chromophores, especially MA2 where the metal ion bridged two such resonating ligands. The role of chloride in these complexes could not be studied by the present method, but it is probable that chloride ions occupy vacant coordination positions on Sn(1V). It is suggested, also, that chloride-bridge structures are important in the dinuclear, M2A, complex.
RECEIVED for review August 31, 1970. Accepted August 23, 1971. Abstracted, in part, from the Ph.D. thesis of W. D. Wakley, Oklahoma State University, Stillwater, Okla., July, 1970. This work was supported, in part, by the Research Foundation and Computer Center at Oklahoma State University, and by a training grant from the Federal Water Quality Administration.
Determination of Molar Substitution and Degree of Substitution of Hydroxypropyl Cellulose by Nuclear Magnetic Resonance Spectrometry Floyd F.-L. Ho, Robert R. Kohler, and George A. Ward Research Center, Hercules Incorporated, Wilmington, Del. 19899 Two independent NMR procedures for the determination of the number of moles of alkylene oxide combined per anhydroglucose unit (molar substitution, MS) in propylene oxide derivatives of cellulose are described. One of these also is capable of determining the average chain length of the substituent chains and, therefore, the average number of hydroxyl groups substituted per anhydroglucose unit (degree of substitution, DS).
HYDROXYPROPYL CELLULOSE containing at least two moles of combined propylene oxide per anhydroglucose unit is a water-soluble polymer which exhibits an unusual combination of properties-e.g., polar organic solvent solubility, interfacial activity, and thermoplasticity. This polymer is manufactured by Hercules Incorporated under the trade name Klucel. The way in which these properties contribute to performance in a variety of uses depends on the number of moles of combined propylene oxide per anhydroglucose unit (MS) and on the number of hydroxyl groups substituted per anhydroglucose unit (DS). The MS of hydroxypropyl cellulose can be determined by a classical terminal methyl procedure ( I , 2 ) , in which the C-methyl is oxidized by chromic acid to acetic acid, which is then distilled from the reaction mixture and titrated with standard base. Some chromic acids are also distilled from the mixture and are corrected for by an iodometric procedure. Newer approaches for the analysis of substituted celluloses based on instrumental analysis have recently appeared in the literature. A combination chromic acid oxidation-gas chromatography procedure for ethoxyl content of ethyl cellulose has been reported (3). Mass spectrometry has been used to obtain the degradation profile of water-soluble cellulose ethers ( 4 ) to allow direct comparison of different modified cellulosic products. However, despite considerable research efforts, satisfactory methods for determining the D S (1) K. G. Stone, “The C-Methyl Group” in “Treatise on Analytical Chemistry,” Part 11, Volume 13, I. M. Kolthoff and P. J. Elving, Ed., John Wiley and Sons, New York, N.Y., 1966. (2) K. V. Lemieux and C. B. Purves, Can. J . Res., Sect. B , 25, 485 (1947). (3) H. Jacin and J. M. Stanski, ANAL.CHEM., 42, 801 (1970). (4) H. R. Harless and K. L. Anderson, Text. Res. J., 40, 448 (1970).
