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When you pack your sandwiches you probably reach for a polymer. If your
car needs a spare part, you might well do the same. This year chemists will
make enough synthetic polymers, mainly polyethylene and polypropylene, to
fill the Empire State Building at least nine times over. Polymers turn up
everywhere, from food packaging to cars, and from textiles to sports goods
and medical equipment. But industrial chemists still dream of designing
the perfect polymer – one that is as light as paper, as hard as metal, non-corrosive,
resistant to chemicals and heat, and able to take on a host of other desirable
properties. A class of sandwich-shaped catalyst molecules called metallocenes
is helping this dream to come true.

The most promising aspect of these catalysts is their unique ability
to synthesise plastics which have a combination of properties not normally
found together: for instance, stiffness and transparency, high melting point
and transparency, or high melting point and flexibility – a useful combination
of properties for a material used to make components of car engines, for
example. The same combination of heat resistance and flexibility could also
be useful in medical equipment such as syringes, that have to be heat sterilised.
A further advantage of concentrating a range of properties in a single polymer
is that fewer types of material are then needed, making it easier to collect
and separate used polymers for recycling.

Most synthetic polymers owe their existence to catalysts – substances
that speed up chemical reactions or allow them to take place under less
extreme conditions than would otherwise be required, without themselves
being used up in the process. If several reactions are possible, the catalyst
often favours just one of them. Industrial chemists quickly discovered how
important they were for making polymers. The chemists at ICI who in the
early 1930s first polymerised ethylene, to form long chain-like molecules
of polyethylene, had to resort to drastic measures – temperatures of up
to 200 °C and pressures as high as 2000 atmospheres. In 1953, Karl Ziegler
and his group at the Max Planck Institute for carbon research in Mulheim
discovered that if they used a small quantity of titanium tetrachloride
(TiCl₄4) blended with triethylaluminium, Al(CH₂CH₃)₃,
they could polymerise ethylene at pressures lower than 10 atmospheres and
temperatures of no more than 50 to 150 °C.

Chemicals companies in Europe and the US soon recognised the importance
of the Ziegler polyethylene process, but this was just the start. A few
months later Giulio Natta, a chemist working at the Institute for Industrial
Chemistry in Milan, drew on Ziegler’s experiences to polymerise propylene
under similar conditions. The carbon ‘backbone’ of the resulting polypropylenes
always forms linear chains, but there are three different ways in which
the methyl (CH₃) groups can be attached to half the carbon atoms
forming the chain, each arrangement conferring a distinct set of properties
(see Figure 1).

Natta was the first to realise that the catalyst played a crucial part
in determining the type of polymer produced. He discovered that certain
catalysts influence not only the speed and conditions of the reaction, but
also the polypropylene’s ‘tacticity’ – the stereo-chemical arrangement of
the methyl units attached its main chain. Natta found that he could make
‘isotactic’ polypropylene, which was stable to high temperatures. He also
found that a suitable choice of catalyst could increase the polymer’s density
and tensile strength. The modern chemicals industry benefited enormously
from these discoveries, and in 1963 Ziegler and Natta were awarded a Nobel
prize for their work.

Chemists’ early attempts to polymerise propylene required relatively
large amounts of catalyst, and produced mixtures of isotactic polypropylene
and ‘atactic’ polymer in which there is no regularity to how the methyl
groups are arranged. But as they developed better Ziegler-Natta catalysts,
the amount of unwanted atactic polypropylene was reduced, and so little
catalyst was needed that if could be left mixed in with the finished polymer
without affecting its properties. Today all the world’s polypropylene ,
and most polyethylene, is produced using Ziegler-Natta catalysts.

Modern Ziegler-Natta catalysts use various alkylaluminium compounds
in place of the original triethylaluminium, and titanium trichloride (TiCl₃3)
instead of titanium tetrachloride. They also contain substances such as
phthalates and alkylsiloxanes, which act as ‘stereoregulators’, controlling
the arrangement of methyl groups along the chain. But despite the success
of these systems, chemists still have not pinned down exactly how the catalyst
works. They think the active form of the catalyst might be a titanium alkylaluminium
compound, but no one has proved this. Nor can they cannot explain precisely
how stereoregulators work.

