START with the polyester leisure suit tucked in the back of the wardrobe and
make your way around the house. Guaranteed, the place will be full of plastics
and artificial fibres. Shampoo bottles, water pipes, hi-fi casing, kitchen
worktops, guttering鈥攖he 20th century would hardly work without them.
But despite their huge variety, most plastics and artificial fibres have
remarkably similar origins. They are made from long stringy molecules called
polymers which in turn are made from molecular building blocks called monomers.
When making plastics on an industrial scale, chemists can only choose from a
dozen or so groups of monomers and from only one of two ways of joining them
together. But from this surprisingly limited recipe book, chemists can create
almost all the plastics and artificial fibres that create our world.
But the downside is that they have no way of accurately controlling the
length of the molecular chains. And as the 21st century nears, chemists want
more: they want to be able to precision-engineer the lengths of their polymers
so they can create materials with specific or even undreamt-of properties. And
they want to join different polymers together to produce materials with the
properties of both. These tasks are so tough that they can normally only be
achieved at extraordinary cost.
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That is set to change. Chemists in California have stumbled on an
extraordinary molecule that offers them the control they鈥檙e looking for to an
unprecedented degree鈥攁nd in a way that鈥檚 easy to use on the factory floor.
The work opens the way for a new generation of plastics and artificial fibres
with mind-boggling combinations of properties. Chemists today can only guess at
the extraordinary applications that might become possible.
Right now, the two main techniques for making polymers on a large scale are
relatively straightforward鈥攂ut their limitations are inherent in the
processes. The polyester in a leisure suit is made using a technique known as
step-growth polymerisation. This begins with the creation of monomers that are
reactive at both ends. When two of these monomers combine they form a larger
molecule, a dimer, that is also reactive at both ends. Other monomers can join
at either end and the growth continues until the reaction is stopped. This can
be done in several ways. One way is to add a special kind of monomer that is
reactive at only one end. These cap the chains, preventing further growth.
The problem is that any reactive molecule can join up with any other. So a
short chain can link to a longer chain and a long chain can link to an even
longer one, creating a huge chain. The result is a material that contains
polymers of a wide range of lengths. Unfortunately, the material鈥檚 properties,
such as its strength, depend on these lengths. So, if the lengths are hard to
control, the properties will be hard to predict. Yet even the wayward materials
created this way end up in extremely useful products such as shatterproof
bottles, nylon and the polyurethane foam used in sofas.
The second way of creating polymers is called chain-growth polymerisation. It
too is responsible for a long list of plastics that includes the grocery bags in
your local supermarket as well as the polyvinyl chloride known as PVC used to
make water pipes and duct tape. Chain-growth polymerisation starts with
molecules called initiators that split into two parts called free radicals. Free
radicals have a highly reactive, unpaired electron that hunt out and bond with
molecules containing double bonds. This bonding creates a larger molecule that
is also a free radical.
The trick with chain-growth polymerisation is to use a monomer that contains
a double bond that the initiating free radical will attack. Once attacked, the
monomer itself becomes a free radical that will attack and bond to another
monomer creating a free radical dimer. This attacks and bonds to yet another
monomer, building a free radical trimer and so on. There are several ways to end
this reaction, the most obvious being the meeting of two free radical chains
that react to form a stable chain. But since this kind of termination occurs
randomly, the final chain can be almost any length, so these materials suffer
the same shortcomings as those created by step-growth polymerisation.
Not surprisingly, then, research groups the world over have been hunting for
ways to control the lengths of polymer chains. In 1993, chemists at the Xerox
Research Centre in Ontario, Canada, led by Michael Georges made a promising
breakthrough using a variation of the traditional chain-growth
polymerisation.
Georges鈥� team used a specific type of initiator containing a molecule called
tetramethylpiperidine-N-oxide, or TEMPO, that not only started the
polymerisation reaction but controlled it as it went along. Georges reasoned
that nitroxide groups in this initiator somehow ensured that chains linked up
only with monomers and not with each other. Exactly how it worked, Georges was
not sure but clearly it had the potential to create materials consisting of
chains of similar length. He called the technique nitroxide-mediated living free
radical polymerisation.
But TEMPO has since turned out to have drawbacks that prevent its widespread
use. For a start, it only works with styrene monomers to produce polystyrenes.
While these styrenes are used in everything from computer casings to office
coffee cups, TEMPO has steadfastly refused to work well with other monomers. It
also works best at high temperatures which are costly to maintain. Improving the
molecule is not easy because the chemical steps involved are not well
understood. And designing alternative initiators is just as difficult without
this detailed understanding.
Enter Rebecca Braslau, an organic chemist at the University of California at
Santa Cruz, and Craig Hawker, a chemist at the IBM Almaden Research Center in
San Jose. Hawker is interested in this problem because the variation in chain
lengths also turns out to be one of the major obstacles preventing computer
scientists from using polymers to mask areas as small as they would like on
silicon chips. But the edges of these masks simply cannot be well defined if
polymers stick out like spaghetti. 鈥淭he smoother the edge, the better,鈥� he
says.
