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Project Goals:

1) Self-assembly of supramolecular pseudorotaxane polymers,

2) conversion to polymers with permanent mechanical (rotaxane) linkages, and

3) characterization of their physical properties.

Summary of Progress:

During the past year we have addressed goals #1 and #3 with the objective of attaining higher degrees of polymerization than in the past.

We again focused on use of 4,4’-bipyridinium salts (paraquats) as guest species and a bis(m-phenylene)-32-crown-10 derived cryptand as the host.  The alcohol functionalized cryptand was coupled with terephthalic acid to form a homoditopic host, a biscryptand.  The guest was also homoditopic, comprised of two N-methyl-4,4’-bipyridium units linked with a decamethylene spacer and hexafluorophosphate anions.  In a model system involving the biscryptand and N,N’-dimethyl-4,4’-bipyridinium bis(hexaflurophosphate) the average association constant was determined by ¹H NMR to be 5.4 x 10³ M^-1.  Based on the relationship between the degree of polymerization and K_a, this value is sufficient to afford a degree of polymerization of 73 between the two complementary homoditopic monomers at 1 M concentrations. In experiments with the two monomers degrees of polymerization estimated by ¹H NMR are indeed high, e. g., n = 50 ± 11 at 0.25 M (theoretical prediction n = 36). More importantly above this concentration a log viscosity vs. log concentration plot had a slope of 2.66; this is the highest slope reported for any pseudorotaxane-based supramolecular polymer.  Furthermore, long, regular, flexible fibers could be pulled from concentrated solutions, definitive proof of the formation of polymeric species sufficiently long enough to entangle. A flexible, creasible film with a glass transition temperature of 57^oC was also formed by solution casting; the yellow color was consistent with the charge transfer interactions in the cryptand-paraquat pseudorotaxane complex.

We have prepared several paraquats with relatively long spacers linked to bulky stoppers and fitted with terminal functional groups as building blocks for semirotaxanes, which will be converted to “slip-link polymers” via reactions of the functional groups on the host and end of the guest with suitable difunctional linkers.

We have also pursued new “stoppers” or blocking groups to be used in preparing the corresponding rotaxane systems (goal #2) and related slip link polymers. Starting with “butylated hydroxy toluene”, 3,5-di(t-butyl)-p-cresol, we prepared the formyl derivative, 3,5-di(t-butyl)-4-hydroxybenzaldehyde.  The phenolic group can be functionalized via Williamson ether syntheses to introduce bromo-, hydroxy- or azido- moieties as dual role species: blocking groups with polymerizable functionalities.  The formyl group will be converted to the diazo moiety and used to esterify psuedorotaxanes possessing terminal carboxy groups.

Desymmetrization reactions of crown ether diesters have continued, because we have high yielding routes to these compounds, but often require monofunctional analogs. We prepared dialdehydes via reduction followed by PCC oxidation.  Then Cannizzaro reactions with Ba(OH)₂ lead to the corresponding carboxy-alcohols, which can ultimately be converted to the corresponding methyl-alcohols, overall resulting in a monofunctional crown ether from a difunctional one.