the absorbance of solutions of this material was measured, its normal benzophenone content, WEIS always taken into account. Procedure. T h e absorbance of a benzophenone solution at' a cert,ain point was the difference bet'ween its absorbance and t h a t (of the pure solvent in the same cell, a t this frequency. Each of these absorbances was the mean value of t,wo measurements, all measurements being carried out during a very short period. T h e estimated mean experimental error in absorbance was about 1 0 . 0 0 5 . RESULTS
The results obtained by the three spectroscopic methods of analysis described above, on solutions of known 0 ' 8 content, are collected in Table I. For enrichments of S-61y0 0 1 8 , both methods I and I1 gave very satisfactory results while for 1 to 4% enrichments both methods I1 and 111 gave results which were only accurate to 11% (absolute). The general mean
Table I. Determination of O18-Benzophenone Content of Known Solutions
Known content,
%
61 55 44 19 8 3
1
0 5 8
4 5 1 4 1 0
Found, Method I I I I I1 I1 I11 I11
56
Dlff.
61 4 55 0 44 5
+O 3
20 3 a
-8 4 1 1
7 6 5 9
0 0 0 0
+o
+o
.5
3 + 1 1 +01 t o 9 0.4
pended somewhat on the total concentration when in excess of 1.5 mg. per ml. I t is hoped that by using 20-mm. cells, and keeping this concentration within the 1.5 ma. per ml. limit, considerably more accurate results will be obtained with enrichments as low as 0.5%. ACKNOWLEDGMENT
The authors thank D. Samuel for the sample of 018-benzophenone.
~
mean a After correcting for the absorption of the normal compound a t 1637 cm.-'
error of all these methods however was about +0.5%. The main reason for the bigger error in the determination of lower 0l8 contents &-a5 the interference of the now relatively high concentration of the normal species a t the key frequency of the labeled benzophenone, which de-
LITERATURE CITED
( 1 ) Halmann, SI., Pinchas, S., J . Chem. Soc. 1958,p. 1703. ( 2 ) Lapidot, A., Pinchas, S., Samuel, D., Proc. Chenz. Soc. 1962, p. 109. ( 3 ) Laulicht, I., Pinchas, S., Israel Journ. Chem. 1, 404 (1963).
RECEIVEDfor review March 3, 1964. Accepted May 21, 1964. A grant from the National Institutes of Health, Bethesda, Washington I>. C., supported this investigation. Presented in part before the XIXth IUPAC Congress, London, U. K., July 1963.
End-Group Analysis and Number-Average Molecular Weight Determination of Some Polyalkylene Glycols and Glycol Polyesters Using Nuclear M a gnetic Kesonance Spectroscopy THOMAS F. PAGE, Jr., and WARREN E. BRESLER Battelle Memorial Institute, 505 King Avenue, Columbus, Ohio 4320 7
b Using nuclear magnetic resonance spectroscopy (NMR), it has been found that methylene and methine groups attached to a hydroxyl group can b e differentiated from those attached to an ether oxygen for some polyalkylene glycols and glycol polyesters. In the specific case of glycols, this differentiation makes it possible to calculate number-average molecular weights without using the -OH resonance area and thus circumvents the problem of obtaining low molecular weight values because of the presence of a small amount of water. By being able to determine the -CHzOH content of a glycol polyester, it is possible to correct the area of the resonance due to the acid and hydroxyl protons for the hydroxyl content, and water i,F present, and thus obtain the acid en'd-group content by difference. Number-average molecular weights of the glycol esters can also b e determined. The technique involves taking advaintage of the
complex formed between pyridine and -OH groups to shift the -CHzOH resonance from the -CHz-Oresonance.
I
that if one can perform a quantitative analysis of the end-groups of a polymer, this information can be used to calculate a number-average molecular weight (NAMW) of the polymer ( I , 11). Xuclear magnetic resonance (NSIR) spectroscopy is especially suited to this kind of determination if the resonance of the end-group protons is chemically shifted from that of the internal group protons and if impurities do not interfere with the respective resonances. The S M R spectra of polyethylene glycols (PEG) fulfill the former of these qualifications but do not usually fulfill the latter because the mall impurity of water which is usually present in conimercial P E G samples contributes to the -OH resonance area. This interference T IS WELL KNOWN
results in the measurement of a high -OH content and thus the calculation of a correspondingly low NAMW. The purpose of this paper is to report a technique which has been developed so that end-group analyses and S X M W determinations can be carried out fcr hydrosyl terminated polymers such as glycols by using only data from the
CH~-, -cH,-,
-CH
resonances.
