Chemistry Reference
In-Depth Information
FIGURE 4.3
Hand drawn sketches of the cross-sectional geometries of a charged dendro-
nized polymer as single chain (left), hypothetic lengthwise opened-up single chain (center), and
duplex structure (right). The hydrophobic contact area in the duplex provides the driving force
for the duplex formation and is correlated with “thickness” of the individual chain.
as an example. Upon bending, the charges in the bend's inner part can be pushed
toward the periphery away from the area with the highest curvature. This increases the
interaction between the hydrophobic interiors of the two polymer segments next to the
bend. The thus increased hydrophobic interaction upon bending is a key factor making
it attractive for the system to continue the back-folding process until a complete
duplex is formed. It also compensates the energy needed for the bending. In addition,
the back-folding leads to an improved protection of the hydrophobic parts against the
surrounding water by the increased charge density at the surface of the developing
duplex. Figure 4.3 shows a sketch illustrating this responsiveness that is based onwhat
one could call a responsive or “dendritic” thickness of linear polymer chains.
Neither for bottle brushes nor for conventional polyelectrolytes such effects have
been reported, which supports the view that it is the unprecedentedly large overlap
area of the hydrophobic parts and thus the unique thickness of the dendronized
polymers that makes this unusual behavior possible. Similar considerations apply to
explain the network formation at higher concentrations. If such and other thickness
effects should be more systematically explored, it would be highly desirable to have
homologous series of dendronized polymers at hand all of which have the same main
chain length but differ by the dendron generation and, thus, their cross-sectional
diameter. In order to allow for quantitative structure/property relations, the quanti-
tative knowledge of the branch work's structural fidelity is essential and any synthetic
method must account for this.
Figure 4.4 shows the core steps of the synthetic sequence developed for this
purpose. It is a powerful, gram-scale attach-to protocol that in principle can be applied
to all polymers with primary amines in the side chain [14]. The advantages of this
strategy aremanifold. Monomer
are known
from earlier work that facilitates their characterization [15]. Also the deprotection
chemistry, using neat trifluoroacetic acid, has been employed numerous times and
found to be highly reliable [16]. The key building block for dendronization,
compound
MG1
and the two polymers
PG1
and
PG2
1
(Scheme 4.1), is new. However, its synthesis rests upon commercially