Braslau, on the other hand, was unaware of the frantic search for better
initiators. She and her postdoctoral student, Vladimir Chaplinski, had been busy
making an entire library of nitroxide molecules similar to TEMPO, but for
unrelated research. In 1997, Braslau and Hawker met at a conference of the
American Chemical Society and immediately recognised the potential of each
other鈥檚 work. 鈥淚 didn鈥檛 even know that they would be useful in polymerisation,鈥�
says Braslau. Last year, they began a collaboration in which she prepared and
sent samples of two dozen nitroxides for Hawker to test.
Huge promise
In Hawker鈥檚 lab on a hill above Silicon Valley, he and postdoc Didier Benoit
went to work testing the samples and found one with huge promise. The so-called
Vladimir Initiator works not only with styrenes but with the whole family of
monomers that styrenes belong to鈥攙inyl monomers. These include
acrylamides, water-soluble polymers which are used in paints, and acrylates
which can be used as adhesives.
The results have been so encouraging that Hawker believes that the Vladimir
Initiator may lead to the development of entirely new materials. 鈥淲e鈥檙e talking
about dominating technology here,鈥� he says. 鈥淯sing the new molecules means we鈥檙e
able to do all these things we couldn鈥檛 do before.鈥� They plan to announce their
results next month at the annual meeting of the American Chemical Society in
Anaheim, California.
One task ahead is to find out exactly how the Vladimir Initiator works. It鈥檚
best thought of as a nitroxide molecule linked to a monomer. It can be made to
split in two simply by heating it to a relatively low temperature. One part is a
free radical monomer and the other is a free radical nitroxide molecule that
Braslau calls the nitroxide plug (see Diagram). Braslau believes the
polymerisation occurs when a free radical monomer attacks and bonds to an
ordinary monomer, thereby creating a free radical dimer. At this point, the
nitroxide plug snaps back into place, ending the reaction. It only starts again
when heat forces the nitroxide plug away from the dimer, creating a free radical
that attacks and bonds to another nearby monomer. This creates a free radical
trimer and the nitroxide plug snaps back into place.
This opening and closing of this bond is like a pair of hungry jaws snapping
for food鈥攂ut only certain types of food. Somehow the nitroxide plug
prevents the free radical polymer from biting onto another chain. This point is
crucial. It means that the polymers grow only by 鈥渆ating鈥� monomers and so all
grow at more or less the same rate. When there are no more monomers left, the
jaws close and the nitroxide plug simply remains in place. The result is a
material in which the polymers are all more or less the same length.
In the immediate future, Hawker sees applications in the silicon chip
manufacture. The features on these chips have to be carved in precise patterns
created by a process called photolithography. This involves applying a polymer
called a photoresist to the silicon wafer. Then the desired pattern is projected
onto the polymer layer using ultraviolet light which chemically alters the
polymer so that it becomes susceptible to solvents that wash it away. The
resulting cavities, which are in the desired pattern, can then be used to create
the features on the chip.
These features are tiny. And to make chips faster they must be made smaller
still, less than 0.2 micrometres across, hundreds of times smaller than the
width of a human hair. By making all the polymer chains in the photoresist
material similar in length, it is possible to create very sharp features on an
even smaller scale than is possible now. Chips made with these new polymers
could be up to 40 per cent faster than those available today.
One of the reasons the process is called 鈥渓iving,鈥� according to Braslau, is
that the plug at the end of the chain can be reactivated. If this is done in the
presence of a different monomer, then it is possible to grow an entirely
different chain on the end of the first. These molecules are called block
copolymers and they have extraordinary potential because they are materials with
combinations of properties.
A good example of block copolymers are used in inkjet printers. These contain
inks made from molecules that are soluble at one end but insoluble at the other.
The insoluble portion of the polymer attaches to an insoluble pigment called
carbon black. The soluble portion forms a shell around the pigment so that it
can be suspended in water and applied as an ink. Without the soluble portion,
the pigment would simply float on the water. The process currently used to
create these block copolymers is expensive, and does not work well with as many
types of monomers as the new process using the Vladimir Initiator.
One big potential application is the creation of polymer nanospheres that
might be used to deliver drugs into the body. At Washington University,
Missouri, chemist Karen Wooley says that the Vladimir Initiator allows her to
create suitable block copolymers in a cost-effective way. The soluble end of
polymer chains surrounds a core of insoluble material that could one day be
filled with drugs. The solubility of the outside shell means the nanospheres
could be suspended in the bloodstream.
Today, the cost of producing block copolymers means that they are not well
understood, says Georges. He believes that once they are widely available at
reasonable prices, new applications will follow at an extraordinary rate. 鈥淭hey
haven鈥檛 been exploited. If this area is successful, innovative companies will
find applications that aren鈥檛 there right now.鈥�
Hawker and Georges believe the technology will take around three years to
perfect before it鈥檚 ready for the commercial world. Clearly, the second plastics
revolution is just beginning. Who knows what the leisure suit tucked in the back
of your wardrobe will be made from in the next few years.