Poly monoliths

The concept of fibrous polymer formulations was extended to the delivery of aquatic herbicides (56). Several herbicides including Diquat, Fluridone, and Endothal were spun into biodegradable poly-caprolactone. Monolithic fibers and a modified monolithic system were produced with levels of herbicide from 5 to 60% by weight. Laboratory and field trials showed efficacious delivery of the active agent. Fibers provided both targeted localized delivery and controlled release of the herbicide to the aquatic weed. 

Several narcotic antagonists, including naloxone, naltrexone, L-methadone, and cyclazocine, have been incorporated in lactide homopolymers and lactide/glycolide copolymers. Cyclozocine was incorporated in poly(L-lactide) in the form of films (81,82). Lamination of drug-polymer films with a drug-free film created a reservoir device and eliminated the burst observed with the monolithic films originally tested. 

Hydrocortisone microspheres (108,109) and films (110) based on poly(lactic acid) have been investigated. A cage implant technique was used to study the performance of monolithic poly (DL-lactide) films loaded with hydrocortisone acetate (110). Films 1.5 x 0.6 cm were inserted into titanium wire-mesh cages 3.5 x 1.0 cm. The cages were implanted in the backs of rats and the inflammatory exudate was sampled periodically. The white cell concentration in the samples was lower than that of controls at all times during the 21-day test. 

This section provides an overview of properties of polymer monolith columns related to 2D-HPLC. Monolithic organic polymer columns, having longer history than silica monoliths, have been reviewed in detail recently by S vec and by Eeltink including their preparation methods and performance (Eeltink et al., 2004 Svec, 2004a). Polymer monolith columns commercially available include polyfstyrene-co-di vinyl benzene) (PSDVB) columns and poly(alkyl methacrylate) columns. 

A rather limited range of mesopores in terms of size and volume were observed in the skeletons of polymer monoliths. The porosity of the polymer monolith seems to be lower than that of silica monolith. The total porosity of these monoliths is in the range of 0.61-0.73, whereas interstitial (through-pore) porosity and mesopore porosity are 0.28-0.70 and 0.03-0.24, respectively. In the case of poly(butyl methacrylate-co-ethylene dimethacrylate), the observed porosity is around 0.61-0.71, resulting in permeability 0.15-8.43 x 10 14 m2, whereas the observed porosity of silica monoliths prepared in a capillary is 0.86-0.96 and the permeability is 7-120 x 10 14 m2. Higher permeability will be advantageous for 2D applications, as mentioned later. 

Oberachert, H., Premstaller, A., Huber, C.G. (2004). Characterization of some physical and chromatographic properties of monolithic poly(styrene-o-divinylbenzene). J. Chromatogr. A 1030, 201-208. 

Schley, C., Altmeyer, M.O., Swart, R., Muller, R., Huber, C.G. (2006). Proteome analysis of Myxococcus xanthus by off-line two-dimensional chromatographic separation using monolithic poly-(styrene-divinylbenzene) columns combined with ion-trap tandem mass spectrometry. J. Proteome Res. 5, 2760-2768. 

Virklund, C., Nordstrom, A., Irgum, K. (2001). Preparation of porous poly(styrene-co-divinylbenzene) Monoliths with controlled pore size distributions initiated by stable free radicals and their pore surface functionalization by grafting. Macromolecules 34, 4361-... 

Although Fields already mentioned the possible preparation of monolithic silica-based CEC columns, the lack of experimental data leads to the assumption that this option has not been tested [111]. In fact, it was Tanaka et al. who demonstrated the preparation of monolithic capillary columns using a sol-gel transition within an open capillary tube [99,112]. The trick was in the starting mixture that in addition to tetramethoxysilane and acetic acid also includes poly(ethylene oxide). The gel formed at room temperature was carefully washed with a variety of solvents and heated to 330 °C. The surface was then modified with octadecyl-trichlorosilane or octadecyldimethyl-A N-dimethylaminosilane to attach the hy-... 

Zhang developed a monolithic poly(styrene-co-divinylbenzene) CEC column in which the EOF is supported by carboxyl groups of polymerized methacrylic acid [ 133]. Using benzene as a probe, column efficiencies of 90,000 -150,000 were observed within a flow velocity range of l-10cm/min (0.2-1.7 mm/s). Different families of compounds such as phenols, anilines, chlorobenzenes, phenylendi-amines, and alkylbenzenes were well separated typically in less than 5 min using 20 cm long columns. 

