Polymerization Kinetics with Multiple-site Catalysts

Polymerization Kinetics with Multiple-site Catalysts 

All heterogeneous Ziegler-Natta and Phillips catalysts have two or more active-site types and many soluble Ziegler-Natta and metallocene catalysts may also show multiple-site behavior [36, 37]. In addition, several metallocene catalysts, when supported on organic and inorganic carriers, may behave like multiple-site catalysts even if they behaved as single-site catalysts in solution polymerization. Therefore, several of the catalysts used industrially for polyolefin manufacturing have in fact two or more active-site types. 

The kinetics of polymerization with multiple-site catalysts is generally considered to be the same as with single-site catalysts, as described by Eqs. (1)-(14) for homopolymerization and in Table 8.1 for copolymerization, with different polymerization kinetic parameters assigned to each site type. In some cases, the polymerization mechanism may be extended to include site transformation steps, where sites of one type may change into sites of another type, such as the one described with the reversible reaction in Eq. (23), where D could be a catalyst modifier such as an electron donor, for instance. 

Since these additional site transformation steps may affect the relative ratio of site types present in the reactor but do not influence the general polymerization behavior with multiple-site catalysts, for the sake of simplicity they will not be further considered in this chapter. 

It is usually straightforward to detect the presence of multiple-site types on a coordination catalyst because these catalysts will produce polymer with polydispersity higher than 2 even under invariant polymerization conditions. The simplest way to visualize this phenomenon is to assume that every different site type on a multiple-site catalyst produces polymers that follow a distinct Flory s distribution that is, those with a distinct number-average chain length, [38]. In this way, the chain length distribution for the whole polymer is a combination of distinct Flory s distributions weighted by the mass fraction of polymer made on each site type, mj [Eq. (24)]. 

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Polymerization Kinetics with Multiple-site Catalysts... 

There is vast experimental and theoretical evidence supporting the existence of multiple site types on heterogeneous Ziegler-Natta catalysts. From a mathematical modelling point of view, each site type has distinct polymerization kinetic constants and consequently produces polymer chains with different average properties. 

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