A New Mechanistic Diagram for Molecularly Imprinted Polymers
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A New Mechanistic Diagram for Molecularly Imprinted Polymers David A. Spivak Department of Chemistry Louisiana State University Baton Rouge, LA 70803
ABSTRACT A new mechanistic diagram describing the non-covalent molecular imprinting process is put forth in the text. A significant consequence of the new mechanistic picture is that the prepolymer complex structure does not necessarily reflect the structure of the final binding sites in the polymer. Two independent studies are presented in combined form that support the suggested changes to the mechanistic diagram. In the first study, the maximum number of functional groups surrounding the template molecule in solution are shown to be less than the average number of functional groups in the binding sites of the polymers. In the second study, shape selectivity is shown to be an important contributor to molecular recognition by the imprinted polymers; which is significant because contributions of shape cannot be predicted by the solution phase pre-polymer complex.
INTRODUCTION The modern era of molecular imprinting was ushered in by Wulff and coworkers.1 Early on to explain the molecular imprinting process, they used a diagram similar to that presented in figure 1.2 The figure indicates that the initial step in molecular imprinting is the pre-organization in solution of functionalized monomers with the template molecule via either covalent or nonpre-polymer
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complex co-polym.
+ Figure 1. Classical outline of the molecular imprinting strategy.
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covalent interactions. This so-called pre-polymer complex is then copolymerized with an excess of cross-linking monomer in the presence of an equal volume of inert solvent (porogen) and a free radical initiator. Thermal or photochemical initiated polymerization results in a highly cross-linked insoluble polymer. Removal of the template, in most cases by extraction or hydrolysis, leaves sites in the polymer that have incorporated the functionalized monomers. These sites are complementary in size and shape to the template molecule, resembling the "lock and key" paradigm of enzymes. Moreover, the functional groups are specifically positioned to converge on the template molecule in a reciprocal fashion. Recently, our group has published two independent studies which suggest a change in the classical representation of the noncovalent molecular imprinting method.3-5 This change reflects the findings that the structure, and thus behavior, of the binding sites in the molecularly imprinted polymer is not necessarily predicted by the pre-polymer complex structure. Although seemingly minor, even slight changes in the represented mechanism for molecularly imprinted site formation can have a profound effect on the way in which researchers approach solutions to problems with the molecular imprinting method. Herein we present the data and concepts in a unified format that support a change in the classical molecular imprinting diagram.
EXPERIMENTAL DETAILS Materials and methods for the synthesis of compounds and polymers,
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