EXTERNAL SILANE DONORS IN ZIEGLER-NATTA CATALYSIS - A 2-SITE MODEL SIMULATION OF THE EFFECTS OF VARIOUS ALKOXYSILANE COMPOUNDS

The results of experiments with a supported Ziegler-Natta catalyst wer e interpreted by a two-site model of the active sites, in order to enh ance the understanding of multiple types of catalyst active sites and in particular the role of the external donor. The applicability of the two-site model is discussed and the stochastic parameters of the mode l are correlated with both the experimental and molecular modelling da ta. The two-site model satisfactorily explains the major part of the a ctive sites of this highly isospecific catalyst and is a suitable tool for analysing the effects of the donors on catalyst active sites. Nev ertheless, it does not completely describe the behaviour of Mg-support ed Ziegler-Natta Ti catalyst. There is probably more than one kind of active site producing the part of polypropylene that is soluble in boi ling heptane. This study also supports the earlier suggestion that som e of the changes in the population of active sites can be explained on ly through the formation of new sites, and more specifically through t he existence of fluctuating sites that can be stabilized by external d onors. The stability of these new sites depends on both the structural and electronic properties of the external donors.