Atactic polypropylene, spectra

Figure 7. Methyl region of 50.3 MHz C NMR spectrum of atactic polypropylene ran at 100°C.

To date, no Raman spectrum of syndiotactic polypropylene has been published although vibrational analyses have been issued by Schacht-schneider and Snyder and also by Peraldo and Cambini during 1965. Recently we have had the opportunity to examine polypropylene in three forms atactic, isotactic and syndiotactic. The results for the last specimen are included in Fig. 6. It will be seen that there is an enormous emission in the 1350—1400 cm-1 region. The nature of this is not known — it may be fluorescence but it cannot be checked, as the anti-Stokes band at v0+ 1350 cm-1 would be vanishingly weak due to the low Boltzmann population 1350 cm-1 above the ground state. A coordinate analysis is available for syndiotactic polypropylene and currently we are working on an assignment of the observed results. 

Fig. 4.17 illustrates the potential of carbon-13 NMR to detect the presence of isotactic (a), syndiotactic (b), and atatactic (c) vinyl polymers with polypropylene as sample [521], The spectrum of atactic polypropylene (Fig. 4.17(c)) displays the signals of all possible stereosequences including iso- and syndiotactic ones. Using the empirical increment systems for alkane carbon shift prediction [85, 201, 202] and including y effects of Zy = — 5 ppm specifically obtained by analysis of stereoisomeric polypropylene partial sequences between 3,5-dimethylheptane and 3,5,7,9,11,13,15-heptamethylheptadecane as a heptad model, the methyl carbon-13 shifts of all 36 possible heptads can be calculated... 

Fig.4.17. Proton broadband-decoupled l3C NMR spectra of polypropylene ((a-c) 25 MHz 200 mg/mL 1,2,4-trichlorobenzene at 140 JC (d-e) 90.52 MHz 200 mg/mL heptane at 67 X) (a) isotactic (b) syndiotactic (c) atactic sample (d) methyl carbon spectrum, simulated for calculated carbon shifts and Lorentzian signals of < 0.1 Hz width at half-height (e) experimental spectrum [521]. Numbers in (d) refer to the 36 possible heptads ...

Fig. 7.28. 2D CPMAS NMR spectrum of atactic polypropylene acquired with mixing times of (a) 5 and (b) 500 ms [38].

For atactic polypropylene (a-PP), studies still continue, because it is difficult to interpret the CP/MAS NMR spectrum of a-PP, which is more broad that of isotactic or syndiotactic polypropylene. In Fig. 11.12, the... 

Hgure 6 The 90 MHz methyl spectrum of atactic polypropylene in 1,2,4-trichlorobenzene at 100°. Empirical shift predictions for different stereoisomers reflecting the meso (m) or racemic (r) relative orientation of neighboring methyl groups. (Reprinted with permissbn from Schilling FC and Tonelli AE (1980) Carbon-13 nuclear magnetic resonance of atactic polypropylene. Macromolecules 13 270 American Chemical Society.)... 

The 220-MHz H NMR spectra of three polypropylene (PP) samples are presented in Fig. 20.8 [5-7]. Note the apparent greater resolution of the spectra in (a) and (c) recorded for the stereoregular samples (isotactic and syndiotactic) than for atactic PP in (b). The impression of degraded resolution in the spectrum for atactic PP is a consequence of the overlapping of many slightly different resonance frequencies or chemical shifts corresponding to the various triad and tetrad stereosequences present in the atactic sample. Only the rr (rrr) and mm (mmm) triads (tetrads) are present in the stereoregular syndiotactic and isotactic samples, respectively. 

Figure 6 NMR spectrum of atactic polypropylene assignment of the pentad sequences of the methyl group signal.

As another example, see the WAXD pattern for a monoclinic isotactic polypropylene (PP) in Figure 2. In this case, the amorphous halo of the semiciys-talline material was obtained by scaling the diffraction pattern of noncrystalhne atactic PP to obtain the best fit to the experimental spectrum. The authors of Reference 2 demonstrated that the WAXD pattern of isotactic PP at a temperature (T) greater than the melting point (T. 180°C in this case) was the same as that from atactic PP at the same temperature. The entire diffracted intensity from the crystalline reflections (/c) was determined, and the crystallinity determined from... 

