The conformational structures of defect-containing chains in the crystalline regions of isotactic polypropylene

Calculations were performed to assign defect-resonance patterns observed in solid-state ¹³C NMR spectra obtained from the crystalline regions of isotactic polypropylene. The spectral features of interest are associated with stereo, regio, and comonomer-type defects, which can typically be found in metallocene-synthesized polymers. The calculations were carried out as follows: a model of the crystalline region of defect-free isotactic polypropylene was constructed from available X-ray data corresponding to the α-lattice. A series of irregularities including ethylene comonomer, stereo-mrrm, regio 2,1-erythro, and butylene-comonomer defects were introduced one at a time at various positions in a specific stem occupying a central position in the model crystallite. Low-lying conformations were then obtained from simulated annealing calculations that were initiated from these structures. Finally, quantum mechanical calculations were performed on the representative segments of the defect-containing chains excised from the annealed crystallites and the calculated chemical shifts were compared to the observed resonances. The results of the calculations were used as a basis for interpreting the NMR intensities of defect-related resonances in terms of the partitioning of defects and to help establish the conformational structures of the defect-containing stems.