Page 200 - Chiral Separation Techniques

178 6 Enantiomer Separations using Designed Imprinted Chiral Phases

6.8 New Polymerization Techniques

As indicated above, MIPs have so far been prepared in the form of continuous blocks
that need to be crushed and sieved before use. This results in a low yield of irregu-
lar particles, a high consumption of template, and a material exhibiting low chro-
matographic efficiency. There is therefore a need for MI-materials that can be pre-
pared in high yield in the form of regularly shaped particles with low size dispersity
and a controlled porosity. These are expected to be superior in terms of mass trans-
fer characteristics and sample load capacity compared to the materials obtained from
the monolith approach. However, the results obtained so far using alternative
approaches, although showing some improvements, have been disappointing.
Bead-sized MIPs have been previously prepared through suspension polymeriza-
tion techniques either using fluorocarbons (Fig. 6-14) [88] or water [89] as continu-
ous phase, dispersion polymerization or precipitation polymerization [90, 91]. This
resulted in spherical particles of a narrow size distribution. These procedures have
the limitation of being sensitive to small changes in the manufacturing conditions
and the type of solvents and polymerization conditions that can be applied, but once
appropriate conditions have been found they should offer an economic alternative for
up-scaling. An alternative to this procedure is the coating of preformed support
materials [92-94]. MIPs have been prepared as grafted coatings on metal oxide sup-
ports [92, 93] on organic polymer supports [94] and on the walls of fused silica cap-
illaries [95-97]. These techniques however involve many steps and are thus associ-
ated with larger batch-to-batch variations. In addition, problems appear in achieving
homogeneous coatings and to suppress secondary interactions with the support sur-
face.

Fig. 6-14. Suspension polymerization technique for noncovalent imprinting.

Much effort has been devoted to the development of a multi-step swelling poly-
merization technique using water as suspension medium [98]. This has resulted in
polymers showing similar selectivities but slightly improved mass transfer charac-
teristics compared with the corresponding monolithic polymers. Of particular rele-