Ethene polymerisation, metallocenes

Figure 10.5. The first homogeneous metallocene catalyst for ethene polymerisation...

Group 4 metallocene complexes were reported by Wilkinson in early 1953 [4], a few months before Ziegler s seminal discovery that mixtures of TiCl4 and AlEt3 catalysed the polymerisation of ethene. It was not long before the new compounds were tested as potential ethene polymerisation catalysts, not least because these... 

Metallocenes with High Activity for Ethene Polymerisation.. .. 5... 

The LCB in metallocene-catalysed ethene polymerisation is considered to occur via a copolymerisation reaction where a vinyl-terminated polyethene chain is reinserted into a growing chain. Thus, the choice of the catalyst used will be extremely crucial. When the prerequisites of LCB are fulfilled, the process conditions will then be even more important [44, 60]. 

The results demonstrate the degree to which ethene polymerisations tend to be mass-transport limited. They also highlight the effect that can be achieved by altering the mode of catalyst activation a catalyst produced by activating rac-Me2Si(Ind)2ZrMe2 with AlBu j /14 proved 36 times more active than the same metallocene activated with MAO under closely comparable conditions. 

As said in the introduction there are many more polymers than can be discussed within the limits of this chapter, but we want to add just one example of a group of high-value polymers that is made using the same principles of coordination polymerisation as shown above for the commodity polymers. We mentioned metallocene catalysts that can be used to copolymerise ethene and norbomene to give Topas type products. 

By contrast to the polymerisation of hexene with 64, which can be followed conveniently by variable-temperature NMR, the polymerisation of smaller monomers like ethene and propene illustrate the limitations of spectroscopic methods since with most metallocene catalysts they are too fast. The kinetic behavior of (SBI)ZrMe2/AlBu 3/[CPh3][CN B(C6F5)3 2] at 25 °C was therefore investigated by quenched-flow techniques to estimate the rates of initiation, chain propagation and chain termination [SBI = rac-Me2Si(Ind)2] [97]. The results are summarised here for comparison with the results on 1-hexene polymerisation discussed above. 

In the early 1990s supported metallocenes were introduced to enable gas phase polymerisation. Also ethene/a-olefin copolymers with high comonomer content, cycloolefin copolymers and ethene-styrene interpolymers became available. In 1990 Stevens at Dow [22] discovered that titanium cy-clopentadienyl amido compounds (constrained geometry catalysts) are very beneficial for the copolymerisation of ethene and long-chain a-olefins. 

Metallocenes, in combination with the conventional aluminum alkyl cocatalysts used in Ziegler systems, are indeed capable of polymerising ethene, but only at a very low activity. Only with the discovery and application of methylaluminoxane (MAO) it was possible to enhance the activity, surprisingly, by a factor of 10000 [113]. Therefore, MAO plays a crucial part in the catalysis with metallocenes. 

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