On the sensitivity of metallocene catalysts toward molecular hydrogen during ethylene polymerization

Polymerization of ethylene has been performed with heterogenised metallocene catalysts in the presence of hydrogen as chain transfer agent in a slurry semi-batch reactor. On-line GC analyses show that both Cp2ZrCl2/MAO/SiO2 and Cp-2*ZrCl2/MAO/SiO2 are much more reactive toward hydrogen as compared to commercial catalysts such as CrO/SiO2 and CrCp2/SiO2. While narrow molecular weight distributions ate obtained for the polymer produced with the metallocene catalysts in the absence of hydrogen, broad distributions are obtained when hydrogen is present. The broadening is at least partly a consequence of the way hydrogen is introduced, but especially for Cp-2*ZrCl2/MAO/SiO2, more than one active site might be present. Polymerizations with deuterium show that both metallocene catalysts react cleanly with deuterium, forming -CH2D fragments.