Biomimetic Polymers for Chiral Resolution and Antifreeze Applications
339
Fig. 5.4. SEM images of (a) poly-D-phenylalanine microspheres formed by precipitating in
water/2-propanol ratio of 0.1.(b) Crystal morphology of D, L-threonine crystallized from
pure water (no chiral polymer additives), (c) chiral microsphere of poly-L-phenylalanine
collected after crystallization of DL-threonine (scale bar = 10 μm)^140.
chlorobenzene) droplets in water. water. The polystyrene particles merged with the organic
droplets, which led to their swelling. In a sequential step chiral monomers, initiator and a
crosslinker (divinyl benzene) were dissolved in the swollen polystyrene particle template.
Polymerization initiated with the rise in temperature. After the polymerization, the
polystyrene particles were dissolved by dry acetone. The outcome of this process was
remarkable; mono-disperse microspheres of 5.4 μm ± 2%, i.e., whole chiral, were obtained
with a perfectly defined structure. The partials morphology was controlled by varying the
polymerization conditions such as swollen solvents and monomer/crosslinker ratio. For
example, the use of chlorobromobenzene as swelling solvent resulted in hollow particles
while the use of chlorobenzene resulted in porous polymeric material [Figure 5.5].
Poly vinyl L-phenylalanine particles with a porous structure were used to test the chiral
selectivity by enantiomeric adsorption from racemic and enantomerical pure D- and L-
valine solutions. The polymeric particles showed chiral selective adsorption toward the L-
valine enantiomer in time-resolved polarimetry experiments. Crystallization experiments of
racemic D, L-valine in the presence of the chiral particles (10 mg/mL) resulted in
enantioselective crystallization on the chiral hollow particles [Figure 5.6]. The crystallization
experiment was quenched when a solid phase appeared on the particles surfaces that could
be observed by a light microscope. In order to analyze enantioselective crystallization on the
chiral hollow microspheres powder X-ray diffraction (XRD) and differential scanning
calorimetry (DSC) were used. The X-ray diffraction pattern of pure valine enantiomers is
a bc