EXPERIMENTAL
Apparatus. A11 spectra were obtained using a Varian Associates Model HR-60 nuclear magnetic resonance spectrometer operating a t 60 Me. per second. Varian 1Iotlel V-3521 electronic integrator was used for integration; the capacitor charge was monitored with a Hewlett-Parkard Model 3440A digital voltmeter equipped with a Hewlett-Packard Model 3442.1 automatic ranging unit. Chemical shifts are reported in csycles per second don nfield froin the internal VOL. 36, NO. 10, SEPTEMBER 1964
1981
I All 0 7 Ct-l2Cli,O-
sample was determined to be 598. How-ever, if one integrate. the -OH proton resonance (170 c.p.s.) cs. the -OCH2CH20proton re\onance (219 c.p.s.)T a %AllK equal to 422 ib calculated from
Protons
HO-CH,CH,-0-
CH,CH,-0-CH,CH,-OH
11”
Central - o C ~ , C ~ , O -
1
X=-OH o r a Y=-0- or-OH
, where
Y X
219cps Figure
170 cps vs. TMS 1. Carbowax 600 in CDCh
reference, Si(CH3)4 (abbreviated T l l S in the figures), and aere measured by the well-known side-band t’echnique. Frequencies were read with a Helylett-Packard Model 521C fivedecade frequency counter. All quantitative measurements are the results of averaging 10 or more independent measurements. Reagents. Reagent’ grade pyridine, which was freshly distilled and stored over barium oxide to minimize wat,er contamination, was used for all pyridine runs shown in the figures. S o apparent spectral differences were noted when special precautions were taken t.o exclude all detectable water. The glycols and glycol esters st’udied n ere commercial samples obtained from a variety of chemical supply houses. RESULTS A N D DISCUSSION
Figure 1 is the ?;AIR spectrum given by a sample denoted as Carbowax 600 when run in CDC13. By boiling point elevation of benzene, the S - i l l W of this
Figure 2.
1982
1 -Chloro-2-bromoethane ANALYTICAL CHEMISTRY
= =
area of -OCH2CH20--- resonance area of -OH resonance.
Equation 1 can be derived in a straightforward manner if one assumes the general structure of a PEG to be HO-(CHzCH,O) .-H. The accuracy of a S . i M W determined using Equation 1 is directly dependent, however, upon two factors: accurate integration of a very small area (-OH) relative to a large area (-OCH2-CH20-); and the absence of water. It should also be pointed out that the concentration dependence of the -OH resonance, together with its low signal intensity ( 5 1% of the total proton resonance signal a t S.-IM.Iw >_ 2218) sometimes requires the expenditure of a significant amount of time to ensure correct assignment of the -OH .- proton resonance. To aroid the difficulties noted above-especially the interference of water-a technique was sought which would permit the differentiation of the two methylene groups in the terminal monomer unit of a PEG-i.e., the -CH20H protons from the -CH*Oprotons. That one could expect to distinguish between these methylene groups is demonstrated by the S l l R spectrum of I-chloro-2-bromoethane (Figure 2). Such a spectrum is typical of X-CH,CH,-Y type molecules, where X and Y a r e slightly different in electronegativity. The same general kind of spectrum is given by diethylene glycol. These kinds of spectra are classified (10, 16) as being of the .-l.A’BB’ type provided that rotation about the central carbon-carbon bond is not completely free, but in the limit of free rotation, they are classified as ..t2Bz. Spectra of the ;L-t’BB’ type are also known to be symmetrical about the center (denoted 0 in Figure 2) ; with two protons giving rise to each half of the spectrum. I n fact, the outermost components of the .-t..l’BB’ spectrum o f the terminal unit of triethylene glycol-i.e., HOCH2CH2OCH,CH?OCH,CH,OH, could barely be observed in the presence of the interfering central -0CHXHzOgroup (220 c.p.3. singlet) when this compound was examined in CDC13 (Figure 3). Since others haire used forms of complexing for the direct study of alcohol groups by X I R (S), it was
-CH_,Y
. 220cps
Figure 3.
vs.