178
of hydroxyalkylcellulose have not as yet been developed (5, 6). A method for determining DS by the reaction of hydroxyethyl cellulose with phthalic anhydride in pyridine was reported by Senju (7). Froment (8) used this method on hydroxyethyl and hydroxypropyl cellulose based on the observation that this reaction only occurs on hydroxyalkyl hydroxyl groups. In our laboratory it has been shown that the results obtained using this procedure vary significantly with changes in the reaction conditions. This report describes rapid and specific NMR experiments designed t o measure both DS and MS of hydroxypropyl cellulose. EXPERIMENTAL
All spectra were obtained on a Varian A-60A N M R spectrometer at an ambient probe temperature of 40 “C in standard 5-mm 0.d. glass tubes (Wilmad Glass Co.). Deuterated chloroform (Stohler Isotope Co.) was used as a solvent throughout this work. Tetramethy lsilane (Matheson, Coleman & Bell Co.) was used as an internal standard for chemical shift reference. The trichloroacetylisocyanate used was reagent grade, from Eastman Kodak Co., and was found by proton N M R to contain no detectable impurity other
9 than a trace amount of its proton analog, CHC-N=C=O. Samples of hydroxypropyl cellulose (Klucel E, Hercules Incorporated) were dried at 110 “C for 2 hours before each use. A weight loss of about 4 to 5 % upon drying was noted. Procedure. In the “direct ratio” experiment, samples were dissolved in CDC13 in 5-ml vials to about 5% in concentration. Generally the solution was heated at 50 “C for about 30 minutes t o complete dissolution of the sample. The solution was then cooled to room temperature and filtered through a cotton plug into an N M R sample tube in which the spectra were run. Generally, spectra were obtained with 500 H z and 1000 H z sweep widths and integrated 10 times with both upfield and downfield sweeps. The precision of the peak area measurement for each individual N M R peak was found t o be better than 3 % at the (5) E. D. Klug, Food Technol., 24, 51 (1970). (6) M. G. Wirick and M. H. Waldman, J . Appl. Polym. Sci.,14, 579 (1970). (7) R.Senju, J . A g r . Chem. Soc. Japan, 22, 58 (1948). (8) G. Froment, Znd. Chim.Belge, 23, 115 (1958).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 1, JANUARY 1972
Figure 1. NMR spectrum of hydroxypropyl cellulose in CDC& at 5 % concentration; sweep width 500 Hz
e -&
F i g u r e 3. E x panded scale spectrum in the methyl region of poly(propylene glycol), sweep width = 100 Hz.
/
Weight ratio of iso-
cyanate to polyether (6) 0.132, (c) 0.319,( d ) 0.524 ( a ) O.OO0,
63 8
Figure 2. Expanded scale spectrum in the methyl region of hydroxypropyl cellulose (sweep width 100 Hz) Weight ratio of isocyanate to cellulose sample (a) 0.00, ( 6 ) 0.226, (c) 0.477, (d) 0.840, (e) 1.065, (f) 1.390
---+I95% confidence level. A typical spectrum is shown in
Figure 1. In the “special titration” experiments, a series of 7 solutions were prepared in a similar fashion. After the sample was completely dissolved in a 5-ml vial, the calculated amount of trichloroacetylisocyanate was added to each solution via a syringe. The transfer of isocyanate was carried out in a dry nitrogen atmosphere. Weights of sample and isocyanate added were measured t o 0.00005 gram. The NMR spectra of the methyl doublets of these solutions were run on an expanded scale at 100 Hz sweep width. A typical set of traces of spectra is shown in Figures 2a to 2f, arranged in order of increasing amount of isocyanate added. In a control experiment, polypropylene glycol, PPG, of known molecular weight (GPC standard from Waters Associates, Catalog No. 41993) was used t o react with isocyanate in exactly the same manner. The methyl doublet regions are shown in Figures 3a t o 3d.
80.4
67.8
propyl groups carries a hydroxyl which is capable of undergoing further reaction. A possible structure is shown in I, illustrating a product of MS 4.0 and DS 2.5. OH
9” 0,-C$CHCH3
I
I
l
0-CH2C HCH3 bH
y 2 0-CH2CHCH3 b-cH2CECH 3 CH$HCH3
d-
RESULTS AND DISCUSSION
Hydroxypropyl cellulose is produced by reacting the reactive hydroxyls of cellulose with propylene oxide at elevated temperatures and pressures. Each of the hydroxy-
I
OH
The maximum value of DS is three while there is no theoretical maximum limit of MS.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 1, JANUARY 1972
179
Figure 5. A titration plot of hydroxypropyl cellulose with trichloroacetylisocyanate ISO/PPG
Figure 4. A titration plot of poly(propy1ene glycol) with trichloroacetylisocyanate
Proton N M R is a useful technique for analysis of these compounds because each different proton structural group gives rise to peaks a t a characteristic magnetic field strength, and the peak intensity (area) is directly proportional to the concentration of the proton group. In Figure 1 , a strong doublet a t high field (1.12 ppm from internal T M S reference) can be readily assigned t o methyl protons, while the broad peak from 2.5 t o 6.0 ppm results from methylene and methyne protons of the hydroxypropyl substituents and all the protons of the cellulosic skeleton. Therefore, the intensity of the methyl doublet gives a direct measurement of the extent of hydroxypropyl substitution, which in the classical C-methyl procedure was obtained by a lengthy combination of oxidation-distillation-titration procedures. Two independent N M R procedures for measuring MS and DS values are discussed below. Direct Ratio Method. As seen in structure I, each of the hydroxypropyl substituents brings to the cellulose skeleton 3 methyl protons and 3 other protons from methylene and methyne groups. Also, there are 10 protons resulting from each cellulosic unit regardless of its MS value, with 7 C-H protons and 3 0-H protons. Therefore, if one designates the integrated area from methyl protons as A and the area from the remaining protons as B, one can readily calculate M S of the hydroxypropyl cellulose by the equation.
