Syntheses of Polyrotaxanes

Similar to those for rotaxanes, different approaches have been employed for poly-rotaxane syntheses these will be summarized in the next section. The most important parameter in polyrotaxanes, the min value, is often employed as a measure of the effectiveness of the preparation method. Because this value mainly depends on the strength of the attractive force between the cyclic and the backbone, this section is again divided into subsections according to the types of driving forces rather than the types of polyrotaxanes. 

---

Harada et al. were the first to synthesize a polyrotaxane. Using the process shown in Scheme 1, they obtained an inclusion compound in which many a-CDs are threaded by a PEG chain and named it molecular necklace . Wenz et al. [132] reported polyrotaxanes containing polyamines and a-CDs. Because of its significance and interest, the approach used by Harada et al. to obtain the molecular necklace is worth reproducing here in some detail. 

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. 

A strong attractive force between the cyclic and the linear species is necessary to achieve high yield syntheses of rotaxanes and high min values for polyrotaxanes [6-8,12]. Different types of driving forces have been explored. Because the results from rotaxanes often provide the basis for polyrotaxanes, this section will briefly summarize various driving forces used in rotaxane syntheses so that we can understand polyrotaxane systems more completely. For specific details regarding rotaxane syntheses, interested readers are referred to those publications cited here and in other reviews [6-9]. 

In early work no such NMR chemical shift changes relative to those of the parent components were observed for polypseudorotaxanes with aliphatic backbones and aliphatic crown ethers as the cyclic species [108, 109]. Model studies were performed with 18-crown-6 (18C6), which is so small that it cannot be threaded. The recovery of intact 18C6 under conditions identical with those for the syntheses of the polyrotaxanes ruled out the possibility of side reactions. The effective removal of the small crown ether by precipitation into a solvent which was poor for backbone but good for the cyclic demonstrated the effectiveness of the purification procedure. In addition, reaching a constant min value after multiple precipitations and the absence of the peak for free crown ether in GPC traces indicated that the larger crown ethers detected by NMR in the purified polymeric products were indeed threaded rather than simply mixed. 

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... 

Wenz and Keller also synthesized similar polyrotaxanes (5) (Figure 7) by threading a-CDs onto poly(iminomethylene) chains [20], Instead of attaching two bulky stoppers at the ends, however, they introduced blocking groups at arbitrary positions on the chain. More specifically, nonspecific reaction of the threaded polymer chain with nicotinyl chloride resulted in trapping of the CD rings located between the two nicotinamide units on the polymer backbone. 

A variety of polyrotaxane structures can be envisioned, as shown in Scheme 1 [1-3]. True rotaxanes polymers possess bulky groups at the ends of the linear species or along the backbone to prevent diffusive loss of the ring component. However, it has been shown that main chain polypseudo-rotaxanes dethread extremely slowly due to random coiling of the backbones [4,5], so that the distinction between pseu-dorotaxane and rotaxane polymers of the main chain type is somewhat blurred for these systems. Main chain polyrotaxanes are the most common type, although all of the other types shown in Fig. 43.1 have been synthesized. 

Comparing the results of synthesis of polyrotaxanes based on the two cyclourethanes demonstrated that the degree of penetration of rings by the growing polymer chain in the polyrotaxanes tends to decrease by passing from CU-A to CU-B. For polyrotaxanes produced by the statistical method (PR-A2 and PR-B2) no changes are practically observed, but for the polymers synthesized in die presence of complexes with ZnClj the difference is marked (PR-Al and PR-Bl)... 

Takata, T., Kihara, N. and Furusho, Y. Polyrotaxanes and Polycatenanes Recent Advances in Syntheses and Applications of Polymers Comprising of Interlocked Structures. Vol. 171, pp. 1-75. 

In the last few years there have been new creative methods of preparation of novel hydrophilic polymers and hydrogels that may represent the future in drug delivery applications. The focus in these studies has been the development of polymeric structures with precise molecular architectures. Stupp et al. (1997) synthesized self-assembled triblock copolymer, nanostructures that may have very promising applications in controlled drug delivery. Novel biodegradable polymers, such as polyrotaxanes, have been developed that have particularly exciting molecular assemblies for drug delivery (Ooya and Yui, 1997). 

---