Fourth generation external

Currently, most of the commercial iPP is made using fourth-generation Ziegler-Natta catalysts with different internal and external donors that increase the productivity to 25 kg polypropylene/g catalyst and isotacticity from 95% to 99%. Shell Oil Co. and Montedison started commercializing these catalytic systems in the early 1980s. Shell reported the use of benzoic acid as internal and external donors, and Montedison utilized alkyl phthalates and silyl ethers as internal and external donors, respectively [3]. 

For MgCl2-supported catalysts (the third- and fourth-generation types see Table 9 the electron donors are classified as internal and external based on their sequence of addition during catalyst preparation or during activation, respectively. For the preparation of the MgCla-supported catalyst an electron donor is added... 

To recap, there are two types of donors, internal and external. For the third-generation catalysts the internal/external pair is an ester/ester pair, while for the fourth-generation it is the phthalate/alkoxy silane pair. The fifth-generation Ziegler-Natta catalysts are comprised of an internal donor, which is a diether compound and may be used with or without the additional external donor (see Table 9). 

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