TMS
Triethylene glycol in CDCI,
reasoned that’ the desired enhancement in chemical shift between the two -CH2groups of the terminal ethylene unit might be obtained if the -OH end-groups were very strongly complexed. The most convenient way of complexation which suggested itself was that of using the solvent as the complexing agent. Pyridine was chosen because it was known to complex strongly with alcohols (2, 4,6, 9, 1%’))because it has no resonance signals in the spectral region of interest, and because it had been successfully used (14) in the same kind of study with steroids. Figure 4 s h o m the rather dramatic changes which take place in the spectrum of triethylene glycol when pyridine. to which has been added a catalytic amount of HC1 gas, is used as solvent. The d.-t’BB’ spectrum of the terminal ethylene groups is now clearly discernible, the strong resonance of the central ethylene group being superimposed on the high field portion at 222 c.11.s. When pyridine which contains no HC1 gas is used as solvent: much the saiiie general kind of spectrum as that
235
222 cps vs. TMS
Figure 4. Triethylene glycol in pyridine HCI gas
+
1yna\-O+CHC 2H~OTerminal Ether - 0 C k l ~ I
where X = resonance area of -CH20H protons Y = resonance area of -CH20protons This value (592) is obviously in much better agreement with the NAIMWof 598 which was measured for this sample by the ebullioscopic method using benzene than is the KAMW of 422 which was calculqted by attempting to measure the hydroxyl protons. Assuming the difference between a molecular weight of 592 (measured in pyridine HCl gas) and 422 (measured in CDC13) is due to water, it is readily calculated that the Carbowax 600 sample contains 1.8 mt. % H2O. However, if KMR measurements for a polyethylene glycol of S A M W = 10,000 which contained a 2% water contaminant were carried out, the niclecular weight one would calculate by NMR using CDCls as a solvent would be only about 800 unless rigorous corrections for the water impurity were made. This illustration points out that a low weight per cent water represents a h’igh mole per cent water in higher molecular weight glycols, and thus induces a large error in the calculated
+
Figure 5. Carbowax 600 in pyridine HCI g a s
+
shown in Figure 4 is obtained. However, the resolution in the latter spectrum is very poor. The addition of a trace of HC1 gas increases the exchange rate between -OH protons on adjacent molecules and effectively decouplea the spin of the hydroxyl proton from those of the adjacent methylene protons-thus giving a sharper, better resolved spectrum. Other solvents such as aniline, substituted pyridines, and pyrazine in pyridine were tried in an attempt to enhance even more the chemical shift between the methylene groups of the terminal monomer unit. While all gave the desired effect, none was so effective as pyridine alone. Pyridine-OH complexes have received wide attention (2,4,6,9,IW). However, their exact nature is still unknown. I t is believed that if the complex could be characaterized, one (could devise solvent systems which would not only extend the limits of the present work but which could be easily adapted to the study of other systems by this same general technique. The nature of the complex is now being further studied. Application of Technique to Polymers. The S M R spectrum of the Carbowax 600 in t’he pyridine solvent system demonstrates the application of the pyridine technique to polymers. T h e methylene resonances of this spectrum are shown in Figure 5 , where the low field series of resonances centered near 235 c.p.8. are the -CH2OH protons and the high field resonances 1222 c.11.s.) arise from the --C‘H20- protons. Although much weaker than that of the triethylene glycol, the one half of the ,l.-L’BB’ spectrum associated with the -CH,OH protons is clearly visible. h XANW of 592 was calculated for the saniple shown in Figure 5, using the equation
-
‘
SAMW. Advantages of Pyridine ; HCl Solvent System. Four major advantages of the pyridine : HC1 solvent can be cited. First, the resonance of the - C H2 0 H protons is always in the same spectral region, unlike that of the -OH proton in nonpolar solvents like C D C K Second, only data from the methylene resonances are used in molecular weight calculations. Therefore, water does not interfere. Third, t,wice the number of protons are being observed for each end group as compared to measuring the -OH- proton content directly-thus increasing the magnitude of the molecular weights which can be determined by a factor of two. Finally, the speed with which molecular weights can be determined, which, in many cases, cannot be obtained a t all by other means, is great,ly increased. The pyridine technique has been used successfully in these laboratories for SalMWmeasurements of polyethylene glycols up to 6000. However, sufficient signal intensity of the -CHaOH multiplet is observed a t niol&!ular weight 6000 to indicate that molecular weights of 15,000 to 20,000 could be measured using the pyridine technique just described. Present data indicate that SAlIW’s measured by the pyridine technique are accurate to about 1% of their true values if the results of 10 or more measurements are avemged. .Ambiguous results were obtained by both ebullioscoliic methods and val)or pressure osmometrj- for all polyethylme
