studies conducted in Japan (see, e.g., [19, 93–100]). Yet another possibility for
the preparation of cryogels of the polymerization type is the use of electron
beams [101] or photoinitiation [102–104]. An essential point in this case is that
the penetration capability of both these kinds of radiation is not very high, which
limits the thickness of the samples. Therefore, the simplest approach, applicable
in most laboratories, for the synthesis of polymerization-type cryogels is the use
of a suitable chemical initiator system, where no specific radiation source is
required and the amount of the added initiating substances can easily be
controlled.
In the context of historical aspects of the development of polymerization-type
cryogels, Table 1 summarizes the reaction components, e.g., the monomers,
crosslinkers, initiator systems, solvents, and the temperature for cryogelation reactions starting from monomeric precursors. The data were taken from the pioneering
reports on such gelation systems as well as from the most significant studies
revealing the basic mechanisms of the key processes or ascertaining specific
properties of the corresponding cryogels. Naturally, this selection is subjective,
from the viewpoint of the author, and some works of good quality might have been
missed.
Cryogels synthesized via polycondensation reactions are also known. These are
mainly inorganic cryogels prepared by the sol–gel transformation in non-deeply
frozen precursor systems, where the condensation of certain hydroxides accompanied by water liberation leads to the formation of polyoxides, as detailed in [173].
Polycondensation-type organic cryogels have also been reported in a few publications. Obviously, the first examples of such cryogels are those synthesized in frozen
aqueous medium at À15
C from the mixture of lysine (a trifunctional amino acid
bearing two NH 2 groups and one COOH group) with water-soluble carbodiimide or
from the mixture of lysyl-lysine (a dipeptide having three NH 2 groups) with
glutaraldehyde [35]. In any case, the basic principle is that one of the precursors
must be at least trifunctional or higher in order to ensure the branched 3D character
of the forming polymer and its crosslinking. One recent example realizing this
principle is the polycondensation-type cryogel prepared via the reaction of threearm amino-terminated oligo(ethylene glycol) star polymers with dithio-bis
(maleimido)ethane in the medium of non-deeply frozen dioxane at À8
C
[174]. Here, a 3D polymeric network forms as a result of the Michael addition of
the primary amino groups to the double bonds of maleimide residues.
2.3 Preparation of Cryogels by Covalent Crosslinking
of High Molecular Weight Precursors
As for the systems discussed in Sect. 2.2, the requirement of a good solubility of the
high molecular weight precursors in the medium of the unfrozen liquid microphase
is also important. If a decrease in the temperature and resulting freezing of the
8
V.I. Lozinsky
the preparation of cryogels of the polymerization type is the use of electron
beams [101] or photoinitiation [102–104]. An essential point in this case is that
the penetration capability of both these kinds of radiation is not very high, which
limits the thickness of the samples. Therefore, the simplest approach, applicable
in most laboratories, for the synthesis of polymerization-type cryogels is the use
of a suitable chemical initiator system, where no specific radiation source is
required and the amount of the added initiating substances can easily be
controlled.
In the context of historical aspects of the development of polymerization-type
cryogels, Table 1 summarizes the reaction components, e.g., the monomers,
crosslinkers, initiator systems, solvents, and the temperature for cryogelation reactions starting from monomeric precursors. The data were taken from the pioneering
reports on such gelation systems as well as from the most significant studies
revealing the basic mechanisms of the key processes or ascertaining specific
properties of the corresponding cryogels. Naturally, this selection is subjective,
from the viewpoint of the author, and some works of good quality might have been
missed.
Cryogels synthesized via polycondensation reactions are also known. These are
mainly inorganic cryogels prepared by the sol–gel transformation in non-deeply
frozen precursor systems, where the condensation of certain hydroxides accompanied by water liberation leads to the formation of polyoxides, as detailed in [173].
Polycondensation-type organic cryogels have also been reported in a few publications. Obviously, the first examples of such cryogels are those synthesized in frozen
aqueous medium at À15
C from the mixture of lysine (a trifunctional amino acid
bearing two NH 2 groups and one COOH group) with water-soluble carbodiimide or
from the mixture of lysyl-lysine (a dipeptide having three NH 2 groups) with
glutaraldehyde [35]. In any case, the basic principle is that one of the precursors
must be at least trifunctional or higher in order to ensure the branched 3D character
of the forming polymer and its crosslinking. One recent example realizing this
principle is the polycondensation-type cryogel prepared via the reaction of threearm amino-terminated oligo(ethylene glycol) star polymers with dithio-bis
(maleimido)ethane in the medium of non-deeply frozen dioxane at À8
C
[174]. Here, a 3D polymeric network forms as a result of the Michael addition of
the primary amino groups to the double bonds of maleimide residues.
2.3 Preparation of Cryogels by Covalent Crosslinking
of High Molecular Weight Precursors
As for the systems discussed in Sect. 2.2, the requirement of a good solubility of the
high molecular weight precursors in the medium of the unfrozen liquid microphase
is also important. If a decrease in the temperature and resulting freezing of the
8
V.I. Lozinsky