Polymeric stationary phase solution polymerization

The mixture of deprotected amino acid derivatives in solution was then immobilized onto a polymeric solid support, typically activated 5-)xm macroporous poly(hydroxyethyl methacrylate-co-ethylene dimethacrylate) beads, to afford the chiral stationary phases with a multiplicity of selectors. Although the use of columns... 

We think, therefore, that the conformation, chain and segment mobilities in the attached macromolecules can play a significant role in the shielding behavior of the polymeric stationary phase as well as in the processes of its formation of complexes with solutes. Obviously, the chromatographic studies relevant to composite supports suffer from a lack of information on the structure of the attached polymer. Nevertheless, we will attempt to point out some relevant data from independent studies on polymer adsorption and/or graft polymerization. 

Temperature has an influence on the retention and consequently on the capacity factors of carotenoids in HPLC columns. Usually, as the column temperature increases, the retention decreases however, in a polymeric C30 column, after an initial decrease of the t values of cis isomers of carotenoids, the retention of cis isomers actually increases at temperatures above 35°C. This different behavior can be explained by the increased order and rigidity of the C30 stationary phase at lower temperatures that in turn induce preferential retention of long, narrow solutes as the trans isomer and partial exclusion of bent and bulky cis isomers. The greater chain mobihty and less rigid conformation of the C30 at higher temperatures may increase the contact area available for interaction with the cis isomers and also may lower... 

The problem of transport of molecules through swollen gels is of general interest. It not only pertains to catalysis, but also to the field of chromatographic separations over polymeric stationary phases, where the partition of a solute between the mobile phase (liquid phase) and a swollen polymeric stationary phase (gel phase) is a process of the utmost importance. As with all the chemical and physicochemical processes, the thermodynamic and the kinetic aspect must be distinguished also in partition between phases. 

The third sorption phenomenon is that of ion-exchange. Here, the stationary phase is a permeable polymeric solid containing fixed charged groups and mobile counter-ions which can exchange with the ions of a solute as the mobile phase carries them through the structure. 

Besides silica, silica-based and polymeric stationary phases, porous graphitized carbon (PGC), zirconium oxide and its derivatives, alumina and its derivatives have been used for the solution of special separation problems which cannot be easily solved by using traditional HPLC stationary phases. 

Reversed-phase liquid chromatography shape-recognition processes are distinctly limited to describe the enhanced separation of geometric isomers or structurally related compounds that result primarily from the differences between molecular shapes rather than from additional interactions within the stationary-phase and/or silica support. For example, residual silanol activity of the base silica on nonend-capped polymeric Cis phases was found to enhance the separation of the polar carotenoids lutein and zeaxanthin [29]. In contrast, the separations of both the nonpolar carotenoid probes (a- and P-carotene and lycopene) and the SRM 869 column test mixture on endcapped and nonendcapped polymeric Cig phases exhibited no appreciable difference in retention. The nonpolar probes are subject to shape-selective interactions with the alkyl component of the stationary-phase (irrespective of endcapping), whereas the polar carotenoids containing hydroxyl moieties are subject to an additional level of retentive interactions via H-bonding with the surface silanols. Therefore, a direct comparison between the retention behavior of nonpolar and polar carotenoid solutes of similar shape and size that vary by the addition of polar substituents (e.g., dl-trans P-carotene vs. dll-trans P-cryptoxanthin) may not always be appropriate in the context of shape selectivity. 

FIGURE 5.5 Synthesis schemes for chromatographic stationary phases (a) monomeric synthesis where X represents reactive (e.g., chloro or alkoxy) or nonreactive (methyl) substituents, (b) solution polymerization, in which water is added to the slurry and (c) surface polymerization, in which water is added to the silica surface. 

Sander et al. [63] investigated the effect of microparticulate silica pore size on the properties of solution-polymerized Cig stationary phases and observed both an increase in bonding density and shape recognition for wider pore (>120 A) silica. A size-exclusion mechanism was proposed, in which the reaction of the silane polymer on the surface is enhanced for wide pores and reduced for narrow pores. Polymeric Ci8 phases prepared on substrates with narrow pores exhibited monomeric-like chromatographic properties. This effect may be the result of an increase in competitive surface linkage with the less sterically hindered monomers that coexist with the bulkier oligomers that have polymerized in the reaction solution (Figure 5.13). 

FIGURE 5.26 (See color insert following page 280.) A representation of the slot model illustrating potential constrained-shape solute (BaP) interactions with the conformational ordered cavities of a polymeric Cjg stationary-phase simulation model. Also included on the chromatographic surface is an identical-scale molecular structure of 1,2 3,4 5,6 7,8-tetrabenzonaphthalene (TBN). 

Despite the problems with silica, it has remained dominant as a stationary phase for the analysis of bases for the same reasons as it has for the separation of other classes of solute. Polymeric phases still give lower efficiency than silica phases, and at low pH seem to suffer the same overloading effect as silica-based phases. lonogenic groups seem to be introduced into polystyrene-divinyl-based phases during their manufacture, and these can lead to tailing of bases at intermediate pH where these groups become ionized. Other phases, such as those made from zirconia, show some promise for the analysis of bases but have not been fully evaluated as yet. 

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