1 ti
li
-Ctj-0-
+ -cy;o,
-238 cps
208 cps
Figure 6. in CDC13
Polypropylene glycol 150
glycols with molecular weights higher than 1500 and for this reason no table of molecular weights measured by ?;AIR us. those measured by an independent method has been included in this report. Extension of Technique. Since the pyridine technique worked so well for the polyethylene glycols, some polypropylme glycols and polymeric ethylene glycol esters were also examined by the same method to see if these systems could be analyzed more completely when the hydroxyl groups are complesed. Figures 6 and 7 show the spectra obtained for polypropylene glycol 150 in CDC1, and in pyridine: HC1, respectively. The strong doublet a t 69 c.p.s. in the CDCls spectrum arises from the methyl protons while the complex multiplets a t about 238 and
I
208 c.p.s. come from the -CHOand -CH20protons, respectively. In HC1 gas, several marked pyridine differences are noted. The sextet of lines (251 c,p,s,)which have been shifted
+
to the low field side of the -CHO-and -CH20resonances has been as-
I
signed to --CHOH and -CHiOH protons by analogy with the e t 6 l e n e glycols. This assignment was confirmed by double resonance esperiinents. In the methyl region of the spectrum, the new doublet (81 c.ps.) which appears a t a slightly lower applied field than the original doublet due to methyl protons (77 c.11.s.) has been assigned to methyl groups alpha to terminal hydrosyl g r o u ] ~ as , noted in the figure. I3eing able to resolve the resonances of these proton. from those attached to internal monomer units makes it 110ssible to peiform not only a calculation of the ratio of primary -OH to secmdary --OH end-groul~s but also to 11erfom a S.1lIW calculation without cowerting for watcr content. This calculation can be cosily carried out in t h e i‘ollon.ing way. VOL. 36, NO. 10, SEPTEMBER 1964
1983
-0-qH-CHbO-X , CH, (AREAZ)
~EA~AR:A]
, I
II
-CH-OL
-c&o-\
X=RorH
CU3 (AREA Y ) HO-CH_,-
+
HO-CH-
-L i ,
cps77cps vs. TMS Figure 7. Polypropylene glycol 150 in pyridine HCI gas
+
309cps 258 234219cps Figure 8. HCI gas
163cps vs
TMS
Polydiethylene glycol succinate in pyridine
+
Let
A
=
Xumber of R-OCH,CHO-R
groups
I
CH3
B
=
Number of R-OCH,CHOH
+
groups
+
I
CH3
C
=
?;umber of R-OCHCH20H groups
CH3 W = Relative area of --CH20H -
The ratio of C to B is equal to the ratio of primary --OH to secondary -OH end groups. However, to obtain a XA111iT'one must api)ly the additional relation,5hiii, B C = 2--i.e., each niolecde contains two end groups. In iiractice, the sum B C as calculated from Equations 8 and 9 rarely equals 2 . However, this sum can be adjusted to equal 2 if one finds a factor, f>such that f = 2t'B C. The number average niolerular weight of the polypropylene glycol can then be calculated from
+
+
~
-CHOH -
SAlITT'
protons
,Y Relative area of -CH2O-
I
and
=
58f(B
+ C + A ) + 18.
(10)
Obviously, the polypropylene glycol end-group ratioa and SA1lIW's one calculates from K l I R data are very Y = Relative area of dependent upon the acruracy with CH,CHOH protons which one can resolve and nieasure the I resonance area of HOCH(CHJCHZCH2 relative to the remainder of the tncthyl resonance areas. For loiv molecular weight glycols (.\Iff 5 500) this can be 2 = Relative area of remaining done with reasonable accuracy by using -CH,- protons. peak intensities. H o w v e r , even though The areas corresponding to If7, X, Y l this technique yield:: results which hecome less quantitative as one tries to and Z have been appropriately noted in analyze higher molecular w i g h t samples, Figure i. the saving in tinie in determining a Then TI' = B 2C (3) SA1llW usually offsets the lnss in accuracy and allom one to determine s = 3'4 2B (4) quickly whether niore exact but inore I- = 3B (ti) time-consuming determinations ( 5 , 7 , 13) are justified. 2 = 3.4 3c (6) Polymeric Glycol Esters. -11though to date only a limited amount Solving for -4, B , and C one obtains of n.ork ha? been done on the end-_ 277' group analysis of polyrneric glycol 4 = s 2_ (7) 6 ester..ridine t c ~ h n i q u e developed in these laboratories shows great 1)romi.e in allo\ving one t o gain B = -I' (8) 3 a hettrr innight into the itructure of these compounds. Figure 8 shows the S l I R spec'trum c = 271' Z of a polydiethylene glycol succinate 6 -CHO-
