Lianbing Ren, Zhen Liu, Yunchun Liu
Aug 24, 2009
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Angewandte Chemie
Abstract
Polymeric monoliths, which show fast convective mass transfer between the monolith bed and the surrounding solution, have already become important materials in separation science and (bio-)catalysis. Although some purely polymeric monoliths (such as acrylate-based monoliths) are excellent separation media, functionalization is of utmost importance in most situations. Conventional functionalization can be classified into two strategies: 1) copolymerization of functional monomers and 2) postpolymerization functionalization. In both cases, the designed functionalities depend on the structure and properties of functional monomers. To obtain a certain functionality, a high-purity monomer with appropriate structure and properties is indispensable. Boronate affinity chromatography (BAC) has been a useful means for the specific capture and separation of cisdiol-containing biomolecules, such as saccharides, nucleosides, and glycoproteins, since the early 1980s. The principle relies on reversible covalent complex formation/dissociation between boronic acids and cis diols in an alkaline/acidic aqueous solution. Recently, boronate-functionalized monoliths were synthesized by copolymerization and postpolymerization functionalization. However, as for other BACbased techniques, an apparent disadvantage is that the chromatography in aqueous solution has to be performed in alkaline media and can lead to the degradation of labile compounds. Thus, boronate-functionalized monoliths that function at neutral pH would be highly desirable for physiological samples. A conventional solution to this problem is to decrease the pKa value of the ligands by synthesizing novel boronic acids with exquisite structures through: 1) the introduction of an electron-withdrawing group, such as a sulfonyl group, into the ligand molecules, 2) the introduction of a neighboring amino group capable of B–N coordination into the ligand molecules (Wulff-type boronic acids), or 3) the replacement of intramolecular B–N coordination with intramolecular B–O coordination (improved Wulff-type boronic acids). According to these strategies, to make a boronate-functionalized monolith that is functional under neutral conditions, a boronic acid monomer with a low pKa value, if not commercially available, must be synthesized and purified first through tedious procedures. Herein, we present a new approach— ring-opening polymerization with synergistic co-monomers— for the preparation of a boronate-functionalized polymeric monolith that functions under neutral conditions, without the synthesis and purification of a single functional monomer. The main synthetic route is based on the ring-opening polymerization protocol established recently by Tanaka and co-workers. An epoxy resin, a diamine curing agent, and a porogenic solvent are required for the preparation of the monolith. To obviate the inconvenience of the synthesis of a new diamine monomer, we took advantage of the coordination of m-aminophenylboronic acid (mPBA, 1) with 1,6hexamethylenediamine (HMDA, 2) to form a stable complex 3 with a B N bond (Scheme 1). The B–N-coordinated complex was used as a diamine curing agent in a ring-opening polymerization reaction with the epoxy resin tris(2,3-epoxypropyl)isocyanurate (TEPIC, 4) to form a macroporous monolith. Since the conformation of the coordinated complex is “frozen” by the polymerization, the coordinated complex