough estimate may be made of the E t 0 content of polyalkylene glycols of the type HO(EtO),(PrO),(EtO)zH which contain less than 20% EtO. The R, values of polypropylene glycols increased with increasing molecular weight, similar to their elutions from
TLC Zone Migrations of Oxyalkylated Glycols on Aluminum Oxide Precoated Plates No. av Migration dist., mm Structure mol wta Mol P r o Mol EtO” Major zone Minor zone 2930 100 0 30-65 64-7 1 HO(PrO),H (I) 3530 91.8 8.2 0-52 54-61 HO(EtO),(PrO),(EtO),H (11) 1940 87.2 12.8 c-11 11-18, HO(EtO),(PrO),(EtO),H (111) Table VIII.
20-26
HO(PrO),H (IV) 1860 100 HO(PrO),H (V) 2600 100 a Measured with a vapor pressure osmometer at 65‘ in toluene solution. b Measured from NMR spectrum.
alumina columns ( 2 1 , 2 2 ) . About an hour is required for the solvent t o travel 120 mm. The R, values of HO(EtO)z(PrO),(EtO),H decreased with increasing E t 0 content until with Pluronic L-63, for example, almost the entire sample remained in the starting spot. There was relatively little effect of molecular weight upon zone elution distance in this type of oxyalkylate. For example, Pluronics L-62, L-72, and L-92 gave almost identical zones within 10 mm of the origin while Pluronics L-61 and L-101 gave three zones between 0 t o 25 and 0 to 15 mm, respectively. The zone migration distances, molecular weights, and E t 0 contents of three polypropylene glycols and two Pluronic-type polyalkylene glycols are given in Table VIII. The same, visually identical pattern was obtained on five different TLC plates run consecutively. For a given compound, measurements of the five major zone fronts gave differences of only about 1-3 mm, which is within the uncertainty in selecting a point as origin and a second point as end of the zone. Polypropylene glycols I and V, which differ by 340 in number average molecular weight (about six P r o units) are readily distinguished from each other, the major zone fronts differing by 20 mm. In contrast, the major zone fronts of polypropylene glycols IV and V, which differ by 740 in number average molecular weight, traveled almost the same distance on alumina. The minor leading zone in compound V, however, traveled about 10 mm farther than that in compound IV. The major zone in compound IV commenced about 8-10 mm closer t o the origin than that in compound V, indicating more lower molecular weight components in IV than in V. Hence, these compounds probably differ mainly in their molecular weight distributions. Oxyalkylated glycols I1 and 111, which differ 4.6 mole per cent in E t 0 content, had appreciably different zone migrations. Both compounds contained about 5 5 % primary hydroxyl groups; the uncertainty in this value for I1 (molecular weight 3530) is greater than that for 111 (molecular weight 1940). Hence, it appears likely that I1 probably contains unreacted, high molecular weight polypropylene glycol. For the purpose of rapidly determining the reproducibility of different syntheses of a given oxyalkylated glycol, TLC System 3 has proved t o be very useful. Under the TLC conditions of System 3, the EtO, P r o , and EtO-Pro adducts of p-alkylphenols exhibited characteristically different elution patterns. This is shown in Figure 5 where the numbers refer to the following compounds: (1) p-t-CdH 9C6H40(Pr0)4H, (2) p-t-C4H9CeH40( Et0)4H, (3)
0 0
15-40 22-45
43-53 44-56
p-t-CdH9C6H40(Pr0)2(Et0)~. 4H, (4) P-C~H&H~O(PTO)~(Et0)7.5H, and ( 5 ) p-C9H19C6H40(Et0)4(Pr0)2H. The oxyethylated p-alkylphenols had R, values of less than 0.1, and the oxypropylated p-alkylphenols were not resolved, but eluted as a single zone located at a distance 30-40z of the solvent front. The P r o - E t 0 adducts of p-alkylphenols were separated partially, those terminated by P r o having larger R,values. As with Systems 1 and 2, it appears that structural types can be identified in certain mixtures of oxyalkylates using TLC System 3. In all three systems using commercial precoated TLC plates, it is recommended that compounds of known structure be run on the same plate as an unknown to avoid the well known variations in R, values with slight changes in adsorbent activity. Gel Filtration Chromatography. Studies of the use of Sephadex LH-20 to measure molecular weight distributions of oxyalkylates and to separate oxyalkylates of different molecular weight ranges have just commenced in our laborawas ~ Oac)~H tory. A good separation of ~ - C ~ H I ~ C S H ~ O ( E complished using methanol as eluant. The plot of the absorbance at 278 m l of fractions as a function of elution volume was found t o be similar in shape t o that of the GLCmolecular weight distribution of the trimethylsilyl ether. In a second experiment, a known mixture of polypropylene glycol 2000 and p-l-C1HgC6H10(Pr0);.,~Hwas separated into 5-ml fractions with methanol as eluant. About 80% of the polypropylene glycol was recovered in the initial fractions free of the second component. Lower molecular weight components of the oxypropylated p-tbutylphenol were isolated in the latter fractions. Some overlap occurred in the middle fractions. These preliminary results demonstrate that this technique should be of value in measuring oxyalkylate molecular weight distributions and in separating mixtures of oxyalkylates which differ in molecular weight. ACKNOWLEDGMENT
The author thanks 0. W. Griffin for technical assistance, F. E. Mange for helpful discussions about these studies over a period of several years, and N. Shoolery, Varian Associates, for suggestions about NMR measurements on the trifluoroacetate esters.
RECEIVED for review April 11, 1968. Accepted June 5, 1968.
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