Porogens columns

It is of much interest to compare polymer monoliths with monolithic silica columns for practical purposes of column selection. Methacrylate-based polymer monoliths have been evaluated extensively in comparison with silica monoliths (Moravcova et al., 2004). The methacrylate-based capillary columns were prepared from butyl methacrylate, ethylene dimethacrylate, in a porogenic mixture of water, 1-propanol, and 1,4-butanediol, and compared with commercial silica particulate and monolithic columns (Chromolith Performance). 

This technology was extended to the preparation of chiral capillary columns [ 138 -141 ]. For example, enantioselective columns were prepared using a simple copolymerization of mixtures of O-[2-(methacryloyloxy)ethylcarbamoyl]-10,11-dihydro quinidine, ethylene dimethacrylate, and 2-hydroxyethyl methacrylate in the presence of mixture of cyclohexanol and 1-dodecanol as porogenic solvents. The porous properties of the monolithic columns can easily be controlled through changes in the composition of this binary solvent. Very high column efficiencies of 250,000 plates/m and good selectivities were achieved for the separations of numerous enantiomers [140]. 

Although inorganic, monolithic columns attracted considerable attention in the last 10 years, the preparation of silica-based monoliths does not yet offer the broad chemical variety of precursors and porogens for specific adjustment of separation compared with their organic counterpart. The preparation of silica monoliths uses the classical sol-gel process of hydrolysis and polycondensation of organosilicium compounds. 

The in situ polymerisation consists of filling a capillary or a column with the prepolymerisation mixture containing the template, the functional monomer, the crosslinker, the initiator and the porogenic solvent (Fig. 11). Then the column is heated or submitted to UV radiation for polymerisation. In the in situ thermally initiated polymerisation process, the tube with the pre-polymerisation mixture is submerged in a controlled-temperature water bath, whereas for in situ photoinitiated polymerisation, a UV-transparent capillary or column is needed. The resulting continuous rod of polymer is washed with an appropriate solvent to remove the template and the excess of monomer. 

Horvath s group has recently reported the preparation of porous rigid monolithic capillary columns for CEC by polymerizing mixtures of chloromethylstyrene 21, divinylbenzene 22 and azobisisobutyronitrile in the presence of various porogenic solvents such as methanol, ethanol, propanol, toluene, and formamide [49]. The capillary wall was silanized using a 50% dimethylformamide solution of 3-(trimethoxysilyl)propyl methacrylate 8 at a temperature of 120°C for 6 hours. In order to avoid the spontaneous polymerization of the functional methacrylate, a stable free radical (DPPH) was added to the solution. The SEM micrographs of Fig. 6.16... 

Fig. 6.20. Schematics for the preparation of monolithic capillary columns. First, the bare capillary is filled with the polymerization mixture (step a) that contains functional monomer, crosslinking monomer, initiator, and porogenic solvent. Polymerization (step b) is then initiated thermally or by UV irradiation to afford a rigid monolithic porous polymer. The resulting monolith within the capillary is washed (step c) with the mobile phase using a pump or electroosmotic flow and used as for the CEC separations.

Fig. 6.30. Column efficiencies for (S) enantiomer determined from DNZ-(R,S)-Leu separations on quinidine-functionalized monoliths as a function of pore diameter. (Reprinted with permission from [60]. Copyright 2000 American Chemical Society). Conditions polymerization mixture, chiral monomer 8 wt%, 2-hydroxyethyl methacrylate 16 wt%, ethylene dimethacrylate 16 wt%, porogenic solvent 60 wt% (consisting of 1-dodecanol and cyclohexanol in different proportions), UV initiated polymerization for 16 h at room temperature ( ) and thermally initiated polymerization for 20 h at 60°C ( ), capillary columns 335 mm (250 mm active length) x 0.1 mm i.d., mobile phase 0.4 mol/L acetic acid and 4 mmol/L triethylamine in 80 20 acetonitrile-methanol, separation temperature 50°C, voltage -25 kV.

In contrast, photoinitiated free radical polymerization of glycidyl methacrylate and trimethylolpropane trimethacrylate in the presence of porogenic solvent affords a monolithic plug within the column that serves as a frit. This procedure represents a simple approach to reproducible fabrication of frits even in capillaries with large inner diameters. 

---