Coordination Catalysts 1 Polymerisation with Heterogeneous Ziegler-Natta

Subsequent investigations revealed that, in principle, styrene undergoes isospecific polymerisation in the presence of heterogeneous Ziegler Natta catalysts [1-4], Although polystyrene of isotactic structure was also prepared with the use of homogeneous nickel-based coordination catalysts, it appeared to be of low molecular weight [22,23]. 

1 Polymerisation with Heterogeneous Ziegler-Natta Catalysts 

Although the isospecific polymerisation of styrene monomers has much less steric demands for the Ziegler-Natta catalysts than that of x-oldins, it proceeds with much lower propagation rate constants by comparison with the polymerisation of x-olefins for example, on a molar basis, styrene is ca 100 times less reactive than propylene in the polymerisation [30,33], Also, compare the relatively slow polymerisation of styrene and other vinylaromatic monomers with the relatively fast polymerisation of vinylcyclohexane [20,31,34-36]. 

The mechanism of the isospecific polymerisation of styrene and its ring-substituted analogues in the presence of heterogeneous Ziegler-Natta catalysts is the same as for x-olefins. As regards the mode of monomer insertion, it has been reported as primary (1,2) insertion [37,38]. 

The steric control is attributed to the presence of chiral centres on the catalyst particle surface [39], 

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Table 4.1 Relative reactivity of vinylaromatic monomers in coordination polymerisation with heterogeneous Ziegler-Natta catalyst ...

No precise information about the olefin polymerisation mechanism has been obtained from kinetic measurements in systems with heterogeneous catalysts analysis of kinetic data has not yet afforded consistent indications either concerning monomer adsorption on the catalyst surface or concerning the existence of two steps, i.e. monomer coordination and insertion of the coordinated monomer, in the polymerisation [scheme (2) in chapter 2], Note that, under suitable conditions, each step can be, in principle, the polymerisation rate determining step [241]. Furthermore, no % complexes have been directly identified in the polymerisation process. Indirect indications, however, may favour particular steps [242]. Actually, no general olefin polymerisation mechanism that may be operating in the presence of Ziegler-Natta catalysts exists, but rather the reaction pathway depends on the type of catalyst, the kind of monomer and the polymerisation conditions. 

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