sufficiently rapid analog to digital conversion for recovery of signals disappearing in less than 40 ws. The former may be avoided by the use of the receiver described herein, which is easily constructed from commercially available units a t a cost less than $700. The latter problem may be circumvented by the use of an appropriate transient recorder, a number of which are commercially available with sampling rates useful in the microsecond range of data accumulation. ACKNOWLEDGMENT
The authors are indebted to H: L. Retcofsky for supplying two of the coal samples used in the present analysis, and to C. R. Dybowski for conversations regarding anlysis of the data. The magnet and power supply were donated by Dow Chemical Company.
LITERATURE CITED L. B. Schreiber and R. W. Vaughan, J. Catal., 40, 226 (1975). (2) J. D. Ellet, M. G. Gibby, V. Haeberlen, L. M. Huber, M. Mehring, A. Pines, and J. S . Waugh, Adv. Magn. Res., 5 , 117 (1971). (3) R. W. Vaughan. D. D. Ellman, L. M. Stacy, W.-K. Rhim. and J. W. Lee, Rev.
(1)
Sci. Instrum.. 43, 1356 (1972).
(4)
D. J. Adduci, P. press,.
A.
( 5 ) VV.-K. Rhim, D. D.
Hornung. and D. R. Torgeson, Rev. Sci. Instrum. (in
Elleman, and R. W. Vaughan, J. Chem. Phys.,59, 3740
(1973). (6) B. C. Gerstein, Chee Chow, R. G. Pembleton, and P.I C. Wilson. J. Phys. Chem. (inpress). (7) D. E. Barnaal and I.J. Lowe, Phys. Rev. Lett., 11, 258 (1963). (8) A. Abragam, "The Principles of Nuclear Magnetic Resonance", Oxford Univ. Press, London and New York, 1961. Chap. 4. (9) C. R. Dybowski, personal communication.
RECEIVEDfor review November 12, 1975. Accepted September 24,1976. A portion of this work was sponsored by the Iowa Coal Project.
Compositional Analyses of Methyl Methacrylate-Methacryl ic Acid Copolymers by Carbon- 13 Nuclear Magnetic Resonance Spectrometry Duane E. Johnson,* James
R. Lyerla, Jr.,
Teruo T. Horikawa, and Lester A. Pederson
IBM Research Laboratory, San Jose, Calif. 95 193
The use of carbon-I3 Fourier transform nuclear magnetic resonance for quantitative analysis of the composition of methyl methacrylate-methacrylic acid copolymers has been Investigated. It was found that in pyridine solutions of these copolymers, the resonances arising from acid carboxyl and ester carbonyl carbons were Sufficiently resolved to allow the determination of relative Integrals. Compositlonal data obtained using 13C NMR compared favorably with those obtained by standard titration techniques.
In titrating copolymers of methyl methacrylate and methacrylic acid P(MMA/MAA), with standard base to determine composition, a number of deficiencies were encountered. For example, there were two common sources of "contamination'' that gave rise to underdetermination of the acid content: 1) the copolymers tended to be hygroscopic and hence, could contain ca. 5% absorbed moisture and 2) they could, on occasion, retain solvents and/or monomers. Additionally, the method was found to be inapplicable to P(MMA/MAA) of high molecular weight (MW Z 1 000 000) and/or high acid content (>6W?h acid) because of the tendency of such systems to reprecipitate during the titration procedure. Because of the nondestructive nature of the technique and the potentially routine character of analyses, proton nuclear magnetic resonance (IH NMR) appeared to be a viable alternative. By using the integral of the ester methoxy protons and combining this result with the total integral for CH2 and ( Y - C Hprotons ~ (the overlap between CH2 and a-CH3 resonances was enough at 100 MHz to prevent separate determination of these integrals), the copolymer composition could be ascertained; however, it was necessary to carry out the determinations at 100 "C or higher to attain resolution sufficient for reliable integrals. Although the temperature requirement was an unattractive feature of these analyses, the
central difficulty was that the reaction solvents (e.g., toluene and hexane) and comonomers had resonances that overlapped those of the CH2 and a-CH3 of the copolymers. Thus when solvents and/or monomers were retained, considerable inaccuracy was introduced in the total CH2/(r-CH3 integral. Because of the greater spectral dispersion and narrower resonance lines (1-3) obtained with carbon-13 NMR relative to proton NMR, problems associated with resonance overlap in lH spectra are often resolved. Thus, we examined the use of Fourier transform I3C NMR as a probe for the compositional analysis of P(MMA/MAA) and herein are reported the results. EXPERIMENTAL Synthesis of Poly(methy1 methacrylate-co-methacrylic Acid). The copolymers were synthesized by using well known free radical techniques. Reactions were carried out in pressure bottles which were oven dried, cooled under Nz, and all reagents were added under a Nz blanket. As one example, 100 ml of THF, redistilled from LiAlH4,14 g (0.14 M) MMA, passed through neutral A1203,12.2 g (0.14 M) MAA, vacuum redistilled, and 0.1 g tert-butyl peroxide were added to a bottle. The capped bottle was placed in a 70 "C oven over the