Polymers reversible covalent chemistry

Keywords Constitutional changes Covalent changes Dynamic covalent chemistry Molecular machines Molecular motors Molecular switches Polymer/monomer switches Reversible polymers Size switches... 

It was recently suggested that carbamate bonds could be employed for a wider variety of dynamic covalent chemistry (DCC) experiments (88) and DCC is quickly emerging as a promising alternative to noncovalent self-assembly (91). This experiment offers an elegant opportunity of performing supramolecular chemistry with covalent bonds. One of the most important advantages here is the robustness of covalently organized structures, which on the other hand can be reversibly broken, at will. Of particular interest are supramolecular polymers and supramolecular materials. 

Wulff, G. Dederichs, W. Grotstollen, R. Jupe, C. On the chemistry of binding sites. Part II. Specific binding of substances to polymers by fast and reversible covalent interactions. Anal. Chem. Symp. Ser. 1982. 9, 207-216. (Affinity Chromatogr. Relat. Tech.). 

The second key element of DCC-reversibility-is a feature that is intrinsic to many polymers, specifically those that are formed by step polymerizations. Although the conditions under which bond formation takes place for the most common functional groups (esters, amides) are quite harsh, a large variety of bonding forming reactions that are reversible under mild conditions have been studied recently and have become known under the name of dynamic covalent chemistry. Reversibility is an even more prominent feature of supramolecular polymers - polymers in which the monomeric units are held together by noncova-lent bonds. 

Similarly, using hydrazone chemistry, modified nucleobases were incorporated into a dynamic analog of peptide nucleic acid [20]. Recently, the relatively weak covalent bonds of ene tetramines have been employed to develop reversible covalent polymers based on carbene dimerization [38]. 

A relatively new field called supramolecular chemistry has been developed over the last three decades. Supramolecular assemblies and supramolecular polymers differ from macromolecules, where the monomeric units are covalently linked. In a supramolecular polymer, the monomeric units self-assemble via reversible, highly directional, noncova-lent interactions. These types of bonding forces are sometimes called secondary interactions. Hydrogen bonding is the secondary force most utilized in supramolecular chemistry, but metal coordination and aromatic tt-tt electronic interactions have also been used. From a materials standpoint, supramolecular assemblies are promising because of the reversibility stemming from the secondary interactions. The goal is to build materials whose architectural and dynamical properties can respond reversibly to external stimuli. Solid phases are prepared by self-assembly from solution. In the solid-state, supramolecular polymers can be either crystalline or amorphous. 

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