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Line Width of Nuclear Magnetic Resonance High Resolution Spectra of Vinyl Polymers

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Line Width of Nuclear Magnetic Resonance High Resolution Spectra of Vinyl Polymers VoI. I, No. I, Junuary-Februury I968 NhlR SPECTRA OF VINYL POLYMERS 93 are nine distinct branching possibilities (A, B, or C constants would be required for complete description branching from A,, B, or C), as many as nine additional of the system. kinetic constants would be required to specify the B. Two major simplifications were made in the system, not considering whatever constants are neces- derivation of eq 15-20. The first was that the effect of sary to describe the branching mechanism. penultimate and ante-penultimate units was negligible, 2. Nearest Neighbor Effects. In the model, the sta- the second assumed constant feed ratios of monomers bility of any link was assumed to be independent of the (fi). If the first assumption is invalid, alternate treat- presence and type of neighboring links. If, however, ments are available.18 If the second is invalid (as, for the effect of a given linkage extends beyond this linkage, example, in a batch or semibatch process), then eq then additional (constants are necessary. For a ter- 15-20 may be integrated numerically to give appropriate polymer, in the case of an A-A linkage, for example, values ofJi. the following scission constants need to be included: (18) E. Merz, T. Alfrey, and G. Goldfinger, J. Polymer Sci., 1, ka~~a,kaaab, k,,,,, kbaab, kbaac, k,,,,. A total of 36 75 (1946). Line Width of Nuclear Magnetic Resonance High Resolution Spectra of Vinyl Polymers A. L. Segre Centro Nazionale di Chiniica delle Macromolerole del C.N.R., Sezione I .Milan, Italy. Receiced Nofiember 21, 1967 ABSTRACT: The line widths of high resolution nmr spectra of vinyl polymers were studied with particular refer- ence to isotactic polypropylene. It was observed, by deuterium decoupling experiments, that for polypropylene it is the coupling constants through several bonds that mainly determine the broadening of the lines. By eliminat- ing them, we obtained spectra with resonance line widths of the same order of magnitude as observed on small organic molecules. et us consider a nondeuterated vinyl polymer in flow on the test tube walls. If a polymer is not steri- L solution. The nmr spectrum of this solution cally pure, its spectrum consists of the superimposition generally does not exhibit well-resolved lines. In the of the spectra of various component^,^ owing to the presence of suitable solvents, the widths of the reso- presence of different types of tetrads and pentads that nance lines can be decreased by increasing the tem- may give partly superimposed bands; thus an artificial perature and decreasing the concentration. However, broadening of the lines is created, due only to the even in the best circumstances, at high temperature, poor resolution of spectrometers. To exclude this the width of the resonance lines of a polymer ranges difficulty, we mainly used isotactic polypropylene be- from a minimum of 1 to several cycles per second. The cause this polymer can be obtained sterically p~re.~,b purpose of this research is to establish why the bands Isotactic Polypropylene. Products A-E have been undergo this residual broadening. studied. All isotactic polymers considered were high A resonance signal may be broadened as a result of several reasons: I, insufficient average of magnetic dipole-dipole interactions in the solutions, due to relatively slow motion of the chains;' 11, excessive viscosity of the solutions;' 111, spectra that are not completely of the first order, with consequent broaden- ing of the experimental bands, which are actually multiple; IV, small long-range coupling constants ; and V, not sufficiently rapid exchange among the various conformers so that the same proton may be C D situated in magnetically nonequivalent surroundings. It was shown that the line width of a polymer in solution is independent of the molecular weight of the so the viscosity of the solution will not be considered here. Nevertheless, we operated under experimental conditions such that the solution might (4) F. A. Bovey, E. W. Anderson, D. C. Douglass, and J. A. (1) F. A. Bovey, G. V. D. Tiers, and G. Filipovich, J. Polymer Manson, J. Chem. Phys., 39, 1199 (1963). Sci., 38, 73 (1959). (5) A. Zambelli, A. L. Segre, M. Farina, and G. Natta, (2) P. L. Corio, Chem. Rec., 60, 363 (1960). Makromol. Chem., in press. (3) A. Odajirna, .I. Phjs. soc. Japan, 14, 777 (1959). (6) A. Zambelli, private communication. 