Fig. 3. Effect of dodecanol in the porogenic solvent on the differential pore size distribution of molded poly(glycidyl methacrylate-co-ethylene dimethacrylate) monoliths (Reprinted with permission from [62]. Copyright 1996 American Chemical Society). Conditions polymerization time 24 h, temperature 70 °C, polymerization mixture glycidyl methacrylate 24%, ethylene dimethacrylate 16%, cyclohexanol and dodecanol contents in mixtures 60/0 (curve 1), 57/3 (curve 2), 54/6 (curve 3), and 45/15 vol.% (4)...

Fig. 6. Reaction of poly(glycidyl methacrylate-co-ethylene dimethacrylate) monolith with diethylamine...

Grafting of these preformed monoliths with dormant radicals is achieved by filling the pores with a monomer solution and heating to the desired temperature to activate the capped radicals. For example, a functionalization of poly(styrene-divinylbenzene) monolith with chloromethylstyrene and vinyl-pyridine to obtain material with up to 3.6 mmol/g of functionalities has been demonstrated [88]. 

Fig. 8. Effect of linear flow velocity of an L-benzoyl arginine ethylester solution (0.2 mol/1) on the enzymatic activity of trypsin immobilized on poly(glycidyl methacrylate-co-ethylene dimethacrylate) beads (curve 1) and monolith (curve 2) (Reprinted with permission from [90]. Copyright 1996 Wiley-VCH). Reactor 50 mm x 8 mm i.d., temperature 25 °C...

Table 1. Porous properties and enzymatic activities of monolithic poly(2-vinyl-4,4-dimethyl-azlactone-co-acrylamide-co-ethylene dimethacrylate) reactors3... 

In contrast, monolithic materials are easily amenable to any format. This has been demonstrated by using short monolithic rods prepared by copolymerization of divinylbenzene and 2-hydroxyethyl methacrylate in the presence of specifically selected porogens [93]. Table 2 compares recoveries of substituted phenols from both the copolymer and poly(divinylbenzene) cartridges and clearly confirms the positive effect of the polar comonomer. 

Table 2. Recovery of phenols from porous poly(divinylbenzene) (DVB) and poly(2-hydrox-ylethyl methacrylate-co-divinylbenzene) (HEMA-DVB) monoliths [93]...

Fig. 10. Scanning electron micrographs of monolithic poly(divinylbenzene) capillary column. Note that the porous monolith is surrounded by an impervious tubular outer polymer layer resulting from copolymerization of the monomer with the acryloyl moieties bound to the capillary wall. This layer minimizes any direct contact of the analytes with the surface of the fused-silica capillary...

Recent chromatographic data indicate that the interactions between the hydrophobic surface of a molded poly(styrene-co-divinylbenzene) monolith and solutes such as alkylbenzenes do not differ from those observed with beads under similar chromatographic conditions [67]. The average retention increase, which reflects the contribution of one methylene group to the overall retention of a particular solute, has a value of 1.42. This value is close to that published in the literature for typical polystyrene-based beads [115]. However, the efficiency of the monolithic polymer column is only about 13,000 plates/m for the isocratic separation of three alkylbenzenes. This value is much lower than the efficiencies of typical columns packed with small beads. 

Fig. 12. Separation of styrene oligomers by reversed-phase (left) and size-exclusion chromatography (right) (Reprinted with permission from [121]. Copyright 1996 American Chemical Society). Conditions (left) column, molded poly(styrene-co-divinylbenzene) monolith, 50 mm x 8 mm i.d., mobile phase, linear gradient from 60 to 30% water in tetrahydrofuran within 20 min, flow rate 1 ml/min, injection volume 20 pi UV detection, 254 nm (right) series of four 300 mm x 7.5 mm i.d. PL Gel columns (100 A, 500 A, 105 A, and Mixed C), mobile phase tetrahydrofuran, flow rate, 1 ml/min injection volume 100 pi, toluene added as a flow marker, UV detection, 254 nm temperature 25 °C,peak numbers correspond to the number of styrene units in the oligomers...

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