Conclusion

In this chapter, the synthesis, structures and functions of helical polymers are described. A large number of helical polymers with an excess of a preferred helix-sense have been synthesized by the helix-sense selective polymerization of achiral monomers, thus producing static helical polymers, or by the predominantly one-handed helicity induction in dynamic helical polymers via covalent or noncova-lent bonding of chiral pendants. These helical polymers differ through their helix inversion barriers; as a result, the former helical conformations are locked during the polymerization under kinetic control, while the latter helical conformations are under thermodynamic control. However, as described in this chapter, helical conformations of dynamic helical polymers can also be locked, as evidenced by the memory effect of an induced helical poly(phenylacetylene). Either static or dynamic helical polymers with an excess one-handedness have also been prepared by the polymerization of analogous monomers bearing different substituents. The history of synthetic helical polymers extends back to the 1960s; at that time the structural elucidation at a molecular level was a laborious task. However, recent significant developments in spectroscopic and microscopic instruments, coupled with precise polymerization techniques, have made it possible to observe directly the helical structures of certain helical polymers including helical pitch and handedness. This more detailed information leads to a better understanding of the principles underlying the generation of helical conformations. In addition, biological helical polymers further hierarchically assemble into complicated supramolecular structures, such as the coiled coil (helix bundle) superstructure, which are responsible for their elaborate functions. The next important and attractive challenge, which has implications for biological helices, superstructures and functions, will be not only to mimic biological helices, but also to develop supramolecular helical assemblies with a controlled helix-sense, and this may also provide a clue for the construction of advanced chiral materials [130].

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