No abstract

The chain-end structures were studied on the basis of 13C NMR and INEPT (insensitive nuclei enhanced by polarization transfer) spectra for polypropylene (PP) polymerized with S-TiClg/EtgAlCl catalytic system in the presence of molecular hydrogen. The chain-end structure I 10' 8' 6'4'

13C NMR chemical shift assignments of comonomer sequences in a 1-butene-propylene copolymer were obtained from the 13C two-dimensional INADEQUATE NMR experiment and from the calculated chemical shifts due to the y effect. By tracing carbon-carbon connectivities in the 2D-INADEQUATE spectrum, the validity of previous assignments of triad and tetrad sequences was confirmed. Referring to the confirmed assignments, the chemical shift differences among comonomer sequences longer than pentad were predicted by the chemical shift calculation (the y-effect method) based on the y effect of the 13C chemical shift and Mark's rotational isomeric state model modified by considering the side-chain conformation in a 1-butene unit. Assignments provided in this study agree well with Cheng's assignments by a reaction probability model. Further, the conformational probability of the side chain in a 1-butene unit was evaluated through the chemical shift calculation.

Two-dimensional (2D) heteronuclear lH-13C, homonuclear lH, and spin-lock relay NMR experiments have been used to provide 13C NMR chemical shift assignments of stereosequences in poly(viny1 chloride). However, relative areas of methylene tetrad peaks based on the assignments given by these 2D NMR experiments do not agree with the relative areas predicted by Bernoullian propagation statietica. Since the methods of the 2D experiments are correct, this is a puzzle. In thie study, tetrad and pentad assignments are proposed on the basis of the carbon-carbon connectivities revealed by the 2D-INADEQUATE (twodimensional incredible natural abundance double quantum transfer experiment) spectrum. The tetrad assignments given by 2D-INADEQUATE experiments show that there are two mmr peaks and that one of them overlaps the mrm peak. These assignments differ from the previous assignments and resolve the contradication in relative peak areas. The relative areas of methylene peaks based on the assignments from the 2D-INADEQUATE spectrum agree with those predicted by Bernoullian propagation statistics. Introductionl3C NMR is the most powerful method to analyze the sequence structures in vinyl polymers. Complicated split peaks arising from pentad or hexad sequences are usually observed in the high-resolution NMR spectra. Recently, two-dimensional (2D) NMR experiments have been developed and used as a reliable method of making assignments in high-resolution spectra.In the field of 13C NMR spectral analysis of poly(viny1 chloride) (PVC), 2D heteronuclear lH-l3C, homonuclear 'H, and spin-lock relay NMR experiments have been used to provide 13C NMR chemical shift assignments of stereosequences in PVC.1*2 Crowther et aL2 have demonstrated (from the spin-lock relay experiment) that the mrm and mmr tetrads are reversed from previous assignment in a similar solvent.314However, the authors found that the relative areas of methylene tetrad peaks based on the assignments given by these 2D experiments1P2 do not agree with the relative areas predicted by a Bernoullian propagation model (selection between m and r). The sample used for this

No abstract