The stereosequence distribution of the “atactic” and “isotactic” fractions of a polypropylene sample made with a MgCl2-supported catalyst was determined by means of high-resolution 13C NMR and analyzed in terms of statistical models of increasing sophistication. Two-site models, including the one normally used for the interpretation of “routine” 13C NMR data at pentad level, were shown to be inconsistent with the much finer high-resolution data. A good agreement between experimental and calculated distributions could be obtained only in terms of a three-site model, describing each fraction as a mixture of highly isotactic, weakly isotactic (“isotactoid”) and syndiotactic sequences. According to such model, the two fractions comprise the same three building blocks (the configuration of the three different types of stereosequences being almost invariant) and differ merely in their relative amounts (which indicates a stereoblock nature). The correlations with the physical properties of the materials and the implications on the nature of the catalytic species are also briefly discussed.

Possible structures of TiCl4 molecules and TiCl3 fragments adsorbed on ( 110) and ( 100) faces of MgCl2, simulated by clusters of different size and shape, have been studied in the framework of density functional theory. For both monomeric TiCl4 and TiCl3, coordination on the (110) face is favored relative to coordination on the (100) face. TiCl3 fragments can bind together on the (100) face either forming or not Ti-Ti bonds, resulting in the formation of polynuclear TinCl3n species. The steric environment of the ending Ti atoms of such polynuclear species, with n > 2, is extremely similar to that of the C2 symmetric sites proposed several years ago for TiCl3-based catalytic systems and presents a strict analogy with the well-established models for isospecific polymerization with catalytic systems based on C2 symmetric metallocenes. This analogy holds also in the case of a TiCl3 fragment adsorbed on the (110) cut when both of its vicinal positions are occupied. When just one of the vicinal positions is occupied, sites of C1 symmetry can be formed, which have two minimum-energy structures with distinct positions (inward and outward) for the dangling chlorine. These inward and outward geometries can be expected to interconvert easily, the inward arrangement being favored.

By partitioning the bond current strength (current susceptibility) into plane symmetric and plane antisymmetric contributions, it is shown that 91% of the diatropic ring current of benzene is transported by the π electrons and the remaining non-negligible 9% is sustained by the σ electrons. In planar cyclooctatetraene 94% (6%) of the paratropic ring current is transported by the π (σ) electrons. In cyclopropane 95% (5%) of the diatropic ring current is transported by the σ (π-like) electrons. The 85% fraction of the diatropic ring current of Al4(2-) is transported by the σ valence electrons and 15% by the π valence electrons. In the nonaromatic borazine system the nitrogen-centered π electron circulations are surrounded by a weak diatropic "ring current" 6.5 times smaller than that of benzene.

13C NMR spectroscopy is the main source of information on the stereochemistry of Ziegler−Natta and related transition metal catalyzed propene polymerizations. In simple cases, like those of polypropylenes formed under pure enantiomorphic-site or chain-end control, the origin of the stereoselectivity can be easily recognized from the steric pentad distribution obtained from routine 13C NMR spectra. On the other hand, the variety of innovative polymers that can now be prepared with “high-yield” heterogeneous and metallocene-based homogeneous catalysts under hybrid, multiple, or oscillating stereocontrol represent very complex systems, which are beyond the possibilities of configurational analysis by routine 13C NMR. In such cases, high-field 13C NMR can be highly advantageous. Indeed, in this paper we show that from the methyl and methylene regions of 150 MHz 13C NMR spectra of polypropylenes of various tacticities, the stereosequence distribution can be determined at a much finer level of detail, so as to obtain an adequate experimental basis for the investigation of the many complicated mechanisms of stereocontrol presently encountered in Ziegler−Natta catalysis.