Controlled Radical Polymerization Guide - Sigma-Aldrich

Copper(I)-mediated Living Radical
Polymerization in the Presence of Pyridylmethanimine Ligands
10
ATRP
the steric bulk of the ligands. 14-15 The combination of these effects is to
stabilize copper(I) towards oxidation such that it is even possible to
bubble air through a solution without causing oxidation.
Figure 3. Crystal structure of [L2Cu]Br, with L = N-ethyl 2-pyridylmethanimine.
In addition to the ligands complexing in a tetrahedral way during
polymerization, the monomer can act as a competing ligand (Figure 4).
This will occur for all methacrylates and acrylates, which inherently
contain good coordinating groups. This is most profound with tertiary
amine-containing ligands which result in competitive binding that can
be observed by a small change in reactivity ratios when compared to
free radical polymerization values.
N
Cu
N
N
R
N
R
R = alkyl group
+
N
O
O
O
O
Cu
N
+ N
N N
Figure 4. Equilibrium involving DMAEMA (Aldrich Prod. No. 234907)/N-propyl 2-pyridyl
methanimine / copper(I) complexes.
It must be remembered that the copper complex formed with all ligand
types is always a salt with the transition metal complex forming the
cation. This can produce a solubility problem in non-polar solvents. Alkyl
pyridylmethanimine ligands help circumvent this by allowing for a facile
change in the alkyl group. As the alkyl chain is lengthened, the solubility
in non-polar solvents increases. For non-polar monomers such as
long chain acrylates and styrene, longer chain ligands need to be used
(n-butyl, n-pentyl, n-octyl). However, for polar monomers, it is desirable
to use the smallest alkyl chain possible for atom economy, and
purification reasons. An interesting, but very useful consequence of this
solubility is that during polymerization at high solids the polarity of the
reaction medium will decrease markedly. Thus, during the polymerization
of methyl methacrylate (MMA) at 30-50% solids the polarity
decreases and often the catalyst will precipitate from solution, generally
as a highly colored oil throughout. This can be avoided by the use of a
more non-polar ligand with a longer alkyl group.
However, this potential problem can also have a positive effect with
regards to catalyst removal, which can be cited as an unfortunate aspect
of ATRP. The solubility of these ligands decreases as the polymerization
progresses and upon lowering the temperature from reaction temperature
(typically 60-80 °C) to ambient. This can result in almost complete
precipitation of the catalyst as sticky oil which adheres to the reactor,
allowing the catalyst to be removed via trituration/decanting. Alternatively,
the reaction mixture can be filtered through a small column/
pad of fine filter agents such as Celite®, basic alumina, basic silica which
remove the particulate catalyst, ligand and complex, since they adhere
to the medium. It is noted that care needs to be taken with certain
polymers which might also complex with the filtration medium.
Aldrich.com
R
N R
Polymerization is typically carried out under an inert atmosphere with
reagents being freeze, pump, thaw degassed. However, on scale-up it is
acceptable to bubble nitrogen through the reagents for 15-30 minutes
or to heat under nitrogen; 100 kg of MMA has been polymerized
effectively in this way.
Typical Reaction Conditions
Up to 66 weight % solids
Range of solvents from water and alcohols to toluene and cyclic ethers
60-80 °C
Inert atmosphere, usually nitrogen
Robust to many different types of functional group 19-21
Summary
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The family of N-alkyl-2-pyridylmethanimines offers an alternative to
aliphatic amine ligands and bipyridine for copper(I)-mediated living
radical polymerization or ATRP. They yield very stable copper(I) which
allows for excellent polymerization of most methacrylate monomers in
polar and non-polar solvents. There is little evidence of oxidation or
disproportionation even with the most polar monomers and in the most
polar solvents 16-17 which is often the case for the aliphatic amine type
ligands. 18 Preparation of the ligands and their use in methacrylate
polymerization is facile. Typically more than 80% of researchers will
achieve polymers with PDI