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Studies On The Selectivity

STUDIES ON THE SELECTIVITY

OF METHACRYLATE POROUS POLYMERS

B. Gawdzik, M. Maciejewka

Faculty of Chemistry, Maria Curie-Sklodowska University,

Maria Curie-Sklodowska sq. 3, 20-031 Lublin, Poland

Stationary phases for gas chromatography (GC) have been characterized in a number of ways. Rohrschneider [1] classified stationary phases by their ability to retard polar solute probes. McReynolds [2] substracted the dispersive contribution to the retention index of these probes to yield the difference, DI, and used the sum of DI values to describe the total polarity of the phases. In these methods, selectivity of the phase is characterized by five constants represented by the differences in the Kováts retention indices of test substances (benzene, n-butanol, 2-pentanone, 1-nitropropane, and pyridine) on the phase under study and on a reference column packed with squalane, at the same temperature.

According to Smith et al [3] graphitized thermal carbon black (GTCB) is a more suitable reference for classifying solid phases in chromatography than squalane because with this phase none of the modified McReynolds’ constants is negative. Using this modification of McReynolds’ system it is possible to obtain a good measure of the selectivity of polymeric packings. This is known as a general selectivity system. Castello and D’Amato [4] proposed to use Porapak Q as a "standard polymeric phase" in the selectivity studies of polymeric phases, because it is the least polar porous polymer commercially available. In order to achieve a detailed classification of the possible interaction of polymeric phases with different functional groups of solute molecules they used the following polarity reference substances: benzene, n-butanol, 2-pentanone, 1-nitropropane, pyridine, ethanol, 2-butanone, and nitromethane. Among them the first five represent the test substances recommended by McReynolds.

To distinguish the contribution of the common retention mechanism (hydrogen-bond donor, hydrogen-bond acceptor, and dipole interaction) to the selectivity of porous polymers, Hepp and Klee [5] adopted the selectivity

triangle developed earlier by Snyder [6] for liquid chromatography. Retention indices of n-butanol, 1,4-dioxane, and 1-nitropropane, in reference to those obtained on GTCB are used for determining hydrogen bond donor, hydrogen-bond acceptor and dipole characteristics of porous polymer. As retention is governed by the total energy of interaction, the extent to which any selectivity is exhibited depends on the amount of polar interaction relative to nopolar dispersive interaction. In this method GTCB is also used as a reference phase.

The aim of this work is to study the influence of chemical structure of the monomers used in the synthesis of porous polymers on their selectivities. Besides nonpolar styrene-divinylbenzene (ST-DVB) porous copolymer, three other copolymers containing ester functional groups are used. They were:

1,4-phenylene dimethacrylate–divinylbenzene (PDM-DVB), ethylene glycol dimethacrylate–divinylbenzene (DMGE-DVB), and bis(4-methacryloylmethyl phenyl)sulphone–divinylbenzene (MMPS-DVB). All copolymers were synthesized in our laboratory.

To determine the selectivities of these copolymers, two procedures were applied: the selectivity triangle and the general selectivity. For more complete evaluation of their selectivities, elution orders of the test mixture separated on the columns packed with the studied copolymers were determined [7]. The mixture contains the following compounds: 2-propanol (hydrogen-bond donor), triethylamine (hydrogen-bond acceptor), 1,2-dichloroethane (weak dipole), acetonitrile (strong dipole), and octane (which has no polar interactions).