LITERATURE CITED
(1) Bailey, M. E., Hass, H. B., J . Am. Chem. SOC.63, 1969 (1941).
(2) Bowman, M. S., Rice, D. E., Switzer, B. R., Ibid., 87,4477 (1965). (3) R‘l G’l Gil-Avl Israel J . Chem. 1, 234 (1963). (4) Corey, E. J., Cassanova, J., Chem. and Ind. (London)1961, 1664.
(5) Dal Nogare, S., Juvet, R., “Gas-
Liquid Chromatography,” pp. 70-86, Interscience, New York, 1962. (6) Gault, Y., Felkin, J., Bull. SOC.Chim. France, 1965,742. (7) Gil-Av, E., Nurok, D., Proc. Chem. SOC.1962, 146. (8) Halpern, E., Westley, J. W., Chem. Commun. 12, 246 (1965). (9) Karger, B. L., Cooke, W. D., ANAL. CHEM.36, 985 (1964). (10) Pollock, G. E., Oyama, V. I., Johnson, R. D., J . Gas Chromatog. 5 , 174 (1965).
(11) Stern, R., Atkinson, E. R., Jennings, F. C., Chem. and Ind. (London) 1962,
1758. (12) Weygand, F., Rox, A., Schmidhammer, L., Konig, W., Angew. Chem. Int. Ed. Engl., 2, 183 (1963).
RECEIVED for review December 6, 1965. Accepted January 12, 1966. Presented at the 150th meeting of the ACS, Atlantic City, N. J., September 1965. Work supported by theBasic ResearchFund, Northwestern University.
Analysis of Alkylene Oxide Polymers by Nuclear Magnetic Resonance Spectrometry and by Gas-Liquid Chromatography ALAN MATHIAS and NORMAN MELLOR Imperial Chemical lndusfries Ltd. (Dyesfuffs Division), Hexagon House, Blackley, Manchester, The proportions of oxyethylene and oxypropylene in alkylene oxide polymers (polyethers) can be determined either by nuclear magnetic resonance spectrometry or by splitting the polyether with HBr and analyzing the bromination product by gas chromatography. The initiating polyol incorporated in such polyethers can also b e identified and determined by gasliquid chromatographic (GLC) examination of the bromination product under appropriate conditions; in this way, polyethers based on glycerol, 2,2di(hydroxy methyl)-1 -propanol (trirnethylol ethane), 2,2-di(hydroxy methyl)-1 -butanol (trimethylol propane), 1,2,6-hexane triol, pentaerythritol, sorbitol or mannitol, and triethanolamine have been analyzed.
E
THYLENE OXIDE/PROPYLENE
OXIDE
adducts of polyfunctional hydroxy compounds-polyethers-are used extensively as the polymeric alcohols for reacting with diisocyanates to make polyurethane foams. The physical properties of the foam depend to some extent on the chemical structure of these polyethers, and it is therefore important that a method of analysis be established for controlling their manufacture. Such a method would also be useful for elucidating the composition of unknown polyethers. Nadeau and Seumann (2) analyzed polyoxyethylene and polyoxypropylene compounds by pyrolysis a t 360’ C. and gas chromatographic analysis (GLC) of a sample of the gaseous product; Graham and Williams ( 1 ) split the polyethers with phosphoric acid to give acetaldehyde and propionaldehyde, which were determined colorimetrically. Nadeau and Waszeciak (5) reacted the 472
ANALYTICAL CHEMISTRY
polyethers with acetyl chloride, in the presence of ferric chloride, to give chloroethylacetate (from the oxyethylene groups) and a mixture of chloroisopropyl acetate and isopropyl diacetate (from oxypropylene groups) , and in this way determined the ratio oxyethylene/ oxypropylene in the range 15-50% oxyethylene. Analysis of the polyether sample for oxyethylene/oxypropylene ratio, without prior chemical conversion, can be carried out by nuclear magnetic resonance spectrometry (NMR), as detailed below : however, the apparatus is expensive and an alternative method, more suited to general laboratory practice, is desirable. Pyrolysis/GLC analysis is not readily controlled, so that reproducibility of the order required (say i 1 unit a t 20% oxyethylene content) is difficult to obtain. Chemical splitting of the ether linkages can be carried out in a number of ways, and the method of choice will obviously be that which gives complete breakdown of polyether t o give only one product from each different grouping in the polyether molecule. Such a method, involving splitting the polyether with HBr and GLC analysis of the products, is described below. DETERMINATION OF OXYETHYLENE CONTENT
