PART I FUNDAMENTALS OF SUPRAMOLECULAR POLYMERS COPYRIGHTED MATERIAL

PART I FUNDAMENTALS OF SUPRAMOLECULAR POLYMERS COPYRIGHTED MATERIAL

CHAPTER 1 A BRIEF INTRODUCTION TO SUPRAMOLECULAR CHEMISTRY IN A POLYMER CONTEXT RAYMOND J. THIBAULT and VINCENT M. ROTELLO 1.1. INTRODUCTION AND BACKGROUND Self-assembly of molecular and macromolecular systems is a versatile and modular tool for the creation of higher order structures (Lehn 1993). Nature employs selfassembly extensively using both phase segregation and lock and key specific interactions to generate the diverse range of highly ordered systems observed in living organisms. Applying biologically inspired self-assembly strategies to synthetic macromolecules provides access to a wide range of desirable structural and dynamic properties. Controlled noncovalent interactions are a particularly attractive strategy for controlling polymer aggregation, using the array of recognition elements developed by supramolecular chemists. The modularity and tunability of these recognition elements makes this approach versatile, because the assembling units can be synthetically tuned to enhance or minimize selectivity, directionality, and the affinity of the interaction. Likewise, the affinity of noncovalent interactions is thermally dependent, imparting reversibility to the assembly process and providing unique material properties such as defect correction and self-healing capabilities. The toolkit of interactions available for supramolecular polymers can be divided into the six categories listed in Table 1.1 that includes hydrophobic interactions, which are dealt with in more detail in Chapter 2. In this chapter, we present a brief overview of supramolecular polymers and polymerization and supramolecular interactions of polymer side chains. We provide examples of the control over the solution state polymer structure that can be achieved at the molecular level and then extended to micro- and macroscale assemblies. Molecular Recognition and Polymers: Control of Polymer Structure and Self-Assembly. Edited by V. Rotello and S. Thayumanavan Copyright # 2008 John Wiley & Sons, Inc. 3

4 INTRODUCTION TO SUPRAMOLECULAR CHEMISTRY TABLE 1.1 Six Categories of Noncovalent Intermolecular Interactions Interaction Description and Bond Strengths Selected Example (London) dispersion forces Stacking.1 kcal mol 21 ; dynamic induced dipole dipole interactions 2 3 kcal mol 21 (face face), 3 5 kcal mol 21 (edge face); attractive forces between electron-rich interior with electron-poor exterior Hydrophobic 1 10 kcal mol 21, association of nonpolar complements in aqueous or polar media Hydrogen bonding.1 kcal mol 21 (weak), 1 4 kcal mol 21 (moderate), 5 10 kcal mol 21 (strong); donor acceptor interaction involving hydrogen atom as donor and base (electron pair) as proton acceptor Electrostatic 1 10 kcal mol 21 (dipole dipole), 10 30 kcal mol 21 (ion dipole),.45 kcal mol 21 (ion ion); Coulombic attraction between opposite charges, highly dependent on media Dative bonding 5 90 kcal mol 21 ; metal ligand coordination, ligand donates electron pair(s) to center Association strengths are for systems in chloroform. 1.2. MAIN-CHAIN VERSUS SIDE-CHAIN SUPRAMOLECULAR POLYMERS The concept of supramolecular polymers containing multiple hydrogen bonding units was introduced over a decade ago by Jean-Marie Lehn (Lehn 1993). In this study, three-point hydrogen bonding between bifunctional diamidopyridine and thymine

