Conspectus
Atom-transfer radical polymerization (ATRP)
is a well-known technique
for the controlled polymerization of vinyl monomers under mild conditions.
However, as with any other radical polymerization, ATRP typically
requires rigorous oxygen exclusion, making it time-consuming and challenging
to use by nonexperts. In this Account, we discuss various approaches
to achieving oxygen tolerance in ATRP, presenting the overall progress
in the field.
Copper-mediated ATRP, which we first discovered
in the late 1990s,
uses a CuI/L activator that reversibly reacts with the
dormant C(sp3)–X polymer chain end, forming a X–CuII/L deactivator and a propagating radical. Oxygen interferes
with activation and chain propagation by quenching the radicals and
oxidizing the activator. At ATRP equilibrium, the activator is present
at a much higher concentration than the propagating radicals. Thus,
oxidation of the activator is the dominant inhibition pathway. In
conventional ATRP, this reaction is irreversible, so oxygen must be
strictly excluded to achieve good results.
Over the last two
decades, our group has developed several ATRP
techniques based on the concept of regenerating the activator. When
the oxidized activator is continuously converted back to its active
reduced form, then the catalytic system itself can act as an oxygen
scavenger. Regeneration can be accomplished by reducing agents and
photo-, electro-, and mechanochemical stimuli. This family of methods
offers a degree of oxygen tolerance, but most of them can tolerate
only a limited amount of oxygen and do not allow polymerization in
an open vessel.
More recently, we discovered that enzymes can
be used in auxiliary
catalytic systems that directly deoxygenate the reaction medium and
protect the polymerization process. We developed a method that uses
glucose oxidase (GOx), glucose, and sodium pyruvate to very effectively
scavenge oxygen and enable open-vessel ATRP. By adding a second enzyme,
horseradish peroxidase (HPR), we managed to extend the role of the
auxiliary enzymatic system to generating carbon-based radicals and
changed ATRP from an oxygen-sensitive to an oxygen-fueled reaction.
While performing control experiments for the enzymatic methods,
we noticed that using sodium pyruvate under UV irradiation triggers
polymerization without the presence of GOx. This serendipitous discovery
allowed us to develop the first oxygen-proof, small-molecule-based,
photoinduced ATRP system. It has oxygen tolerance similar to that
of the enzymatic methods, exhibits superior compatibility with both
aqueous media and organic solvents, and avoids problems associated
with purifying polymers from enzymes. The system was able to rapidly
polymerize N-isopropylacrylamide,
a challenging monomer, with a high degree of control.
These
contributions have substantially simplified the use of ATRP,
making it more practical and accessible to everyone.