The purpose of this work is to ascertain what proportion by weight of the polyether chain consists of oxyethylene and oxypropylene groups; for con-
r
9, England
venience, this has been expressed as the percentage by weight of oxyethylene calculated on the sum of oxyethylene and oxypropylene, and this has been
EO
+
abbreviated to %EO PO’ Determination by NMR Spectrometry. N M R spectrometry provides a quick and easy method for t h e analysis of oxyethylene/oxypropylene copolymers. Figure 1 shows the typical spectrum at 60 Mc./sec. obtained from such a sample examined as a 10% solution in carbon tetrachloride, with tetramethylsilane (TMS) as internal reference compound. dl1 the N M R results quoted in this paper were obtained using a Varian A60 spectrometer; the chemical shifts were expressed in 6 units-Le., p.p.m. downfield from the ThIS reference signal. The spectrum contains only two resonances, (9) the doublet centered at 1.08 6 due to the methyl groups of the oxypropylene units, and ( B ) a composite band from 3.2 to 3.8 6 due to the CHzO groups of the oxyethylene and oxypropylene units and also the CHO of the oxypropylene units. The resonance due to the hydroxyl group protons, which terminate the polyether, also occurs in band ( B ) . However, if a small amount of trifluoroacetic acid is added to the sample, exchange processes cause the hydroxyl resonance to move to low field. The composition of the copolymer is now obtained readily from the relative areas to bands (A) and ( B ) . Thus for a copolymer
1
I . . . . I . . , . I . . . I I . . . . I . . . . I . . I . I . . . . I . . . . I . . . . , . I . . I . . . . , . , , . 1 . . . . , . . . . l . . , 70
10
10
Figure 1.
A 3b B 4af3b where A and B are the areas of bands (A) and ( B ) respectively. From this, the weight percentage of the polymer is easily obtained.
EO
33a
-- x 100 + PO - 33a + 58 where a
=
IO
1.0
30
O
m
Typical NMR spectrum of an oxyethylene/oxypropylene copolymer
then
% ' EO
a0
(0
B -1 (2) A
-
The required areas can be obtained directly using the electronic integrator of the A60 spectrometer. At least five integrations were performed on each sample. For polyethers in which a small amount of a different poly01 has been used to initiate polymerization, corrections have to be applied to Equation 2 in order to take into account the resonances due to the protons introduced in the initiating polyol. For initiators such as ethylene glycol, glycerol, and pentaerythritol, the only resonances are those due to the CHzO and CHO protons which fall in band ( B ) with the similar resonances from the oxyethylene and oxypropylene units. Equation 2 would therefore overestimate the percentage of oxyethylene by an amount which obviously increases as the molecular weight of the copolymer falls. However, the nature of the initiator may be known or it can be identified and determined by the HBr/GLC method described below, and slight corrections can be made to take into account the contributions made by the initiating units to
the area of band ( B ) . Similar corrections can be made in the cases where the initiator contributes to both bands (A) and ( B ) , e.g. 2,Z-di-(hydroxymethyl)-1-butanol. These corrections require that the approximate number average molecular weight (mol. wt) of the polyether be known, i.e. that the hydroxyl value be determined and the mol. wt. calculated by multiplying the equivalent weight by the expected number of hydroxyl groups per molecule; the correct equation thus becomes
EO
?6 EO 330
-
PO 638 ( B X r )
$.
