Poly -based monoliths

Fig. 6.16. SEM micrographs of poly(styrene-divinylbenzene) based monolithic capillary. (Reprinted with permission from [49]. Copyright 1999 Elsevier).

The most widely used polymer monoliths are poly(styrene-co-divinylben-zene)-based monoliths and moderately polar monoliths based on methacrylic acid esters. While the first ones are strongly hydrophobic materials prepared by polymerization of styrene and its derivatives with divinylbenzene as the cross-linking agent [323,324], methacrylate-based monoliths are synthesized by polymerization of butyl methacrylate or other methacrylic acid esters with ethylene dimethacrylate as the cross-linking agent [325,326]. These types of monolithic media in capillary format are the main focus of a very detailed review by Urban and Jandera [327]. 

A boronate affinity monolithic column first appeared in 2006. Hilder and co-workers first prepared an epoxy-functionalized monolithic capillary through copolymerization of glycidyl methacrylate (GMA) and EDMA. The poly(GMA-co-EDMA) base monolithic column was then chemically modified through nucleophilic attack of the epoxide with p-hydroxyphenylboronic acid (Scheme 11.6). Since then, various boronate affinity monolithic columns have been rapidly developed because of the merits of both boronate affinity and monolith columns. Liu and co-workers has made many studies in recent They synthesized the hydrophilic bor-... 

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. 

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-... 

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. 18a-c. Base peak chromatograms for the LC/MS analyses of a cytochrome c Lys-C digest (0.7 pmol injected) on a a poly(styrene-co-divinylbenzene) monolith-filled needle b Vydac C18-packed needle c Poros R2-packed needle. (Reprinted with permission from [128]. Copyright 1998 American Chemical Society)... 

In addition to classical reverse phase separation of peptides on octadecyl derivatized silica monoliths, sugars and peptides as well as proteins and nucleosides have been analyzed on a 20-cm-long silica-based poly(acrylic acid) column (ID. 200 pm), employing HILIC and weak cation-exchange chromatography, respectively [194]. Furthermore, HILIC fractionation of polysaccharides delivered remarkable and promising results [84,194]. 

PNIPAM and its copolymers are not the only options for thermoresponsive stationary HPLC phases. Poly(acrylates) and poly(methacrylates) bearing OEG groups in the side chains are known as thermoresponsive polymers that offer some advantages over PNIPAM [40]. Such polymers were recently employed for the modification of silica monoliths, which then served as stationary phase in the HPLC separation of steroids [193], Unsurprisingly, the results were qualitatively similar to those obtained for PNIPAM-based systems, but the separation of relatively hydrophilic steroids was superior. 

A monolithic silica-based CIS stationary phase was used under high flow rate condition (2 mL/min) without significant back pressure in IPC analysis of a recently discovered new drug candidate for the treatment of Alzheimer s disease [15]. Nanoscale IPC using a monolithic poly(styrene-divinylbenzene) (PS-DVB) nanocolumn coupled to nanoelectrospray ionization mass spectrometry (nano-ESl-MS) was evaluated to separate and identify isomeric oligonucleotide adducts. Triethylammonium bicarbonate was used as the IPR. Interestingly, the performance of the polymeric monolithic PS-DVB stationary phase significantly surpassed that of columns packed with the microparticulate sorbents CIS or PS-DVB [16]. 

A typical mobile-phase composition is an acetonitrile-water gradient with a fixed concentration of trifluoroacetic acid (TFA), formic, or acetic acid (typically 0.05-0.5%). TFA acts as an ion-pairing agent and masks secondary interactions with the silica-based stationary phase. TFA may significantly suppress the ESI response in positive-ion mode. To avoid this, either formic acid is preferred or a mixture of 0.02% TFA and 0.5% acetic acid can be used. Some silica-based RPLC materials can be used with a lower TFA concentration (PepMap ). Alternatively, poly(styrene-divinylbenzene) polymeric materials (PS-DVB) can be applied. With a monolithic PS-DVB column, only a small decrease in separation efficiency on the monolithic column was observed when the TFA concentration was reduced from 0.2%to0.05%[51]. 

Slentz BE, Penner NA, Lugowska E, Regnier F. Nano-hter capillary electrochromatography columns based on collocated monolithic support structures molded in poly(dimethyl siloxane). Electrophoresis 2001 22 3736-43. 

The most common polymer supports used for chiral catalyst immobilization are polystyrene-based crosslinked polymers, although poly(ethylene glycol) (PEG) represents an alternative choice of support. In fact, soluble PEG-supported catalysts show relatively high reactivities (in certain asymmetric reactions) [le] which can on occasion be used in aqueous media [le]. Methacrylates, polyethylene fibers, polymeric monoliths and polynorbornenes have been also utilized as efficient polymer supports for the heterogenization of a variety of homogeneous asymmetric catalysts. 

One approach to surface erodibility is to prepare a polymer that contains linkages that are stable in base but are very labile in acid. Because one such linkage is an ortho ester, poly(ortho esters) are currently under intensive development as monolithic devices for zero order drug release 1). 

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