Abstract
13C nuclear magnetic resonance (n.m.r.) spectra were obtained at 67.8 MHz for highly isotactic polypropylene polymerized with a Ziegler-Natta catalyst. The tactic heptad peaks observed in the methyl region of the spectrum were assigned on the basis of the gamma effect of chemical shift and by using the Suter-Flory RIS model of polypropylene conformations. The mechanisms of propylene polymerization were analysed from the values of triad and pentad tacticities. The analysis of the propylene polymerization mechanism was based on the two-site model, where polymerization at one site proceeds according to the Bernoullian model of selection between meso and racemo configurations, and at the other site according to selection between dextro and laevo. As a result, the two-site model, with its three parameters optimized, reproduced well the observed relative areas of pentad and heptad peaks in the region of methyl carbon resonances.
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