Propylene polymerization has been studied with a use of MgCl2 supported titanium solid catalyst in combination with triethylaluminium (TEA) and a monoamino- or diamino-dimethoxysilane as an external electron donor. The ammosilanes having a bulkier amino group, or two amino groups show a tendency to give lower isotacticity and broader molecular weight distribution of polymers obtained. Specifically the aminosilanes with piperidinyl group(s) were found to be highly isospecific external donors. C-13 and H-1 NMR studies on the mixtures of TEA and di(piperidinyl)dimethoxysilane (DPIP) indicate that TEA forms a Lewis acidibase adduct through the interaction of aluminum and oxygen atom of methoxy group in the silane as have been reported in the case of TEA and di(hydrocarbyl)dimethoxysilanes. (C) 2002 Elsevier Science B.V. All rights reserved.