CMU’s Krzysztof Matyjaszewski on
A ScienceWatch.com Featured Interview
Krzysztof Matyjaszewski of Carnegie Mellon
University discusses his work on living radical
polymerization, a means of creating highly ordered
polymers for use in coatings, sealants, drug delivery,
and other applications. Matyjaszewski himself remains
one of the most influential scientists in all of
In the world of chemistry, there are polymers and then there are
living polymers. The former are long-chained molecules, generally
with inactive chain ends, composed mostly of the same uniform elements
known as monomers. The latter are polymers with one or more active chain
ends that will continue to grow, adding monomer after monomer until the
chemist decides it’s time to terminate the process.
Living polymerization allows chemists to tailor-make polymers to suit the
needs of whatever application they have in mind, from biomedicine to
robotics to smart materials that respond to the most subtle imaginable
changes in the environment. "The possibilities are infinite," says chemist
Krzysztof Matyjaszewski, "and it all depends on conceiving and designing
the molecules in the proper way and then synthesizing them precisely. One
needs to control every aspect of the molecular structure to target the
Living radical polymerization became a viable technology in 1995 when
Matyjaszewski published his seminal paper on what he called "atom transfer
radical polyermization," or ATRP (J.S. Wang, K. Matyjaszewski, Journal
of the American Chemical Society, 117: 5614-5, 1995). That paper
has since been cited nearly 1,700 times.
"This level of control over
radical polymerization was the Holy Grail in synthetic
polymer chemistry," says Kris Matyjaszewski of
Carnegie Mellon University, Pittsburgh,
Matyjaszewski himself remains one of the most influential scientists in all
of chemistry. In the current update to Thomson Scientific’s
Essential Science IndicatorsSM database, Matyjaszewski
is the third-most-cited author in chemistry, with nearly 14,000 total
citations to papers published since 1997. His 2001 Chemical
Reviews paper on ATRP, for example, has been cited over 1,300 times
(see table), and nearly 50 of his papers have each received more than 100
Matyjaszewski, 57, received his bachelor’s degree in 1972 from the
Technical University of Moscow and his Ph.D. four years later from the
Polish Academy of Sciences. Since 1985, he has been at Carnegie Mellon
University in Pittsburgh, Pennsylvania, where he now holds the title of
J.C. Warner University Professor of Natural Sciences. He is also director
of the Center for Macromolecular Engineering, as well as director of the
CRP Consortium, which focuses on interacting with industrial corporations
to commercialize products based on controlled radical polymerization.
Matyjaszewski spoke to
ScienceWatch.com from his office in
Have you always been interested in the problem of
living polymerization, or was that a relatively new theme in your
research in the mid-1990s?
My research has always focused on polymer synthesis, on making well-defined
polymers. When I came to Carnegie Mellon in 1985, I started working on
living ionic polymerization and applying this concept to the synthesis of
well-defined organometallic and inorganic polymers. And then, in 1992 or
1993, I began thinking about the principle of extending living
polymerization to radical polymerization.
What does radical polymerization do for you that
living polymerization does not?
Radical polymerization, as with any chain-growth polymerization, consists
of several elementary reactions. Chain growth is first initiated, and then
the chain continues to grow until eventually it terminates by interaction
with a different radical. So you have the "birth" of the macromolecule,
which is its initiation, then its growth or propagation, and then
eventually it "dies" by termination. It can also "divorce" or generate
"offspring," by chain transfer, in which case the growth transfers to
Cited Papers by
(Ranked by total citations.)
K. Matyjaszewski, J.H. Xia,
"Atom transfer radical
polymerization,"Chem. Rev., 101(9): 2921-90,
K. Matyjaszewski, T.E. Patten, J.H.
radical polymerization. Kinetics of the
homogeneous atom transfer radical
polymerization of styrene,"J. Am. Chem. Soc., 119(4):
T.E. Patten, K. Matyjaszewski,
"Atom transfer radical
polymerization and the synthesis of
Materials, 10(12): 901, 1998.
V. Coessens, T. Pintauer, K.
polymers by atom transfer radical
Polymer Sci., 26(3): 337-77, 2001.
J.H. Xia, K. Matyjaszewski,
radical polymerization. Atom transfer
radical polymerization using
multidentate amine ligands,"Macromolecules, 30(25):
The main advantage of radical polymerization over prior-art living
polymerization is that one can polymerize a very broad range of monomers.
Radical polymerization today accounts for 50% of all commercial
polymers—perhaps 100 million tons worth each year.
In 1956, Michael Szwarc invented living anionic polymerization,
which is a process that proceeds without termination and transfer. These
reactions, however, require very stringent conditions—practically
zero moisture, zero air, and no impurities. This process has been
successfully applied to the synthesis of block copolymers that enable
commercial production of thermoplastic elastomers, an advanced rubber that
can be recycled and reprocessed many times.
