Process for preparing propylene copolymers

The present invention relates to an olefin polymerization process, wherein propylene and 1-butene and optionally ethylene are reacted in the presence of a Ziegler-Natta catalyst system so as to obtain a polypropylene, wherein the polypropylene comprises 1-butene-derived comonomer units in an amount of from 0,5 to 15 wt% and optionally ethylene-derived comonomer units in an amount of up to 3 wt%, and the Ziegler-Natta catalyst system comprises an external donor of the formula (I) (R3)z(R2O)YSi(R1)x (I). 本发明涉及烯烃聚合方法，其中在齐格勒-纳塔催化剂体系的存在下使丙烯和1-丁烯和任选的乙烯反应得到聚丙烯，其中聚丙烯包括0.5至15重量％范围内的量的1-丁烯衍生的共聚单体单元和任选的多达3重量％的量的乙烯衍生的共聚单体单元，且齐格勒-纳塔催化剂体系包括式(I)(R)(RO)Si(R)的外部给体。(R)(RO)Si(R)(I)。