A study of the effect of "trimethylaluminium (TMA)-depleted" methylaluminoxane (MMAO) and alkylaluminiums on propylene polymerization at high temperature in the use of a TiCl4/MgCl2/aromaticdiester-alkoxysilane catalyst has shown that, MMAO was not the actual cocatalyst. However, at low Et3Al/Ti mole ratio addition of MMAO to catalyst improved the catalyst activity and stereospecificity and the catalyst showed a high proportions of isotactic polypropylene (95%) and high activity with the mixture of MMAO and Et3Al at 100 degrees C. Addition of MMAO reduced the molecular weights of isotactic polypropylene (iPP) but had no effect on the crystallinity of the iPPs. The appearance of beta modification in the iPP obtained with Et3Al crystallization process implied that the microstructure of iPP obtained with Et3Al was different from that of iPP obtained with i-Bu3Al and Hex(3)Al at 100 degrees C. (c) 2005 Elsevier B.V All rights reserved.