MIPs polymerization

MIPs can be synthesized in the pores and on the surface of pre-made porous particles. Porous silica particles have been applied for this purpose. To ensure that the imprinted polymer is attached firmly to the particle, the particles are often chemically modified by coupling of polymerizable groups or initiator molecules to the particle surface prior to the MIP polymerization [100-102]. The use of immobilized initiators is often referred to as the iniferter (initiator-transfer agent-terminator) approach [103]. The method has been applied to the imprinting of a range of templates [104—107]. 

Recently, an in-depth review on molecular imprinted membranes has been published by Piletsky et al. [4]. Four preparation strategies for MIP membranes can be distinguished (i) in-situ polymerization by bulk crosslinking (ii) preparation by dry phase inversion with a casting/solvent evaporation process [45-51] (iii) preparation by wet phase inversion with a casting/immersion precipitation [52-54] and (iv) surface imprinting. 

Several selective interactions by MIP membrane systems have been reported. For example, an L-phenylalanine imprinted membrane prepared by in-situ crosslinking polymerization showed different fluxes for various amino acids [44]. Yoshikawa et al. [51] have prepared molecular imprinted membranes from a membrane material which bears a tetrapeptide residue (DIDE resin (7)), using the dry phase inversion procedure. It was found that a membrane which contains an oligopeptide residue from an L-amino acid and is imprinted with an L-amino acid derivative, recognizes the L-isomer in preference to the corresponding D-isomer, and vice versa. Exceptional difference in sorption selectivity between theophylline and caffeine was observed for poly(acrylonitrile-co-acrylic acid) blend membranes prepared by the wet phase inversion technique [53]. 

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 template 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, covalent polymer-template [12]. 

Various novel imprinting techniques have also been presented recently. For instance, latex particles surfaces were imprinted with a cholesterol derivative in a core-shell emulsion polymerization. This was performed in a two-step procedure starting with polymerizing DVB over a polystyrene core followed by a second polymerization with a vinyl surfactant and a surfactant/cholesterol-hybrid molecule as monomer and template, respectively. The submicrometer particles did bind cholesterol in a mixture of 2-propanol (60%) and water [134]. Also new is a technique for the orientated immobilization of templates on silica surfaces [ 135]. Molecular imprinting was performed in this case by generating a polymer covering the silica as well as templates. This step was followed by the dissolution of the silica support with hydrofluoric acid. Theophylline selective MIP were obtained. 

After polymerization, the MIP or functionalized polymer matrix is dried, groimd, sieved, and packed. 

FIGURE 1.6 Influence of the polymerization temperature on the porosity of polyfglycidyl methacrylate-co-ethylene dimethacrylate) monoliths determined by MIP. (a) Differential pore size distribution curves of the polyfglycidyl methacrylate-co-ethylene dimethacrylate) rods, prepared by 22 h polymerization at a temperature of 55°C ( ), 12 h at 70°C ( ), and a temperature increased during the polymerization from 50°C to 70°C in steps by 5°C lasting 1 h each and kept at 70°C for another 4h ( ). (Reprinted with permission from Svec, F. and Frechet, Chem. Mater., 1, 707, 1995. Copyright 1995, American Chemical Society.) (b)... 

The polymerization time as a polymerization parameter for adjustment of the porous properties of thermally initiated copolymers has recently been characterized [111]. A polymerization mixture comprising methylstyrene and l,2-bis(p-vinylbenzyl)ethane as monomers was subjected to thermally initiated copolymerization for different times (0.75, 1.0, 1.5, 2, 6, 12, and 24h) at 65°C. The mixtures were polymerized in silanized 200pm I.D. capillary columns as well as in glass vials for ISEC and MIP/BET measurements, respectively. 

The results of the MIP analyses of the bulk polymers are illustrated in Figure 1.7. It could be demonstrated that the polymerization time is capable of influencing the shape of the pore distribution itself, rather than shifting a narrow macropore distribution (and thus the pore-size maximum) along the scale of pore diameter (see effect of the porogenic solvent in Section 1.3.2.2 and Figure 1.5). On... 

FIGURE 1.7 Influence of the polymerization time on the porosity of monolithic MS/BVPE polymer networks, determined by MIP. Reduction of the polymerization time converts a narrow monomodal pore distribution into a broad bimodal distribution, comprising mesopores. 

Even if MIP and BET are widely accepted regarding the characterization of HPLC stationary phases, they are only applicable to the samples in the dry state. In order to investigate the impact of polymerization time on the porous properties of wet monolithic columns, ISEC measurements of 200 jm I.D. poly(p-methylstyrene-co-l,2-bis(vinylphenyl)ethane) (MS/BVPE) capillary columns (prepared using a total polymerization time ranging from 45 min to 24 h) have been additionally evaluated (see Table 1.2 for a summary of determined e values). On a stepwise decrease in the time down to 45 min, the total porosity (St) is systematically increasing to about 30% in total (62.8% for 24 h and 97.2% for 45 min). This is caused by a simultaneous increase in the fraction of interparticulate porosity (e. ) as well as the fraction of pores (Cp). The ISEC measurements are in agreement with those of the MIP as well as BET analyses, as an increase in should be reflected in an increase in 8p and as the relative increase in the total porosity (caused by decreasing the polymerization time... 

Influence of the Polymerization Time on the Porous Properties of Monolithic MS/BVPE Networks, Considering Capillary Columns (80x0.2 mm I.D.) for ISEC and Glass Vial Bulk Polymers for MIP and BET Measurements... 

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