A catalyst for change - a look at olefin polymerization catalysts past, present, and future
Tracy Davis, Manager, Global Sales Support, Thomson Scientific
November 2004
Reproduced with kind permission from the Chemical Daily, Japan
We probably could not imagine life in the 21st century without polymers.
Almost everything today can be, and is, made from “plastic”. But
this is an inaccurate term, since plastics are only a sub-set of the world of
polymers.
The most common polymers are polyolefins, especially polyethylene (better known as Polythene, although this is a trade name owned by DuPont) and polypropylene. However, efficient ways of producing these vital materials are only the result of recent discoveries and have been dependant on the chemistry of catalysts.
Ziegler-Natta catalysts
Since the 1950s, the production of polyolefins has depended on the use of Ziegler-Natta catalysts. These are based on discoveries made by Karl Ziegler and Giulio Natta [1], which were so important that Ziegler and Natta jointly won the Nobel Prize for Chemistry in 1963. However, federal courts have since decided that Robert L Banks and J Paul Hogan were in fact the first to discover these catalysts [2].
Ziegler-Natta catalysts are based on a mixture of a transition metal, commonly a titanium compound, and an alkali metal, most commonly aluminium oxide. Whilst these systems have extremely high activity, their products have variable physical properties. To this day, the systems are little understood, but the monomers (polymer starting materials) react through a number of reaction sites on the catalyst. Unfortunately, this means the polymer can grow from many sites and at different rates, leading to a very wide distribution in the molecular weight, based on the polymer chain length. As the need to control the chain lengths of polymers has grown, the need for a new type of catalyst has also grown. This has given rise to a new breed of catalyst: the metallocene.
The rise of the metallocenes
Modern life has demanded more of the humble polymer. We want polymers that are stronger than steel, lighter than aluminium, and can be dyed any colour imaginable. The Ziegler-Natta system, with its waxy, variable results, could not reliably supply these attributes. Metallocene catalysts are, in fact, just as old as the Ziegler-Natta systems [3], but the first systems using them were found to have low activity. It wasn’t until 1980, when they were put together with a methyl aluminoxane cocatalyst, that their full potential was realised [4].
Metallocenes are positively
charged metal ions, most commonly Titanium or Zirconium, sandwiched between
two negatively charged cyclopentadienyl rings (see fig). Their big advantage
over the Ziegler-Natta systems is that they catalyse the reaction of olefins
through only one reactive site. Due to this “single site” reaction,
the polymerization continues in a far more controllable fashion, leading to
polymers with narrow ranges of molecular weight and, more importantly, predictable
and desirable properties. Also, it has been found that changing the ligands
(functional groups attached to the metal) upon the metallocene molecule can
controllably affect the properties of the polymer. This is very attractive to
chemical companies trying to keep up with the demand for engineered plastics.
Patenting in the area has been plentiful, with most of the big chemical and oil companies having at least some patents (see table):
| Top 10 companies patenting metallocenes from 1994-2003 | |
| Company | No of patent families |
| Mitsui Chemicals | 446 |
| Exxon Mobil | 338 |
| Basell | 219 |
| Mitsubishi Chemicals | 172 |
| Dow | 161 |
| Tosoh | 148 |
| Hoechst | 135 |
| Sumitomo Chemicals | 134 |
| AtoFinaElf | 128 |
| Phillips | 115 |
The patenting of new metallocene catalysts hit a peak in 1999, with 295 new metallocenes out of a total of 572 new catalysts for polyolefins [5]. Since that time, the number of new metallocenes has fallen. But so has the total number of new catalyst inventions, probably reflecting the downturn in the chemical industry during that time.
What is more significant is that the share of metallocenes of all novel polymerization catalysts that have been patented has fallen in recent years – from a peak of 55% of all new patents families in 1997 to 42% in 2003:
The drawback of metallocene catalysts is that they are unable to polymerize polar molecules, such as common acrylics or vinyl chloride. This is due to the metallocenes’ oxophilicity – their propensity for binding to oxygen. Introduction of a polar monomer into a reaction system will kill the catalyst activity to almost zero. So polymer chemists have started searching for new types of single site catalysts.
Post-metallocenes
A lot of research is now being directed at other types of chemical that can produce polyolefins with the desired properties. Currently, work has progressed using metals from all over the Periodic Table. But one area in particular is proving advantageous: late transition metal compounds.
Late transition metal compounds
Catalysts using late transition
metals – those metals from groups 6 and higher in the Periodic Table –
have become increasingly utilized. These compounds have good polymerization
activity, although slightly less than metallocenes. However, crucially they
can polymerize reactions with polar monomers. The most commercially advanced
of this type of catalysts are the Brookhart catalysts [6], which are diimine
complexes of palladium or nickel (see fig).
Patenting of late transition metal catalysts has been increasing over the last 10 years, although again with a slight decrease along with all catalyst patenting since 2000 [7].
What is interesting here is that the share of late transition metal compounds of all novel polymerization catalysts that have been patented has significantly risen – from 21% of all new patents families in 1994 to 42% in 2003:
We also can see a change in the top patenting companies (see table below), with Exxon Mobil and Mitsui Petrochemicals relinquishing their tight hold on the metallocenes market:
| Top 10 companies patenting late transition metal catalysts from 1994-2003 | |
| Company | Number of patent families |
| Mitsui Chemicals | 167 |
| BASF | 119 |
| Mitsubishi Chemicals | 115 |
| Phillips Petroleum | 110 |
| Basell | 94 |
| BP | 88 |
| Nippon Zeon | 80 |
| Tosoh | 68 |
| Exxon Mobil | 67 |
| Shell | 50 |
Conclusion
Although it is a little early to be sounding the death knell of the metallocene catalyst, it would seem that the next generation of olefin polymerization catalysts are likely to be late transition metal catalysts.
References
[1] Patent US 2,691,647 (12 October 1954) – Conversion of ethylene and/or
propylene to solid polymers in the presence of group 6a metal oxides and alkali
metals.
[2] Patent US 2,642,467 (16 June 1953) – Production of high-octane fuel
components
[3] Patent US 2,827,446 (18 March 1958) – Polymerization of ethylene
[4] Patent DE3007725 (1980) – Verfahren zur Herstellung von Polyethylen,
Polypropylen und Copolymeren
[5] Source data – Derwent
World Patents Index®
[6] Patent WO 96/23010 (01 August 1996) – Alpha-olefins and olefin polymers
and processes therefore
[7] Source data – Derwent
World Patents Index