Full Assignment of the ¹³C NMR Spectra of Regioregular Polypropylenes: Methyl and Methylene Region

Abstract

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.