+ - [T P 33a + 58
-s
1x
lecular weight of 3500, but only 90% of the alleged glycerol content is actually present. It can be seen that the variations in the corrected values are quite small. Because of the speed of operation and the ease of calculation, NMR is particularly useful for the routine analysis of oxyethylene/oxypropylene copolymers. However, in the cases of completely unknown samples, the HBr/GLC method has the advantage that the initiating polyol is also identified and determined. DETERMINATION BY HBr/GLC EXPERIMENTAL
100 (3)
where r = number of protons tributing to band from initiator s = number of protons tributing to band from initiator p = mol. wt. of polymer wt. of initiator
con-
(A)
METHOD
Conversion to Dibromides. The reaction of polyether with 4 5 4 0 % HBr/acetic acid reagent was carried out in thick walled borosilicate glass tubes 8 cm. long, 4 mm. i d . , having a capacity of about 1 ml.; these were
con(B) mol.
Results obtained from samples which have been examined both by the HBr/ GLC method and by NMR are discussed later. In Table I the corrections for initiating units are examined more fully. Uncorrected oxyethylene content values are shown for six polyethers of mol. wt. 3500. In order t o assess the effect of any uncertainty in the molecular weight, corrected values are then shown assuming molecular weights of 3300, 3500, and 4000. Values are also given assuming t h a t the samples have the correct mo-
Table 1.
Effect of Applying Correc-
tions to NMR Results for
EO
% EO + PO
EO 'EO
+ PO Corrected for only
Un-
90% of alleged
cor- Corrected mol. wt. rected 3500 3300 4000
glycerol content
11.9 1 0 . 5 10.4 10.7 11.4 10.0 9 . 9 1 0 . 2 6.9 5.5 5.4 5.6 9.2 7.7 7.6 7.9 9.0 7.6 7 . 5 7 . 8 1 9 . 8 18.6 1 8 . 4 1 8 . 8
10.7 10.1 5.6 7.8 7.7 18.7
VOL. 38, NO. 3, MARCH 1966
473
Table II. GLC Results on Synthetic Mixtures of lI2-Ethylene Dibromide and 1 ,2-Propylene Dibromide
WR
AR
Wt. ratio ethylene dibromide/propylene dibromide
Area ratio ethylene dibromide/propylene dibromide
2.225 1,190 0.401
1.83 1.015 0.343
V'R/AR 1.215 1.175 1.17 Mean 1 . 1 9
charged with 20 mg. of sample (using a glass melting-point tube for transferring) and 0.4 ml. of reagent, sealed, mixed, and placed in a protective metal case, and put in an oven at the required temperature. After the required time, the tubes were cooled and opened, and the contents were diluted with 0.7 ml. of water and extracted with 1.5 ml. of carbon disulfide (called Solution I). Solution d (0.25 ml.) was diluted with 1.0 ml. of carbon disulfide (called Solution B). The GLC tests were carried out on 4 p1. of Solution B. The amount of HBr-acetic acid reagent used is in appreciable excess of the theoretical amount required for reacting 20 mg. of polyether. Variations in the weight ratio for sample/ reagent showed no significant effect on nn L" determination up to the % EO PO a limit of about 45 mg. per 0.4 ml. of reagent; however, in the polyol determination described later, significantly low results were obtained if the weight of sample per 0.4 ml. of reagent exceeded about 28 mg., and for this reason all the tests on polyether samples were limited to 20 =t4 mg., accurately weighed. Under the optimum reaction conditions as detailed below (2 hours a t 150" C.) the oxyethylene and oxypropylene groups were converted to ethylene dibromide and propylene dibromide. GLC Apparatus. The separation of ethylene dibromide and propylene dibromide was carried out on a GLC apparatus equipped with a flame ionization detector and a 1-mv. potentiometer recorder. The column
+
Table 111.
