Chemical shift polymer structure determination

A phenomenological study was performed to determine the effect of solvent on Sn NMR spectra of these organoraetallic polymers. Samples were dissolved in chloroform, benzene, n-hexane, acetone, tetrahydrofuran, methanol, and pyridine. The Sn NMR spectra in these solvents are given in Figure 1. The appearance and location of the H Sn resonance changes drastically over the range of selected solvents. The chemical shift moves upfield in the order chloroform, benzene, n-hexane, acetone, tetrahydrofuran, pyridine, and methanol. The amount of structural information and, conversely, the broadening of the resonance increases in the same order with methanol and pyridine reversed. 

In donating solvents the subtle effects determining the chemical shift in chloroform, benzene, and hexane are apparently masked. In hexane, which is considered a poor solvent, self-association is possible and would explain the appearance of the Sn spectrum. Chloroform and benzene are excellent solvents for organometallic polymers, and the structure and downfield position support a well-solvated, unassociated environment. 

In the solid, dynamics occurring within the kHz frequency scale can be examined by line-shape analysis of 2H or 13C (or 15N) NMR spectra by respective quadrupolar and CSA interactions, isotropic peaks16,59-62 or dipolar couplings based on dipolar chemical shift correlation experiments.63-65 In the former, tyrosine or phenylalanine dynamics of Leu-enkephalin are examined at frequencies of 103-104 Hz by 2H NMR of deuterated samples and at 1.3 x 102 Hz by 13C CPMAS, respectively.60-62 In the latter, dipolar interactions between the 1H-1H and 1H-13C (or 3H-15N) pairs are determined by a 2D-MAS SLF technique such as wide-line separation (WISE)63 and dipolar chemical shift separation (DIP-SHIFT)64,65 or Lee-Goldburg CP (LGCP) NMR,66 respectively. In the WISE experiment, the XH wide-line spectrum of the blend polymers consists of a rather featureless superposition of components with different dipolar widths which can be separated in the second frequency dimension and related to structural units according to their 13C chemical shifts.63... 

The polymer obtained from 9 by y-radiation was soluble in chloroform despite a high crystallinity. The alternating molecular stacking of 9 led to stereoregular polymer formation with a disyndiotactic structure. The racemo and meso structures of the resulting polymers were confirmed by NMR spectroscopy. A comparison of the NMR data of related polymers concludes that the chemical shifts for a series of the polymers are predominantly determined by the meso-racemo structure rather than the diisotactic-disyndiotactic one. 

Empirically determined chemical shift additivity parameters have been determined- for diene-type polymers. The shift contribution of a quaternary carbon which is fllto the carbon in question was not determined by those authors. However, using their additivity parameters and the shift positions of the ( carbons in Figure 8, a value of +15.4 ppm can be estimated for the contribution of a neighboring (0() quaternary carbon. Using this value, the shift positions of the carbons in structures VII and VIII are calculated as shown. If the first 1,2 unit were on the chain in a 2,1 manner, the methylene carbon resonance would be at a considerably higher field, but it would be difficult to estimate its position with any certainty because the quaternary effect... 

C. A. Stortz and A. S. Cerezo, The 13C NMR spectroscopy of carrageenans Calculation of chemical shifts and computer-aided structural determination, Carbohydr. Polym., 18 (1992) 237-242. 

Cl -Pf-C=C-Y-C=C]-n-Pf-Q can be replaced by other functional groups by a substitution reaction shown in Eqs. 18 and 19. The polymer structure is confirmed by 31P-NMR based on the chemical shift of phosphorus on the terminal platinum bearing an end group different from that of the internal groups and the peak ratio is used to determine the degree of polymerization (Table 4)40). 

Of considerable importance in the determination of polymer composition and structure is nuclear magnetic resonance (NMR) spectroscopy. A large number of literature reports are available regarding the application of this technique in the study of polymers (see e g. [5]). This technique allows the identification of various structural units in polymers based on the chemical shift and spin-spin coupling either in proton NMR spectra or... 

P NMR spectra detected a hydrogen-bonded phosphate polymer in calcium phosphate composites.40 19F MAS-NMR spectra were used to determine isotropic 19F chemical shifts in various environments in CaF2-AlF3 and BaF2 A1F3 systems.41 137Ba NMR data were used to probe the structure of a new... 

Most polymer characterization is accomplished with H and NMR although N, F, Si, and NMR are also routinely used. H NMR is most commonly used because it is present in most chemical structures, has 100% natural abundance and a high gyromagnetic ratio, making it the most sensitive of all the NMR observable nuclei to detect. Moreover, because of its fast relaxation and the ability to extract information from spin-spin coupling constants, H NMR is one of the most useful techniques for determining chemical structure. However, because of its small chemical shift range (only 10 ppm) and the complexity of each proton resonance, severe peak overlap occurs for H atoms situated in similar chemical environments. 

The estimation of chemical shifts by examining the spectra of model compounds is not always feasible, and the prediction models fail to distinguish between two or more stereosequences as they cannot always be distinguished on the basis of intensity alone. To overcome these limitations, large numbers of organic compounds have been analyzed by NMR and their chemical shifts have been used to determine a set of empirical correlations that are used to determine the structure based on the polymer s NMR spectrum. The carbon chemical shifts of hydrocarbon-based polymers such as polyethylenes can be predicted by empirical additivity rules such as ... 

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