Pentad resonance

Although the aromatic C-l carbon resonance patterns observed for partially epimerized polystyrenes are readily interpreted using the above considerations, this is not the case for the pattern observed for polystyrene (or for the completely equilibrated polymer). The six-area pattern that is so clearly evident in the spectra of the partially epimerized polymers is not evident in the spectrum of polystyrene. It seems that resonances of heptads or nonads with high r-contents have chemical shifts that correspond to valleys observed in the spectra of the partially epimerized polymers. This causes the demarcation between pentad resonance patterns to become obscure when the r-content is about 0.5. This complication should disappear as the r-content increases from 0.5 and resonances due to heptads or nonads with high m-contents diminish in concentration. Unfortunately the unavailability of polystyrenes with high r-contents at the present time prevents this possibility from being pursued. 

Matsuzaki and coworkers [71] assign both mm and (mr+rm) centered pentads to them and conclude that polystyrene is highly syndiotactic. On the other hand, theoretical calculations by Fujiwara and Flory [72] suggest that the mm centered methine proton pentad resonances should occur considerably downfield from the (mr+rm) and rr centered pentads. On this basis the three lowest field... 

The methine proton resonance pattern of poly(vinyl thiophene) is the same as that observed for polystyrene. The lowest field resonance component amounts to 25 percent of the total methine proton resonance and is consistent with poly(vinyl thiophene) being atactic if this lower field component can be assigned to mm triads. Finally, the C-1 carbon resonance spectrum of poly-(vinyl thiophene) (Figure 11) seems to consist of triad resonances, with the lower field area being further subdivided into pentad resonances. This spectrum also suggests that poly(vinyl thiophene) is atactic. If we can conclude on the basis of the results presented here that poly(vinyl thiophene) is indeed atactic, then it seems reasonable to expect the same thing to be true for polystyrene, since the benzene and thiophene rings have very similar steric requirements and similar chemical behavior. 

The various structural features outlined above can give rise to a large number of resonance lines in the nmr spectrum of a stereoregular copolymer. It is not a simple matter to make assignments for these resonances since there are ten possible tetrads and twenty possible pentads, and since the various tetrad or pentad resonances are usually overlapped to a considerable extent. The case can be even more complicated if all of the nuclei in a given environment do not have the same chemical shift, as for example, the methylene protons in isotactic copolymers. 

Fig. 7.14. The 150-MHz C-NMR spectra are shown of the methyl region of four model polypropylene samples A-D dissolved in l,l,2,2-tetrachloroethane-d2 at 70°C. Polymer A was obtained in the presence of the catalyst system bis(cyclopentadienyl)TiCl2/methylalumoxane at low temperature (m 0.80). Polymer B ws prepared with the catalyst system mc-ethylene-bis(4,5,6J-tetrahydro-l-indenyl)ZrCl2 (m 0.70). Polymer C was prepared in the presence of (methyl)2C(cyclopentadienyl)(9-fluorenyl)ZrCl2 (r 0.80). Polymer D is a hemiisotactic polypropylene obtained from the catalyst system (methyl)2C(cyclopentadienyl) (9-fluor-enyl)ZrCl2/methylalumoxane. The Roman numerals at the bottom of the spectra represent the regions of the pentad resonances and the numbers on the peaks are for the heptad resonances. (Source Ref. [20, fig. I].)...

The nmr spectmm of PVAc iu carbon tetrachloride solution at 110°C shows absorptions at 4.86 5 (pentad) of the methine proton 1.78 5 (triad) of the methylene group and 1.98 5, 1.96 5, and 1.94 5, which are the resonances of the acetate methyls iu isotactic, heterotactic, and syndiotactic triads, respectively. Poly(vinyl acetate) produced by normal free-radical polymerization is completely atactic and noncrystalline. The nmr spectra of ethylene vinyl acetate copolymers have also been obtained (33). The ir spectra of the copolymers of vinyl acetate differ from that of the homopolymer depending on the identity of the comonomers and their proportion. 

Statistical analysis is important and relatively easy. Suppose we have a fully atactic polymer which we analyse for the triad content for isotactic polymer. Only three methyl resonances due to triads are observed, and the statistical ratio of mm, rr, and mr is 1 1 2. Thus even in the atactic polymer our isotactic content is 25% Pentad analysis, however, would give only 8% mmmm isotactic content Especially for catalysts with low enantiospecificity it is worthwhile keeping this in mind. 