94 A. L. SEGKE Mucromolecules m the bands from less than 0.5 (small organic molecules) m to 1 cps may be only due to I, IV, or V. The spectra 0 2 10cps of polymers C and D consist of singlets having width 9 t- higher than 3 cps. However, under conditions of D decoupling, the width of these singlets is <0.6 cps f (Figure 2). Considering that in polymers C and D only cause IV of line broadening has been completely excluded, if line broadening were due to I or V, this 0 broadening should be still present. The fact that this u' 0, 2 does not occur, except to a very small extent, indicates h6 that long-range coupling is the predominant cause of line broadening in polypropylene. The extreme sharpness of the backbone protons in high molecular weight isotactic polypropylene, obtained under these conditions, and with this type of deuterated polymers, shows that all the motions are fast compared with the nmr observation time. This sharpness of peaks raises the signal-to-noise ratio to such an extent that we I I. observed the C13 side bands for polymer D. The = 124.4 =k 0.5 cps is quite in agreement with the data obtained on ethane (126 cps) and cyclohexane (123 cps).* Moreover, the observation of this band, which is 0.56% of the main signal, reveals the limit of J sensitivity to impurities present and indicates that the steric purity of isotactic polypropylene is higher than 9937.9 in CPS dai HHDS After these results, which show that the broadening Figure 1. Spectrum of isotactic polypropylene-2,3,3,3-d4 of resonance signals in isotactic polypropylene is sample E. mainly due to the presence of long-range coupling constants, we studied other polymers in order to check molecular weight heptane residues, whose intrinsic vis- whether this conclusion might be extended. cosity, measured in tetralin at 135", ranged between 3 Syndiotactic Polypropylene. lo We ran the spectra and 5 (100 cm3/g). The widths of the lines forming of syndiotactic polypropylene under the same experi- the spectrum of polymer A are, respectively, CH3 mental conditions as for the isotactic ones. The D 1.1 cps, Ha =2 cps, Hb e2cps, and H, >3 cps. The decoupled spectrum of coupling constants of this polymer are lJABl = 13.5, JAC1. 7 cps, JBCN 6 cps, and JHC-CH~= 6.5 cps. The spectrum of polymer B consists of a singlet having width greater than 2 cps. This broadening may be easily explained by considering the vicinal coupling shows a singlet having width <0.6 cps and other small constant JCD-C"~. bands originating from steric impurities. This result YD confirms that, even in syndiotactic polypropylene, all Jcu-CH, = ~ JCH-GII~ = (0.154)(6.5) = 1 C~S YH motions are fast in comparison with the nmr observa- tion time. This behavior is completely different from Deuterium was then decoupled. The D-decoupled what was observed on polymethyl methacrylate, singlet has a band width less than 0.5 cps; this value but since in this polymer line widths are very broad, may be fully compared with those found in small we think that the results may be compared only on organic molecules. Cause V, therefore, does not seem suitably deuterated samples. to be present. This example, however, does not dis- Polyvinyl Chloride. We ran D-decoupled spectra of tinguish between the causes of broadening I, 111, IV. a high molecular weight polyvinyl chloride-/3,/3-d2 in In fact, from wide nmr line studies,' we know that the pentachloroethane at 150" (Figure 3). The spectrum methyl group, at the spectrum-running temperature, shows that the three syndiotactic pentads are un- rotates far faster than the nmr observation time. resolved; the four heterotactic pentads are resolved Therefore it does not supply information on I. More- over, the broadening of the methyl doublet of sample A (8) J. W. Emsley, J. Feeney, and L. H. Sutclife, "High Resolu- can be easily understood by 111. tion N.M.R. Spectroscopy," Vol 11, Pergamon Press Ltd., Lon- don, 1966, p 1012. Let us now consider the spectrum of polymer E; (9) We note in addition that we succeeded in detecting the iso- it is a spectrum of the AB type which, decoupled from topic impurities of the methyl group, which can be estimated to deuterium (Figure l), shows band widths of the order be 1.5 %, since the isotopic purity of the monomer was evaluated as 99.5% on the whole by mass spectrometry. Now if we sup- of 1 cps. This polymer supplies us with precise in- pose a random distribution of the isotopic impurities, 1.5% of formation, since cause I11 cannot be present in a the methyl group will contain one proton. spectrum of the AB type. Therefore the broadening of (IO) A. Zambelli, G. Natta, and I. Pasquon, Pol),mer S~~mp., Part C, 4, 41 1 (1963). (I I) S. Browiistein and D. M. Wiles, Cu/i.J. Chem., 44, 153 (7) J. G. Powles, Poij.rrier, 1, 219 (1960). (1966) Vol. I, No. 1, Januury-Februury 1968 NMRSPECTRA OF VINYLPOLYMERS 95 P n 10 cps H 10- cps w in CPS dal HNDS 11ill 'I Figure 2.
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