Scientists led by Carnegie Mellon Univ. chemist Krzysztof
Matyjaszewski are using electricity from a battery to drive atom transfer
radical polymerization (ATRP), a widely used method of creating industrial
plastics. The environmentally friendly approach, reported in Science, represents a breakthrough in
the level of control scientists can achieve over the ATRP process, which will
allow for the creation of even more complex and specialized materials.
ATRP, first developed by Matyjaszewski in 1995, allows
scientists to easily form polymers by putting together component parts, called
monomers, in a controlled piece-by-piece fashion. Assembling polymers in such a
manner has allowed scientists to create a wide range of polymers with highly
specific, tailored functionalities. ATRP has been used to develop cosmetics,
coatings, adhesives, and drug delivery systems, and is used to develop “smart”
materials—those that respond to environmental changes, such as changes in
temperature, light, pressure, or pH.
The current study represents the latest in a series of
advances Matyjaszewski’s research group has made since ATRP’s inception that
make the technique more precise and more environmentally friendly. In a process
they are calling electrochemically mediated ATRP, or eATRP, the researchers
used a computer-controlled battery to apply an electrochemical potential across
the ATRP reaction.
“This marks the first time that we’ve paired
electrochemistry with ATRP, and the results were startlingly successful,” said
Matyjaszewski, the J.C. Warner Professor of Natural Sciences at CMU. “We found
that by adjusting the current and voltage we could slow and speed up, or even
start and stop the reaction on-demand. This gives us a great deal more
flexibility in conducting our reactions that should lead to the development of
precisely engineered materials.”
In traditional ATRP reactions scientists use a copper
catalyst to grow a complex polymer structure by adding a few monomeric units at
a time to the polymer chain. The process relies on paired reduction-oxidation
(redox) reactions between two species of copper—the activator CuI and
deactivator CuII—where the two catalysts exchange electrons back and forth.
Occasionally, one of the exchanges will spontaneously stop, called a radical
termination, resulting in the accumulation of CuII. To keep the polymerization
going, researchers must rebalance the system by compensating for the excess
CuII.
In the early ATRP experiments, scientists addressed this
problem by adding more CuI to the system. This generated materials with high,
sometimes toxic, levels of copper, reaching around 5,000 ppm. Such levels of
copper are hard to remove using current industrial equipment. As an alternative,
Matyjaszewski and colleagues developed novel methods for using activators and
reducing agents to reactivate the CuII. Most notably, they found that
environmentally friendly reducing agents like sugars or vitamin C were highly
effective in reducing the amount of copper catalyst used in ATRP reactions.
In the current study, Matyjaszewski and Visiting Assistant
Professor of Chemistry Andrew Magenau looked to electrochemistry as a means for
maintaining balance in ATRP reactions. They found that adding electricity
capitalized on the redox reaction by moderating the transfer of electrons. This
allowed them to compensate for the radical terminations and reduce the amount
of copper needed to run ATRP. As a result the amount of copper in the system
was reduced to 50 ppm, a 100-fold decrease. In terms of creating a greener,
less toxic form of ATRP, this amount rivaled Matyjaszewski’s previous studies
that used vitamin C and sugars as reducing agents, but has the added benefit of
not requiring the addition of any additional organic or inorganic reducing
agents.
The researchers found that applying electricity to the
system also gave them more precise control over the reaction. The
computer-controlled battery allowed them to manipulate the ATRP process in
real-time by changing the current or voltage.
The researchers have used this process to create the
standard types of polymers made with ATRP: star, brush, and block copolymers.
They believe that the meticulous control eATRP gives them over the rate of
polymerization will allow for the creation of polymers with even more complex architectures.
Co-authors of the study include Nicholas Strandwitz from the
Kavli Nanoscience Institute and Beckman Institute at the California Institute
of Technology and Armando Gennaro of the Univ.
of Padova, Italy.
The study was funded by the National Science Foundation and
the CRP Consortium at Carnegie Mellon.
