NATEIllAI~ Today

METAL CATALYSTS FIGHT BACK In recent years organometallic catalysts, especially metallocenes, have been a major focus of attention in terms of polymerisation chemistry. But the news earlier this year of a family of iron-based catalysts able to rival the effectiveness of both conventional and metallocene catalysts in the polymerisation of ethylene has excited the plastics industry. Because of the impact of this discovery and its potential as a route to lower-priced commodity plastics in the future, it may be useful at this stage to summarise the recent developments and stress once more their importance. George Marsh reports.

Polyethylene, otherwise known as polythene (PE) the ubiquitous plastic used in everything from packaging through household buckets and bowls to heavy duty pipes, is produced by polymerising the olefin monomer ethylene. The efficiency of this process and the quality of the resulting product depend crucially on how the molecules of ethylene gas are persuaded to link up with each other. Catalysts can greatly affect the extent and nature of the linking process determining, for instance, whether straight or branch-chain polymer molecules are formed. Branched chains tend to confer better mechanical properties and long- chained plastics tend to be of higher density and tougher than those with short chains.

Metal-based catalysts have been used in polythene production since the 1950s when Karl Ziegler first introduced his well-known titanium- based system, a modified version of which was exploited by G. Natta to make isotactic polypropylene. The Ziegler-Natta process, while still an industry standard, tends to produce plastic of uneven quality with a random mix of short and long chains, and substantial catalyst contamination.

Since the Ziegler-Natta process much research has been concentrated on new catalyst systems for the polymerisation of olefins. This is not surprising as polyolefins are the basis for about half of all synthetic polymers. Notable developments in this area include silica supported chromium systems developed by Philips and Unipol, which are now another industry standard for polyethylene production.

Another type of catalyst, which proved remarkably effective in polymerising olefins, are the metallocenes, compounds made up of a transition metal, e.g. zirconium or titanium sandwiched between cyclopentadienyl or 5-membered rings. These offered much improved control over the structuring of large polymer molecules. Using them, it proved possible to produce polythene having uniformly long branched structures, resulting in physical properties superior to those of 'standard' polythenes.

Enhancing the production and quality of polythene is regarded as a significant prize for the plastics industry. Large companies such as Dow, DuPont, Exxon, BASF and BP are interested in exploring existing and new catalytic processes in order, not only to improve control over branching in the polymer chains, but also to give greater control over the molecular weight distribution, tacticity and to extend the process to new monomer combinations in particular to allow polar co-monomer incorporation. Extending the process to new monomers could allow the synthesis of new materials with potentially different properties and new applications.

So, when announcements were made earlier this year of British and American discoveries that a new type of metal catalyst can, after all, be just as effective as metallocenes, much

excitement ensued in industry and the polymer synthesis community. These new discoveries demonstrated that certain complexes of iron and cobalt can mediate the polymerisation of ethylene with excellent results in terms of economy, yield and product quality. Such relatively simple catalysts would be preferred since metallocenes are sophisticated molecules based on relatively exotic metals, and are therefore expensive.

Europe

A team of British chemists from Imperial College and BP Chemicals made their announcement this Spring in a paper presented to the Royal Society of Chemistry's National Congress held in Durham, UK, and subsequently reported in Chemical Communications. Team leader, Professor Vernon Gibson, said he had been surprised that an iron-based catalyst could be so active since iron had "no previous track record of being used in this way."

The team had synthesised their catalysts by treating ferric and cobalt chlorides with 2,6-bis (imino) pyridines. In the case of the iron, the resulting catalyst comprises an iron atom bonded to two chlorine atoms and surrounded by a tridentate ligand. The synthetic pathway to these catalysts is shown below, Figure 1, together with the X-ray crystal structure of one of the catalysts, Figure 2.

There are a number of control possibilities open to chemists. Influencing the way the aryl rings around the metal complexes are substituted offers a route to governing the molecular weight of the polymer product. Some substituent groups are more bulky than others, tending to 'screen' the active site and hence constraining the extent of the polymeric reaction. As Imperial

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MATIERIA~ Today

The Synthesis of o n e of the Iron Complexe~

Me

N ~N iPr

2,6 -Bis(imino)pyridyl ligand

FeCI2, n-BuOH

EIOH, MeCO2H ~

The Catalyst Complex

Fig. 1.

0

Fig. 2. 'X-ray crystal structure of one of Professor Vernon Gibson's iron catalysts'

Professor Gibson commented to New Scientist,

"The beauty of these new iron catalysts is that by modifying the groups around the iron centre, we can control the activity of the catalyst and select the size of the polythene molecules we want. This catalyst system is readily available, relatively cheap and more environmentally

friendly than some of the alternatives."

North America

Another team, working independently in America, has made similar discoveries. The US workers presented their findings at an American Chemical Society national meeting in Dallas this Spring. Like

their British counterparts, they produced precursor complexes of iron and cobalt with 2,6-bis (imino)pryidyl ligands. According to Maurice Brookhart, professor of chemistry at the University of North Carolina, the iron and cobalt complexes proved able to produce high density polythene in high yield when activated by the co-catalyst methylalumoxane (MAO).

Andrew R. Barron, professor of chemistry and materials science at Rice University, Houston, suggests that the work could be the first really new development in this area since the discovery in the mid 1970s by Professor Walter Kaminsky of Hamburg University that zirconocenes provide high catalytic activity for olefin polymerisation when used with MAO co-catalyst. As he noted, it also demonstrates that the search for new catalysts need not be limited to the traditional group four metals.

Overall, the new catalysts have shown that they compare with the best of the metallocenes in promoting polymerisation, with iron apparently more active than cobalt. Maurice Brookhart says that the system is additionally active in propylene polymerisation and, with modifications, in oligomerising ethylene to alpha-olefins. That, to say the least, is indicative of future potential.

Bill Tallis, Technology Director of BP Chemicals in UK, perhaps best sums up the current situation. Referring to the discoveries as a 'breakthrough technology', he says,

"The new catalyst family shares many of the advantages of metallocene catalysts in terms of activity and control of polymer properties. In addition, it offers the potential for producing a much broader range of polymeric materials at low cost."

George Marsh

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