protons
+
+
+c
+
+
+
1984
x
ANALYTICAL CHEMISTRY
prepared by reacting buccinic anhydride with diethylene glycol. The assignments of the various resonances are noted in Figure 8. .1X.1lITT ran be calculated if one ran deterniine the ratios of succinate glycol '--OH/--COOH. This ran be done in the following way. 13y intpgrating the 163 c.11.s. resonance relative to the 200-265 c.11.s. region, one ran determine directly the ratio of glycol and acid moieties in the product. From the relative areas of the 234 r.p.s. rnultiplrt and the 219 c.;).s. niulti~~let. a measure of the relative amount of glycol -OH termination ran lie obtained. By determining the water by an independent method ( 3 ) , a1)propriate corrections to the 309 c.p,s. resonance area can be applied for H20 and glycol -OH termination to give the relative amount of acid termination present by difference. Glycol polyesters are extensirel)- used as polar gas liquid chromatographic (GLC) suhstrntes. As siich, it is possible that one ni the relative amoun cnd groulis and glycol e.*ters to such GLC column prol)ertie-. as stability and resolving power. At the sanie time. insight niay be gained as to the fundamental processes occurring on the column and their relationshill to substrate structure. hat the pyridine : HC1 ibed in this communication provides the nieans for accurately measuring the structural parameters just mentioned and work directed toward rclating substrate structure and rolunin 1)roi)ertiesis presently in process in these laboratories. It is h e l i e i d that the technique of forniing complexes (whether by solvents or reaction with a suitable reagent such L L ~13F3)to shift the resonance signals of 1)riitnn': adjacent to a slmifii. group is a new aiipoach to structure charac-terizatinn by S l I R and that it should find
wide application fcir NhlR studies of polymer systems which have functional groups which can be comi)lesed. LITERATURE CITED
(1) Bilrneyer, F. W.,Jr., “Textbook of Polymer Science,” pp. 53-4, Interscienve, Sew York, 1962. (2) Errera, J., Gaspart, R., Sack, H., J . C‘hem. Phys. 8, 63-’71 (1940). (3) Fiwher, Karl, .4ngrew. Chem. 48, 394 (1935). (4) Gordy, Walter, .1. Chem. Phys. 7, (33-9 (1939).
(5) Hanna, J. G., Siggia, 567 297 (1962).
S.,J . Polymer
(6) Hatern, Simone, Yalladas-Ihbois, Suzanne, Bull, sot, Chim, France 604-7 (1949). ( 7 ) Hendrickson, J. G., ANAL CHEM.36, 126-8 (1964). 18) J . A m . Chem. SOC.86, . . Kina, 1%.W., 1256-8 (1964): (9) Martin, Maryvonne, Herail, Frangoise, Compt. Rend. 248, 1994-6 (1959). (10) Pople, J. A., Schneider, IT. G., Bernstein, H. J., “High Resolution Nuclear Magnetic Resonance,” Chap. 6, McGraw-Hill, Xew York, 1959. (11) Price, G. F., “Techniques of Polymer Characterization,” P. W. Allen, ed.,
Chap. 7, Academic Press, Yew York, 1959. (12) Shukla, 11. P., Bhagwat, W. V., Agra. Univ. J . Res Sci. Suppl. 4, 645-50 (1955). (13) Siggia, S., Hanna, J . G., ANAL. CHEM.33, 896 (1961). 114) SlornD. G., J . Am. Chem. SOC.82. 999-1 oob’( 1980). (15) Whitman, I]., J . Mol. Spectry. 10, 250-62 (1963). RECEIVEDfor review April 1, 1964. Accepted June 19, 1964. Presented in part at the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, March 2, 1964.
Extractability of Selected Metal Ions with High Molecular Weight Amines Application to the Separation and Determination of Iron and Cobalt in Ferrous Alloys B. E. McCLELLAN’ arid V. M. BENSON Department o f Chemistry, The University o f Mississippi, University, Miss.
b A number of transition metal ions were tested for extractability from hydrochloric acid solution with several high molecular weight amines dissolved in carbon tetrachloride. Only aliphatic amines of high molecular weight were found to b e useful extractants for iron(lll), cobaIt(ll), manganese(ll), and copper(l1). Primary amines were found to b e somewhat effective as extractants for manganese (11) and copper(l1). Iron(ll1) and cobalt(l1) were quantitatively separated b y extraction from i!M HCI with trioctylamine dissolved in carbon tetrachloride. Under these conditions the iron(II1) extracts almost completely, while no cobalt(l1) e.ntracts. Several ferrous alloys were analyzed for iron and cobalt by separation using solvent extraction followed b y spectrophotometric analysis. An empirical method was developed whereby 100% extraction was not essential for a quantitative determination. The method is rapid and accurate for a large variety of ferrous alloys.
S
and Page (14) were first to repoit the use of high molecular \%eightamines as acid
