In this project funded by the Macromolecular, Supramolecular and Nanochemistry Program of the Chemistry Division, Harry Gibson of the Virginia Polytechnic Institute and State University will synthesize and study macromolecules in which branch points, crosslinks or repeat units are mechanical connections in the form of rotaxanes or catenanes. A variety of polymerization techniques (e. g., anionic, atom transfer radical, nitroxide mediated, classical free radical and ring opening metathesis) will be employed to prepare well-defined macromolecules with terminal, central or multiple host and guest functionalities. These polymers will then be self-assembled into a variety of topologies (star, H-branched, poly[2]rotaxane, bottle brush, etc.) in both homopolymeric and copolymeric forms, all involving rotaxane linkages, some topologies not readily accessible otherwise. These supramolecular rotaxane polymers are expected to exhibit enhanced properties relative to covalent polymers because of the restoring force between the cyclic and linear components present in the rotaxane units. Morphological, rheological, thermal and mechanical studies will elucidate the novel properties and responses of these systems that distinguish them from normal covalently linked macromolecules, providing new insights into the ultimate properties that are achievable through supramolecular science. The broader impacts involve training undergraduate and graduate students as well as postdoctoral researchers, an ongoing commitment to training students from groups underrepresented in the chemical sciences, and disseminating research results through publications in journals and presentations at conferences.
This work will enhance our fundamental understanding about polymers linked mechanically via interlocking loops and chains and their physical behavior. Polymer chains are the fundamental units of many plastic materials, and the development of new types of polymers could lead to new applications in biomedical materials, coatings, and composites.
Supramolecular science is based on non-covalent interactions among molecules; these interactions include hydrogen bonding, stacking of complimentary electron rich and electron poor aromatic units and dipole-dipole interactions. In general, greater control results from stronger non-covalent interactions. The Intellectual Merit of this project derives from a host-guest system that displays very strong binding, whose utility was enabled by developing efficient methods for attachment of various functional groups. The host is a cryptand, a bicyclic molecule with a built-in cavity that accepts the guest, forming a pseudorotaxane, a mechanically threaded structure (see Figure 1). Moreover, very effective template methods were developed for nearly quantitative syntheses of crown ethers that serve as starting materials for the cryptands and are host molecules themselves. In concert with this, very effective template syntheses (~90% yield) of the cryptands were developed and generalized (e. g., see Figure 2). Then using these host-guest systems a variety of polymeric (macromolecular) architectures were constructed starting from pre-formed polymers that contain a host species that complexes a guest fitted with initiator species for growth of a second polymer molecule through the cavity of the host, forming a mechanical linkage between the two polymers, resulting in new types of copolymers (see Figure 3). Conventional covalently linked block copolymers are commercially used to provide a unique combination of properties, such as rigidity with impact resistance exemplified by the system used in "plastic" pickup truck caps. It is anticipated that these new types of copolymers will not only behave similarly, but also possess additional useful properties because of the flexible nature of their mechanical linkages, such as response to external stimuli and damage recovery. Moreover, for the first time starting from small molecules containing the host-guest units a high molecular weight (containing large numbers of repeat units) supramolecular pseudorotaxane-type polymer self-assembled in solution (see Figure 4); such polymers are widely expected to display extreme sensitivity to external stimuli, such as temperature, pH, shear, etc., and provide applications in drug and gene delivery, improved processability and self-repair. In terms of Broader Impacts, in addition to the introduction of new paradigms for manipulation of polymer architecture and properties noted above, this grant partially supported research by a total of 5 PhD students (1 graduated, 3 pending, 1 in progress), 1 postdoctoral fellow and 3 undergraduates; additionally a visiting graduate student from Japan spent 6 months working on the project. The interdisciplinary nature of this work provided valuable training in terms of organic chemistry, supramolecular chemistry and polymer science to this diverse group of students (including 3 females). During this time 16 publications resulted. The impressive results of these efforts were incorporated into a graduate course taught by the PI.
