External silane donors in ziegler-natta catalysis. A three-site model analysis of effects on catalyst active sites

A three-site model of the active sites in a MgCl₂-supported Ziegler-Natta catalyst was used to simulate the effects of various external alkoxysilane donors on the catalyst active sites. The simulations correlated well with the experimental data, and it is evident that the polypropylene produced by this catalyst system originates mainly from three types of active sites. One site is a highly isospecific enantiomorphic site producing boiling heptane insoluble polymer. Two sites are responsible for the major part of the polymers of the heptane soluble firaction: a moderately isospecific enantiomorphic site (the fluctuating site) and a symmetric Bernoullian syndiospecific site. All the alkoxysilanes tested, including monoalkoxysilanes, exhibited a stabilizing role, making fluctuating sites more isospecific. In addition, the silanes having more than one alkoxy group were capable of deactivating active Ti-sites, and the selectivity of deactivation depends mainly on the size of the hydrocarbon part.