Supramolecular polymers
Introduction
A recent review article [2]on supramolecular polymer chemistry entitled `Polymer Chemistry Comes Full Circle' describes the ironic turn of events that has recently been witnessed in the synthetic polymer community. Over the past decade or so, there has been growing activity to develop supramolecular polymers — macromolecules whose monomeric units are held together by non-covalent forces. The irony is that the father of modern polymer science, Hermann Staudinger fought many scientific battles to prove that polymer molecules were covalently bonded entities, not colloidal aggregates of small molecules as believed by the colloid chemists of that day. Given the advances that have been made over the past 30 years in supramolecular chemistry and the knowledge that has come from the study of biological self-assembly, it is no wonder why there is great enthusiasm for synthetic supramolecular polymers. From molecular biology we learn that large molecules provide fertile ground for seeding sophisticated supramolecular design. For example, sequence-specific peptide chains store an enormous amount of information that can direct the complex assembly of multi-molecular components.
At least three aspects of supramolecular chemistry need to be considered in discussing polymer molecules. First, there is a topological aspect that deals with the self-assembly of chains and networks by reversible association of bifunctional or multifunctional monomers through specific non-covalent interactions (e.g. hydrogen bonds). Second, supramolecular polymers can be formed by multi-molecular self-organization of mesoscale molecules based on `non-specific' interactions. A biological example would be the aggregation of quasi-equivalent protein molecules to form microtubulin `polymers'. The third aspect involves intramolecular supramolecular polymer chemistry and deals with the creation of order within a single molecule, i.e. the folding of chains into defined conformations. Internal order can bestow a macromolecule with precise size and shape as well as high-definition molecular surface features. These solvent exposed surfaces in turn can be used to guide higher order multi-molecular assembly. The most sophisticated supramolecular constructions will probably rely on combinations of these various aspects of supramolecular chemistry.
Section snippets
Self-assembly of bifunctional monomers and telomers
This section covers the construction of chains and networks by reversible association of bifunctional or multifunctional monomers (either small molecules or telomers) through specific non-covalent interactions. Early efforts focused entirely on the use of hydrogen bonding interactions owing to their moderate strength, selectivity, and directionality [2]. Recent work from Meijer's laboratory has taken this approach to a new height [3]⋅. The readily available 2-ureido-4-pyrimidone was found to
Shape control and self-organization of mesomolecules
In recent years, premium attention has been paid to the design and study of mesoscale molecules possessing well-defined shapes and high-definition surfaces leading to self-organized structures. From the standpoint of pure covalent bond chemistry, some of the most impressive recent examples have come from Müllen's laboratory (Fig. 2) [9]⋅. These giant polycyclic aromatic hydrocarbons are constructed by combinations of (1) Diels–Alder reactions involving alkynes and tetraphenylcyclopentadienone;
Folded macromolecules in solution
Chain molecules provide the unique opportunity for an intramolecular variant of supramolecular chemistry — folded macromolecules in solution. While ordered conformations in solution are commonplace among biopolymers, only recently have significant strides been made with synthetic chain molecules 38, 39, 40, 41. Why are macromolecules so well suited for this type of supramolecular chemistry? Clearly the local concentration of interacting segments within a giant chain can be much higher than the
Conclusions
Complex supramolecular organization demands information-rich molecules that display high-definition molecular surfaces. The mesoscale molecules illustrated above are recent examples from the synthetic world. Folded polymers on the other hand, whose chemistry is determined by sequence, composition, chain-length, stereochemistry, and conformation, are the building blocks of biological assembly. The supramolecular chemist has only recently begun to explore this approach, perhaps in part because of
Acknowledgements
I gratefully acknowledge the National Science Foundation for support of my research.
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