)*( To whom correspondence should be addressed. E-mail: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Abstract Key Words promoter, Ziegler-Natta catalyst, polymerization, polyethylene, polypropylene, Polymerization Quarterly, 2014 Volume 4, Number 4 Pages 22-31 ISSN: 2252-0449 n.bahri@ippi.ac.ir Effect of Modifers on the Performance of Ziegler-Natta Catalysts in Olefn Polymerization Reza Bazvand, Naeimeh Bahri-Laleh * , Mehdi Nekoomanesh-Haghighi, Hossein Abedini Polymerization Engineering Department, Iran Polymer and Petrochemical Institute,P.O. Box 14975/112, Tehran, Iran Received: 17 May 2014, Accepted: 18 August 2014 C atalyst activity is one of the most important criteria in polymerization processes. In a catalyst system, the activity may depend on the polymerization conditions such as temperature and pressure. However, at a fxed condition of polymerization, it would be undesirable to change the temperature and pressure of the system in order to enhance the activity of the catalysts. Accordingly, scientists are encouraged to increase the amount of catalyst and/or prolong the polymerization time which are not economically approved. In recent years, modifers have been used to enhance the activity of Ziegler-Natta catalysts. They do not have any impact on the polymerization mechanism but affect the fnal properties of the fnal polymer including number-average molecular weight and weight- average molecular weight. They have also played a role in rising melting temperature, crystallinity, bulk density and particles size to a smaller extent. Because of the special role of the modifers in Ziegler-Natta based olefn polymerization processes, different types of promoters are introduced here. In this respect, the effect of different modifers including organo-halides as well as metal-halides and metalloids on the activity of Ziegler-Natta catalysts and fnal properties of polymer is explained in detail.