35a-c/MAO

At low propylene concentrations, isolated m stereoerrors were also observed for 35a-c/MAO and found to be more numerous than mm stereoerrors. The isolated m stereoerrors were found to have a strong dependence on propylene concentration. This gave further support for site epimerization as the major stereoerror-forming mechanism for 35a-c/MAO. However, since chain epimerization occurring with simultaneous site epimerization can also lead to isolated m stereoerrors, the authors undertook a deuterium labeling study to probe the importance of chain epimerization as a potential mechanism for forming both single m and double mm stereoerrors. 

For catalyst 35b/MAO, a kinetic isotope effect experiment was devised using the monomers do-propylene and 2-<7i-propylene for polymerization. 2- /i-Propylene was chosen for this experiment since P-hydride elimination is a component of the mechanism for chain epimerization. Polymerizations were carried out at 25 °C in toluene solution with 1 atm of propylene (0.8 M) (Figure4.17). The deuterated polymer was examined by NMR spectroscopy this technique showed that most of the deuterium was incorporated as -CD(CH3), with 2-3% incorporation as -CH(CH2D). The latter can be attributed to chain epimerization, which appears to be consistent with the 2-3% mm triads observed under similar conditions for propylene polymerization with 35a-c/MAO. 

The effect of propylene concentration on polymer molecular weight was inspected for Cj-symmetric precatalysts 35a-c and Ci-symmetric precatalysts 40b-c, all activated with MAO. For 35a-c, as propylene concentration increases, there is a roughly linear increase in polymer molecular weight. For 40b-c (in particular, 40b), as propylene concentration increases, polymer molecular weight increases however, there is slightly less than first order dependence of polymer molecular weight on propylene concentration. 

Based on H NMR analysis of low molecular weight polypropylene formed by 40b/MAO at 25 C with a propylene concentration of 0.5 M (in toluene solution), the predominant chain termination pathway appears to be f)-H elimination (i.e., vinylidene end groups are present). The primary kinetic isotope effect observed for 35b/MAO (vide supra) is also indicative of P-H elimination operating as the predominant chain termination pathway. No fl-CH3 elimination was observed for these catalysts specifically, no vinylic end groups are present in the H NMR spectra. For 35a-c and 40c, chain transfer was found to be primarily unimolecular, derived from f)-H elimination. For 40b, a small bimolecular contribution (chain dansfer to monomer) was also suggested, owing to the less than first order dependence of molecular weight on propylene concentration. 

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