MS
=
0
I1
1 OA 3(B - A)
By use of this procedure for the analysis of a typical commercial Klucel sample, a n MS of 4.2, with a confidence limit of +0.2 at the 95z confidence level, was obtained. The precision for the measurement of MS under different experimental conditions is thus about 6 %. This precision for the measurement of MS is relatively low, and is caused by the fact that uncertainties in the measurement of each individual peak area are multiplied in the mathematical calculation. However, the advantage of the N M R method is the speed of analysis. Moreover, it does not require any primary standards for sample titration or for instrument response calibration. It does not even require knowledge 180
of the sample weight. A complete analysis for a single sample can be done within one hour. As a check on the accuracy of this approach, a n analysis by the classical Cmethyl oxidation procedure was performed and a n MS of 3.8 was obtained on the same sample. The agreement between these two results is fairly good, in view of the precision of the N M R method. On the other hand, the smaller M S obtained by the chemical method might have been caused by the fact that a n empirical accounting for only 95 recovery of C-methyl was assumed in the calculation for MS. Special Titration Method. This approach is based o n the measurement of the relative concentration of methyl groups at the end of the substituent chains and those methyls inside the chain. From this ratio, the length of the average substituent chain, and thus the DS, can be calculated. Although the terminal methyl groups and those inside the chain are magnetically nonequivalent, the difference is too small to be resolved at 60 MHz under normal conditions. In a n attempt to increase the resolution, we first attempted t o apply a europium complex of dipivalomethane, as a shift reagent. This has been shown to increase N M R spectral resolution dramatically on simple alcohols (9) as well as on poly(propylene glycols) (10). Unfortunately, this reagent forms a stiff gel with the hydroxypropyl cellulose sample. Lines are broadened and only slightly shifted. Next, we applied a derivatizing reagent, trichloroacetylisocyanate, which reacts with hydroxyls as
CClsC-N=C=O
0
I1
+ R O H e CC13C-N-C=0
I 1
(2)
H OR This derivitizing reagent was first applied by Goodlett ( I I ) in 1965 to shift a-hydrogen proton signals toward lower magnetic field by virtue of the influence of the electron withdrawing carbonyl group. Protons several bonds removed from the hydroxyls are not appreciably shifted. In our experiments, the terminal methyl peak was shifted downfield from the methyl peaks inside the side chain. Examples of the ~~
(9) J. K. M. Sanders and D. H. Williams, Chem. Commun., 422 (1970). (10) F. F.-L. Ho, Polym.Lett., 9, 491 (1971). (11) V. W. Goodlett, ANAL.CHEM., 37,431 (1965).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 1, JANUARY 1972
effect are shown in Figures 2 and 3. These are the expanded scale spectra in the methyl regions of hydroxypropyl cellulose and poly(propy1ene glycol), respectively. I n the latter, a methyl doublet a t lower field (80.4 Hz from TMS) grows in intensity and reaches a constant value as the concentration of isocyanate added increased. This can be assigned to end methyl protons. Figure 4 shows a plot of the ratio of the intensity of the low field doublet to that of the one at high field, 6s. the weight ratio of isocyanate to poly(propy1ene glycol). This is a typical titration plot, with two straight lines intersecting at the end point, a t which all the hydroxyls in the polymer chains are reacted. Goodlett ( I / ) found that trichloroacetylisocyanate reacted very rapidly with a variety of types of hydroxyl groups. I n our work, it appeared that the reaction was complete by the time the first NMR spectrum of the sample could be run, since n o change in the spectra was observed on standing for longer reaction times. Furthermore, addition of excess reagent to the sample caused no additional change in the spectrum. Therefore, the ratio of terminal methyls (END) to internal methyls (IN) in the polymer chain can be readily obtained and used t o calculate the number average molecular weight through : IN M - -__ X 116 134 (3) - END
(an)
+
