Ziegler polymerization of olefins. IX. Donor effects in metal alkyl‐free and Ziegler‐type stereospecific catalysts

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Abstract
Electron donors, especially trialkylamines and azulene, have been examined in aluminum alkyl‐, CH3TiCl3‐ and hydrogen‐activated TiCl3 catalysts for the polymerization of propylene to isotactic polymer. A comparison and an evaluation were made with findings which were established earlier with zinc alkyl‐based TiCl3 catalysts. We find that the donor, when it is present in low concentrations in all of the above catalysts, can inactivate preferentially the less stereoregulating sites. In this way the isotactic content and the molecular weight of the polymer are increased, but only at the expense of a lower catalyst activity. The addition of hydrogen to the TiCl3–donor catalyst at −78°C produced a threefold effect: (1) the activity of the catalyst was increased about 5 to 15 times and higher, (2) the polypropylene formed with this more active catalyst was more isotactic (ca. 10–15%), and (3) the polymer had a lower molecular weight. It is proposed that the increase in catalyst activity was due to the generation of Ti‐H bonds to which propylene molecules then added, the Ti‐H bonds thus being transformed into active Ti‐C centers.

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