Biomedical Engineering Reference
In-Depth Information
The postmodii cation strategy makes use of a monolithic support with reactive moi-
eties that are derivatized and chirally functionalized, respectively, in a second step.
Pioneering works for chiral polyacrylamide gels were performed by Koide and
Ueno [175,176], for enantioselective polymethacrylate monoliths by Svec's group
[184] and for monolithic MIPs by Lin et al. [190] and Nilsson and coworkers [191,192].
A few details are outlined hereafter.
In 1998, Peters et al. prepared Pirkle-type polymethacrylate monoliths by
in situ copolymerization of
N
-[(2-methacryloyloxyethyl)oxycarbonyl]-(
S
)-valine-
3,5-dimethylanilide as chiral monomer with ethylenedimethacrylate (EDMA),
butylmethacrylate or glycidylmethacrylate (GMA) as comonomer and 2-acrylamido-
2-methyl-1-propanesulfonic acid (AMPS) as an ionizable comonomer for EOF gener-
ation in the presence of a porogenic solvent consisting of 1-propanol, 1,4-butanediol,
and water. The polymerization was carried out by thermal initiation with
'-
azobis(isobutyronitrile) (AIBN) as radical initiator. These experiments toward
enantioselective monolithic capillary columns were successful in that they enabled
the baseline separation of a model test solute. However, only the monolith synthe-
sized from the GMA-comonomer yielded reasonable efi ciencies when its epoxide
groups were hydrolyzed into a diol surface.
The above in situ molding approach is very l exible and allows the tuning of the
polymer in terms of porosity, macropore diameter, and surface chemistry in order
to meet the specii c requirements of CEC and enantiomer separation simultane-
ously. This was shown by detailed studies on chiral anion-exchange type poly-
mer monoliths [185-188]. Copolymerization of a hydrophilic comonomer such as
2-hydroxyethylmethacrylate (HEMA) reducing nonspecii c interactions with the
polymer matrix as well as the preclusion of the ionizable comonomer from the
polymerization mixture and use of an ionizable chiral monomer (
1
) or (
2
) (see
Figure 14.17) constituted major advancements toward efi cient in situ prepared
polymethacrylate type chiral monoliths for CEC application. Highly suitable
monolithic capillary columns for the separation of chiral acidic compounds could
be obtained by a single-step thermally initiated or photo-polymerization reaction
of chiral monomer, HEMA, and EDMA. Characteristically, every monomer sys-
tem requires specii c optimization of the porogen composition and detailed studies
in this regard have been performed with monomers
1
[185] and
2
[188]. Changes
in the porogen composition, e.g., variations of the 1-dodecanol/cyclohexanol ratio
which served as pore-forming agents during the preparation of these monolithic
columns, allow for adjusting macropore diameter of the materials (Figure 14.17c).
Characteristic morphologies for such organic polymer monoliths [68] are shown in
Figure 14.17b. Individual polymer microglobules are irregularly clustered together
and cross-linked. By decreasing the macropore diameter (e.g., in the given example
of Figure 14.17b by decreasing the dodecanol content in the polymerization mix
from 60% (w/w) to 47% (w/w) yielding materials with 3.2
α
,
α
m pore size),
the microglobules get smaller and the polymer morphology appears more homoge-
nous. Associated with such a decrease in the pore diameter is thus an improvement
in the chromatographic efi ciencies (mainly due to a reduced
A
-term) (see below).
These morphological aspects are therefore of key importance because they deter-
mine the kinetic behavior and l ow properties of the monolithic column.
μ
m and 0.8
μ
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