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Coleen Pugh, Kaitian Xu, Carin A. Helfer, Guoqiang Xu,Sagar S. Rane, And Wayne MatticeDepartment Of Polymer Science, Maurice Morton Institute of Polymer ScienceThe University Of Akron, Akron, Oh 44325-3909 USA
One of the primary driving forces for synthesizing rotaxanes and polyrotaxanes has been to study the predicted "interesting" solution and solid-state properties of molecules that are simply a physical entrapment of non-interacting cyclic and linear molecules. However, only heteropolyrotaxanes with extremely strong interactions between the two components are readily accessible. We are therefore developing a new route for preparing rotaxanes and polyrotaxanes, especially homopolyrotaxanes based on poly(ethylene oxide), that lack an enthalpic driving force for threading, and correlating our synthetic studies with simulations. This approach is based on the ability of amphiphilic macrocrown ethers, in which a hydrophobic tail is attached to a hydrophilic macrocrown ether, to form organized phases in solvents that selectively solvate the hydrophobic tails. When a hydrophilic polyether is then added to the organized solution, it is forced into the interior of the micelle where it threads the crown ethers. If a hydrophilic polyether that is normally not soluble in these solvents, is then added to the organized solution of the macrocrown ethers, it will be forced into their interior, thereby threading the crown ethers (Scheme 1). Threading will be driven by the high concentration of macrocycles and their optimum alignment, as well as by the enthalpic driving force for aggregation in a poor solvent. According to our simulations, the efficiency of threading increasing as the solvent quality for the macrocycle decreases, as well as if the macrocycles are organized at a surface. We initiated this study using"3,4-(42-crown-14)benzy dodecyl ether" (MC-12), which is a hydrophilic macrocrown ether with an average ring size of 42 atoms (pdi=1.02) functionalized with a hydrophobic 12-carbon tail.
In order to optimize both our isolation procedure and the MALDI-MS conditions for identifying (poly)rotaxanes, we are beginning with the synthesis of low molecular weight rotaxanes, and systematically increasing the molecular weight to polyrotaxanes. The MC-12 used for these studies was free of linear and cyclic oligomers. It forms organized solutions in aliphatic and aromatic hydrocarbon solvents if small amounts of water (eg. H2O]:[EO]<0.5 in toluene) are added. The optimum endcapping reaction for PEG-bisamine threads in the water-induced micellar solutions of MC-12 is the addition reaction with 2-p-[tris(p-tbutylphenyl)methyl]phenoxymethyl-4,4-dimethylazlactone. A threading experiment using 2 equivalents of MC-12 with a bisamine-terminated poly(THF) with nominally an average of three repeat units of THF produced a rotaxane of poly(THF) threaded through one MC-12 ring, as shown by the MALDI-MS spectrum in Figure 1. The spectrum shows four molecular weight distributions, with the components present in the highest concentrations separated by 72 mass units, which is the molecular weight of one THF repeat unit. The four distributions therefore correspond to oligo(THF) with two, three, four and five repeat units. The components within each of these four distributions are separated by 44 mass units, which is the molecular weight of one ethylene oxide repeat unit. The mass of each component corresponds to a rotaxane of oligo(THF) threaded with one MC-12 ring and endcapped with the t-butyltrityl azlactone. The rotaxane present in the highest concentration is an MC-12 ring with ten ethylene oxide repeat units and a thread with three THF repeat units.
论文来源：International Symposium on Polymer Chemistry,June,2004 |
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