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without any significant interference of the different metal centers (Loveless et al.
2005).
Meier and Schubert (2003) used terpyridine-based metal coordination for polymer
cross-linking. They postpolymerized a commodity plastic such as poly(vinyl chlor-
ide) to introduce terpyridine moieties in the side chains. The functionalized
polymer was then cross-linked by complexation of the terpyridine groups with
ruthenium to form a metal cross-linked polymer (Fig. 5.20, 17). This example
illustrates the importance of using postpolymerization functionalization in converting
easily available commodity plastics into high value materials. Furthermore, Hofmeier
and Schubert (2003) cross-linked a terpyridine functionalized poly(methacrylate)
polymer using Fe(II) and Zn(II) and demonstrated that the addition of Fe(II)
resulted in more efficient cross-linking than the addition of Zn(II). They also
completely decross-linked the Zn(II)-based network by the addition of hydroethyl-
(ethylenediaminetetraacetic acid), thus demonstrating the complete reversibility of
metal cross-linked polymers.
Multifunctional Polymer Networks: Combining Cross-linking and Polymer
Functionalization.
Side chain functionalized polymers offer a strategy to use mul-
tiple noncovalent interactions that can be used to reversibly and simultaneously
cross-link as well as functionalize the polymeric scaffolds to form highly functiona-
lized cross-linked polymers with unprecedented complexity. This strategy involves
the employment of an orthogonal functionalization and cross-linking strategy. The
functionalization is achieved by using monofunctionalized moieties that are noncova-
lently anchored to the scaffold whereas cross-linking is achieved by using bifunc-
tional cross-linking agents to cause interchain cross-linking. When orthogonal
noncovalent interactions for cross-linking and functionalization results are employed,
both processes should be mutually independent and noninterfering. The Weck group
synthesized highly functionalized noncovalently cross-linked polymers prepared
from a single “universal polymer backbone” via directional self-assembly processes
using a combination of metal coordination and hydrogen bonding (Pollino et al.
2004). They report a functionalization/cross-linking strategy in which the polymeric
scaffold can be noncovalently cross-linked by employing one of the self-assembly
motifs and the second one is used for the noncovalent functionalization. The Weck
system is based on a terpolymer functionalized with 2,6-diaminopyridine as the
hydrogen bonding receptor moieties, palladated SCS pincer complexes for metal
coordination, and a third inert spacer monomer to increase the polymer solubility
and to dilute the recognition units (Fig. 5.21).
Two distinct cross-linking/functionalization schemes were investigated: 1) cross-
linking via hydrogen bonding interaction employing either a bis-thymine or
bis-perylene unit with the palladated SCS pincer centers used for polymer functiona-
lization by metal coordination or 2) cross-linking via metal coordination using a
bis-pyridine cross-linking agent with the 2,6-diaminopyridine along the polymer
scaffold functionalized with thymine derivatives. Extensive polymer cross-linking
was observed in all cases as investigated by viscometry. However, the metal coordi-
nated cross-linked scaffold exhibited significantly higher viscosity than its hydrogen