To produce uniquely shaped polymer particles the catalyst is supported
on an inert, insoluble material – usually magnesium dichloride. However,
‘heterogeneous’ systems like this have the disadvantage that not every active
site has the same structure, so they produce polymer chains with a variety
of different lengths and molecular weights. The properties of the polymer
depend in large part on these factors, and it takes a lot of trial-and-error
research to develop a heterogeneous catalyst that gives the required combination.

To avoid this, chemists would like to be able to control the properties
of polymers at will, simply by varying the catalyst used to make them. But
for this to be possible they need to know the structure of the catalyst
at the point where the polymerisation reaction takes place, as well as
the mechanism of the desired reaction and of any unwanted side reactions.
They knew none of these things for Ziegler-Natta catalysts. So some chemists
began to look for systems which they could understand more easily.

Even while Ziegler and Natta’s discoveries were revolutionising the
industrial chemistry of the 1950s and 1960s, researchers were investigating
other ways of polymerising ethylene. One of these involved catalyst systems
based on trialkylaluminium plus a metallocene – a compound in which a central
metal atom is sandwiched between a pair of flat, five-carbon cyclopentadiene
rings.

For the researchers, metallocenes had the advantage of being soluble
in hydrocarbon solvents, forming a ‘homogeneous’ system which was easier
to study by common chemical analytical methods than heterogeneous systems.
The first member of the metallocene family, ferrocene or bis(cyclopentadienyl)
iron, was discovered in 1951. But metallocenes made a slow entry into the
world of polymer manufacture. The first homogeneous systems based on a titanium
metallocene, could not be used to catalyse polymerisation on an industrial
scale because they were less efficient than the Ziegler-Natta catalysts.
Then in 1976, Hans Sinn and Walter Kaminsky of the University of Hamburg
discovered that one metallocene did act as an efficient catalyst in the
polymerisation of ethylene. It was a ‘bent metallocene’, in which a zirconium
atom bound to two chlorine atoms lay at the centre of the sandwich
(see Figure 2). The homogeneous catalyst system was made by combining a zirconium-based
metallocene called biscyclopentadienylzirconium dichloride with an aluminium
compound called alumoxane. Alumoxanes are made up of a chain of alternating
oxygen and aluminium atoms to which methyl groups are bound. Such a structure
is commonly known as MAO (methylalumoxane).

Under control

Interesting though this was, chemists were more concerned with finding
a catalyst that would give them control over the polymerisation of propylene,
a reaction in which Kaminsky’s metallocene-MAO system proved to have poor
activity and no stereoselectivity. Then in 1984, Kaminsky and Hans Brintzinger
of Konstanz University in southern Germany succeeded in polymerising propylene
with a bridged or ansametallocene called ethylenebis(4,5,6,7-tetrahydroindenyl)zirconium
dichloride. In combination with MAO, this catalyst produced highly isotactic
polypropylene. Researchers at several chemicals companies, including our
research group at Hoechst, recognised the industrial importance of this
breakthrough and turned their attention to the metallocenes.

We began by looking closely at the structure of the Brintzinger-Kaminsky
catalyst. Two arrangements of the cyclopentadienyl groups are possible in
this molecule, the difference being in their symmetry. The symmetry of the
metallocene is important because it affects the structure of the polymer
produced. Arrangement A has C₂ symmetry: in other words, the
structure of the molecule emerges unchanged when rotated through 180 degrees
around an axis that passes through the metal atom and the centre of the
ethylene bridge. The complex is chiral (it has handedness), so a pair of
enantiomers (the structure in Figure 3 and its mirror image) is possible.

Using structure A with MAO for the polymerisation of propylene gives
isotactic polypropylene. The molecule can also exist as B, but this form
gives atactic polypropylene. To understand this difference, we looked in
more detail at the polymerisation mechanism . The type of polymer produced
seems to depend on the orientation of propylene at the metal.

