Chemistry Reference
In-Depth Information
Figure 4.14 Calixarene synthesized by Castellano et al. (2000). Reprinted from Castellano
et al. (2000). Copyright 2000 National Academy of Sciences USA.
led to the observation of a C
in the solution viscosity versus concentration profile as
well as the ability to draw fibers from solution. Furthermore, the introduction of a tet-
rafunctional calixarene resulted in noncovalent networks in which greater than 99%
of the material was associated at only 5 wt% concentration. The gels behaved as
solids on short time scales but flowed over longer time scales.
4.5. COMBINING HYDROGEN BONDING WITH OTHER
NONCOVALENT INTERACTIONS
The combining of multiple noncovalent interactions is currently a topic of intense
study. DNA, which is an exquisite example of supramolecular assembly, is com-
posed of interactions including hydrogen bonding, electrostatic interactions
(Herbert et al. 2006) due to the charged phosphodiester linkages, hydrophobic
interactions, and p-p stacking of the nucleobases (Vanommeslaeghe et al.
2006). This combination of noncovalent interactions enables DNA to perform
functions such as replication and protein synthesis (Voet and Voet 1995).
Combining multiple noncovalent assembly mechanisms results in versatility
and tunable properties, particularly when the interaction mechanisms respond to
different stimuli. Recently, Nair and coworkers (2006) introduced both metal -
ligand and hydrogen bonding interactions into ROMP block copolymers
(Fig. 4.15). The polymers consisted of blocks of palladium sulfur-carbon-
sulfur (SCS) pincer complexes (which bind nitrile groups) and diacyldiaminopyr-
idine (which binds to thymine) functionalized ROMP monomers. The presence of
the metal coordinating groups did not affect the hydrogen bonding capabilities of
the diacyldiaminopyridine containing block copolymers, suggesting orthogonality
of the associative mechanisms.
Recently, Hofmeier et al. (2005) reported the use of both metal-ligand and
UPy hydrogen bonding interactions in the main chain using a heterotelechelic
poly(1 -caprolactone). Highly associated noncovalent polymers were formed
upon the addition of iron salts to a poly(1 -caprolactone) containing UPy and ter-
pyridine end groups. Sudden decreases in melt viscosity at temperatures ranging
from100to1278C were attributed to the dynamics of the metal-ligand associ-
ations. Replacing iron with zinc salt decreased the temperature of this transition to
70-80 8C.