The NMR chemical shift of a given CH3 group is influenced by its neighbors. Most modem NMR instruments will allow resolution at the pentad level, i. e. the resonance of a methyl group is determined by the orientation of the two monomer... 

The study of the stereoregularity of the polymers prepared, provides also Information about the stereoregulating mechanism. The probability of formation of the different types of sequences, was determined on the basis of the resonance of the quaternary carbon of pVP (12). The NMR spectrum performed at 15 MHz allows one to determine the concentration of triads. The values summarized In Table 4 do not agree with those expected for bernoullllan statistics. Hence, more than the last unit of the living chain Is Involved In the process. In order to obtain more precise Information about the process, It is necessary to measure the probability of formation of pentads. Such measurements are possible with spectra performed at 63 MHz (Figure 18). In spite... 

C NMR spectra are recorded for a low molecular weight atactic PP dissolved in a variety of solvents over a broad temperature range [293 - 393 K). Comparison of chemical shifts calculated via the y effect method with the observed resonances, whose relative chemical shifts are solvent independent, permits their assignment to most of the methyl heptad, methylene hexad, and methine pentad stereosequences. Agreement between observed and calculated chemical shifts requires y effects, he., upfield chemical shifts produced by a gauche arrangement of carbon atoms separated by three bonds, of ca. - 5 ppm for the methyl and methine carbons and ca. - 4 ppm for the methylene carbons. 

C NMR chemical shifts are calculated for the carbon nuclei in PVA to the pentad and hexad levels of stereosequence for the methine and methylene carbons, respectively. The RIS model developed by Wolf and Surer (A 069) is employed to calculate the frequencies. The relative orders of the observed methine pentad and methylene hexad resonances agree with the calculated chemical shifts, in addition to the agreement between the overall chemical shift dispersions measured and predicted for the methylene carbons. 

Methane as the prototype of spherical tops was the subject of a great number of investigations, of which only some can be mentioned here. At high resolution the structure of rovibrational bands becomes very complicated due to tensorial splittings. Moreover Fermi and Coriolis resonances lead to interactions between fundamentals and overtone and combination bands of CH4 therefore theoretical models for the dyade 1/2 and 1 4 (Champion, 1977) and the pentade i/j, i/3, 2j> 2, 1 2 + and 21/4 (Lolck et al., 1982 and Poussigue et al., 1982) have been developed and sets of molecular con.stants were determined by adjustment to all available experimental high resolution IR and Raman... 

In a separate paper Harris and Kimber (30) examined mixed polymeric siloxanes of the type MDj.D. M where M = Me3SiOo.s, D = Me2Si(Oo.5)2, and D = HMeSi(Oo 5)2- The Si spectrum of these polymers shows three distinct areas of resonance, M at +9-71 to + 7 07 ppm, D from —18-81 to —22 03 ppm, and D from —35-19 to -37-35 ppm. The resolution at 2-35 T is sufficient to show triad and pentad fine structure at the D and D resonances, respectively, and chemical shift sensitivity at the M resonance to structural changes occurring up to six bonds away (Table X). 

PMPS, PMHS, PMDS, and PDHS. Individual resonances are observed for each carbon type, except for the dodecyl polymer in which C-4 to C-9 in the side chain are not completely resolved. The carbon resonances can also be assigned without the use of model compounds by using standard two-dimensional NMR techniques (2). C-1 and, in some cases, C-1, C-2, or both in the three asymmetrically substituted polymers show shift dispersion, which results from the many different stereochemical sequences along the polymer chain. The methyl carbon in PMPS is resolved to at least the pentad level of stereosequences. This chemical shift information can be analyzed to provide a description of the chain statistics resulting from a particular polymerization. 

Rinaldi et al. introduced the use of correlations for the characterization of microstructures in fluoropolymers by demonstrating that highly dispersed 2D- F, C HMQC and HMBC spectra allowed the identification of different triads and pentads in a 1-chloro-l-fiuoroethylene/isobutylene copolymer, and 3D- H, C, F experiments permitted assignment of the tacticity in polymeric l-chloro-l-fluoroethylene. The latter experiment is interesting because it exploits the unique chemical shift dispersion of F nuclei for the separation of resonances originating from different stereosequences, and then uses the cross-peak multiplicities in the slices... 

Quantitative measurement shows about 11% of the monomer units to be inverted. The principal spectrum shows splitting into mm, mr, and rr triad resonances with some pentad fine structure. The polymer is nearly atactic. Assignment of inversion "defect" resonances is made easier by reference to spectrum (b), which is that of poly (vinyl fluoride) prepared by the following route (17) ... 

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