New method allows for more precise control over ATRP
http://www.rdmag.com/News/2011/04/Manufacturing-Materials-New-MScientists 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 coetics, coatings,
adhesives, and drug delivery systems, and is used to develop "art"
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.