weekend and the product recovered by pouring the viscous solution into excess n-hexane or diethyl ether. The product was washed with nonsolvent and vacuum dried a t 60 "C. The acid content was 48%. Other polymerizations were carried out in toluene, which had been redistilled from Na. Acid contents were varied by adjusting monomer ratios in the monomer feed stocks. The emulsion polymerizations were carried out by forming an emulsion with 20 g Triton X-200,100 g MMA, and 86 g MAA in about 1 1. of DI water. The reaction was initiated by adding 3.8 ml of an F e S 0 ~ 7 H 2 0solution (0.15 g of sulfate in 100 ml HzO), 1.7 g of (NH&Sz08 in 25 ml HzO, and 0.85 g NaHS03 in 10 ml HzO. The copolymer was precipitated by adding i-PA. Titration. Samples of the copolymers were analytically weighed into a beaker and dissolved in 50 ml ethanol, followed by the addition of 50 ml HzO. Sample weights were chosen to require a titer of 1.5 mequiv or more. The solution was titrated to a phenolphthalein end point (5 min) with standardized 0.15 N aqueous KOH. Carbon-13 NMR Analyses. Samples were prepared by dissolving ANALYTICAL CHEMISTRY, VOL. 49, NO. 1, JANUARY 1977
77
h
1I ‘I
1
b)
1I I
200
I
I
I
I
I
I
I
I
I
150
I
I
l
100
I
I
I
50
P(MMAIMPAh (75125) + TEA
il
TMS
I
I
I
/
I
I
Ester
I
0
182
180
178
176
PPM Downfield from TMS
Figure 1. The 20 MHz I3C NMR spectrum of P(MMA-co-MAA)-33% acid dissolved in (50/50)v/v pyridine/pyridine-d5. Assignment of the various resonance regions is given in the Figure. The I, H, S notation refers to isotactic, heterotactic, and syndiotactic stereochemical triads; A and E refer respectively to acid and ester
I UI
VI
H
I
-C-
-C-
-OCH3
H
I
H
I
s
wCH,
Figure 2. Scale expansions of the various resonance regions of the 13C spectrum of P(MMA-co-MAA)-33% acid (see Figure 1)
or swelling ca. 0.3 g of copolymer in 2g solvent-both components being weighed directly into a 10-mm NMR sample tube. A 50/50 mixture of pyridine and pyridine-db was used as the solvent and provided the deuterium internal lock for the spectrometer. Spectra were obtained on a Varian CFT-20 pulse Fourier transform spectrometer operating a t 18.7 kG (20.0-MHz 13Cfrequency). T h e pulse power delivered to the single coil 10-mm o.d. probe was sufficient to rotate the 13Cmagnetization by 90° in 15 pus. Probe temperature under the experimental conditions was 38 “C. Spectrometer parameters for the determination of spectra were: ca. 65O (10 ps) pulse, 4 kHz spectral width, 1-s data acquisition time, and 3.0-9 delay between repetitive pulses (Le., a total experiment recycle time of 4 s). Typically, 12 000-15 000 free induction decays (FID) were accumulated (i.e., an “overnight” run mode was employed) for each quantitative deter78
ANALYTICAL CHEMISTRY, VOL. 49, NO. 1, JANUARY 1977
PPM Downfield from TMS
Figure 3. The 20-MHz 13C NMR spectrum of P(MMA-co-MAA)-25 % acid dissolved in (a) pyridine and (b) pyridine plus triethylamine (TEA) added to 20% excess of acid units present. (Note there is a change in vertical scale between the two spectra)
mination. The accumulated FID was digitally-filtered with a time constant that produced a 1.7-Hz line-broadening. The 8K data table was then Fourier transformed to yield a 4-kHz spectrum defined by 4K real points (i.e., digital resolution ca. 1 Hz). Spin-lattice relaxation times (TI) for the carbonyl carbons were estimated from 180-t-90 pulse sequences ( 4 ) and found to be in the range 0.8-1.1 s. (It should be noted that the solutions were not degassed and that the presence of dissolved oxygen may influence the observed relaxation rate. If samples were degassed, the carbonyl relaxation could belonger in which case a longer delay between repetitive pulses would be required.) The imposition of a 3.0-s pulse delay and employment of a 65’ pulse ensured the recovery of the carbonyl magnetization during the experiment recycle time, thus eliminating possible errors in quantitation resulting from differential relaxation times ( 5 ) .Spectra were obtained under fully proton-decoupled conditions using a gated-decoupling scheme in which the proton decoupler was off during the 3.0-s delay time and gated on a t the start of the 1.0-s acquisition period. In this manner, problems associated with differential nuclear Overhauser enhancements ( 5 )(NOE) for different carbonyl carbons were avoided. (Recent arguments by Opella e t al. ( 6 )concerning the use of gated-decoupling to determine NOE values indicate that if significant differences existed between ester and acid carbonyl NOE values, a pulse delay greater than that required to negate the effects of differential relaxation on quantitation would be required. For the (MMA/MAA) copolymers, the NOE values of the carbonyl carbons are all ca. equal (1.7-1.9), thus the pulse delay of 3.0 s was sufficient for quantitation.) One method of checking that the spectrometer conditions were sufficient to yield quantitative data was to compare the total integral of the carbonyl region with those obtained for OI-CHQ, >CC
