Polyrotaxane

As oligomeric or polymeric species of rotaxanes, polyrotaxanes are also capable of converting energy input to molecular movements because the ring motions can be controlled by external stimulus.

Also, rings are capable of shuttling along and rotating around the axes freely, which leads to a huge amount of internal degree of freedom of polyrotaxanes.

Due to this topologically interlocked structure, polyrotaxanes have many different mechanical, electrical, and optical properties compared to conventional polymers.

In slide-ring materials, crosslinks of rings can pass along the axes freely to equalize the tension of the threading polymer networks, which is similar to pulleys.

Because the shape and location of the assembly are capable of showing different responses to changes in temperature, pH or other environment conditions, polyrotaxanes have many distinctive properties.

This method requires that the monomeric rotaxane units are stable in the solvent and have active groups that can be polymerized, which means the rings will not dethread from the main chain.

Designed monomers are polymerized to obtain special linear polymers with precursors of cyclic compounds.

The disadvantage of this method is the complex chemistry needed in the process of design and synthesis of the special linear polymers with precursors and the transitions to polyrotaxanes, e.g. the selective chemical bond cleavage.

Kinetic limitations due to the low concentration of chain ends and entropic effects also need further consideration.

Compared with synthesis route 1, rings are a major constituent of the system instead of the rotaxanes, so the high dilution conditions are not required for this methods.

An example of the "statistical approach" is that a polyrotaxane was synthesized through polymerizing the rotaxane monomer that was assembled by oligomeric ethylene glycols (string) and crown ethers (ring) and naphthalene-1,5-di-isocyanate (stopper), which involves the threading equilibrium in the chain-ring system.

The models confirm that the distance of form zig-zag structure of repeat units corresponds to the size of cavity in α-cyclodextrins.

This is a classical example of "template threadings" which also explains why poly(ethylene glycol)s are not able to form rotaxanes with β-cyclodextrin.

In this method, metal ions are employed as the synthesis templates to determine the coordinating sites of rotaxane structures.

(1) Ring-threading of performed graft polymer[15] (2) Ring-grafting[16] (3) Rotaxane-grafting[17] (4) Polymerization of macromonomer with rings (5) Polymerization of rotaxane-monomer (6) Chemical conversion Similarly, the positions of chain and rings can be switched, which results in corresponding side-chain polyrotaxanes.

Also, the rings are capable of rotating on or shuttling around the axles, resulting in the large amount of freedom of polyrotaxanes.

However, specific salts, changes of pH condition or temperature that can disturb or interrupt the interactions between ring-ring, ring-backbone, or backbone-backbone will destroy the structural integrity of polyrotaxanes.

For example, dimethylformamide or dimethyl sulfoxide is able to interrupt the hydrogen bonding between cyclodextrins in the cyclodextrin-based polyrotaxanes.

As the stoppers cut from the chain, the rings will dethread from the axles, which leads to the dissociation of the polyrotaxanes.

In addition, the complexation process is exothermic in thermodynamic tests, which is also corresponding with the existence of hydrogen bonding.

One of the properties of polytorotaxanes involves the photoelectronic response when introducing photoactive or electrionic-active units into the mechanically interlocked structures.

[19] Amplification of a fluorescence chemosensory can be achieved by using polyrotaxane structure, which enhances the energy migration in the polymer.

However, in the slide-ring materials, the polymer chain are able to pass through the figure-of-eight crosslinks which is like pulleys, and equalize the tension of network.

There are two main advantages for polyrotaxanes applied to drug/gene delivery: Because the mechanically interlocked structures are maintained by bulky stoppers at the ends of the strings, if the bulky stoppers are removed, such as removed by a chemical stimulus, rings dethread from the axes.

As the network is further swollen in the water-based environment, part of the carrier will be dissolved gradually, so the capsulated drug or gene can be released from the hydrogels over a long period of time.