Page 175 - Chiral Separation Techniques

6.2 Molecular Imprinting Approaches 153

6.2 Molecular Imprinting Approaches

Molecularly imprinted polymers (MIPs) can be prepared according to a number of
approaches that are different in the way the template is linked to the functional
monomer and subsequently to the polymeric binding sites (Fig. 6-1). Thus, the tem-
plate can be linked and subsequently recognized by virtually any combination of
cleavable covalent bonds, metal ion co-ordination or noncovalent bonds. The first
example of molecular imprinting of organic network polymers introduced by Wulff
was based on a covalent attachment strategy i.e. covalent monomer–template, cova-
lent polymer–template [12].
Currently, the most widely applied technique to generate molecularly imprinted
binding sites is represented by the noncovalent route developed by the group of Mos-
bach [13]. This makes use of noncovalent self-assembly of the template with func-
tional monomers prior to polymerization, free radical polymerization with a
crosslinking monomer, and then template extraction followed by rebinding by non-
covalent interactions. Although the preparation of a MIP by this method is techni-
cally simple, it relies on the success of stabilization of the relatively weak interac-
tions between the template and the functional monomers. Stable monomer–template
assemblies will in turn lead to a larger concentration of high affinity binding sites in
the resulting polymer. The materials can be synthesized in any standard equipped
laboratory in a relatively short time, and some of the MIPs exhibit binding affinities
and selectivities in the order of those exhibited by antibodies towards their antigens.
Nevertheless, in order to develop a protocol for the recognition of any given target,
all of the alternative linkage strategies must be taken into account.
Most MIPs are synthesized by free radical polymerization of functional monoun-
saturated (vinylic, acrylic, methacrylic) monomers and an excess of crosslinking di-
or tri- unsaturated (vinylic, acrylic, methacrylic) monomers, resulting in porous
organic network materials. These polymerizations have the advantage of being rela-
tively robust, allowing polymers to be prepared in high yield using different solvents
(aqueous or organic) and at different temperatures [14]. This is necessary in view of
the varying solubilities of the template molecules.
The most successful noncovalent imprinting systems are based on commodity
acrylic or methacrylic monomers, such as methacrylic acid (MAA), crosslinked with
ethyleneglycol dimethacrylate (EDMA). Initially, derivatives of amino acid enan-
tiomers were used as templates for the preparation of imprinted stationary phases for
chiral separations (MICSPs), but this system has proven generally applicable to the
imprinting of templates allowing hydrogen bonding or electrostatic interactions to
develop with MAA [15, 16]. The procedure applied to the imprinting with l-pheny-
lalanine anilide (L-PA) is outlined in Fig. 6-2. In the first step, the template (L-PA),
the functional monomer (MAA) and the crosslinking monomer (EDMA) are dis-
solved in a poorly hydrogen bonding solvent (porogen) of low to medium polarity.
The free radical polymerization is then initiated with an azo initiator, commonly azo-
N,N’-bis-isobutyronitrile (AIBN) either by photochemical homolysis below room
temperature [16, 17] or thermochemically at 60 °C or higher [15]. Lower thermo-