288
spacers between their two reactive groups can lead to macromolecular networks
with increased porosity, which are also more flexible and less compact. In these
networks, smaller molecules can increase their diffusion rates, and bulkier substances may also become entrapped (Mocanu et al. 2001).
Even though the clean polymer resulting from the synthesis using toxic reagents,
such as acid dichlorides, epichlorohydrin, or diisocyanates, should be harmless, the
use of nontoxic cross-linkers like the polycarboxylic acids is also feasible. Martel’s
group described the synthesis of soluble and insoluble polymers (Martel et al. 2005)
and the production of cotton-bound cyclodextrin using these cross-linkers (Martel
et al. 2002).
Several parameters are of great interest when characterizing these materials:
cyclodextrin and cross-linker amounts and ratios, water contents (intrinsic moisture of the polymer), the water sorption capacity (swelling processes), the particles size and shape and their porosity, e.g., for suspension of polymerized beads,
and, obviously, the sorption capacity of organic molecules of interest. In the last
years of the past century, a comprehensive bibliography concerning the synthesis
and sorption studies for cyclodextrin polymers could be gathered, but, in contrast,
not much information on their characterization was available at that time. The
analysis of cyclodextrin polymers cross-linked with epichlorohydrin is complicated when it comes to infrared or Raman spectroscopic techniques (Crini et al.
2000) because, as mentioned above, both the cross-linked cyclodextrin units and
the self-polymerized epichlorohydrin possess hydroxyalkyl and ether groups.
Nevertheless, the interpretation of the infrared spectra of starch cross-linked with
epichlorohydrin was resolved at that time (Dumoulin et al. 1998; Delval
et al. 2004).
While the spectroscopic characterization of cyclodextrin-epichlorohydrin polymers
is not easy, that of cyclodextrin polymers cross-linked with diisocyanates seemed to be
simpler. Qualitative characterizations by infrared spectroscopy (Li and Ma 1999;
Bhaskar et al. 2004) or Raman (Lee et al. 2002) were attempted, and the successful
quantitative analysis was achieved, thanks to the intense carbonyl band of the crosslinker (García-Zubiri 2005). Thermal and thermogravimetric analysis were also used in
most of those studies, as well as nuclear magnetic resonance spectroscopy (Asanuma
et al. 1998; Lee et al. 2002). For polymers with other cross- linkers, such as maleic
anhydride, there were also some nuclear magnetic resonance results of interest (Girek
et al. 2000).
As for the cyclodextrin content of the polymer, the most common procedures
used already in the 1990s were the colorimetric methods using chlorotetrazolium
blue (Crini et al. 1995, 1998b; Janus et al. 1999) or iodometry (Renard et al. 1997)
and phenolphthalein (Mäkelä et al. 1987). For soluble cyclodextrin polymers, proton nuclear magnetic spectroscopy could be used (Renard et al. 1997), and for the
insoluble resins,
1
H or
13
C solid-state nuclear magnetic resonance were employed
(Crini et al. 1998b, 2000). Thus, the β-cyclodextrin-epichlorohydrin (or, rather,
2-hydroxypropyl ether) ratio can be determined because the signal corresponding to
the hydrogen atoms in the 2-hydroxypropyl ether segments is displaced below the
two wide peaks of the glucopyranose units.
M. Petitjean et al.
spacers between their two reactive groups can lead to macromolecular networks
with increased porosity, which are also more flexible and less compact. In these
networks, smaller molecules can increase their diffusion rates, and bulkier substances may also become entrapped (Mocanu et al. 2001).
Even though the clean polymer resulting from the synthesis using toxic reagents,
such as acid dichlorides, epichlorohydrin, or diisocyanates, should be harmless, the
use of nontoxic cross-linkers like the polycarboxylic acids is also feasible. Martel’s
group described the synthesis of soluble and insoluble polymers (Martel et al. 2005)
and the production of cotton-bound cyclodextrin using these cross-linkers (Martel
et al. 2002).
Several parameters are of great interest when characterizing these materials:
cyclodextrin and cross-linker amounts and ratios, water contents (intrinsic moisture of the polymer), the water sorption capacity (swelling processes), the particles size and shape and their porosity, e.g., for suspension of polymerized beads,
and, obviously, the sorption capacity of organic molecules of interest. In the last
years of the past century, a comprehensive bibliography concerning the synthesis
and sorption studies for cyclodextrin polymers could be gathered, but, in contrast,
not much information on their characterization was available at that time. The
analysis of cyclodextrin polymers cross-linked with epichlorohydrin is complicated when it comes to infrared or Raman spectroscopic techniques (Crini et al.
2000) because, as mentioned above, both the cross-linked cyclodextrin units and
the self-polymerized epichlorohydrin possess hydroxyalkyl and ether groups.
Nevertheless, the interpretation of the infrared spectra of starch cross-linked with
epichlorohydrin was resolved at that time (Dumoulin et al. 1998; Delval
et al. 2004).
While the spectroscopic characterization of cyclodextrin-epichlorohydrin polymers
is not easy, that of cyclodextrin polymers cross-linked with diisocyanates seemed to be
simpler. Qualitative characterizations by infrared spectroscopy (Li and Ma 1999;
Bhaskar et al. 2004) or Raman (Lee et al. 2002) were attempted, and the successful
quantitative analysis was achieved, thanks to the intense carbonyl band of the crosslinker (García-Zubiri 2005). Thermal and thermogravimetric analysis were also used in
most of those studies, as well as nuclear magnetic resonance spectroscopy (Asanuma
et al. 1998; Lee et al. 2002). For polymers with other cross- linkers, such as maleic
anhydride, there were also some nuclear magnetic resonance results of interest (Girek
et al. 2000).
As for the cyclodextrin content of the polymer, the most common procedures
used already in the 1990s were the colorimetric methods using chlorotetrazolium
blue (Crini et al. 1995, 1998b; Janus et al. 1999) or iodometry (Renard et al. 1997)
and phenolphthalein (Mäkelä et al. 1987). For soluble cyclodextrin polymers, proton nuclear magnetic spectroscopy could be used (Renard et al. 1997), and for the
insoluble resins,
1
H or
13
C solid-state nuclear magnetic resonance were employed
(Crini et al. 1998b, 2000). Thus, the β-cyclodextrin-epichlorohydrin (or, rather,
2-hydroxypropyl ether) ratio can be determined because the signal corresponding to
the hydrogen atoms in the 2-hydroxypropyl ether segments is displaced below the
two wide peaks of the glucopyranose units.
M. Petitjean et al.