Solid-state C-13-nmr Investigation of the Disorder In Crystalline Syndiotactic Polypropylene

A quench-precipitated sample of highly syndiotactic polypropylene (s-PP) (with 86% fully syndiotactic pentads), which shows an X-ray spectrum near that calculated for an ''ideal'' orthorhombic C-centered structural model of packing as proposed by Corradini et al. (with chains in a helical (TTGG)(n) conformation), has been analyzed by high-resolution solid-state C-13-NMR spectroscopy. The C-13-NMR CP MAS spectra of this sample, in addition to the resonances already reported in the literature for s-PP (in the B-centered structure), exhibit additional peaks in the regions of the methyl (at 18.9 and 22.4 ppm) and methylene (at 44.9 ppm) groups. It is established that these additional resonances belong indeed to nuclei in well-defined conformational environments fixed in rigid structures placed at the interface of the crystallites and within the strained or disordered area of the crystallites themselves. In particular, the peak at 44.9 ppm in assigned to CH2 carbon atoms in a TX.YG conformational environment, whereas the resonance at 18.9 ppm is assigned to CH3 in trans-planar portions of the chain (gamma-gauche effect). This indicates local disorder in the C-centered crystalline s-PP sample, due to the presence of conformationally disordered chains. Models of s-PP chains comprising defective but energetically feasible portions of chains in a trans-planar conformation (disturbing the (TTGG), dominating conformation) are imagined as packed according to a C-pseudocentered structure. In order to keep the methyl groups in a crystalline register and the chains well interdigitated, the defects should be clustered on planes which cross 3D ordered portions in the crystals. The chemical shifts of the carbon atoms in the regions comprising the defect for model chains have been calculated on an ab initio level through the IGLO method, and the results are compared with the experiments. The calculations provide a good explanation of the peculiarities of the experimental C-13-NMR spectra of the s-PP sample under study. Thus, detailed information on chain defects can be obtained by combining X-ray scattering, solid-state MMR, and computer simulations.