AbstractThe effects of two alkoxysilane donors, triethoxyphenylsilane (PTES) and dimethoxydi‐tert‐butylsilane (DTBDMS), on the propene/1‐hexene copolymerization with MgCl2/DIBP/TiCl4(DIBP: diisobutyl phthalate) are compared. Two series of copolymers of different compositions were produced and the copolymers with the lowest hexene content, were fractionated. An analogous series, produced with the same catalyst without the use of an external donor, is the point of reference. All the fractions were characterized by13C NMR. The data reported show that there is a noticeable difference in the behaviour of ethoxy‐and methoxysilanes in the copolymerization. The copolymers prepared with ethoxysilanes can be separated into fractions characterized by increasing isotacticity and decreasing comonomer content as long as the fractionation temperature increases. The copolymer obtained with DTBDMS is nearly completely isospecific while the comonomer content distribution is even wider than with ethoxysilanes. The sum of these data provides a deeper insight into the alkoxysilane activation me