In a way, this was the very beginning of nanotechnology. Living
polymerization allows incorporation of incompatible and immiscible polymer
segments together in the same macromolecule; these segments phase-separate
spontaneously into regular domains on the scale of nanometers. Commercial
products based on living anionic polymerization started appearing in the
1980s, but were based on polymerization of a very limited range of
non-polar monomers—such as styrene, isoprene, or butadiene.
The challenge has always been to make this polymerization process more
user-friendly, to be able to use other monomers, and to relax the process
conditions, which is what radical polymerization allows us to do.
So what’s the catch? Why isn’t radical
polymerization itself sufficient to allow for designing polymers from
the ground up?
The problem is that chain growth in a radical polymerization is very rapid;
a monomer unit is added to the growing chain every millisecond. After
perhaps 1,000 monomer units are added, the chain terminates.
So the life of a propagating chain—the time during which you can do
some chemistry—is only about one second. That’s a very short
time, and it makes radical polymerization very difficult to control. The
question was, how could one extend the life of the growing chains from one
second to one day, to provide enough time to do some chemistry and
functionalize the chain ends?
How did your 1995 JACS paper solve the
The idea I had in the early 1990s was to allow chains to add maybe one or
two monomer units and then send them to sleep. They would grow for one or
two milleconds and would then become inactive or dormant. Then, after a few
seconds or perhaps a minute, they would wake up again, add a couple of
polymer units, and again go back to sleep.
Thus, a one-second lifetime of a propagating chain is divided into a
thousand pieces of one millisecond each, and, between these one-millisecond
bursts of activity, a one-minute period of dormancy is inserted. This way,
a one-second chain lifetime is now expanded to approximately 1,000 minutes,
or one day. Therefore, we now have a day to perform a number of complex
One can add a second monomer and controllably incorporate it; one can use
multifunctional initiators to prepare very sophisticated structures, such
as star polymers, brushes, or even dendritic or hyper-branch polymers; and
one can make block copolymers, graft copolymers, or gradient copolymers.
This is the basic advantage of controlled or living radical
In the paper published originally in JACS in 1995, we applied this
concept to copper-mediated atom transfer radical polymerization, or ATRP.
We used very inexpensive alkyl halides as initiators and copper complexes
with simple commercially available ligands as the catalytic system.
ATRP adapts a known organic-chemistry reaction, atom transfer radical
addition, to polymer synthesis. Other transition metals can be applied as
catalysts for ATRP, most notably ruthenium, used by Sawamoto. Other
controlled/living radical polymerization systems developed in the past 20
years, based on nitroxides or dithioesters, were introduced by Rizzardo,
Georges, Hawker, Tordo, and other groups.
Did you realize how big this was going to
Yes, because this level of control over radical polymerization was the Holy
Grail in synthetic polymer chemistry. There are thousands of possible
monomers, and so many possible structures, and there is a real need to
control how they are incorporated into functional materials.
How has your research and the field itself evolved
in the dozen years since your breakthrough?
Well, early on it was very easy for us. Whatever we did in ATRP, we were
always first. We used new ligands, new initiators, applied new conditions.
We made many new structures—various block copolymers, stars,
hyperbranched polymers--and whatever we touched, it was something new.
Now we’re reaching a level of saturation in the field; there are many
groups throughout the world working on ATRP. Approximately 1,000 papers on
ATRP are published every year. At the same time, a deeper understanding of
the process allows new possibilities for precise macromolecular
engineering. The interest now is not only in understanding how these
systems work, but also in making new copolymers with properties targeting
Can you can give us an idea of the range of these
Well, we can start with biomedical applications—say, for drug
delivery, or signaling, or the engineering of tissue or bone. The polymers
might carry a particular drug to a bio-target and release the agent with a
controlled speed. A very interesting area is bio-conjugation, which
covalently links natural products and synthetic polymers. There are many
publications on applying ATRP to synthesis of "smart" materials that
respond to changes in temperature, pH, salt concentration, or even to
light. They can be designed to expand or collapse, and can be used as
artificial muscles in soft robotics.
Some materials made by ATRP can be very tough but also super-soft, a
material a thousand times softer than rubber. There are many new
opportunities in optoelectronic applications, in advanced coatings, in
sealants—but also in health-and-beauty products.
What’s been the biggest challenge in turning
these applications into reality?
We can now make many new polymers that didn’t exist before, but we
need to understand and correctly determine their properties. One of the
biggest challenges, therefore, is the precise correlation between the
molecular structure and the final material’s properties. These
properties depend not only on molecular structure but also on processing.
