Side-Chain-Functionalized Supramolecular Polymers

In this chapter we will focus on side chain functionalized supramolecular polymers as well as main chain noncovalent functionalized polymers, which are the two main areas of supramolecular polymers. We will initially discuss the design principles and methodology of side chain functionalization, in particular, multifunctionalization. In the later part of the chapter, we will discuss in detail two important applications of side chain functionalized supramolecular polymers. The first application involves the use of noncovalent interactions to yield highly functionalized materials, whereas the second application involves the reversible noncovalent cross-linking of polymers to yield responsive materials. 

Week M. Side-chain functionalized supramolecular polymers. Polym Int 2007 56 453-460. 

This chapter reviews main-chain- and side-chain-functionalized supramolecular polymers that were synthesized by ROMP and acyclic diene metathesis (ADMET) polymerization. Emphasis is placed on polymers assembled using directional supramolecular interactions, such as H-bonding, M-L coordination, and inclusion complexation, or a combination thereof. 

Rotello and coworkers [30] reported the first PNBE-based random copolymers with nucleobase-functionalized side chains, wherein longer linkers were utilized to achieve solubility. The system relied upon the complementary three-point H-bonding motif between uracil and diacyl DAP pendant groups on two respective polymer chains (figure 4.5a). The resulting side-chain-functionalized supramolecular polymers self-assemble into spherical polymersomes (figure 4.5b). 

The previous section described the supramolecular side chain functionalization of polymers based on a single recognition motif. However, biological systems use a wide variety of noncovalent interactions such as hydrogen bonding, metal coordination, and hydrophobic interactions in an orthogonal fashion to introduce function,... 

Figure 4.1 Schematic representation of dif- polymer, (c) Supramolecular block copolymer, ferent classes of supramolecular polymers. (d) Side-chain-functionalized supramolecular (a) Small-molecule main-chain supramolec- polymer, (e) Supramolecular network, ular polymer, (b) Branched supramolecular...

Pollino JM, Week M. Supramolecular side-chain functionalized polymers synthesis and self-assembly behavior of polynorbornenes bearing Pd SCS pincer complexes. Synthesis 2002 9 1277-1285. 

An interesting dimension of metal-coordinated self-assembly that is often ignored, or at least not exploited to its fullest extent, occurs when the resulting coordination complex is a charged species and, as such, in need of a counterion. This counterion itself presents yet another subtle instance of ionic self-assembly, which often is overshadowed by its partner, the coordination complex. The second multi-functional side-chain supramolecular polymer system is based on this simple but important concept [14, 106-111]. In 2003, Ikkala and coworkers reported a study in which they exploited (1) a side-chain functionalized polymer, poly(vinyl-pyridine), (2) metal-coordination self-assembly via a tridentate Zn2+ complex and (3) ionic self-assembly through functionalized counterions, i.e. dodecylbenzene-sulfonate ions, to form multiple self-assembled complexes which adopted a cylindrical morphology (Fig. 7.23) [112]. 

Physical cross-linking is easily incorporated into the structures of supramolecular side-chain polymers by using bi- or multifunctional molecules [9,31,93]. The complexation of the functionalized polyacrylates, a stilbazole, and bipyridine molecules form supramolecular side-chain cross-linked polymer 37 (Figure 27) [31,93]. Bis-imidazolyl compounds are also used for such cross-linking [105]. Reversible association-dissociation of the hydrogen bonding leads to the reversible thermal order-disorder transitions, which is an important feature of supramolecular materials. Liquid-crystalline main-chain supramolecular networks have been prepared from multifunctional components [32,123-126], which will be described in Section IV. 

Having discussed self-assembly strategies toward noncovalently functionalized side chain supramolecular polymers as well as studies toward the orthogonahty of using multiple noncovalent interactions in the same system, this section presents some of the potential applications of these systems as reported in the literature. The apphcations based on these systems can be broadly classified into two areas 1) self-assembled functional materials and 2) functionalized reversible network formation. 

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