Chemistry Reference
In-Depth Information
SCHEME 16.12
blocks the first-generation dendrimers
41-G
1
(carboxylic acid terminal groups) and the structurally well-defined Ti-organo-oxo
cluster Ti
16
O
16
(OEt)
32
. The hybrid interface is created by trans alcoholysis in the case
of the alcohol tips, and by bridging carboxylates in the case of acidic tips. In both cases
a bicontinuous hybrid gel in which the titanium clusters are regularly spaced by the
dendrimers was obtained (Scheme 16.12) [49].
Another type of organization was observed when some polycationic dendrimers
such as
40-G
1
(alcohol terminal groups) or
42-G
n
(both possessing amide terminal groups) were left for days in
water. Indeed, solid hydrogels can be obtained in both cases, even when using
amounts of dendrimers as small as 0.25% in weight in water; the gels thus obtained are
solid at room temperature though they contain 99.75% of water (Figure 16.11, left).
During the gelation process, large amounts of various types of hydrosoluble sub-
stances (for instance up to 30% in weight of nickel acetate) can be incorporated.
Around 70,000 water molecules are estimated to be gelled by amolecule of dendrimer
of generation 4 (
37-G
4
,
37-G
n
or
1400 molecules of
water per terminal ammonium or pyridinium unit, regardless of the generation
considered [50]. The same gelation phenomenon was observed with nanolatexes
coated with dendrons possessing the same types of terminal groups as
42-G
4
), that is gelation of about 1200
42-G
n
[51].
Dendrimers
42-G
n
were also used for obtaining macroscopic fibers. As shown in
Figure 16.11 (right), a concentrated solution (10% in weight) of dendrimer in water
was put in a syringe equipped with a kinked needle and linked to a step motor.
The dendrimer solution contained in the syringe was continuously injected into a
FIGURE 16.11
Hydrogels and fibers obtained from pyridinium-ended dendrimers.