The early Britzinger-Kaminsky catalyst had two disadvantages compared
with conventional polypropylene catalysts. It produced polymer chains of
a lower molecular weight. And it was less stereospecific – not all the methyl
groups along the chain appeared on the same side – so the polymers it produced
had melting points about 30 °C lower. But because the catalyst’s structure
was known, chemists thought they could adjust its design to improve the
polymer’s qualities.

Picking properties

This holds out the alluring possibility of being able to pick and choose
a polymer’s properties – its transparency, tensile strength, stiffness,
heat resistance and brittleness, for example. But such developments depend
on the design of the catalyst. Our group at Hoechst envisages changing the
catalytic effect of metallocenes by manipulating their structure. This could
include changing the structure of the rings themselves – replacing cyclopentadienyl
rings with indenyl or fluorenyl rings, for example – or altering the chemical
groups that are attached to the rings or to bridges between them. Using
such alterations, we hope eventually to be able to design the catalytic
sites needed for specific polymerisation processes. We are currently investigating
new structures with high activities which produce polypropylenes having
a molecular weight of about 1 million daltons and melting points above 160
°C – some 30 °C higher than those produced using the Brintzinger-Kaminsky
catalyst.

Other researchers are also looking at ways of improving metallocenes.
James Stevens and his colleagues at the American chemicals company Dow ,
and Jo Ann Canich of the rival company Exxon, have independently developed
a type of metallocene in which one cyclopentadienyl ring is replaced by
a nitrogen atom attached to an alkyl group (see 6 in Figure 4). These catalysts
have already proved useful in the production of polyethylene, though the
researchers are not yet certain whether they will have advantages over the
more thoroughly studied metallocenes.

John Ewen of the Fina petrochemicals company in Houston, Texas, used
a zirconium metallocene with a different symmetry (see 5 in Figure 4) in
combination with MAO to produce large amounts of ‘syndiotactic’ polypropylene,
in which the orientation alternates in a regular way along the chain. This
reaction also seems to fit in with our model (described in Box 2) of how
metallocenes work. During the past few years there has been much interest
in this new polymer, which is more transparent then isotactic polypropylene
but less stiff.

Researchers at Hoechst have also used metallocenes to copolymerise cyclic
alkenes (such as cyclopentene, cyclohexane or nobornene) with ethylene,
to produce a ‘copolymer’ molecule made up of chains containing two types
of repeating units. The resulting polymer is an amorphous, highly transparent
thermoplastic material with a high melting point that has interesting potential
for optical applications such as coatings for compact discs.

So far, researchers have devoted most of their attention to metallocenes
based on zirconium. Catalysts in which zirconium is replaced by hafnium
produce longer polymer chains without any loss of stereoselectivity. But
to make these hafnium compounds, exceptionally pure metal is required, and
hafnocenes are much less active catalysts than their zirconium equivalents,
and so are needed in larger quantities. Titanocenes are too unstable to
survive the conditions needed for polymerisation.

If the number of chemicals companies beginning to use the new zirconium
metallocenes is anything to go by, metallocene catalysts have a bright future.
Both Exxon and the Japanese petrochemicals company Mitsui Petro plan to
build manufacturing plants that will use metallocene catalysts to produce
100 000 tonnes of polyethylene per year by the mid-1990s. Dow plans to adapt
an existing polyethylene plant to use a metallocene catalyst, and expects
it to go into full-scale production next year. Fina has gone as far as pilot-scale
production of syndiotactic polypropylene, and Hoechst is cooperating with
Mitsui Petro in the development and commercialisation of cyclic copolymers.

Many small but important problems will have to be solved before metallocenes
replace conventional catalysts for making polymers. But their potential
advantages include a high activity, comparable to that of enzymes, the ability
to produce customised polymers over a narrow range of molecular weights,
and eventually the possibility of designing polymers with specific properties.
The prospect of these invaluable gains is driving an enormous worldwide
research effort.

Frank Kuber is a member of the metallocene catalysts project at the
central research laboratories of Hoechst AG in Frankfurt am Main, Germany.

