EXTERNAL SILANE DONORS IN ZIEGLER-NATTA CATALYSIS - AN APPROACH TO THE OPTIMUM STRUCTURE OF THE DONOR

Our earlier work about alkoxysilanes as external donors in high-activity Ziegler-Natta catalysts is broadened with studies on the performance of some other oxygen-containing silane compounds and with some studies on the effect of the structure of the hydrocarbon part of the alkoxysilane compounds: There are new informations about the optimum structure of an external silane-type donor. The alkoxy group proved to be the best electron-donating group, whereas the other oxygen-containing electron-donating groups tested worked less effectively. In some structures the oxygen atoms were sterically too hindered for effective deactivation. Some increase in the isotacticity was nevertheless observed even with the poor performance donors, which indicates that there could be a population of active centres that are very easy to deactivate. All the para-substituted alkoxy phenyl silane donors tested gave polypropene with high isotacticity and activity. Therefore, we conclude that the nature of the substituent does not markedly affect the performance of the donor - at least as long as the substituent has not a sterically free electron-donating group.