Chemistry Reference
In-Depth Information
Figure 5.13 Self-associative polymer network formed via the dimerization of 2-ureido-
4[1H]-pyrimidone groups attached to the polymer backbone.
Although self-complementary polymer networks clearly show distinct advantages
over conventional covalent cross-linked polymers, a limitation of this system is that
the system always remains “cross-linked” and would exhibit “uncross-linked” beha-
vior only at temperatures above the hydrogen bond dissociation temperature. Because
this system relies on the dimerization of the functional groups that are covalently
attached to the polymer chains, the system displays some of the same limitations
as covalent ones. As a result, to tune network properties, such as the degree of
cross-linking or the cross-linking density, new generations of optimized polymeric
material have to be redesigned and resynthesized. Therefore, this strategy does not
allow the full exploitation of the advantages of supramolecular self-assembly.
Polymer Networks Based on the Use of Complementary Linkers.
In poly-
meric networks that are based on the employment of complementary linkers (a two
component system), the hydrogen bonding recognition units attached to the
polymer chains undergo little or no self-association and hence cannot effectively
“cross-link” the polymer chains. Such a system represents an “open” system. An
external chemical agent or a “cross-linking agent” has to be added that is able to
undergo hydrogen bonding with the recognition groups attached to the polymer
side chains, resulting in the effective cross-linking of the polymer chains through
interchain hydrogen bonding (Fig. 5.14b). In such a system, a polymer can be con-
verted from being a completely “uncross-linked material,” that is, no cross-linking
agent present, to a completely “cross-linked system” with the addition of exactly 1
equiv of cross-linking agent based on the recognition motifs along the polymer back-
bone. Changes of the stoichiometry of the recognition site/linker ratio can be used to
tailor the physical properties of the resulting polymeric network and therefore obtain
a range of materials ranging from highly viscous fluids to highly viscoelastic solids,
all resulting from the same parent polymer.
The network strength of these systems can also be altered by varying the stability
of the hydrogen bonded complex formation between the cross-linking agent and the