Therefore, we progressively focus more on the entire macromolecular
engineering process that includes not only the rational design of polymers
with controlled architecture, but also the precise synthesis of these
structures—controlling the processing in such a way that we can
achieve the final, targeted properties of these advanced materials. In
addition, for commercial production, the appropriate balance between cost
and performance is needed, and the recent advances in reducing the
concentration of catalyst in an ATRP will contribute to lower costs.
Until now, we focused on making all polymers exceptionally uniform.
However, another challenge is to actually control the
disorder—controlled heterogeneity, or controlled chaos, if you wish.
Copolymers with irregular (but still controlled) branching, segment size,
and monomer sequences (like in gradient copolymers) can provide materials
with entirely new morphologies and new properties.
Another challenge is to continuously increase control over the
polymerization and to exceed 99.99% selectivity. If you think about typical
organic chemistry, when people get a 90% yield, they’re happy. But in
polymer chemistry, 90% selectivity is a disaster. You can only add 10
monomer units before chain transfer or termination occurs. The selectivity
is only 9 to 1. So we need to go from 90% selectivity to 99.9% or more.
Naively, one would think that 99.99% selectivity
is impossible, more wishful science than reality. What makes you think
This has already been demonstrated. Polymers with very high molecular
weight, exceeding one million, have been prepared. This means that the same
reaction occurred repeatedly, 10,000 or more times, without error. But
doing that for one single polymer, for a homogenous polymer, is boring.
What’s exciting is to be able to switch to another monomer as
desired, so that the end result is, say, a block copolymer, with two
different segments in the same chain. These segments will phase-separate,
depending on their composition and size, or length, of each segment. There
might be five-nanometer separation between the two segments, or 100
nanometers between them.
Then, depending on the mole fraction of each monomer, the phase-separated
block copolymers adopt various regular structures: spheres or cylinders,
like wires, or lamellae—layered structures—but also more
complex morphologies with many potential applications in nanotechnology.
You recently had a breakthrough in lowering the
amount of catalyst needed. Can you tell us about that?
This was an idea based on the concurrent use of reducing agents. Even in
living radical polymerization, a small amount of unavoidable
termination occurs, and this process irreversibly consumes the catalyst.
Thus, relatively large amounts of catalysts were added to overcome this
The novel thought was to use a very small amount of catalysts, essentially
a few parts per million, together with environmentally friendly reducing
agents, which could be vitamin C, ascorbic acid, or a little sugar like
glucose to regenerate the catalyst.That kind of environmentally friendly
solution is what industry needs to reduce catalyst removal/recycle costs,
and that’s what we reported just last year.
How difficult is it to do ATRP in the laboratory?
It’s so simple that undergraduate students can do it without special
training. When I was a graduate student 30-odd years ago, doing living
anionic or cationic polymerization was extremely difficult. We had to learn
how to do glass blowing and solvent purification, and it would take a month
of training just to do the initial experiment.
We can now teach undergraduate students how to make almost-perfect block
copolymers on a bench in two hours.
What are your hopes for the future of radical
It’s now 12 years since this discovery and our first patent on ATRP.
Right now, this is still a process that’s only used commercially for
highly specialized products. I hope it will be used in more applications
and also for larger-volume applications.
I’d like to see people like physicists begin making these polymers
themselves, because it’s so easy. The same goes for biologists, who
can make new bio-conjugates: you take a natural product—say, a
segment of DNA or a peptide—and then link it together with one of the
polymers made by ATRP. Because you can design these polymers intelligently,
and force them to do what you want and when you want, you can attach such a
polymer to an enzyme and either shut it down or activate it on demand. You
can steer its activity this way.
Modification of surfaces is another tremendously important area. By
grafting well-defined polymers from a surface, you can change its behavior
and materials with totally new properties. Imagine growing polymers from a
surface by adding units one by one to create the desired responsive
surface. This allows preparation of extremely dense polymer films that can
change the lubrication properties of the surface, get a different
compressibility, prevent corrosion, and so on.
You can create such a dense polymer film layer that nothing can get beneath
it and detect the nature of the original surface. This will be very
important for biomedical applications and preparation of biocompatible and
Bridging between these sibling technologies—let’s say between
surface science and engineering, including optoelectronicss, robotics , and
bioengineering—is opening up exciting new areas of research at
surprising speed. I recall my very first discussions with some engineers,
and they couldn’t believe it was possible to create block copolymers
from almost any two monomers. Now we can do it easily and mareally useful
materials that never existed previously.
RELATED>Krzysztof Matyjaszewski answers a few questions
about his fast moving front paper (from the archives of ESI-Topics.com)
for January 2008, "Macromolecular engineering: From rational design
through precise macromolecular synthesis and processing to targeted
macroscopic material properties," (PROG POLYM SCI, 30 (8-9):
858-875 AUG-SEP 2005).
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