Catenanes and rotaxanes consist of two or more separate components that are interlocked together and held in place by the mechanical or topological bond. Macromolecular versions of these species are especially intriguing as they offer new polymeric architectures to explore. Interlocked macromolecular structures such as poly[n]catenanes and poly[3]rotaxanes have the mechanical bond as an integral part of the polymer backbone which should impart unusual properties on these materials. For example, they have the ability to expand/contract without significantly altering bond or torsion angles and as such, the ability to control such motions could result in new macromolecular actuators. Furthermore, unusual viscoelastic properties, e.g. very large modulus loss, low activation energy for viscous flow and rapid stress relaxation, are all predicted for such structures. The Macromolecular, Supramolecular and Nanochemistry Program of the Chemistry Division of the National Science Foundation supports the research group of Prof. Rowan of Case Western Reserve University to develop novel synthetic protocols to allow access to these interesting macromolecules. The synthetic approach utilizes metal-ligand coordination as the thermodynamic driving force for the polymerization step and different covalent 'fixing' steps to access either the poly[n]catenanes or poly[3]rotaxanes.
The proposed research is truly a mix of different disciplines within the chemistry arena and the integrated research approach will provide students, at high-school, undergraduate and graduate levels, a broad research education base. The outreach activities are designed to increase the fraction of underrepresented minorities in science and engineering, integrate research and education, provide an exciting learning environment, and create teaching opportunities for the student researchers. Part of this is the expansion and growth of successful outreach programs initiated by the Rowan group, for example the "Natures Materials" program which is part of the Cleveland Museum of Natural History's MLK day Discovery Day.
The goal of this two year proposal was to develop a robust, high yielding synthetic methodology to the interlocked ring compounds, called catenanes, with the goal of laying the foundation to access polymeric versions of the interesting class of compound. Main-chain poly[n]catenanes (a linear series of interlocked rings) should exhibit high flexibility and segment mobility on account of its freely-rotating topological/mechanical bond. For example, a polycatenane chain can deform from a linear extended conformation into a highly coiled one (Figure 1) without any significant deformation of covalent bonds or bond angles. This unusual behavior is very attractive as it should lead to interesting polymer properties such as excellent energy dampening properties. However, the synthesis of poly[n]catenanes is extremely challenging and their confirmed synthesis has yet to be achieved. In this grant we employed metal ion templating to access a [3]catenane (3 interlocked rings) in high yield. To achieve this a linear metal-ion binding thread component, two macrocyclic metal-ion binding components and two metal ions were self-assembled to form a supramolecular structure called a [3]pseudorotaxane (that consists of the linear component threaded through two rings), which upon ring closing of the thread resulted in the desired [3]catenane (Figure 2). It was shown that the yield of the [3]catenane was dependent on the preorganization and conformational flexibility of the thread-like component and the size of substituents on the macrocyclic component. Through judicious design of both components the resulting reaction distribution can be altered to almost exclusively favor the formation of the [3]catenate (metal-ion containing [3]catenane). Such efficient formation of a [3]catenate opens the door to accessing larger interlocked architectures, an area we are actively pursuing. Furthermore, we also demonstrated that the metal ion template could be removed to yield the [3]catenanes. This grant funded three graduate students, two undergraduates and a high school student. It also helped to fund a number of outreach programs. One program, called "Natures Polymers", was held at the Cleveland Museum of Natural History as part of their MLK Day "Winter Discovery Program". Over the two years of the grant more than 1,800 visitors (of all ages) embarked on a tour that typically lasted 20 minutes and took them along five science stations, where graduate and undergraduate students took them through a series hands-on demonstrations (Figure 3) describing different aspects of how nature employs polymers. In addition, the CWRU researchers held an outreach program entitled "Science For Kids" program to bring our polymer road?show into classrooms. This program was focused on students in the 4th grade. Here over 100 students (split into 5 groups of ca. 20) participated in this program designed to stimulate interest in science and engineering careers. For each group the program starts with the PI describing to the students what a polymer by using a magic trick and showing the examples of the different items that are made from polymers that the students experience in everyday life (Figure 4). Next the students are further split into 4 subgroups of 5?6 and visit science stations manned by graduate students where they learned about making polymers, polymer recycling, and the concept of glass transition temperature using liquid nitrogen.
