Polyrotaxanes - Syntheses and Properties

As depicted in structures 1 and 2, rotaxanes are molecular composites consisting of cyclic and linear molecules in which the two components are connected together without any covalent bond, i.e., mechanically. Structure 1 is a pseudorotax-ane because the cyclic, represented by an open ring, can be disassociated from the linear species by external forces, e.g., dilution and heating in the solution state. Structure 2 is a true rotaxane, because the cyclic is permanently confined between two bulky blocking groups (BG), represented by the two balls, and can not slip off the linear molecule. 

On the other hand, polymer science has rapidly reached a stage where special and new properties are urgently needed to broaden the applications of polymeric materials. In addition to chemical compositions, the properties of polymers are 

Different types of polyrotaxanes, depending on how the cyclic and the linear units are connected, have been conceived [6-8, 12], According to the location of the rotaxane unit, polyrotaxanes can be defined as main-chain systems, Types 4, 5, 6, 7, and 8 (rows one and two in Table 1), and side-chain systems, Types 9, 10, 11, and 12 (rows three and four in Table 1). In main-chain polyrotaxanes the rotaxane unit is part of the main chain. In side-chain polyrotaxanes, the rotaxane moiety is located in the side chain as a pendant group. Polyrotaxanes can also be classified as polypseudorotaxanes and true polyrotaxanes, depending on their thermal stability toward dethreading. Polypseudorotaxanes are those without BG (column one in Table 1), in which the rotaxane components can be disassociated from each other by external forces. True polyrotaxanes are those with BG at the chain ends or as in-chain units (column two in Table 1), in which the rotaxane units are thermally stable unless one or more covalent bonds is/are broken. 

Therefore, polyrotaxanes can be simply defined as polymeric materials containing rotaxane units. They are different from conventional linear homopolymers because they always consist of two components, a cyclic species mechanically attached to a linear species. They also differ from polymer blends as the individual species are interlocked together and from block copolymers since the two components are noncovalendy connected. Thus new phase behavior, mechanical properties, molecular shapes and sizes, and different solution properties are expected for polyrotaxanes. Their ultimate properties depend on the chemical compositions of the two components, their interaction and compatibility. This review is designed to summarize the syntheses of these novel polymers and their properties. 

---

This chapter describes the structures and properties of supramolecular silver complexes with specific topologies such as cages, tubes, catenanes and polycate-nanes, rotaxanes and polyrotaxanes, and multidimensional frameworks, as synthesized by reactions of various silver salts with predesigned organic ligands. 

It is vital that simple and cheap synthesis of interlocked polymers is achieved in order to make progress in the chemistry of polyrotaxanes and polycatenanes. Since bulk property is essential in polymer science, difficulty in synthesis of interlocked polymers should be avoided, this being different from the case of molecular materials such as molecular devices functioning at a molecular level. Both polyrotaxanes and polycatenanes as well as both rotaxanes and catenanes are becoming easy to synthesize with the progress... 

Other polyrotaxanes containing a thiophene-based conjugated backbone have been synthesized from the Cu(I)- or Zn(II)-driven assembly between a macrocyclic phenanthroline and a bithiophene-substituted phenanthroline [322] or bipyridine [323, 324]. Swager and co-workers have demonstrated that the contribution of the metal ion to the electronic properties of the polyrotaxane was possible when more electron-rich 3,4-(ethylenedioxy)thio-phene groups were used in place of thiophenes in the polymer backbone [324]. So a 10 -10 -fold increase in the polymer s conductivity was observed after poly(24) was treated with Cu(II) solution. This result was ascribed to the oxidation of the poly(24) backbone by Cu " " ions to generate poly(24,Cu) with charge carriers in the polymer backbone. 

---