Chemistry Reference
In-Depth Information
5.2. STRATEGIES TOWARD NONCOVALENT SIDE CHAIN
FUNCTIONALIZATION OF POLYMERIC SCAFFOLDS
Side chain functionalization of covalently functionalized polymers can take place
either by “prepolymerization functionalization,” which is the functionality that has
been bestowed to the monomer, or by “postpolymerization functionalization,”
which is after the polymerization, when the polymer backbone is subsequently
functionalized with the desired moiety (Pollino and Weck 2005). Although both
approaches have been employed successfully in covalent polymer chemistry, the
first approach can be synthetically more demanding but always yields 100%
functionalization, which is not the case for most postpolymerization functionalization
strategies.
In supramolecular side chain functionalized polymers, the prepolymerization
functionalization strategy is not available. However, using appropriate design
principles and noncovalent functionalization strategies, we can tune the postpolymer-
ization functionalization strategy from fully functionalized polymers to very weak
and nonperfect functionalized polymers, depending on the application in mind.
Clearly, the strength of the noncovalent interaction in the desired medium is key to
this strategy. Noncovalent interactions with the full spectrum of association constants
from very weak (e.g., hydrogen bonding, dipole-dipole, p-pstacking, or hydrophi-
lic interactions) to fairly strong (e.g., metal coordination or coulombic interactions)
have been used to functionalize polymeric scaffolds. It is important to note that the
strength of these association constants is dependent on external factors such as the
temperature and solvent. When we describe the strength of a noncovalent interaction,
unless otherwise noted, we report it at room temperature in a noncompetitive solvent
such as a nonpolar halogenated solvent. In the following subsections we describe the
most common side chain polymer functionalization strategies that are based on
hydrogen bonding, metal coordination, and coulombic charge interactions. These
examples of noncovalent interactions to functionalized polymeric scaffolds along
the side chains are not meant to be conclusive; instead, they demonstrate the basic
design principles behind the functionalization strategies and give some selected
examples from the literature. Wide varieties of recognition motifs for the functiona-
lization step are imaginable and have been used often. Thus, it is not possible to
mention all of them here.
5.2.1. Side Chain Functionalization Using Hydrogen Bonding
Hydrogen bonding interactions are the most widely employed noncovalent
interactions for the functionalization of polymeric scaffolds (Armstrong and Buggy
2005; Wilson 2007). Over the past 20 years, a wide variety of hydrogen bonding
motifs ranging from one-, two-, three-, four-, and six-point recognition motifs to
higher order systems have been developed. Figure 5.1 describes some of the more
common recognition motifs described in the literature. The popularity of hydrogen
bonding is because the strength of the hydrogen bonded complexes can be tuned
easily by using these different hydrogen bonding motifs and by altering the acidity