A value of M , of 762 on the PPG sample was obtained’ which compares favorably with 790 from the supplier’s data and with 771 determined by a n independent N M R procedure (IO). However, because of the slight spectral overlapping of the upfield peak of the low field doublet with the low field peak of the high field doublet, and because of the fact that slight variations in the small value of E N D can cause a large error on it was more accurate to measure the E N D value by doubling the integrated area of the peak at lowest field, which is clearly resolved from spectral interferences. The resolution of end groups in PPG achieved by using this derivatizing reagent is greater than that reported using a solvent system of pyridine-HC1 (12). In the case of hydroxypropyl cellulose, a similar change in the methyl region was observed, as seen in Figure 2. The doublet a t lower field can be assigned t o terminal methyl groups. However, the spectral lines are broader than those observed in Figure 3, due t o the high viscosity of the sample solution and the fact that the methyls of hydroxypropyl cellulose are in slightly different structural environments in the more complex cellulose polymer. Because of this line broadening and overlap, only the resonance intensities from No. 1 (at 63.8 H z from TMS) and No. 4 (at 81.6 Hz) were measured. The ratio of intensities of these two lines was plotted against the weight ratio of isocyanate over hydroxypropyl cellulose sample in Figure 5 . Again, this is a typical titration plot, where N M R is used to follow the progress of titration. These data can be used to calculate MS and DS and the treatment of data is shown below. Treatment of Titration Data. M S VALUEFROM THE XAXIS. The amaunt of isocyanate reagent required per gram of sample is found from the X-axis in Figure 5 to be 1.400 grams. This can be used to calculate the moles of hydroxyls and therefore moles of the cellulose unit per gram of sample from the expression
mn,
Wt isocyanate __ - 1.400 3 X mol wt of isocyanate 188.4 X 3
=
2.48
x
10-3 (4)
(12) T. F. Page, Jr., and W. F. Bresler, ANAL.CHEM., 36,1981 (1964).
since each cellulosic unit carries 3 hydroxyls, regardless of size and extent of hydroxypropyl substitutions. The average molar weight of a unit is therefore Wt of sample l.Oo0 2.48 X No of moles of cellulose unit
=
4.04
x
102 (5)
This is a weight combination of hydroxypropyls (molar weight of 58) and a n anhydroglucose unit (molar weight of 162). Therefore, the average number of hydroxypropyl substituents per anhydroglucose unit or MS is : (Total wt) - (Wt cellulose) - 404 -162 Wt of hydroxypropyl 58
=
4.2
(6)
This value compares favorably with a value of 4.2 f 0.2 from the independent “direct ratio” method discussed earlier. DS VALUE FROM Y-AXIS. From an extension of the horizontal line in the titration plot in Figure 5 , one finds a ratio of peak No. 4/No. 1 of 1.53, which is a measure of the relative amount of terminal methyls and internal methyls, or END/IN
=
1.53
(7)
This together with a M S of 4.2 found earlier, or END
+ IN = 4.2
(8)
gives a value of 2.6 for END. This is the average number of terminal methyls per anhydroglucose unit and therefore is the average number of hydroxyl groups substituted, the DS. Because of the use of line intensity rather than the more accurate integrated area, and the uncertainty involved in measurements, the precision of DS is estimated at 5-10z. The accuracy of the DS determination cannot a t present be tested, since no referee method is available, and samples of known DS cannot be prepared. However, the determination of M , in the polypropylene glycol system can be used as a test of the quantitative accuracy of the procedure for end group analysis. As described previously, good agreement with other techniques was obtained. The precision of the special titration procedure for the determination of M S is comparable t o that of the direct ratio procedure, so that the more complex titration procedure is preferable only when a measurement of DS is required. The results of MS of 4.2 =t0.2 and DS of 2.6 f 0.2 on a Klucel E sample are approximately depicted by the idealized structure shown in Structure I. ACKNOWLEDGMENT
The authors thank E. D. Klug for helpful discussions and C. A. Lewis for the C-methyl analysis.
RECEIVED for review June 1,1971. Accepted August 23,1971.
Correct ion Separation of Uranium from Seawater by Adsorbing Colloid Flotation I n this article by Y. S. Kim and Harry Zeitlin [ANAL. CHEM.43, 1390 (1970)], there is an error in the abstract. Line 5 should read, “At pH 5.7 f 0.1 , , . ”.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 1, JANUARY 1972
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