Chemistry Reference
In-Depth Information
supported by the fact that the hydrophilicity of the various polymers changes with
generation (see log P values in Figure 4.5). These changes however are not mirrored
by the LCST values. If this assumption is correct,
could possibly serve as an
interesting container that in its interior does not collapse and, thus, is capable of
holding its load while the periphery is closed-up above LCST. Further work along
these lines is in progress and should eventually furnish the desired molecular scale
picture of the collapse [26].
The collapse of
PG3
was further investigated by static and dynamic light
scattering, and its aggregation by cryo-TEM. Interestingly, a coil-to-mesoglobule
transition was observed that results in stable and nearly monodisperse mesoglobules
(Figure 4.8). Possible reasons why the aggregation of collapsed chains stops auto-
matically when the mesoglobules have reached a certain size have been discussed [27].
This rather successful journey into thermoresponsive dendronized polymers
triggered further investigations along the lines indicated but at the same time also
on related systems. Dendronized block copolymers (DBCP) combine the structure
characteristics of both dendronized polymers and conventional block copolymers.
They are therefore obvious candidates for stimuli-responsive materials and also, for
example, for bioapplications and supramolecular constructions. poly(N-isopropyla-
crylamide) (PNiPAM) is a prominent thermoresponsive polymer [28]. It shows a
LCSTaround body temperature, is biocompatible, and has therefore been widely used
also in thermoresponsive block copolymers. Comb-like polymethacrylates with OEG
side chains are another type of attractive thermoresponsive polymer [29]. They show
smaller hystereses than PNiPAM due to their much weak ability to form hydrogen
bonds and are also biocompatible. Two different types of DBCPs were prepared
namely those with a PNiPAM and a dendronized block [30] and those with comb-like
OEG units and a dendronized block [31].
PG2(ET)
FIGURE 4.8
Cryo-TEM images of the dendronized polymer
PG2(ET)
at different tem-
peratures. (a) Image taken at a solution temperature of 25
C. The individual dendronized
polymers are visible. (b) At 60
C, the chains aggregate and form stable spherical mesoglobules.
The inset shows an individual single mesoglobule. Reprinted fromRef. [27] with permission of
the publisher.