FIGURE 1.2 The ten possible combinations of five adjacent stereocenters (pentads) in polypropylene and the NMR spectrum of the methyl region of atactic polypropylene. 

FIGURE 2.3 A pentad consists of five repeat units. The NMR chemical shift of a given methyl carbon is sensitive to the relative stereochemistry of the pentad (m = meso and r = racemo). A statistical distribution of pentads is observed in the H NMR spectrum of the methyl region of atactic polypropylene. 

Commercially available polypropylene, in the form of pellets, films, and fibers, exists as isotactic polypropylene, and this is produced by well-controlled stereoregular head-to-tail addition polymerization reaction with Ziegler-Natta-type catalysts. Formed in this manner, isotactic polypropylene is a crystalline polymer. Commercial samples typically contain small amount of atactic and/or syndiotactic polypropylene. Furthermore, blocks with different stereoregularities are also observed. The reference spectra (Reference Spectrum 2) in the Appendix provide the IR and Raman spectra of isotactic polypropylene. 

The infrared band at 875 cm of the syndiotactic polypropylene is used to estimate crystallinity. These three crystallinity sensitive bands (1167, 998, and 875 cm ) do not appear in the IR spectrum of atactic polypropylene. If a sample of polypropylene is drawn to one direction—that is, it is uniaxially oriented—some of the absorption band intensities become dependent on the angle between the drawn axis and the angle of polarization of the infrared radiation. 

Figure 4 A 50.3 MHz NMR Spectrum of the Methyl Region of an Atactic Polypropylene at 125°C in 1,2,4-Trichlorobenzene (Sample courtesy of Walter Kaminsky, Univ. of Hamburg, W.Germany)...

Figure 4.11 Raman spectra of polypropylene particles obtained in situ on three different alumina-supported catalysts by using the Raman microscope (488 nm excitation). The microscope allows fluorescence from he alumina support to be avoided. (Interestingly, these samples fluoresced worse when using the FT-Raman system than with the visible laser system, a result found to be quite common with catalyst samples.) The variation in polymer tacticity is evident in the three spectra the very weak, diffuse upper spectrum is characteristic of atactic material the strong, sharp lower spectrum indicates highly isotactic polypropylene and the middle spectrum indicates...

Fortunately, the IGLO method provides the full chemical shift tensor, and not just the isotropic value. The contour plot in Fig. 28.b shows an isotropic/anisotropic separation spectrum of atactic polypropylene, which has been recorded at T 250 K with a home-built DAS probehead [98] on a BRUKER MSL-300 spectrometer. For every isotropic shift on the x-axis, the intensity distribution along the y-axis displays the corresponding powder pattern the edges correspond to Oxx and Ozz as indicated in the figure. The theoretical values of the xx- and the zz-component are superimposed on the experimental contour plot in Fig. 6 and symbolized by the small rhombi (o). The absolute values were adjusted in the same way as for the isotropic chemical shift. The spectral position of Gyy is not easily seen in the experimental spectrum to keep the figure simple, the theoretical values for Gyy have been... 

If polymers are examined spectroscopically without removing additives such as fillers, plasticisers, stabilisers, lubricants, etc. then their infrared spectra may be affected drastically by the presence of these substances. Also, if care has not been taken during the preparation of a sample, bands due to contaminants such as water, silicate, phthalates, polypropylene (from laboratory ware), etc, may appear in the spectra and so result in some confusion. Hence, the flowcharts given below should be used with some degree of caution. In order to confirm an assignment made by use of the flow chart, it is important finally to make use of known infrared reference spectra. However, it should be borne in mind that stereoregular polymers may have spectra which differ from their atactic form and that sample preparative techniques may also affect the spectrum obtained for a particular polymeric sample. 

Figure 3.6. Simulated NMR spectrum (at 100.6 MHz and with protons decoupling) in the range of CH3 resonance. This spectrum points out the different pentads of an atactic polypropylene.

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