1.2. MAIN-CHAIN VERSUS SIDE-CHAIN SUPRAMOLECULAR POLYMERS 5 derivatives results in the formation of supramolecular polymers featuring liquid crystalline ordering (Fig. 1.1). Lehn and coworkers later extended this strategy to include bifunctional molecules joined by chiral tartaric acid spacers (Gulikkrzywicki et al. 1993) and rigid anthracene-based linkers (Kotera et al. 1995). This approach is quite general, as can be seen in later chapters of this book. The work done by E. W. Meijer using self-complementary ureidopyrimidinones builds upon Lehn s supramolecular polymers (Sijbesma et al. 1997). The quadruple hydrogen bonding system employed in these studies has two major differences from Lehn s polymers: 1) a high degree of association (K dim. 10 6 M 21 ) and 2) selfcomplementarity that eliminates stoichiometric concerns. The high dimerization constant of ureidopyrimidinones makes this recognition element an excellent choice for supramolecular polymerizations, providing a high degree of polymerization in solution. An alternative approach to supramolecular polymers is provided by covalently attaching recognition elements to the polymer backbone. These polymers can then be used as macromonomers for higher level polymer assembly or for plug and Figure 1.1 (a) Supramolecular polymers developed by Lehn using three-point hydrogen bonds between diamidopyridine and thymine residues and ( b) analogous polymers by Meijer employing self-complementary, quadruple hydrogen bonds. (c) A schematic depiction of the extended chain of repeating bisfunctional monomers forming the backbone of supramolecular polymers.

6 INTRODUCTION TO SUPRAMOLECULAR CHEMISTRY Figure 1.2 A schematic representation of the versatility of reversible, supramolecular sidechain modification and selected examples of interactions that can be employed. play noncovalent side-chain modification. Stadler (Stadler and Burgert 1986) initially investigated these systems for their elastomeric properties, serving to increase the miscibility between incompatible polybutadiene and polyisoprene blends using the dimerization of urazole moieties. These motifs have been investigated by a number of researchers, including Weck (Pollino et al. 2004) and Rotello (Deans et al. 1999; Ilhan et al. 2001), for a wide variety of applications (Fig. 1.2). As you will see throughout this book, noncovalent interactions provide an elegant means to reversibly control polymer structures on the nano- and microscale. The lock and key nature, high directionality, and thermal response of these interactions make supramolecular polymer systems an attractive alternative for the fabrication of novel, functional materials. In addition, the wealth of available interactions allows the tuning of the form, function, and interaction strength of the assembling units, providing control in materials processing. Many investigators are discovering the versatility of supramolecular interactions for bottom-up methodology and top-down techniques in nanoscience and nanoscale engineering. Polymer scientists will likewise realize the expansive field available to them for the creation of novel plastics. REFERENCES Deans R, Ilhan F, Rotello VM. Recognition-mediated unfolding of a self-assembled polymeric globule. Macromolecules 1999;32:4956 4960.

REFERENCES 7 Gulikkrzywicki T, Fouquey C, Lehn JM. Electron-microscopic study of supramolecular liquid-crystalline polymers formed by molecular-recognition-directed self-assembly from complementary chiral components. Proc Natl Acad Sci USA 1993;90:163 167. Ilhan F, Gray M, Rotello VM. Reversible side chain modification through noncovalent interactions. Plug and play polymers. Macromolecules 2001;34:2597 2601. Kotera M, Lehn JM, Vigneron JP. Design and synthesis of complementary components for the formation of self-assembled supramolecular rigid rods. Tetrahedron 1995;51:1953 1972. Lehn JM. Supramolecular chemistry. Science 1993;260:1762 1763. Lehn JM. Supramolecular chemistry concepts and perspectives. Weinheim: VCH; 1995. Pollino JM, Stubbs LP, Weck M. One-step multifunctionalization of random copolymers via self-assembly. J Am Chem Soc 2004;126:563 567. Sijbesma RP, Beijer FH, Brunsveld L, Folmer BJB, Hirschberg JHKK, Lange RFM, Lowe JKL, Meijer EW. Reversible polymers formed from self-complementary monomers using quadruple hydrogen bonding. Science 1997;278:1601 1604. Stadler R, Burgert J. Influence of hydrogen-bonding on the properties of elastomers and elastomeric blends. Makromol Chem Macromol Chem Phys 1986;187:1681 1690.