Various new malonate compounds ((RR2)-R-1 C(COOBu)(2)) with different substituents (R-1, R-2) were systematically synthesized and investigated for use as an internal donor (ID) in combination with a MgCl2-supported TiCl4 type catalyst, a triethylaluminum (TEA) co-catalyst and an alkoxysilane external donor (ED) for application in propylene polymerization. The catalytic activity and isotacticity of polypropylene (PP) greatly depended on not only the oxygen electron density of the ED, but also on the molecular volume of the ID. Furthermore, the mechanism of the active site formation was discussed with respect to the composition of the catalyst treated with TEA and ED, and with respect to the temperature rising elution fractionation (TREF) results of PP and so on. It was presumed that the desorption of malonates near Ti species from MgCl, caused the generation of atactic PP sites, and the decrease of the isotacticity of PP. (C) 2007 Elsevier B.V. All rights reserved.