Abstract: The present invention provides a catalyst for the polymerization of olefin that exhibits improved selectivity and broader molecular weight distribution in the polymer product. The catalyst is a conventional supported Ziegler-Natta catalyst in combination with a mixture of at least two electron donors, both having the general formula SiR.sub.m (OR').sub.4-m where R is selected from the group consisting of an alkyl group, a cycloalkyl group, an aryl group and a vinyl group; R' is an alkyl group; and m is 0-3, wherein when R is an alkyl group, R may be identical with R'; when m is 0, 1 or 2, the R' groups may be identical or different; and when m is 1, 2 or 3, the R groups may be identical or different. This catalyst produces polypropylene having xylene solubles of 1-7 to 5.1 wt % and a molecular weight distribution of about 10.

Abstract: The present invention provides a catalyst system and a process for the polymerization of olefin that exhibits improved selectivity and broader molecular weight distribution in the polymer product. The catalyst system includes a conventional supported Ziegler-Natta catalyst in combination with a mixture of at least two electron donors, both having the general formula SiR.sub.m (OR').sub.4-m where R is selected from the group consisting of an alkyl group, a cycloalkyl group, an aryl group and a vinyl group; R' is an alkyl group; and m is 0-3, wherein when R is an alkyl group, R may be identical with R'; when m is 0, 1 or 2, the R' groups may be identical or different; and when m is 1, 2 or 3, the R groups may be identical or different. Preferably, one electron donor is described by the formula: ##STR1## wherein R.sub.1 and R.sub.4 are both an alkyl or cycloalkyl group containing a secondary or tertiary carbon atom attached to the silicon atom, R.sub.1 and R.sub.4 being the same;; R.sub.2 and R.sub.