* * *

1: Parade of polymers

The chains that make up polyethylene molecules are built up from two-carbon
units derived directly from the ethylene (C₂H₄) monomer. The
chains can be linear or branched, and this structural difference affects
the polymer’s properties. Linear polyethylenes are slightly denser than
branched ones, and have a higher melting point. They are used to make durable
components such as tubes, barrels and petrol tanks for cars. Branched-chain
polyethylene turns up in films and foils. There are also special types of
polyethylenes with up to a thousand times as many ethylene units as the
basic polyethylenes. These ultra-high-density polyethylenes are used for
coating skis and for making highly stressed articles such as prostheses
and gears.

Propylene molecules contain three carbon atoms, but only two of them
are incorporated into the linear chains that make up a polypropylene molecule.
The third carbon atom appears as a methyl group attached to every second
carbon atom along the chain. The physical properties of the polypropylene,
such as its transparency, tensile strength, stiffness and brittleness, are
sensitive to the orientation of the methyl groups as well as to the molecular
weight of the molecule – which depends on the length of the chain. The
interactions between chains are also important. Where the orientation of
the methyl groups is regular, the chains align to produce crystalline sections.
In the amorphous, non-crystalline regions between these ‘crystallites’,
the polymer chains are arranged essentially at random. In typical polypropylenes
crystalline sections make up about 50 to 70 per cent of the molecule.

There are three principal types of polypropylene chains: these are isotactic,
syndiotactic and atactic. They differ in the orientation of methyl groups
along the chain (see Figure 1).

Isotactic polypropylene melts at about 165 °C, considerably higher
than the 110 °C at which linear polyethylenes melt. This difference
arises because polyethylene crystallises in extended zigzag chains which
are much more flexible than the helix-like chains of crystalline polypropylenes.
Isotactic polypropylene has excellent mechanical properties: it is used
to make car bumpers and interior fittings, cases for electrical tools, fibres
and foils. Syndiotactic polypropylene, which thanks to metallocene catalysis
has been available since 1990, is flexible, transparent and strongly resistant
to impacts. It is likely to prove most useful where transparency is required
but stiffness is not important – perhaps as film for food packaging. Atactic
polypropylene is an amorphous, waxy material that is used as a component
of additives and in blends with other polymers.

* * *

2: How do they work?

Compared to conventional heterogeneous catalysts such as the well-established
Ziegler-Natta sys-tem, the mechanism of metal-locene-based catalysts is
relatively easy to study because every catalytic site has the same molecular
structure. At Hoechst in Frankfurt, we think that the polymerisation process
follows the path shown in the figure, where the metal centre is chiral and
the metallocene has C₂ symmetry.

Until 1986, no one had any idea how these catalysts worked. Then Richard
Jordan from the University of Iowa and Howard Turner from the petrochemicals
company Exxon isolated some metallocenes in which the metal atom has lost
some electrons. Both groups of researchers discovered that several positively
charged or ‘cationic’ metallocenes are active in the polymerisation of ethylene
and in the stereospecific polymerisation of propylene. These results strongly
suggest that a metal cation is the active part of the catalyst.

Before the metallocene can catalyse the polymerisation it must first
react with methylalumoxane (MAO) with which is it mixed. The details of
this first step are not yet understood, but somehow the chlorine atoms are
removed from the metal atom, and the zirconium complex is transformed into
a methylated cation. MAO acts as a balancing anion, but the structure of
the cation-anion complex has yet to be discovered. We believe that the polymerisation
starts in the second step, when the alkyl group moves to a propylene molecule
that attaches itself at the chiral metal.

There are two possible orien-tations of propylene at the metal centre
(see Figure). If one of these is requires less energy to form, polymerisation
will be stereo-specific. The orientation which requires more energy is
the one where propylene is attached in such a way that the methyl group
comes close to the groups attached to the ring.

In the third step, propylene is inserted into the carbon-metal bond.
The growing polymer chain moves towards the mono-mer, which is still attached
to the metal. Then the chain and monomer exchange positions and the whole
sequence begins again, to give isotactic polypropylene.