Conditions during bromination
Observed
Temp. C. Time (hour) O
EO
Observed EO ethylene dibromide and A p Q
474 *
+
RESULTS AND DISCUSSION
Tests on known mixtures of ethylene dibromide and propylene dibromide are shown in Table 11. From these results, it was evident that the response of the apparatus was linear over the range covered, but equal areas were given by 1 part by mt. of propylene dibromide and 1.19 parts by wt. of ethylene dibromide. However, in practice, the areas would be converted to the corresponding amounts of oxypropylene and oxyethylene groups, assuming that 1 part by wt. of propylene bromide would be derived from 58/202 parts oxypropylene and 1.19 parts by wt. of ethylene dibromide would be derived from 1.19 X 44/188 parts oxyethylene; hence equal areas were given by amounts corresponding to 1part by wt. oxypropylene and 202/58 X 1.19 X 44/188 = 0.98 parts oxyethylene. Thus, within experimental error, equal areas would correspond to equal amounts of oxyethylene and oxypropylene, provided that the conversion of the alkoxy group to dibromide was complete. Preliminary work on polyethers shom-ed that excess of 45-50% rvV./v. HBr in glacial acetic acid a t 100' C. for 0.5 hour a t atmospheric pressure would convert polyethylene glycol (PEG) to bromoethylacetate, and polypropylene glycol (PPG) to bromopropylacetate. Using known copolymers, the ratios of bromoethylacetate to bromopropylacetate, as revealed by GLC, were eignificantly lower than the expected values; although the results were improved by
100" 16 32.3 66.0 54.7
125" 5
...
67.4 54.2
12 34.3 67.6 54.3
2 34.8 67.6 54.5
150" 5 35.1 67.7 56.8
ANALYTICAL CHEMISTRY
AE
~
AE
in
d
5
10
'
Omin.
Figure 2. Separation of ethylene dibromide and propylene dibromide at
65" C. adding zinc bromide and by using longer reaction times, the results were still lorn, and the GLC record still showed that higher boiling compounds were present, presumably the bromoacetates and products from partial fission of the polyether chain. However, tests carried out for 4 hours a t 100' C. in sealed tubes as described above yielded about equal proportions of the bromoacetate and dibromo derivatives, while reaction at 150' C. yielded only 1,2-ethylene dibromide from polyethylene glycol and 1,2propylene dibromide from polypropylene glycol-Le., the reaction products were free from bromoacetates, diacetates, and products from partial fission. Similar tests on oxyethylated polyol (i.e., reaction product of polyol with ethylene oxide) and oxypropylated polyol (Le., reaction product of poly01 lvith propylene oxide) (where the polyol glycerol, 2,2-di(hydroxy methyl)-1butanol or triethanolamine) also gave 1,2-ethylene dibromide and 1,2-propylene dibromide, respectively, together with higher boiling products with GLC retention times corresponding to the derivatives obtained under the same conditions from the individual polyols. The effects of temperature and time of the bromination reaction on the EO quantitative results for % EO PO were ascertained by a series of tests using the CS2solutions (B) obtained from the following samples (in which the number refers to number average mol. wt.) :
+
(a) Polyethylene glycol, 400 (b) Polypropylene glycol, 1000 (c) Mixture of oxyethylated glycerol and oxypropylated glycerol (31.5%
70 EO Eo + PO on Synthetic Mixtures" 16 40.0 70.8 65.3
X 100 where A E = area under peak due to + AP area under peak due to propylene dibromide.
= =
consisted of a 90-cm. length of 4-mm. i.d. glass tubing packed with 30y0 w./w. Silicone Elastomer E301 (I.C.I., Sobel Division) coated on 30- t o 60-mesh acid-washed Celite (G rrCel'J C210 from Gas Chromatography Ltd., Maidenhead, England). The column was operated isothermally a t 65' C., and the flow rate of hydrogen/nitrogen 1:l v./v. carrier gas was 60 ml./ minute; the sample loads were 4 pl. of a carbon disulfide solution of the dibromides injected from a 10-p1. Hamilton syringe into the top of the Celite packing, which mas preheated to 120' C. A typical chromatogram is shown in Figure 2.
Eo calcd. from wts. taken and EO PO hydroxyl value of the oxyethylated glycerol) EO (d) As (c) but 63.5% EO PO (e) 2,2 - di(hydroxymethy1) - 1 - butanol oxypropylated and then oxy-
+
+
ethylated (46.7% EO Eo
calcd.
