Ethene polymerisation

Another, highly selective oligomerisation reaction of ethene should be mentioned here, namely the trimerisation of ethene to give 1-hexene. Worldwide it is produced in a 0.5 Mt/y quantity and used as a comonomer for ethene polymerisation. The largest producer is BP with 40 % market share utilizing the Amoco process, formerly the Albemarle (Ethyl Corporation) process. About 25 % is made by Sasol in South Africa where it is distilled from the broad mixture of hydrocarbons obtained via the Fischer-Tropsch process, the conversion of syn-gas to fuel. The third important process has been developed by Phillips. 

Triazacyclohexane also gives rise to very active catalysts with the use of chromium [13] as do ligands of the type RS(CH2)2NH(CH2)2SR [14], The latter coordinate in a meridional fashion, while the former can only coordinate in a facial fashion. Recently examples using cyclopentadienyl titanium complexes [15] and tantalum have been reported [16], The diversity of the chromium systems and the new metal systems show that very likely more catalysts will be discovered that are useful for this reaction, including 1-octene producing catalysts (1-octene is in high demand as a comonomer for ethene polymerisation for certain grades of polyethylene). 

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

In the last decade an enormous revival of late transition catalysts for the polymerisation of alkenes has taken place [45] (remember that the first discovery of Ziegler for ethene polymerisation also concerned nickel and not titanium). The development of these catalysts is due to Brookhart in collaboration with DuPont (Figure 10.28) [46], Detailed low-temperature NMR studies have revealed the mechanism of the reaction [47], Interestingly, the resting state of the catalyst is the ethene-metal-alkyl complex and not the metal-alkyl complex as is the case for the ETM catalysts. For ETM catalysts the alkene complex intermediates are never observed. Thus, the migratory insertion is the rate-determining step (the turnover limiting step , in Brookhart s words) and the reaction rate is independent of the ethene concentration. 

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... 

Grubbs group [31, 32] developed another type of Ni-based catalyst. This neutral Ni-catalyst, based on salicylaldimine ligands, is active in ethene polymerisation without any co-activator and originated from the Shell higher olefin process (SHOP). Shortly thereafter another active neutral P,0-chelated nickel catalysts for polymerisation of ethene in emulsion was developed by Soula et al. [33, 34, 35]. The historical development of single site catalysts is represented in Fig. 1. 

Gibson, Brookhart (1998) -ethene polymerisation -short chain branches -tolerate polar groups... 

Recent developments in catalyst design have offered more sophisticated catalyst structures with even better copolymerisation ability. Me2Si(Me-Benz(e)Ind)2ZrCl2 is an example of such a catalyst [57, 58]. This catalyst, as mentioned earlier, was originally designed for polymerisation of isotactic polypropene, but it also shows a very high comonomer response in ethene polymerisation [51]. 

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]. 

Bambirra, S., van Leusen, D., Meetsma, A., Hessen, B., and Teuben, J.H. Neutral and cationic yttrium alkyl complexes with linked 1,4,7-triazacyclononane-amide monoanionic ancillary ligands synthesis and catalytic ethene polymerisation, Chem. Comm. (2001), 637-638. 

Table 1 Ethene polymerisations with rac-Me2Si(Ind)2ZrMej 714, 7 bar ethene pressure ...

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. 

Continuing with this study, Delley et investigated CO-reduced Phillips catalyst to study the initial bond formation that occurs during the initiation step of ethene polymerisation. Published literature has included much debate about the initial bond formation, stating that it could result, for example, via C-H or bond activation of ethene or silanol, re-... 

Since the mechanism of ethene polymerisation commonly suggested for transition-metal catalysts involves the formation of metallacycle, development of this type of catalyst was performed in order to elucidate the type of intermediate involved in the reaction. Binuclear Cr(ii) metallacycles showed little production of 1-hexene when reacted with ethene at room temperature, even after reacting for over 24 h. However, mononuclear Cr(m) metallacycles were considered possible intermediates as these complexes are able to trimerise ethene. Thus, Monillas et al. concluded this to be the likely intermediate involved in the reaction. Nonetheless, the use of the Cr(i) dinitrogen complex yields the desired product, 1-hexene, in a mechanism... 

Polymerisation by homogeneous chromium catalysis is overviewed while further developments of vanadium-siloxane ethene polymerisations are also available. Triflate or... 

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