Chain threading

Molecules can link together in one dimension to form chains (threads), or in two dimensions to form membranes, or in three dimensions to form blocks. Hardness has meaning only for the last case. Long chains of molecules constitute polymers and will be discussed in the next chapter. Small molecules formed of a few atoms are gases or liquids at room temperature, so hardness has no meaning for them. 

Pseudopolyrotoxanes and polyrotoxanes are polymers composed of a linear polymer chain threaded through one or more cyclic species [Gihson and Mahan, 2000 Raymo and Stoddart, 1999]. Synthesis of pseudopolyrotoxanes and polyrotoxanes involves polymerization of monomer(s) in the presence of the cyclic species. Cyclodextrins, crown ethers, paraquots, and other macrocyclics are used as the cyclic component. Both step and chain polymerizations have heen successfully used. 

Such rotaxanes incorporate a macrocyclic structure (representative of a bead) on an acyclic chain (thread), with bulky stopper groups at their ends to prevent dethreading. The position of the bead on the thread, and hence representation of information, is controlled by the incorporation of stations on the thread upon which the bead preferentially resides. Requirements of a successful system are that the bead can be controllably shuttled between stations by an external stimulus and that the position of the bead can be subsequently read out. 

Zilkha s group also reported a series of papers describing the synthesis of mechanically crosslinked polymer through vinyl polymerization [223-225]. They employed the radical copolymerization of a 32-membered crown ether derivatives having one polymerizable double bond with styrene or methyl methacrylate (Scheme 49). Sufficient chain threading repeatedly took place... 

PPR hydrogels were obtained for the first time by Harada and coworkers with PEG chains threading a-CD units. Photoresponsive hydrogels were also created from PEG-a-CD systems by addition of l- p-(phenylazo) benzyljpyridinium bromide as competive guest for a-CD. Photoreversible control of sol-gel transition was achieved through trans-cis photoisomerization. (Fig. 17). A light controlled fluorescent PPR... 

The complexation studies described in the previous section clearly indicate that a-CD is threaded by the aliphatic chain of compounds 2" and 3". Since the (ferrocenylmethyl)dimethylammonium group is very bulky, the dissociation of these a-CD complexes takes place by unthreading at the other end of the alkyl chain. We thought that, if we could attach a reactive functional group at this end, it would be possible to trap the chain-threaded CD by reaction of this end with an appropriately bulky capping group. Therefore, we synthesized compounds 4+ and S" " in which the aliphatic chain is terminated in a carboxylic acid group. 

The stability of polyrotaxanes was accounted for by the macrocycle tendency to assume such a conformation which would assure the maximum dipole-dipole interaction between oxyethylene linls of the chain and ring. This vms confirmed by kinetic studies which demonstrated the polyrotaxane dissodation activaticm energy to amount to 15.9 kcal/mole and the ring-on-chain threading i.e.pofyrotaxane formation, activation, energy to 3.4 kcal/mole. 

That a company like Acme will have multiple partnership options, as listed in Chapter 17, is not unusual. When a company reaches a certain size, it will face complexity in deciding where to focus its improvement efforts. This chapter points to ways to "partition" the various supply chain "threads" into manageable parts. We include this discussion in our partnership task because we believe it is a prerequisite to devising sound partnership strategies. 

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