Fabrication of porous polymer monoliths covalently attached to the walls of channels in plastic microdevices

UV-initiated grafting of plastic tubes and microfluidic chips with ethylene diacrylate followed by the preparation of porous polymer monoliths has been studied. The first step affords a thin grafted layer of polymer with a multiplicity of pendent double bonds that are then used in the second step for covalent attachment of the monolith to the wall. As clearly seen on scanning electron micrographs, this procedure prevents the formation of voids at the monolith-channel interface a problem that has always plagued approaches involving bulk polymerization in nontreated channels due to the shrinkage of the monolith during the polymerization process and its lack of compatibility with the material of the device. Irradiation with UV light through a photomask allows precise patterning specifying both the area subjected to surface modification and the location of the monolith within specific areas of the device.     

Rhenium-to-benzoylpyridine and rhenium-to-bipyridine MLCT excited states of fac-[Re(Cl)(4-benzoylpyridine)(2)(CO)(3)] and fac-[Re(4-benzoylpyridine)(CO)(3)(bpy)](+): a time-resolved spectroscopic and spectroelectrochemical study

The lowest allowed electronic transition of fac-[Re(Cl)(CO)(3)(bopy)(2)] (bopy = 4-benzoylpyridine) has a Re --> bopy MLCT character, as revealed by UV-vis and stationary resonance Raman spectroscopy. Accordingly, the lowest-lying, long-lived, excited state is Re --> bopy (3)MLCT. Electronic depopulation of the Re(CO)(3) unit and population of a bopy pi orbital upon excitation are evident by the upward shift of nu(CO) vibrations and a downward shift of the ketone nu(C=O) vibration, respectively, seen in picosecond time-resolved IR spectra. Moreover, reduction of a single bopy ligand in the (3)MLCT excited state is indicated by time-resolved visible and resonance Raman (TR(3)) spectra that show features typical of bopy(*)(-). In contrast, the lowest allowed electronic transition and lowest-lying excited state of a new complex fac-[Re(bopy)(CO)(3)(bpy)](+) (bpy = 2,2'-bipyridine) have been identified as Re --> bpy MLCT with no involvement of the bopy ligand, despite the fact that the first reduction of this complex is bopy-localized, as was proven spectroelectrochemically. This is a rare case in which the localizations of the lowest MLCT excitation and the first reduction are different. (3)MLCT excited states of both fac-[Re(Cl)(CO)(3)(bopy)(2)] and fac-[Re(bopy)(CO)(3)(bpy)](+) are initially formed vibrationally hot. Their relaxation is manifested by picosecond dynamic shifts of nu(C(triple bond)O) IR bands. The X-ray structure of fac-[Re(bopy)(CO)(3)(bpy)]PF(6).CH(3)CN has been determined.     

Extending the array of crosslinkers suitable for the preparation of polymethacrylate-based monoliths

The in situ preparation of monolithic capillary columns comprising copolymers of butyl methacrylate with ethylene dimethacrylate, diethylene glycol dimethacrylate, triethylene glycol dimethacrylate, and pentaerythritol tetraacrylate using thermal polymerization within 250 microm ID capillaries and their application for micro-HPLC separations of proteins has been studied. For all crosslinkers, optimization of the porogenic mixture consisting of 1-propanol and 1,4-butanediol yielded monoliths with pore sizes above 1 microm suitable for rapid separations at low back pressure. Very good separations were achieved for a protein mixture consisting of ribonuclease A, cytochrome c, myoglobin, and ovalbumin with all tested columns.     

Chiral recognition: design and preparation of chiral stationary phases using selectors derived from ugi multicomponent condensation reactions and a combinatorial approach

Combinatorial approaches together with high-throughput screening have been used to develop highly selective stationary phases for chiral recognition. Libraries of potential chiral selectors have been prepared by the Ugi multicomponent condensation reactions and screened for their enantioselectivity using the reciprocal approach involving a chiral stationary phase with immobilized model target compound N-(3,5-dinitrobenzoyl)-alpha-l-leucine. The best candidates were identified from the library of phenyl amides of 2-oxo-azetidineacetic acid derivatives. This screening also enabled specification of the functionalities of the selector desired to achieve the highest level of chiral recognition. The substituents of the phenyl ring adjacent to the chiral center of the selector candidates exhibited the most profound effect on the chiral recognition. The best candidate was then synthesized on a larger scale, resolved into single enantiomers using preparative enantioselective HPLC, and attached to porous poly(2-hydroxyethyl methacrylate-co-ethylene dimethacrylate) beads via an ester linkage to afford the desired stationary phase. Selectivities alpha as high as 3.2 were found for the separation of a variety of amino acid derivatives.     

Preparation of porous hydrophilic monoliths: Effect of the polymerization conditions on the porous properties of poly (acrylamide-co-N,N′-methylenebisacrylamide) monolithic rods

Molded macroporous monoliths with pores sizes up to 1000 nm have been prepared by copolymerization of the hydrophilic monomers, acrylamide, and N,N′-methylenebisacrylamide, in the presence of a porogenic diluent. A combination of dimethylsulfoxide and 2-heptanol was selected from a broad spectrum of solvents and water soluble polymers to achieve the optimum composition of the porogenic mixture. In addition to the composition of the porogen the porous properties of the monolithic rods can also be optimized through changes in the percentage of both N,N′-methylene-bisacrylamide (crosslinking monomer) and azobisisobutyronitrile (free radical initiator) used for the polymerization. The hydrophilic monoliths may be used in the separation of biological polymers, solid-phase extraction, or for immobilization of proteins. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35 1013–1021, 1997     

Chromatography of functional polymers: A new approach to the characterization of reactive polymers obtained by chemical modification

High-performance liquid chromatography (HPLC) has been used to complement size-exclusion (gel permeation) chromatography (SEC) for the characterization of functional polymers. Whereas SEC is unable to detect compositional changes, HPLC in an appropriate interacting medium can provide detailed information on compositional changes occurring during chemical modification of a polymer. The method has been demonstrated using a normal-phase column consisting of porous monodisperse 10 μm poly(2,3-dihydroxypropyl methacrylate-co-ethylene dimethacrylate) beads that have a homogeneous coverage of aliphatic hydroxyl groups for the analysis of brominated poly(isobutylene-co-4-methylstyrene). Differences of well below 1 mol % of bromomethylstyrene units are easily detected and quantified. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 1173–1180, 1997