Pentad epimerized isotactic

The dependence the polymer microstructure on the propylene concentration in the reactor is shown in Table XVI. By varying the monomer concentration in operation with the supported catalyst, one can obtain polypropylenes with relatively low concentration of rrrr pentads. At low propylene concentrations, it is possible that m insertions are formed by epimerization. The yield of these isolated m diads is significantly increased when the catalyst is supported. Similar effects were shown by catalysts producing isotactic polymer. 

All the iPPs produced with this class of complexes had 4/w/4/n around 2, which is consistent with the single-site behavior. The NMR analysis of the iPP revealed equal amounts of mmmr and mmrr pentads, consistent with a mechanism of enantioselectivity controlled by the site chirality.276 To explain the increased isotacticity at increasing propylene pressure, it has been proposed that monomer insertion competes with chain epimerization. At increased propylene pressure, chain-epimerization reactions would be depressed by faster monomer insertion.247,248,276... 

These equations show how a mmmm sequence in polystyrene can be converted into a mrrm sequence by a simple epimerization event. Should the configuration of the fourth carbon from the left in the last structure also be altered, a rmrm pentad would result. Thus, by a series of epimerization steps it is possible to change isotactic polystyrene gradually into a polymer that exhibits the same NMR spectra as the polystyrene that was prepared by free radical polymerization(17-19). A Monte Carlo program has... 

---