Donor atom-stabilized aluminum alkyls as cocatalysts for the Ziegler-Natta polymerization of propene

A number of organoaluminum compounds, stabilized with intramolecular nitrogen- or oxygen-donor functions, have been used as cocatalysts for the MgCl2/TiCl4-catalyzed homopolymerization of propene as well as for the copolymerization of ethene with propene. The polymerization behavior of these aluminum alkyls was examined at different Al/Ti ratios within the range of 2 to 50 and compared with the reference of triethylaluminum (TEA). Especially the alkyls [2-(N,N-dimethylaminomethyl)phenyl]dimethylaluminum, (1) and [2-(NN-dimethylaminomethyl)phenyl]diethylaluminum (2) show the highest activities at very low Al/Ti ratios in the homopolymerization of propene, whereas TEA is almost inactive. The species [8-(N,N-dimethylamino)naphthyl]dimethylaluminum (4) reaches the highest activity of all examined alkyls and is very close to the highest value obtained with TEA. Bulky iso-butyl groups at the aluminum center are responsible for the very poor performance of the nitrogen stabilized cocatalysts [8-(N,N-dimethylamino)naphthyl]diisobutylaluminum (5) and [2-(NN-dimethylaminomethyl)phenyl]diiobutylaluminum (3). The properties of the polypropenes synthesized with the stabilized organoaluminum species are similar to those produced with TEA but with a distinctly higher molar mass. In the case of 1, it was possible to increase the molar mass by a factor of three. For the copolymerizations, the compounds [2-(N,N-diethylaminomethyl)phenyl]diethylaluminum (7) and (2-methoxybenzyl)diisobutylaluminum (8) were found to be most suitable, producing polymers with significantly higher activities than TEA. For all copolymers two fractions were obtained, one crystalline fraction with a low and an amorphous part with a high amount of comonomer. In both fractions, 7 and 8 provide a higher comonomer incorporation than TEA.