+
from gain in weights and OH value of product) EO The results for observed % ___
EO
+ PO
in (c), (d), and (e) are given in Table 111. It will be seen from Table I11 that if the temperature was kept a t 150' C. for more than 5 hours, the observed
EO
increased ; %E5m
this increase
was accompanied by a decrease in yield of total dibromides calculated from the area under the peaks due to ethylene dibromide and propylene dibromide. Furthermore appreciable carbonization was observed in all the reaction products a t 150' C. for 5 and 16 hours, except in the tests on polyethylene glycol 400, and it was deduced that propylene dibromide was less stable than ethylene dibromide. Since it was obviously desirable to use conditions which gave maximum yield and minimum loss of propylene dibromide, it was decided to standardize on 2 hours a t 150' C. As will be shown later, these conditions were also most propitious for determining the poly01 contents. Under these conditions some darkening (carbonization) of the reaction mixture occurred and the observed %
Eo results (see Table E O + PO 111)were higher than the known figures. To correct for the preferential loss of propylene dibromide, a correction was applied in future calculations by introducing an arbitrary factor D to give the expression :
Table IV.
EO
Product Mixtures of oxyethylated glycerol and oxypropylated glycerol Mixtures of polyethylene glycol 400 and polypropylene glycol 1000 Block copolymers from ethylene oxide and propylene oxide 2,2-di(hydroxymethy1)-1butanol ox propylated and then oxyet$lated
Table V.
Sample no.
'>
2
9
5 6 7 8 9
10 11 12 13 14
15 16 17 18 19 20 21
corrected
%
EO
+ PO --
EO
22 23
The values of D can be calculated from the results on known samples using the expression :
D =
(100 - Known % EO) X (100 - Observed % EO) Observed % EO Known % EO
Results for Factor D, Using Known Mixtures
Results for
The observed results a t 2 hours at 150" C. on the products shown in Table 111, and on a variety of block copolymers specially prepared from known weights of ethylene oxide and propylene oxide, and also on mixtures of polyethylene glycol of mol. wt. 400 and polypropylene glycol of mol. u t 1000 gave the results for D shown in Table IV. Considering the range covered (4.8 to 63.5%) the factor is reasonably constant with a mean figure of 1.26 and a spread not greater than 0.12. When Equation 4 is used for the analysis of samples the variation of 1 0 . 1 2 in D will correspond to about k 0 . 6 a t 7% level
Known 31.5 63.5
Observed 34.8 67.5
Calcd. factor D 1.17 1.20
4.8 9.7
5.8 11.8
1.24
9.9 22.0 60.0 46.7
11.7 27.0 67.5 54.5
1.21 1.32 1.38 1.37
1.22
70EO Eo + PO by HBr/GLC Method and by NMR Spectrometry
Poly01 present Known mixtures of polyethylene glycol 400 and polypropylene glycol 200 Copolymer from ethylene oxide and propylene oxide Glycerol Glycerol Glycerol Glycerol Glycerol Glycerol Glycerol Glycerol Glycerol Glycerol Glycerol Glycerol Glycerol Glycerol Glycerol 2,2-Di(hydroxymethy1)-Ibutanol 2,2-Di(hydroxymethy1)-1butanol 2,2-Di(hydroxymethy1)-Ibutanol
and 1 2 . 0 a t 60a/, level-Le., the variation in D becomes progressively EO less important as % ____ increases. EO PO There is no significant evidence that simple polyoxyethylene/polyoxypropylene mixtures behave differently from block copolymers of ethylene oxide and propylene oxide or polyols that have been oxypropylated and then oxyethylated. The method was applied, using a mean figure 1.26 for factor D, to a series of samples, with the results shown in Table V; results for the first five samples are from repeat tests on some of the synthetic products shown in Table IV that contain only oxyethylene and oxypropylene groups. Table V also includes the results by NMR spectrometry. Inspection of Table V shows that results by the HBr/GLC method tend to
+
(5)
%ETTTO
Known Yo 4.8 9.7
HBr/GLC result, 70 (corr.) 4.6
NlIR result,
9.5 3.5
