Polypropylene chemical shift calculation

Asakura, T. Nakayama, N. Demura, M. Asano, A. Carbon-13 NMR spectral assignments of regioirregular polypropylene determined from two-dimensional INADEQUATE spectra and chemical shift calculations. Macromolecules 1992, 25, 4876-4881. 

In liquid state NMR, other than the general reviews noted above, reviews have appeared on chemical shift calculations (32) and on computer-assisted approaches (33). In addition, the NMR of polypropylene (34) and dienes and polyenes (35) has been specifically reviewed. 

Zambelli et al. have studied the effect of incorporating small amounts of ethylene on the stereochemistry of polypropylene prepared with stereoregular catalysts, concluding that for an isotactic catalyst, insertion of ethylene has no effect on the propylene stereochemistry, whereas for a syndiotactic catalyst, the stereochemistry alters. For the isotactic case therefore, the stereochemistry is controlled by the catalyst, a conclusion also reached by Sanders and Komoroski. However Tonelli has questioned the deduction of Zambelli et al., on the basis that chemical shift calculations suggest that the chemical shift of the central ethylene unit in a PPEPP sequence used by Zambelli et al. is in fact independent of the adjacent propylene stereochemistry, and cannot give information on the mechanism. 

The configurational structure (stereoregularity) of 1-butene and of the higher polyolefins up to 1-nonene has been studied by NMR spectroscopy in solution [38, 39], interpreted with the aid of chemical shift calculations, consideration of the y effect and of the rotational isomeric state model of Flory. The evaluation of the results favors the bicatalytic sites model of polymerization [40] over simple Markovian statistics. In contrast to polypropylene, side-chain conformation also has to be considered. Comparison with alkane model compounds indicates that in meso-units of poly-1-butene, trans conformation of backbone is less favored than in isotactic polypropylene because of contiguous ethyl group interactions. Introduction of racemic units in both... 

The chemical shift calculation (y-effect method) based on the y-effect of the chemical shift and the rotational isomeric state model (RIS model) has been developed as a reliable method for predicting chemical shift differences among pentad, hexad, and heptad sequences in various polyolefins [47-49, 14, 50, 51]. chemical shift assignments of tactic pentad and heptad sequences in polypropylene have been provided by this method [47-49]. Hayashi and co-workers [45,46] confirmed that the chemical shift due to the y-effect is also sensitive to different comonomer sequences in ethylene-propylene copolymers. Asakura and co-workers [52] have demonstrated that... 

In Section lU-B we showed that the chemical shift of methyl carbons in polypropylene can be calculated considering the relative position of the adjacent substituents, expressed in configurational terms (m or r, Oor 1). An alternative interpretation, with a better physical basis, takes into account only conformational aspects. Experimental evidence from hydrocarbon spectra has established that the presence of a gauche conformation between carbon atoms separated by three bonds causes an upfield shift(7 effect) (206). For flexible molecules the magnitude of this shift depends on the fraction of conformers having gauche interactions with the observed carbons (207). 

From Mark s RIS model for ethylene-propylene copolymers (J. Chem. Phys. 1972, 57, 2541) it is determined that P(t) = 0.380, P g+) = 0.014, and Pig") = 0.606 in 2,4-dimethylhexane (2,4-DMH). Using this RIS model, furthermore, for all the branched alkanes considered whose isopropyl groups are separated by at least one methylene carbon from the next substituted carbon and the RIS model developed by Asakura et at. (Makromol. Chem, 1976, 177, 1493) for head-to-head polypropylene to treat 2,3-dimethyl pentane, AS s are calculated for a large number of branched alkanes. The agreement between the observed and the calculated nonequivalent 13C NMR chemical shifts is quite good, including the prediction that separation of the isopropyl group from the next substituted carbon by four or more methylene carbons removes the nonequivalence. 

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