Supramolecular Polymers
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for rather high association constants to obtain chain lengths long enough to preserve polymer properties (the equivalent of several lengths between entanglements). Also, purity of the units is a prerequisite for high degrees of polymerization. As a result of these stringent requirements, only a limited number of interactions seem applicable.
Luc Brunsveld, Brigitte J.B. Folmer, and E.W. Meijer
What Defines a Supramolecular Polymer?
Polymers
Introduction What started as a scientific challenge roughly 10 years ago has become a technological reality today, as materials from supramolecular polymers and their many applications as smart materials have emerged. Synthetic polymeric materials are among the most important classes of new materials introduced in the 20th century. They are primarily used for construction, but electronic and biomedical applications are also at the forefront of science and technology. Only after the pioneering work of Hermann Staudinger did it become evident that polymeric properties in both solution and solid state are the result of the macromolecular nature of the molecules. A large number of repeating units are covalently linked into a long chain, and the entanglements of the macromolecular chains are responsible for many of the typical polymer properties.1 Before the theory of macromolecules was generally accepted, the majority of scientists were convinced that polymer properties were the result of the colloidal aggregation of small molecules or particles. The impressive recent progress in supramolecular chemistry,2 however, paved the way to the design of polymers and polymeric materials that lack the macromolecular structure. Instead, highly directional secondary interactions are used to assemble the many repeating units into a polymer array.3 By adequate design, these systems should still have all of those material properties that make polymers so valuable, and furthermore, we may expect some unprecedented behavior as a result of the reversibility of supramolecular design. Figure 1 shows the required directionality in supramolecular interactions as compared with the historical and current macromolecular view on polymers. With supramolecular polymers, the strength of the bonding within the main chain is weak compared with covalent
MRS BULLETIN/APRIL 2000
bonding, and the bonding is reversible with a certain lifetime. However, the bonding should be strong enough, and as a result have a lifetime long enough, to maintain typical material properties. The relationship between the association constant of the bonding, Ka, and the degree of polymerization is given in Figure 2. Due to the reversibility of the bonding, the values are obviously dependent on concentration. The plot critically shows the need
Figure 1. A supramolecular polymer (c) as a redesign of the historical view of polymeric materials (a) and as compared with Staudinger’s macromolecular concept (b).
The term supramolecular polymer is rather popular and is used for a variety of different structures utilizing secondary (or supr
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